32. Estimation of Iron and Alumina in Mineral Phosphates.—When mineral phosphates are to be used for the manufacture of superphosphates by treatment with sulfuric acid their content of iron and alumina becomes a matter of importance. By reason of the poor drying qualities of the sulfates of these bases their presence in any considerable excess of a few per cent becomes exceedingly objectionable. The accurate estimation of these ingredients is not only then a matter of scientific interest but one of great commercial significance to the manufacturer.

The conventional methods so long in use depending on the precipitation of the iron and alumina as phosphates in the presence of acetic acid have been proved to be somewhat unreliable. Not only does the acetic acid fail to prevent the precipitation of some of the lime, but it also dissolves more or less of the iron and aluminum phosphates. The solution of the precipitate and its reprecipitation by the addition of ammonia, may free the second precipitate from lime, but it increases the error due to the solubility of the aluminum salt. The methods recently introduced for the estimation of iron and alumina in presence of excess of lime and phosphoric acid are not entirely satisfactory, but are the best which can now be offered.

33. The Acetate Method.—The principle of this process is based on the fact that in a solution containing iron, alumina, lime, and phosphoric acid the iron and aluminum phosphates can be thrown down in a slightly acid solution by ammonium acetate while the calcium phosphate remains in solution. The acidity in the older methods is due to acetic and can be secured by making the solution slightly alkaline with ammonia and adding acetic to slight acidity. One of the best methods of conducting the operation is that of C. Glaser[23]. Glaser’s modification of the older processes is based on the assumption that at 70° the aluminum phosphate is quantitatively precipitated by ammonium acetate in a dilute hydrochloric acid solution and that the mixed precipitates of iron and aluminum phosphates obtained at this temperature are free of lime. The operation is conducted in the following manner:

The hydrochloric acid solution of the phosphate must contain no free chlorin and is treated with a few drops of a methyl orange solution. Ammonia is added until nearly neutral, but the acid reaction is retained as shown by the indicator. A few cubic centimeters of ammonium acetate are added, which produce a yellow coloration of the liquid and also a complete precipitation of the iron and aluminum phosphates when warmed to 70°. At this temperature the precipitation of any calcium phosphate is avoided. A small quantity of the lime may be carried down mechanically and therefore the precipitate should be dissolved in hydrochloric acid and the precipitation again made as above after the addition of some sodium phosphate. If the original solution contain any free chlorin, as may be the case when aqua regia is employed as solvent, before beginning the separation, ammonia should be added in slight excess and the acidity restored by hydrochloric after adding the indicator. In washing the precipitates, water of not over 70° must be used. As has been shown by Hess in the work cited in the next paragraph, the statement of C. Glaser to the effect that the precipitates obtained as above are free of lime has not been proved to be strictly correct. The process, however, is a distinct improvement over the older methods and forms the basis of the amended process given below, which appears to be sufficiently accurate to entitle the acetate method to favorable consideration.

34. Method of Hess.—Hess has lately made a thorough investigation of the standard methods of determining the iron and aluminum oxids in the presence of phosphoric acid and has shown that the assumption that the composition of the precipitate is represented by the formula Al₂(PO₄)₂ + Fe₂(PO₄)₂ is erroneous[24].

In the washing of the precipitated iron and aluminum phosphates there is a progressive decomposition of the compound with the production of a basic salt. The composition of the precipitate at the end is dependent chiefly upon the way in which the washing takes place. It is quite difficult to always secure a washing in exactly the same way and the final composition of the precipitate varies with almost every determination. It is not, therefore, an accurate proceeding to take half the weight of the precipitate as phosphoric acid or as iron oxid and alumina. In every case it is necessary to dissolve the precipitate and determine the phosphoric acid in the regular way. Hess proposes the following method for carrying out the acetate process of separation:

The mineral phosphate should be dissolved in hydrochloric acid and the solution made up to such a volume as shall contain in each fifty cubic centimeters, one gram of the original substance. This quantity of the solution is diluted with two or three times its volume of water to which a drop of methyl orange solution (1-100) is added, and ammonia added with constant stirring until the solution is just colored and still reacts slightly acid. Without taking any account of the precipitate which is produced by this approximate neutralization of the solution, there are added fifty cubic centimeters of acid ammonium acetate which in one liter contains 250 grams of commercial ammonium acetate. The acidity of the solution is due to an excess of acetic in the commercial salt. The temperature is then carried to 70° and the precipitate produced immediately separated by filtration, washed four times with water below 70°, and again dissolved in dilute hydrochloric acid. The dissolved precipitate is treated with ten cubic centimeters of a ten per cent ammonium phosphate solution and again almost neutralized as described above, twenty-five cubic centimeters of the ammonium acetate solution added and warmed to 70°.

The precipitate obtained is once more dissolved and precipitated as above described, and is then collected upon a filter, washed, ignited, and weighed. The residue after ignition is dissolved in the crucible by heating with a little concentrated hydrochloric acid, and washed into a beaker. Any silicic acid present is separated by filtration, ignited, and weighed, and subtracted from the total weight of the precipitate. To the filtrate is added ammonia to diminish the acidity, but not sufficient to produce a precipitate and the clear solution is treated with thirty cubic centimeters of the ordinary ammoniacal citrate solution and fifteen cubic centimeters of magnesium mixture, and the precipitation of the ammonium magnesium phosphate hastened by stirring with a glass rod.

It is advisable to always make the filtrate from the third precipitation slightly ammoniacal and to boil it for a long time. If the operation have been carried on correctly, there occurs only a slight precipitate of Ca₃P₂O₈ amounting only to a few milligrams. In some cases it may be necessary to dissolve the precipitate and reprecipitate the iron and aluminum phosphates a fourth time.

The whole time required for the triple precipitation, according to Hess, if all the operations be properly conducted, is from three to four hours. It is therefore possible by this variation of the acetate method to secure a determination of the iron and alumina as phosphates in the same time which is occupied by the Glaser-Jones method when the separation of lime is taken into account.

If the solution of the mineral phosphate employed contain any notable quantity of organic material, it must be destroyed by boiling with bromin or some other oxidation agent, before the precipitation by the acetate method is commenced.

The presence of silicic acid need not be taken into special consideration since this can be detected and determined in the phosphate precipitates after they have been ignited and weighed. While the determinations of the phosphoric acid in Hess’ method were made by precipitation in the presence of citrate, he found that they agree perfectly with the previous precipitations with molybdic solution.

35. Method of Glaser.—The principle on which this method rests depends on the preliminary removal of the lime by conversion into calcium sulfate and its precipitation in the presence of strong alcohol.[25] It is conducted as follows:

Five grams of the phosphate are dissolved in a mixture of twenty-five cubic centimeters of nitric acid of 1.2 specific gravity and about 12.5 cubic centimeters of hydrochloric acid of 1.12 specific gravity, and made up to a volume of half a liter, and filtered. One hundred cubic centimeters of the filtrate, equivalent to one gram of the substance, are placed in a quarter liter flask and twenty-five cubic centimeters of sulfuric acid of 1.84 specific gravity added. The flask is allowed to stand for about five minutes and meanwhile shaken a few times. About 100 cubic centimeters of alcohol of ninety-five per cent are then added and the flask filled with alcohol to the mark and well shaken. A certain degree of concentration takes place and this is compensated for by lifting the stopper and filling again with alcohol to the mark and shaking a second time. After allowing to stand for half an hour the contents of the flask are filtered, 100 cubic centimeters of the filtrate being equal to four-tenths gram of the substance. This volume, filtered, is evaporated in a platinum dish until the alcohol is driven off. The alcohol-free residue is heated to boiling in a beaker with about fifty cubic centimeters of water. Ammonia is added to alkaline reaction, but in order to avoid strong effervescence it is not added during the boiling. The excess of ammonia is evaporated, the flask allowed to cool, the contents filtered, precipitate and filter washed with warm water, ignited, and the phosphates of iron and alumina weighed. Half of the weight of the precipitate represents the weight of Fe₂O₃ + Al₂O₃. The estimation, as before indicated, should be carried on without delay, the whole time required not exceeding from one and a half to two hours.

36. Jones’ Variation.—The method of Glaser described above, as practiced by the German chemists, has been found by Jones to be inaccurate on account of the alcohol not being added in sufficient quantity in the precipitation of calcium sulfate and for the additional reason that the amount of sulfuric acid added is more than is actually necessary[26]. Jones modifies the method as follows: Ten grams of the material are dissolved in nitro-hydrochloric acid and the solution made up to 500 cubic centimeters and filtered. Fifty cubic centimeters of this solution, representing one gram, are evaporated to twenty-five cubic centimeters and, while still hot, ten cubic centimeters of dilute sulfuric acid (one to five) added. The mixture is then well stirred and cooled. One hundred and fifty cubic centimeters of ninety-five per cent alcohol are next added and after stirring, the solution is allowed to stand three hours. The calcium sulfate is collected on a filter, washed with alcohol, and the filtrate and washings collected in an erlenmeyer. The washing is completed when the last ten drops, after dilution with an equal volume of water, are not colored with a drop of methyl orange.

The moist calcium sulfate is transferred to a platinum crucible, the filter placed on it, the alcohol burned off, the filter incinerated, and the calcium sulfate ignited and weighed. The contents of the flask are heated to expel the alcohol, the residue washed into a beaker, made slightly alkaline with ammonia, and again heated till all the ammonia is driven off. This treatment is necessary to prevent the precipitate from being contaminated with magnesia. The precipitate is collected on a filter, washed four times with hot water, or water containing ammonium nitrate, dried, ignited, and weighed. One-half of the weight of the precipitate represents the weight of the ferric and aluminic oxids.

37. Estimation of Iron and Alumina in Phosphates by Crispo’s Method.—The phosphate of ferric iron is subject to a slight decomposition in presence of both hot and cold water with a tendency to the production of basic compounds. It is soluble to a slight extent in hot and cold acetic acid, almost insoluble in ammonium acetate, and quite insoluble in ammonium chlorid and nitrate. Aluminum phosphate is likewise soluble, to a slight degree, in acetic acid and ammonium acetate, and insoluble in ammonium chlorid and nitrate. The method of Crispo for the separation of iron and alumina in phosphates is based on the above properties.[27] Five grams of the mineral phosphate are dissolved in fifty cubic centimeters of aqua regia, composed of forty cubic centimeters of hydrochloric acid of 1.10, and ten of nitric acid of 1.20 specific gravity, and this solution is diluted to half a liter. To fifty cubic centimeters of the filtered solution are added two of ammonia (0.96) and fifty of a half saturated solution of ammonium chlorid, and the whole boiled. The liquid should remain clear, but if it become cloudy add a little dilute nitric acid, drop by drop, until the turbidity is removed, and then ten cubic centimeters of a saturated solution of ammonium acetate, and boil for three minutes, cool, and filter. The precipitate is washed twice with a ten per cent solution of ammonium chlorid and redissolved with two cubic centimeters of nitric acid, and the filter washed with hot water. The phosphoric acid is separated by forty cubic centimeters of molybdate solution, and the precipitate washed three or four times with a one per cent nitric acid solution.

To the filtrate are added fifty cubic centimeters of a one-half saturated ammonium chlorid solution, ammonia is added in slight excess to produce precipitation and the mixture boiled for a few minutes. After filtering, the precipitate is washed with hot water three or four times, dissolved in two cubic centimeters of nitric acid, and the filter washed with hot water. Again, fifty cubic centimeters of half saturated ammonium chlorid are added and the precipitate thrown down once more by ammonia in slight excess. The precipitate is washed with hot water and finally ignited and weighed as iron and aluminum oxids.

According to Crispo, the original Glaser method, with its various modifications, is not to be considered reliable, and the choice lies between the molybdic method as usually practiced, and his own for the accurate estimation of iron and alumina. Manganese disturbs the accuracy of the results unless the directions given are carefully followed. Manganese phosphate is soluble at all temperatures below fifty. If then the mixture of the phosphates be allowed to cool before filtering, the iron and aluminum salts are not contaminated with manganese. This method of Crispo is somewhat tedious, but it is claimed that these variations of the molybdic method render it exact in respect of the determination of iron and alumina.

38. Method Employed in Geological Survey.—Chatard gives the following directions for conducting the Glaser-Jones process[28]: The distillation flask containing the alcoholic filtrate is connected with its condenser and heated on a water-bath until no more alcohol comes over. This distillate, if mixed with a little sodium carbonate and redistilled over quicklime, can be used over and over again, so that the expense for alcohol is really very slight, while in the use of the Glaser method, with its large amount of sulfuric acid, all the alcohol is lost.

When the distillation is ended the residue in the flask is washed into a platinum dish and evaporated to a small bulk on the water-bath. The dark brown color produced is due to the presence of organic matter and this must be destroyed, as it prevents the complete precipitation of the phosphate in the subsequent operation.

The organic matter is best destroyed by removing the dish from the bath, adding a small quantity of pure sodium nitrate, and heating very carefully over the naked flame, keeping the dish well covered with a watch-glass to avoid spattering. The mass fuses to a colorless, viscous liquid, becoming glassy when cooled and is readily soluble in a hot very dilute solution of nitric acid. The solution transferred to a beaker is made distinctly alkaline with ammonia and carefully neutralized with acetic acid, diluted with hot water, boiled, and the precipitate allowed to settle, after which it is separated by filtration.

After the precipitate has been completely transferred to the filter, the washing is completed with a dilute solution of ammonium nitrate. The precipitate is dried, ignited, cooled, and weighed.

The determinations should be made in pairs, one portion being used for the estimation of the phosphoric acid by fusing with a little sodium carbonate, and the other, after fusion with sodium carbonate, is dissolved with sulfuric acid and the iron reduced and titrated with potassium permanganate solution. The filtrate from the iron and alumina determination is evaporated to a small bulk, made strongly ammoniacal and allowed to stand for some time when the magnesia present separates as ammonium magnesium phosphate which is determined in the usual way.

If, during the evaporation of the filtrate, any flocculent matter separate, it should be removed by filtration and examined before precipitating the magnesia.

39. Variation of Marioni and Fasselli.—Glaser’s method has been shown to be subject to errors by Marioni and Fasselli[29] in the following respects:

1. The precipitation of a small quantity of calcium phosphate with the ferric and aluminum phosphates.

2. The possible precipitation of basic phosphates if all the iron and alumina are not combined with phosphoric acid in the mineral.

3. The partial solubility of ferric and aluminum phosphates in dilute acetic acid.

4. The decomposition of ferric orthophosphate into soluble acid phosphate and insoluble basic salt by boiling.

To avoid these errors the following procedure is proposed: From one to five grams of the phosphate are boiled in a flask for ten minutes with fifteen cubic centimeters of strong hydrochloric acid, and afterwards diluted with a double volume of water. A few crystals of potassium chlorate are added, and several drops of nitric acid, and the liquid boiled to expel chlorin. It is then filtered and washed until the volume of the filtered liquid amounts to 150 cubic centimeters. After cooling, a half gram of ammonium phosphate in solution is added, and two cubic centimeters of glacial acetic acid, followed by dilute ammonia, drop by drop, until a slight precipitate persists on stirring. Again the same quantity of acetic acid is added as above, well shaken, and left for two hours. The precipitate is collected on a filter and washed with a one per cent ammonium phosphate solution. The precipitate is dissolved by a minimum quantity of hydrochloric acid and the solution collected in the same vessel in which the precipitation took place. A second precipitation is conducted just as described above. The precipitate is washed as above described and ignited at a dull red heat. Half the weight obtained represents the ferric oxid and alumina.

40. Method of Ogilvie.—For the separation of alumina from phosphoric acid Ogilvie recommends that the filtrate from the phosphomolybdate precipitate be neutralized with ammonia, the precipitate thus formed redissolved in nitric acid, again precipitated with ammonia, filtered, ignited, and weighed as aluminum oxid.[30] If iron be present it will, of course, appear in the product. For use in the examination of mineral phosphates the method can not have a wide application without amendment.

41. Method of Krug and McElroy.—Krug and McElroy show that when sufficient alcohol is added to precipitate all of the calcium sulfate in the Glaser method, it will also cause a precipitation of a considerable quantity of iron, by means of which the calcium sulfate will be colored.[31] The presence of potassium and ammonium salts also affects very notably the precipitation of calcium. The method employed by them, in order to avoid these sources of error, is as follows:

One hundred cubic centimeters, equivalent to one gram of the substance, in a nitric acid solution, are placed in a half liter flask and a solution of ammonium molybdate added until all the phosphoric acid has been precipitated. The addition of ammonium nitrate will hasten the separation of the ammonium phosphomolybdate. The liquid should be allowed to stand for twelve hours. The flask is then filled to the mark, the contents well shaken, filtered through a dry filter, and duplicate samples of 200 cubic centimeters each of the filtrate taken for analysis.

A small quantity of ammonium nitrate is dissolved in the liquid, and ammonia cautiously added, keeping the solution as cool as possible. The iron and alumina are precipitated as hydroxids. The mixed hydroxids are collected on a filter, washed with water, the filtrate and washings being collected in a beaker. The precipitate should be dissolved with a small quantity of a solution of ammonium nitrate and nitric acid, again precipitated with ammonia, filtered, washed, ignited, and weighed. This treatment is for the purpose of excluding all possibility of error from the presence of molybdic anhydrid. After weighing, the mixed oxids should be fused with sodium bisulfate, the magma dissolved in water, and the iron determined volumetrically with potassium permanganate after reduction to the ferrous state.

McElroy has further shown by experiments in this laboratory that even the molybdate method of separating the iron and alumina from phosphoric acid with the improvements as first suggested by Krug and himself, may not always give reliable results.[32] In a solution containing ferrous iron equivalent to 56.4 milligrams of ferric oxid, were placed enough of a solution of sodium phosphate to correspond to 100 milligrams of phosphorus pentoxid. The precipitate was dissolved by adding nitric acid, oxidized with bromin water, and the phosphoric acid thrown out with ammonium molybdate. The precipitate was washed with weak nitric acid and the combined filtrate and washings neutralized with ammonia. The resultant precipitate was dissolved in a solution of ammonium nitrate and nitric acid, filtered, and again precipitated with ammonia. In two instances the quantities of material recovered after ignition were 56.9 and 57.3 milligrams, respectively, instead of the theoretical amount, viz., 56.4 milligrams.

When the work was repeated after the addition of 400 milligrams of calcium oxid the weight of the precipitate recovered was 62.3 and 63.1 milligrams in duplicate determinations. Similar determinations were made with a known weight, viz., 35.6 milligrams of alumina. The treatment of the mixture was precisely as indicated above for iron. The quantity of alumina finally obtained was 28.9 and 29.3 milligrams, respectively, in duplicate determinations. When the lime was added, however, the weights of alumina, recovered, fell to 19.8 and 20.6 milligrams, respectively. These results show that the molybdate method for the separation of iron and alumina in the presence of a large excess of lime and phosphoric acid is subject to widely varying results, but that the error due to the excess of iron in the weighed product is partly corrected by the one due to deficiency of alumina.

42. Method of Wyatt.—A method largely used in this country, both in private laboratories and by fertilizer factories, for determining iron and alumina, is described by Wyatt[33]. It is claimed for this method that, while it may not be strictly accurate, yet it is rapid and easy, and in the hands of trained analysts yields concordant results. Fifty cubic centimeters of the first solution of the sample in aqua regia, or an amount thereof equivalent to one gram of the phosphate, in a beaker, are rendered alkaline by ammonia. The resulting precipitate is first redissolved by hydrochloric acid, and a slight alkalinity is again produced with ammonia. Fifty cubic centimeters of strong acetic acid are next added, the mixture stirred and placed in a cool place and left until cold. The precipitate is then separated by filtration and washed twice with boiling water. The vessel holding the filtrate is replaced by the beaker in which the precipitation was made. The precipitate is dissolved in a little fifty per cent hot hydrochloric acid and the filter washed with hot water. After rendering slightly alkaline, as in the first instance, the treatment with acetic acid is repeated as described. The precipitate is washed this time, twice with cold water containing a little acetic acid and three times with hot water. The precipitate is dried, ignited, and weighed as iron and aluminum phosphate. Half of this weight may be taken to represent the quantity of iron and aluminum oxids.

To separate the iron and alumina the precipitate just described is dissolved in hot hydrochloric acid, filtered into a 100 cubic centimeter flask, and made up to the mark by hot wash-water.

The phosphoric acid is determined in one-half of the filtrate and in the remaining half the iron is reduced with zinc and determined with potassium permanganate in the usual way. The phosphoric acid and iron having been thus determined the alumina is estimated by difference. The chief objection to this process is in the excessive quantity of acetic acid used and the danger of solution of the precipitated phosphates caused thereby.

43. Estimation of the Lime and Magnesia.—The filtrate and washings from the first precipitation, (paragraph 41,) of iron and alumina in the method of Krug and McElroy, above described, are collected and sufficient ammonium oxalate is added to precipitate the calcium. The precipitated calcium is very fine and should be collected on a gooch, under pressure. The filtrate and washings from the calcium precipitate are again collected, and a solution of sodium phosphate added to precipitate the magnesia. The solution must be kept cool and slightly alkaline with ammonia during the above operations in order to prevent the separation of molybdic anhydrid.

44. Estimation of Sulfuric Acid.—As a rule, sulfates are not abundant in mineral phosphates. In case the samples are pyritiferous, however, considerable quantities of sulfuric acid may be found after treatment with aqua regia.

The acid is precipitated with barium chlorid, in the usual way, in an aliquot portion of the filtrate first obtained. The precipitate of barium sulfate is washed with hot water until clean, dried, ignited, and weighed. If the portion of the filtrate taken represent half a gram of the original material then the weight of barium sulfate obtained multiplied by 0.6858 will give the quantity of sulfur trioxid in one gram.

45. Estimation of Fluorin by the Method of Berzelius as Modified by Chatard.—The method of estimating fluorin as proposed by Berzelius, has been found quite satisfactory in the laboratory of the Geological Survey, with the modifications given below.[34]

Two grams of the phosphate are intimately mixed in a large platinum crucible with three grams of precipitated silica and twelve grams of pure sodium carbonate, and the mixture is gradually brought to clear fusion over the blast-lamp. When the fusion is complete the melt is spread over the walls of the crucible, which is then rapidly cooled (preferably by a blast of air). If this have been properly done, the mass separates easily from the crucible, and the subsequent leaching is hastened. The mass, detached from the crucible, is put into a platinum dish into which whatever remains adhering to the crucible, or its lid, is also washed with hot water. A reasonable amount of hot water is now put into the dish, which is covered and digested on the water-bath until the mass is thoroughly disintegrated. To hasten this, the supernatant liquid may, after awhile, be poured off, the residue being washed into a small porcelain mortar, ground up, returned to the dish and boiled with fresh water until no hard grains are left. The total liquid is then filtered, and the residue is washed with hot water. The filtrate (which should amount to about half a liter) is nearly neutralized with nitric acid (methyl orange being used as indicator), some pure sodium bicarbonate is at once added, and the solution (in a platinum dish, if one large enough is at disposal, otherwise in a beaker) is placed on the water-bath, when it speedily becomes turbid through separation of silica. As soon as the solution is warm it is removed from the bath, stirred, allowed to stand for two or three hours, and then filtered by means of the filter-pump and washed with cold water.

The filtrate is concentrated to about a quarter of a liter and nearly neutralized, as before, some sodium carbonate is added, and the phosphoric acid is precipitated with silver nitrate in excess. The precipitate is separated by filtration and washed with hot water, and the excess of silver in the filtrate is removed with sodium chlorid.

The filtrate from the silver chlorid (after addition of some sodium bicarbonate) is evaporated to its crystallizing point, then cooled and diluted with cold water; still more sodium bicarbonate is added, and the whole is allowed to stand, when additional silica will separate, and this is to be removed by filtration.

This final solution is nearly neutralized, as before; a little sodium carbonate solution is added; it is heated to boiling and an excess of solution of calcium chlorid is added. The precipitate of calcium fluorid and carbonate must be boiled for a few minutes, when it can be easily filtered and washed with hot water. The precipitate is then washed from the filter into a small platinum dish and evaporated to dryness, while the filter, after being partially dried and used to wipe off any particles of the precipitate adhering to the dish in which it was formed, is burned, and the ash is added to the main precipitate. This, when dry, is ignited, and allowed to cool; dilute acetic acid is added in excess, and the whole is evaporated to dryness, being kept on the water-bath until all odor of acetic acid has disappeared. The residue is then treated with hot water, digested, filtered on a small filter, washed with hot water, partially dried, put into a crucible, carefully ignited, and weighed as calcium fluorid. The calcium fluorid is then dissolved in sulfuric acid by gentle heating and agitation, evaporated to dryness on a radiator, ignited at full red heat, and weighed as calcium sulfate. From this weight the equivalent weight of calcium fluorid should be calculated, and this should be very close to that actually found as above, but should never exceed it. The difference, which is generally about a milligram (sometimes more), is due to silica precipitated with the fluorid. The percentage of fluorin is, therefore, always calculated from the weight of the sulfate, and not from that of the fluorid obtained.

The main improvements in this method are the use of sodium bicarbonate to separate the silica, and the keeping of the earlier solutions as dilute as possible, which can not be done, if ammonium carbonate be used for the separation of the silica. These changes make the fluorin estimation, although still tedious, far more rapid than before, and the results are very satisfactory.

46. Modification of Wyatt.—By reason of the tediousness of the method of Chatard given above, Wyatt has sought to shorten the process by the following modification:[35]

Five grams of the finely ground phosphate are fused in a platinum dish with fifteen grams of the mixed carbonates of sodium and potassium and three grams of fine sand. After fusing very thoroughly with a strong heat for a quarter of an hour, the dish is removed from the fire and cooled. Its contents, dissolved in hot water, are then put into a half liter flask, and a considerable excess of ammonium carbonate is added to the liquid. All the soluble silica falls out of solution, and the flask, after cooling, is made up to the mark with distilled water, well shaken, and then set aside for twenty-four hours to settle. At the end of this time 200 cubic centimeters are carefully decanted through a filter; the filter is well washed, and the filtrate, after being nearly neutralized with hydrochloric acid, is treated with an excess of calcium chlorid solution.

The precipitate, consisting of phosphate, fluorid, and some calcium carbonate, is allowed to settle, and is then carefully washed with boiling water, first by decantation several times, and finally on the filter. After being properly dried in the gas-oven, calcined, and cooled, the residue is treated with acetic acid, placed upon the water-bath, and evaporated to complete dryness.

The calcium acetate is now well washed out by several treatments with boiling water, and the residue is brought upon a filter, dried, calcined, and weighed. The weight represents the calcium phosphate and fluorid contained in two grams of the original sample; and if the calcium phosphate in the residue be determined, according to the usual methods, the difference will be calcium fluorid and may be thus estimated.

The above method, while shorter, is not to be preferred to the Chatard process when great accuracy is desired. All the soluble silica may not fall out of the solution as Wyatt says. Finally the fluorin is calculated from small differences in the weight of very heavy precipitates and all the error of the process may be found affecting the numbers for fluorin. For commercial purposes, however, the method is to be recommended for its comparative brevity.

GENERAL METHODS FOR ESTIMATING
PHOSPHORIC ACID IN FERTILIZERS.

47. Preliminary Considerations.—The chief sources of the phosphoric acid in commercial fertilizers are the mineral phosphates and bones. In respect of the analyses of mineral phosphates detailed directions have been given in the preceding pages. Bones are valuable for fertilizing materials, both because of their content of phosphoric acid and of their organic nitrogen. The methods of treating bones for their phosphoric acid will be found in the general methods for fertilizing materials, and their nitrogen content can be determined by the processes to be described hereafter. Other fertilizing materials also contain phosphorus, as ashes, tankage, oil cakes, and other organic products. In general, the methods for determining the phosphoric acid is the same in all cases, but the means of destroying the organic matter precedent to the analysis vary in different cases. In most cases a simple ignition is sufficient, while, if the phosphorus be found in certain organic products, the oxidation must be accomplished by one of the methods described in the processes adopted by the official chemists, or by the means described in volume first, paragraph 378 or 382. In all cases of acid phosphates and superphosphates, the water and ammonium citrate soluble phosphoric acid is to be determined as well as the total. In basic slags the amount soluble in ammonium citrate or dilute citric acid is also to be ascertained.

In all cases where soluble or so-called reverted acid is to be considered, the analysis must be performed without previous desiccation or ignition. If water content or loss on ignition are to be considered, the operation to determine them must be conducted on a separate part of the sample.

The methods of analysis which have been adopted by associations of chemists should be given the preference in the conduct of the work, although it must be admitted that they may contain sources of error, and may be in no respect superior to processes employed by chemists in their private capacity. In this country the methods adopted by the Association of Official Agricultural Chemists should be followed as closely as possible. The great merit of other methods, however, must not be denied. Especially those methods which shorten the time required or diminish the labor and expense of the analysis are worthy of careful consideration. In factory work, for instance, it is often far more important for the chemist to be able to rapidly determine the phosphoric acid in a great number of samples with approximate accuracy than to confine his work to one with absolute precision. Some of the shorter methods, moreover, notably the citrate process, appear to be quite, if not altogether, as reliable as the molybdate method, while in the case of the uranium volumetric process, it must not be forgotten that it is almost the only one practiced in France. Other volumetric processes are given in full, as, for instance, the one perfected by Pemberton, but data are still lacking to justify their strong recommendation. It should be remembered that this manual is not written for the beginner but rather for the chemist already acquainted with the principles and practice of general chemical analysis, and it is, therefore, expected that each analyst will make intelligent use of the data placed at his disposal.

48. Determination of Phosphoric Acid with Preliminary Precipitation as Stannic Phosphate.—This method, once much in use and highly recommended, is now almost unknown among the processes of fertilizer control. It was first proposed and described by Girard, and rests on the precipitation of the phosphoric acid in a nitric acid solution by means of metallic tin.[36] The stannic acid formed by the oxidation of the tin unites with the phosphoric acid held in a free state by the nitric acid. The precipitation of the phosphoric acid is said to be complete, but a trace of it has been found in the iron and alumina subsequently separated from the solution. The precipitate obtained is dissolved in caustic potash, whereby soluble potassium metastannate and phosphate are obtained. Following is the method of conducting the analysis as described by Crookes:[37]

The phosphate should be dissolved in nitric acid, and any chlorin present be expelled by repeated evaporations with the solvent. Finally, to the evaporated mass the strongest nitric acid is added. Pure tin-foil is added and heat applied. The phosphoric acid is precipitated by the stannic acid formed. The quantity of tin used should be from six to eight times as great as that of the phosphoric acid present.

The precipitate is collected on a filter, washed, and dissolved in caustic potash. The solution is saturated with hydrogen sulfid, and on adding acetic acid in slight excess the tin sulfid is separated and removed by filtration. The whole of the phosphoric acid, supposed to be almost free of tin, is now found in the filtrate. The filtrate is concentrated to small bulk and any tin sulfid present separated by filtering, and the phosphoric acid finally removed from the ammoniacal filtrate by precipitation with magnesia mixture. The chief difficulties of this method are to be found, on the one hand, in the retention of some of the phosphoric acid by the iron and alumina which may be present, and on the other, in the presence of some tin in the final magnesium pyrophosphate. If the tin be all removed as sulfid, the latter source of error will be avoided. It is difficult to secure pure metallic tin, and this is another disturbing element in the process. It can not be recommended for the work which agricultural analysts are usually called on to perform.[38]

49. Water-Soluble Phosphoric Acid.—The method of procedure recommended by the Association of Official Chemists is as follows:[39] Place two grams of the sample in a nine centimeter filter; wash with successive small portions of cold water, allowing each portion to pass through before adding more, until the filtrate measures about 250 cubic centimeters. If the filtrate be turbid, add a little nitric acid. Make up to any convenient definite volume; mix well; take any convenient portion and proceed as under total phosphoric acid.

50. Citrate-Insoluble Phosphoric Acid.—Heat 100 cubic centimeters of strictly neutral ammonium citrate solution of 1.09 specific gravity to 65° in a flask placed in a bath of warm water, keeping the flask loosely stoppered to prevent evaporation. When the citrate solution in the flask has reached 65°, drop into it the filter containing the washed residue from the water-soluble phosphoric acid determination, close tightly with a smooth rubber stopper, and shake violently until the filter paper is reduced to a pulp. Place the flask back into the bath and maintain the water in the bath at such a temperature that the contents of the flask will stand at exactly 65°. Shake the flask every five minutes. At the expiration of exactly thirty minutes from the time the filter and residue were introduced, remove the flask from the bath and immediately filter as rapidly as possible. It has been shown by Sanborn, in this laboratory, that the filtration is greatly facilitated by adding asbestos pulp. Wash thoroughly with water at 65°. Transfer the filter and its contents to a crucible, ignite until all organic matter is destroyed, add from ten to fifteen cubic centimeters of strong hydrochloric acid, and digest until all phosphate is dissolved; or return the filter with contents to the digestion flask, add from thirty to thirty-five cubic centimeters of strong nitric, and from five to ten cubic centimeters of strong hydrochloric acid, and boil until all the phosphate is dissolved. Dilute the solution to 200 cubic centimeters. If desired, the filter and its contents can be treated according to methods (1), (2), or (3), under total phosphoric acid. Mix well; filter through a dry filter; take a definite portion of the filtrate and proceed as under total phosphoric acid.

In case a determination of citrate-insoluble phosphoric acid be required in non-acidulated goods it is to be made by treating two grams of the phosphatic material, without previous washing with water, precisely in the way above described, except that in case the substance contain much animal matter (bone, fish, etc.), the residue insoluble in ammonium citrate is to be treated by one of the processes described below under total phosphoric acid, (1), (2), or (3).

51. Total Phosphoric Acid.—In case of ignition the residual material is to be dissolved in hydrochloric acid. The following methods of treating the raw material, using two grams in each case, may be employed: (1) Evaporate with five cubic centimeters of magnesium nitrate, ignite, and dissolve in hydrochloric acid. (2) Boil in a Kjeldahl flask graduated to 250 cubic centimeters, with from twenty to thirty cubic centimeters of strong sulfuric acid, adding from two to four grams of sodium or potassium nitrate at the beginning of the digestion and a small quantity after the solution has become nearly colorless; or adding the nitrate in small portions from time to time. After the solution is colorless, add 150 cubic centimeters of water and boil for a few minutes, cool, and make up to volume. (3) Digest with strong sulfuric acid and such other reagents as are used in either the plain or modified Kjeldahl or Gunning methods for estimating nitrogen. Do not add any potassium permanganate, but after the solution has become colorless add about 100 cubic centimeters of water and boil for a few minutes, cool, and make up to a convenient volume; two and five-tenths grams of substance and a digestion flask graduated to 250 cubic centimeters are recommended. This method will be found convenient when both the nitrogen and the total phosphoric acid are to be determined in a fertilizer. In this case, after diluting the volume and mixing, a part for the estimation of nitrogen, may be removed with a pipette and the remainder then filtered through a dry filter and a portion taken for the determination of the total phosphoric acid. (4) Dissolve in thirty cubic centimeters of concentrated nitric acid and a small quantity of hydrochloric acid. (5) Add thirty cubic centimeters of concentrated hydrochloric acid, heat, and add cautiously, in small quantities at a time, about five-tenths gram of finely-pulverized potassium chlorate. (6) Dissolve in from fifteen to thirty cubic centimeters of strong hydrochloric and from three to ten cubic centimeters of nitric acid. This method is recommended for fertilizers containing much iron or aluminum phosphate. Boil until all phosphates are dissolved and all organic matter is destroyed; cool and dilute to 200 or 250 cubic centimeters; mix and pass through a dry filter; take an aliquot part of the filtrate corresponding to a quarter, half, or one gram, neutralize with ammonia, and clear with a few drops of nitric acid. In case hydrochloric or sulfuric acid have been used as a solvent, add about fifteen grams of dry ammonium nitrate.

To the hot solutions, for every decigram of phosphorus pentoxid that is present, add fifty cubic centimeters of molybdic solution. Digest at about 65° for an hour, filter, and wash with water or ammonium nitrate solution. Test the filtrate by renewed digestion and the addition of more molybdic solution. Dissolve the precipitate on the filter with ammonia and hot water and wash into a beaker to a bulk of not more than 100 cubic centimeters. Nearly neutralize with hydrochloric acid, cool, and add magnesia mixture from a burette; add slowly (about one drop per second), stirring vigorously. After fifteen minutes add thirty cubic centimeters of ammonia solution of 0.95 density. Let stand for some time; two hours are usually enough. Filter, wash with dilute ammonia, ignite gently at first and then at white heat for ten minutes, and weigh. For the quantity of magnesia mixture to be added see paragraph 21.

52. Citrate-Soluble Phosphoric Acid.—The sum of the water-soluble and citrate-insoluble subtracted from the total gives the citrate-soluble phosphoric acid.

53. Preparation of Reagents.—(1) Ammonium Citrate Solution.—(a) Mix 370 grams of commercial citric acid with 1,500 cubic centimeters of water, nearly neutralize with commercial ammonia, cool, add ammonia until exactly neutral (testing with saturated alcoholic solution of corallin) and bring to a volume of two liters. Test the specific gravity, which should be 1.09 at 20°, before using.

(b) Alternate Method.—To 370 grams of commercial citric acid add commercial ammonia, of 0.96 specific gravity, until nearly neutral; reduce the specific gravity to nearly 1.09 and proceed as follows: Prepare a solution of fused calcium chlorid 200 grams to the liter, and add four volumes of strong alcohol. Make the mixture exactly neutral, using a small amount of freshly prepared corallin solution as a preliminary indicator, and test finally by withdrawing a portion, diluting with an equal volume of water, and testing with cochineal solution. Fifty cubic centimeters of this solution will precipitate the citric acid from ten cubic centimeters of the citrate solution. To ten cubic centimeters of the nearly neutral citrate solution add fifty cubic centimeters of the alcoholic calcium chlorid solution, stir well, filter at once through a folded filter, dilute with an equal volume of water, and test the reaction with neutral solution of cochineal. If acid or alkaline, add ammonia or citric acid, as the case may be, to the citrate solution, mix, and test again as before. Repeat this process until a neutral reaction of the citrate solution is obtained. At the end the specific gravity must be 1.09 at 20°.

(2) Molybdic Solution.—See paragraph 22.

(3) Ammonium Nitrate Solution.—Dissolve 200 grams of commercial ammonium nitrate in water and bring to a volume of two liters.

(4) Magnesia Mixture.—See paragraph 22.

(5) Dilute Ammonia for Washing.—See paragraph 22.

(6) Magnesium Nitrate.—Dissolve 320 grams of calcined magnesia in nitric acid, avoiding an excess of the latter; then add a little calcined magnesia in excess, boil, filter from the excess of magnesia, ferric oxid, etc., and bring to volume of two liters.

54. Official Methods with Norwegian Fertilizers.—The Director of the Chemical Control Station of Norway, expresses the opinion, that for Norwegian, Swedish, Danish, and German conditions, the American methods for the determination of phosphoric acid, notwithstanding their analytical exactness, are quite inapplicable.[40] In those countries are found many, in part, poorly pulverized and badly mixed manures, such as ammonium superphosphate, potassium superphosphate, and potassium ammonium superphosphate, and these can not usually be so well pulverized and mixed that one can take out a true average sample of from two to two and five-tenths grams. Care in the analysis is useless when the material employed does not represent the average condition of the materials investigated. Therefore, in the countries named, often from ten to twenty grams, and almost never less than five grams of substance are taken in the preparation of the solutions, except for instance, in the determination of nitrogen and reverted phosphoric acid.

55. The Molybdic Acid Method, as Practiced by Direction of the Union of the German Experiment Stations.—The method adopted by the German Experiment Stations is essentially that used at Halle.[41] The samples are brought into solution in the following way: For the estimation of phosphoric acid in bone-meal, fish-guano and raw phosphates, and the total phosphoric acid in superphosphates, five grams of the sample are dissolved in fifty cubic centimeters of aqua regia, made of three parts of hydrochloric acid of 1.12 specific gravity and one part of nitric acid of 1.25 specific gravity, or the solution may be made of a mixture of twenty cubic centimeters of nitric acid of 1.42 specific gravity and fifty cubic centimeters of sulfuric acid of 1.8 specific gravity. The boiling should continue for half an hour. The solution is made up to half a liter and filtered. Fifty cubic centimeters of the filtrate containing the phosphoric acid, with double superphosphates twenty-five cubic centimeters, are digested with 200 cubic centimeters of ammonium molybdate solution for three hours at 50° in a water-bath and, after cooling, filtered, so that as little as possible of the precipitate is collected upon the filter, which is made of strong paper.

The yellow precipitate is washed by decantation in the flask nine times with twenty cubic centimeters of molybdic solution diluted with one volume of water and the filter washed out once with the same quantity of liquid. The funnel, with the filter, is immediately placed upon the flask and the portion of the precipitate collected in the filter dissolved in five per cent ammonia, which is easily accomplished by throwing ammonia upon it from a wash-bottle. Afterwards the filter is washed with a sufficient quantity of hot water and finally removed. The contents of the flask are neutralized warm, with hydrochloric acid, the acid being added until the precipitate first formed, after continued shaking, is again dissolved in the liquid. The solution is then cooled and treated, drop by drop, with constant stirring, with twenty cubic centimeters of magnesia mixture. Finally twenty-five cubic centimeters of dilute ammonia solution are added, the precipitate is not shaken, and, after two hours, is filtered through a gooch.

For the filtering of the ammonium magnesium phosphate by the molybdic method, freshly prepared felts are always employed since the remarkably fine crystalline precipitates will pass through a filter which has once been used. It is necessary also that special precautions be taken in the ignition. The crucible should be heated in a platinum cap, which has the purpose of protecting the contents of the crucible from the access of reducing gases during the ignition. After redness has been reached the cap can be removed and the crucible transferred to a blast where it is strongly ignited for ten minutes before weighing. The precipitate should be pure white.

The molybdic solution is prepared as follows: 150 grams of ammonium molybdate are dissolved in a liter of water, and after the solution is completely cooled, poured into a liter of nitric acid of 1.2 specific gravity.

56. Estimation of Soluble Phosphoric Acid.—1. The extraction of the superphosphates is made as follows: Twenty grams of the superphosphates are placed in a liter flask with 800 cubic centimeters of water and shaken continuously for thirty minutes. The flask is then filled with water to the mark and the whole again thoroughly shaken and filtered. For shaking, a machine is recommended, driven by hand or water power. The normal rate of the machine is fixed at 150 turns per minute.

2. The solution of the total superphosphates, obtained as above, must be boiled with nitric acid before the precipitation of the phosphoric acid in order to convert any phosphoric acid present as pyrophosphoric into tribasic phosphoric acid. For each twenty-five cubic centimeters of the superphosphate solution ten cubic centimeters of concentrated nitric acid are added and the mixture boiled.

3. The precipitation of the phosphoric acid is conducted by the molybdenum method as usually practiced.

4. For the estimation of iron and alumina in each of the superphosphates the Glaser method is recommended provisionally.

57. Methods for Phosphoric Acid used in the Norway Stations.[42]—1. Description of the Method for Total Phosphoric Acid.—For determining the phosphoric acid in bone-meal, fish-guano, and superphosphates, five grams of the substance, with twenty cubic centimeters of nitric acid of 1.42 specific gravity, and fifty cubic centimeters of sulfuric acid of 1.8 specific gravity, are boiled half an hour in a half liter flask, diluted with water, and after cooling, made up to the mark. Fifty cubic centimeters of the filtrate are made alkaline with ammonia, then acid with nitric acid, precipitated with fifty cubic centimeters of molybdic solution for every one-tenth gram of phosphorus pentoxid present, heated over the water-bath for one hour, and allowed to stand twelve hours more, when the supernatant liquid is separated by decantation the precipitate washed thoroughly with dilute molybdate solution (1: 4) dissolved in warm dilute ammonia, and the filter washed with hot water. The ammoniacal solution is neutralized with hydrochloric acid, cooled, mixed, drop by drop, with constant stirring, with from ten to twenty cubic centimeters of magnesia mixture, and after a quarter of an hour one-third the volume of ten per cent ammonia is added. This is allowed to stand two hours, is filtered, washed with five per cent of ammonia until the disappearance of the chlorin reaction, dried, burned in an open crucible over a bunsen, and finally for a quarter of an hour, in a covered crucible heated over the blast.

2. Water-Soluble Phosphoric Acid.—To twenty grams of the substance in a liter flask, are added 800 cubic centimeters of water, and shaken every fifteen minutes for two hours; the volume made up to the mark and the phosphoric acid in fifty cubic centimeters of the filtrate, equaling one gram substance, is determined as under total.

3. Reverted Citrate-Soluble Phosphoric Acid.—Two and five-tenths grams substance are rubbed up with water, then washed upon the filter with about 100 cubic centimeters of water, the residue on the filter washed into a flask with a part of the measured citrate solution, and digested one hour at 35° to 40° with 200 cubic centimeters of Petermann’s citrate solution. The water and citrate extracts are made up to a quarter of a liter each, and the phosphoric acid determined in from twenty-five to fifty cubic centimeters, according to the quantity present.

Solutions. 1. Molybdate Solution.—375 grams of ammonium molybdate are dissolved in two and five-tenths liters of water, and the solution poured into two and five-tenths liters of nitric acid of 1.20 specific gravity.

2. Magnesia Mixture.—275 grams of crystallized magnesium chlorid and 350 grams of ammonium chlorid are dissolved in 3250 cubic centimeters of water and filled up to five liters with ammonia of 0.96 specific gravity.

3. Petermann’s Solution.—One kilogram of citric acid is dissolved in about two liters of water and 1350 cubic centimeters of ammonia of 0.925 specific gravity and filled up with water to 5750 cubic centimeters. The solution then has a specific gravity of 1.09; 300 cubic centimeters of ammonia of 0.925 specific gravity are now added.

58. Swedish Official Method for Determination of Phosphoric Acid.[43]—The Swedish chemists determine phosphoric acid in fertilizers both by the molybdate and the citrate methods. These methods carefully conducted according to the directions given below, give very concordant results. In doubtful cases the former method is taken as the deciding one, it having proved by long practice to give very satisfactory results.

Reagents for the Molybdate Method.—1. Molybdic Solution.—Prepared by dissolving 100 grams of finely powdered molybdic acid with heat, in 400 grams of eight per cent ammonia of 0.967 specific gravity and pouring the solution into 1,500 grams of nitric acid of one and two-tenths specific gravity; or else by dissolving 150 grams ammonium molybdate in one liter of hot water, and pouring the solution into one liter of nitric acid of 1.2 specific gravity. Prepared in this way, the molybdic solution will contain, in the former case five per cent, in the latter case from five to six per cent of molybdic acid, and 100 cubic centimeters of it are required for precipitating one-tenth gram of phosphorus pentoxid.

2. Magnesia Mixture.—Prepared from 110 grams of crystallized magnesium chlorid, 140 grams ammonium chlorid, 700 grams of eight per cent ammonia of 0.967 specific gravity and 1,300 grams of distilled water. The mixture is filtered after a few days, if necessary; ten cubic centimeters of the same are required for precipitating one-tenth gram of phosphorus pentoxid.

3. Ten per cent ammonia of 0.959 specific gravity.

(a) Water-Soluble Phosphoric Acid.—1. Preparation of the Aqueous Solution.—Of superphosphates and in general fertilizers containing water-soluble phosphoric acid, a sample of twenty grams is taken, and water poured over it in a mortar; lumps are crushed lightly, but completely with the pestle without pulverizing it finer; the whole mass is then washed into a graduated flask holding one liter, which at once is filled up to the mark. The volume taken up by the residue insoluble in water, is left out of consideration in the calculation. The sample is left standing in the flask (which is occasionally shaken) at the ordinary temperature of the room for two hours, and the solution is then filtered.

2. The Determination.—Take twenty-five cubic centimeters of the superphosphate solution thus prepared (when a twenty per cent sample is taken equal to one-tenth gram phosphorus pentoxid); add a quantity of molybdic solution sufficient for complete precipitation, leave standing for four hours in a beaker covered with a watch-glass; decant the solution through a small filter, wash the precipitate first by decantation, then on filter, with a mixture containing 100 parts molybdic solution, twenty parts nitric acid of 1.2 specific gravity, and eighty parts water, until a few drops put into alcohol, to which some dilute sulfuric acid has been added, does not, any longer, cause turbidity. The molybdic precipitate is now washed with but little water from the filter into a beaker, and particles adhering to the filter are dissolved by a hot mixture of one part ammonia and three parts water, which is allowed to flow into the beaker till the precipitate is, finally, completely dissolved in it. To the clear solution, add dilute hydrochloric acid while stirring, till the yellow precipitate formed by the acid is no longer immediately dissolved; then add from six to eight cubic centimeters of ammonia through the filter. The volume of the solution is not to exceed seventy-five cubic centimeters. It is now cooled completely and one cubic centimeter of magnesia mixture is added from a burette for every centigram of phosphorus pentoxid which it is expected to contain, and finally one-quarter of its volume of ammonia is added. The precipitate may be filtered after four hours. This is washed on the filter, preferably by means of suction, with a mixture of one part ammonia and three parts water till the filtrate is entirely free from chlorin. After drying, heat the precipitate, first gently, then stronger, and finally with a blast for a few minutes and then weigh it.

Treated with hydrochloric acid it must leave no insoluble residue (SiO₂), nor should hydrogen sulfid cause any precipitation in the solution thus formed (MoO₃).

(b) Total Phosphoric Acid.—1. In Superphosphates.—For the determination of total phosphoric acid, treat a weighed quantity of the superphosphate with nitric acid, if necessary to bring a difficultly soluble residue into solution, with addition of hydrochloric acid, or of potassium chlorate, to destroy organic matter present. Dilute the solution to a definite volume, and determine the phosphoric acid in a measured quantity of the same, as directed under (a) 2; if hydrochloric acid or potassium chlorate, be applied in the preparation of the solution, however, not till the measured quantity has been repeatedly evaporated to dryness with concentrated nitric acid.

2. In Bone-meal.—Destroy organic matter in five grams of the sample by ignition, dissolve the residue in nitric acid, filter from the insoluble residue, dilute the filtrate to half a liter, take an aliquot part containing about one-tenth gram phosphorus pentoxid and determine the phosphoric acid as directed under (a) 2.

3. In Fish-guano (and other fertilizing materials of organic origin).—The organic matter cannot here be removed by simple ignition, as in this way a loss of phosphorus may take place; It is therefore destroyed either in the wet way through nitric acid and potassium chlorate or in the dry way by fusion with a mixture of potassium nitrate and sodium carbonate, otherwise the procedure is as in (b) 1.

4. In Mineral Phosphates.—Determine the phosphoric acid in a solution obtained by nitric acid; organic matter is destroyed preferably in the wet way.

5. In Basic Slag.—Dissolve ten grams of powdered slag by treating it with 100 cubic centimeters of fuming hydrochloric acid with heat; wash the solution into a graduated half liter flask, fill to the mark, shake well, and filter. Determine the phosphoric acid in twenty-five cubic centimeters of the clear filtrate, according to (a) 2, after having first, however, evaporated the solution to dryness and then at least three times evaporated the residue to dryness with concentrated nitric acid.

59. Method Employed by the Royal Experiment Station of Holland.—A. Soluble Phosphoric Acid.[44]—The necessary reagents are:

(1) Molybdate solution, made by dissolving 150 grams of ammonium molybdate in a liter of water and pouring the solution into a liter of nitric acid of 1.20 specific gravity.

(2) A ten per cent solution of ammonium nitrate.

(3) Strong and dilute ammonia, the latter being between two and five-tenths and three per cent of 0.988 specific gravity.

(4) Magnesia mixture made by dissolving 110 grams of crystallized magnesium chlorid, 140 grams of ammonium chlorid, and 700 cubic centimeters of ammonia of 0.96 specific gravity in water and bringing the solution to two liters.

(5) Ammoniacal citrate solution, made by dissolving 500 grams of citric acid in a liter of water, and mixing with four liters of ten per cent ammonia of 0.96 specific gravity.

Manipulation.—Place twenty grams of substance in a mortar together with some cold distilled water or pure rain water, stir, and decant the water and suspended matters into a liter flask. After this has been repeated several times, rub up the residual mass and wash it all into the flask. Fill up to about 900 cubic centimeters and allow to stand two hours (twenty-four hours in the case of double phosphates with more than twenty-two per cent of soluble phosphoric acid), shaking repeatedly; or shaking continuously, for half an hour. Fill up to the liter mark and filter through a dry filter. Take portions of twenty-five or fifty cubic centimeters for each determination, add 100 cubic centimeters of molybdate solution for each 100 milligrams of phosphorus pentoxid present, warm to about 80° for an hour, filter, and wash the precipitate with the ammonium nitrate solution. Add a little molybdate solution to the filtrate, warm, and, if a fresh precipitate be observed, it is to be added to the first. The precipitate is to be dissolved in ammonia, and hydrochloric acid carefully added until the precipitate caused by it only slowly redissolves on stirring. The phosphoric acid is precipitated from the clear liquid which is still ammoniacal with magnesia mixture, using ten cubic centimeters for each 100 milligrams of phosphorus pentoxid present. This is added, drop by drop, and the liquid kept stirred during the addition. Allow it to stand at least two hours, filter, wash with dilute ammonia, dry, and ignite. This last is done at first with a very small flame but is finished with the blast-lamp or in a Rössler furnace. To insure burning to whiteness, nitric acid may be used, but not more than one or two drops.

B. Total Phosphoric Acid.—(1) For bone and flesh-meal, fish-guano, and similar fertilizers the reagents necessary are the same as before.

Carefully burn five grams to ash, boil the ash for half an hour with nitric acid of 1.32 specific gravity, dilute with water, and, after cooling, dilute to 500 cubic centimeters. Filter through a dry filter and take fifty cubic centimeters of the filtrate. Add 100 cubic centimeters of the molybdate solution for each 100 milligrams of phosphorus pentoxid present. Treat further as before described.

(2) Phosphates, guanos, bone-black, etc.

One gram of substance, after powdering, and, if necessary, igniting, is covered with four cubic centimeters of hydrochloric acid of 1.13 specific gravity and a little water and heated for an hour and a half. Evaporate to dryness without filtration, making repeated additions of nitric acid until no more vapors of hydrochloric acid are evolved. Boil the residue with nitric acid, cool, make up to 100 cubic centimeters with water, and shake. Filter and treat fifty cubic centimeters of the resulting solution by the molybdate method and proceed further as before described.

60. Sources of Error in the Molybdate Method.—When conducted with proper care, the gravimetric molybdate method is one of the most exact processes known to analytical chemistry.

There are, however, some sources of error in the process which should be avoided as carefully as possible or taken into account.

1. Error Due to Occluded Silica.—When silica passes into solution in the original sample, and this may be the case especially with mineral phosphates, it may appear both in the yellow precipitate and in the final magnesium pyrophosphate. In all such cases the residue, after ignition, should be dissolved in hydrochloric acid, and any insoluble residue weighed as silica and deducted from the first weight. If the silica be removed by evaporating the solution of the original material to dryness, and igniting to destroy organic matter, care must be taken to reconvert all phosphoric acid into the ortho form by long boiling with nitric acid before precipitation.

Another method of avoiding any trouble from silica consists in using sulfuric and a little nitric acid as the solvent for the original substance. Silica is not soluble in hot concentrated sulfuric acid. The volume of the sulfuric should be about ten times that of the nitric acid used, and the boiling be continued until sulfuric vapors are evolved.

2. Error Due to Arsenic.—Only in rare cases will arsenic be found in phosphatic fertilizing materials. In case of pyritic phosphates, the iron disulfid may carry arsenic. The solution in such a case is best accomplished in hydrochloric acid. If aqua regia be used, all nitric acid should be removed by repeated evaporation with hydrochloric. The arsenic can then be precipitated in the hot dilute hydrochloric acid solution by hydrogen sulfid.

3. Error Due to Occluded Magnesia.—The danger of contamination of the yellow precipitate with magnesium oxid has been pointed out by some authors. The re-solution of the precipitate followed by a second precipitation is the usual remedy proposed. Lorenz states that this source of error may be entirely avoided by the addition of two per cent of citric acid to the phosphomolybdate solution.[45]