513. Further Examination of Waters.—Having described in the preceding part the approved methods of determining the oxidized nitrogen in waters and soil extracts there remains to be considered the examination of waters for other substances of importance to agriculture. Rain waters add practically nothing to the soil but nitric acid and ammonia, and, therefore, demand no further discussion here. In drainage and sewage waters, in addition to the oxidized nitrogen, there may be sufficient quantities of phosphoric acid and potash to make their further analysis of interest. But by far the most practical point to be considered is in the case of waters used for irrigation purposes where the continued addition to the soil of mineral matters may eventually convert fertile fields into barren wastes. In irrigated lands there is practically no drainage and the whole of the water is removed by superficial evaporation. It is easily seen how these mineral matters tend to accumulate in that part of the soil in which the rootlets of plants seek their nourishment.

514. Estimation of Total Solid Matter.—The total solid contents of a sample of water are determined by evaporating a known volume or weight to dryness and weighing the residue. For comparative purposes a given volume of water may be taken if the solid contents do not exceed four grams in a United States gallon. The water should be measured at a temperature of about 15°.5. Where the content of mineral matter is greater it is best to weigh the water and calculate the solid contents to parts per one hundred thousand. For practical purposes in the United States it is customary to state the content of solid matter in grains per gallon. Since, however, the gallon has so many different values it is always necessary to indicate what particular measure is meant.

In ordinary spring and well waters the volume to be used is conveniently taken at 100 cubic centimeters. To avoid calculation a volume in cubic centimeters corresponding to some decimal part of a gallon in grains may be taken and the weight in milligrams will then be equivalent to the grains per gallon. Thus in the imperial gallon which contains 70,000 grains of distilled water at 15°.5, seventy cubic centimeters may be taken. If the residue weigh twenty-five milligrams the water contains twenty-five grains of solid matter per gallon. The United States gallon at 15°.5 contains 58,304 grains of distilled water. In this case 58.3 cubic centimeters should be used, or double this amount and the weight in milligrams be divided by two.

The evaporation may be made in a platinum, porcelain, or aluminum dish, preferably with a flat bottom; The dish does not need to hold the whole volume at once, but the water may be added from time to time as the evaporation continues. The dish, however, should, as a rule, hold not less than 100 cubic centimeters. The evaporation is best conducted over a steam-bath, and after the complete disappearance of the liquid the heating should be continued until the residue is perfectly dry.

In the case of mineral waters highly impregnated with inorganic salts, a smaller volume or weight may be taken, and greater care must be exercised in drying the residue. For the purpose of qualitively determining the percentage of special ingredients, quantities of the water should be taken inversely corresponding to the content of the ingredient desired. In general, it will not be necessary to evaporate the sample to complete dryness, but only to concentrate it to a volume convenient for the application of the analytical process. Where a complete quantitive analysis of the solid residue is desired, a sufficient quantity of the water is evaporated to give a weighable amount of the least abundant ingredient. The total solid content of the water having been previously determined, the actual weight or volume of the water taken to obtain the above residue is of no importance.

515. Estimation of the Chlorin.—The chlorin in the solid residue from a sample of water may be determined directly by dissolving the soluble salts in distilled water, to which enough nitric acid is added to preserve the solution slightly acid. After filtering and washing, silver chlorid is added, little by little, with constant shaking until a further addition of the reagent produces no further precipitate. The beaker or flask should be placed in a dark place, on a shaking apparatus which is kept in motion until the precipitate has entirely settled in a granular state. The silver chlorid is then collected on a gooch, washed free of all soluble matter, dried at 150° and weighed. If the precipitate be ignited to incipient fusion, a porcelain gooch should be used.

A more convenient method is to determine the chlorin directly in the water, or, where the quantity is too minute, after proper concentration, volumetrically by means of a titrated solution of silver nitrate, using potassium chromate as indicator. As soon as the chlorin has all united with the silver, any additional quantity of the silver nitrate will form red silver chromate, the permanent appearance of which indicates the end of the reaction. This process is especially applicable to water, which in a neutral state contains no other acids capable of precipitating silver. The chromate indicator does not work well in an acid solution.

516. Solutions Employed.—A quantity of pure silver nitrate, about five grams, is dissolved in pure water and made up to a volume of one liter. For determining the actual strength of the solution, 0.824 gram of pure sodium chlorid is dissolved in water and the volume made up to half a liter. Twenty-five cubic centimeters of this solution are placed in a porcelain dish, and a few drops of the solution of potassium chromate added. The silver nitrate solution is allowed to flow into the porcelain dish from a burette graduated to tenths of a cubic centimeter. The red color produced as each drop falls, disappears on stirring as long as there is any undecomposed chlorid. Finally a point is reached when the red color becomes permanent, a single drop in excess of the silver nitrate being sufficient to impart a faint red tint to the contents of the dish.

The solution of potassium chromate is prepared by dissolving five grams of the salt in 100 cubic centimeters of water. Silver nitrate solution is added until a permanent red precipitate is produced, which is removed by filtration, and the filtrate is employed as the indicator as above described. Water with any considerable quantity of chlorin can be treated directly with the reagents; when the percentage of chlorin is low, previous concentration to a convenient volume is advisable.

In waters containing bromids and iodids these halogens would be included with the chlorin estimated as above. For agricultural purposes such waters have little importance. In the case of soluble carbonates capable of precipitating silver this action can be prevented by acidifying the water with nitric acid and afterwards removing the excess of acid with precipitated calcium carbonate. In this reaction McElroy recommends the use of Congo paper, which is not affected by the carbon dioxid but is turned blue as soon as an excess of nitric acid is added. After the addition of the calcium carbonate the mixture should be boiled to expel carbon dioxid.^[343]

Irrigation waters from natural sources or derived from sewage rarely contain enough chlorin to make their use objectionable. On the other hand, when water is obtained for this purpose from artesian wells it may often contain a quantity of chlorin which will eventually do more harm to the arable soil than the water will do good.

517. Carbon Dioxid.—Free carbon dioxid in water has no significance in respect of its use for irrigation purposes. Such waters, however, are usually of a highly mineral nature and thus are justly open to suspicion when used for farm animals and on the field. The presence of free carbon dioxid as has already been pointed out in paragraph 42, gives to water, one of its chief sources of power as an agent for dissolving rocks and ultimately forming soil. The estimation of the total free carbon dioxid in a sample of water issuing from a spring or well is a matter of some delicacy by reason of the tendency of this gas to escape as soon as the water reaches the open air and is relieved from the natural pressure to which it has been subjected. The actual quantity of the gas remaining in solution at any given time is determined as follows: 100 cubic centimeters of the water are placed in a flask with three cubic centimeters of a saturated solution of calcium and two of ammonium chlorid. To this mixture is added forty-five cubic centimeters of a titrated solution of calcium hydroxid. The flask is stoppered, well shaken, and set aside for twelve hours to allow the complete separation of the calcium carbonate formed.

When the supernatant liquid is perfectly clear an aliquot part thereof, from fifty to one hundred cubic centimeters, is removed and titrated with decinormal acid with phenacetolin or lacmoid as an indicator. From the quantity of calcium hydroxid remaining unprecipitated the amount which has been converted into carbonate can be determined by difference. The difference between the quantity of calcium hydroxid originally present in the solution and that remaining after the above treatment multiplied by the factor 0.0022 will give the weight of carbon dioxid present in the water in a free state or in excess of that present as normal carbonates.

UNUSUAL CONSTITUENTS OF SOIL.

518. Boric Acid.—Boron, while not regarded as an essential plant food, is yet found quite uniformly in the ashes of a large number of plants. It may, therefore, be of some interest to the agricultural analyst to determine the amount of it which may be present in a soil extract or mineral water. For this purpose the following method due to Gooch may be employed.^[344] To one liter of the water supposed to contain boric acid add enough sodium carbonate to produce distinct alkalinity. After evaporation to dryness acidify the residue with hydrochloric acid, apply a piece of turmeric paper and dry at a moderate heat. The usual brown-red tint will reveal the presence of boric acid.

The quantitive estimation of the acid is accomplished as follows: One or more liters of the water rendered alkaline as above are evaporated to dryness. With the aid of as small a quantity as possible of acetic acid the dry residue is transferred to a distillation flask and condenser arranged as shown in Fig. 92. About one gram of recently ignited pure lime, cooled in a desiccator and weighed accurately, is introduced into the flask at the bottom of the condenser and slaked by a few cubic centimeters of water. When the flask is attached, the terminal tube of the condensing apparatus should dip into the lime-water in the flask. The heating-bath is partly filled with paraffin at a temperature of about 120°. The paraffin-bath is raised so that the entire bulb of the flask is immersed therein and the distillation continued until all the liquid has been distilled. The bath is removed and after cooling somewhat, ten cubic centimeters of methyl alcohol are introduced by means of the stoppered funnel-tube and the process of distillation repeated. This operation with methyl alcohol is repeated five times. The boric acid passes off with the distillate and is found in the flask below the condenser as calcium borate. The contents of the distillation flask are evaporated to dryness and ignited conveniently in the same crucible in which the lime was burned. The increase in weight represents the quantity of boric anhydrid, B₂O₂ obtained.

519. Method Of Moissan.—The principle of the method of Gooch, which has just been described, is applied by Moissan in a slightly modified manner.^[345]

In this method the generating flask is made smaller than in the Gooch apparatus, and the funnel at the top is oval and provided with a ground-glass stopper. It is closed at the bottom with a glass stop-cock, and the slender funnel-tube enters through a rubber stopper and ends about the middle of the bulb of the flask. The delivery-tube is longer than in Fig. 91, and is bent upward at its middle part in the form of an obtuse angle. The receiving flask is connected with the condenser by means of a tube-shaped funnel, which prevents any regurgitation into the generating flask. The receiving flask also has attached to it a three-bulb potash absorption tube, through which all vapors escaping from the receiving flask must pass. The bulbs contain a five per cent solution of ammonia. The receiving flask should be placed in a crystallizing dish and kept surrounded with ice-water.

The boron which is to be estimated should be in the form of boric acid. This can readily be accomplished by treating the residue to be analyzed with nitric acid in a sealed tube. The mixture is introduced into the generating flask, washing with a little nitric acid, and evaporated to dryness. The heat is removed, and, by means of the funnel, ten cubic centimeters of methyl alcohol added, and distillation is renewed. This operation with methyl alcohol is repeated four times, taking care to distill to dryness in each case before the addition of a fresh quantity of alcohol. Afterwards, there is introduced into the apparatus one cubic centimeter each of distilled water and nitric acid and the distillation again carried to dryness. The treatment with methyl alcohol, as described above, is then repeated three times. To determine whether all the boric acid have passed over, the receiving flask at the bottom of the condenser is disconnected and a drop of the alcohol taken from the end of the condensing tube by means of a filament of filter paper. On burning, the flame should not show any trace of green. In case a green color is observed, the distillation with nitric acid and methyl alcohol must be repeated.

The ammonia in the potash bulbs serves to arrest any of the vapors carrying boric acid which might escape from the receiving flask. The contents of the bulbs are to be mixed with the liquid in the receiving dish, and the whole poured onto a known weight of recently ignited calcium oxid contained in a platinum dish, and the mixture briskly stirred. If the liquid be very acid the platinum dish should be kept in ice-water to prevent heating. After fifteen minutes the liquid usually becomes alkaline, and it is then evaporated at a temperature below the boiling-point of methyl alcohol (66°). The mass, after the methyl alcohol has disappeared, is dried at a gradually increasing temperature, and finally, the dish is ignited over a blast, at first covered and afterwards open. The dish is covered and weighed and again ignited until constant weight is obtained.

The lime used should be specially prepared by igniting calcium nitrate incompletely, and reigniting a portion of this to constant weight just before beginning each analysis. The calcium oxid is then obtained in a perfectly fresh state. It should be employed in considerable excess, for each half gram of boric acid at least eight grams of the lime. The operation is tedious but the results are quite accurate.

SPECIAL TREATMENT OF MUCK SOILS.

520. General Considerations.—Deposits of muck which are to be used as soil for cultural purposes, or marsh lands, containing large quantities of organic matter, require a special treatment in addition to the general principles of examination illustrated in the previous pages. These soils, essentially of an organic origin, do not permit of the same treatment either chemical or physical as is practiced with soils of a mineral nature. For instance, it would be useless to attempt a silt analysis with organic soils, and the extraction of them with hydrochloric acid for the purpose of determining the materials passing into solution would prove of little utility. The object of the examination is not only to obtain knowledge of the ultimate constitution of the sample, but also, and this is the practical point, to gain some idea of its stores of plant food and of the proper steps necessary to secure a supply of the deficient nutrients. The final analytical processes for the estimation of the constituents of a muck or vegetable soil are the same as those already given, but the preliminary treatment is radically different.

521. Sampling.—First of all the geologic and meterologic conditions of the muck formations must be determined as nearly as possible. It is fair to presume that these formations are of comparatively recent origin, in fact that they are still in progress. The geologic formation in the vicinity of the deposit should be noted. Information should be given in respect of the character of the water, whether running or stagnant, fresh, salt, or brackish, and changes of level to which it is subject, should be noted. It should be particularly stated whether the vegetable growth contributing to the formation be subject to frost or freezing. The character of the growth is to be carefully noted, and observation made of any changes in vegetation due to drainage preparatory to cultivation. It is to the original vegetation that the chief vegetable accretions in the muck must be accredited. In all cases, for purposes of comparison, some samples must be taken from parts of the field which have not been under cultivation or fertilization. The original properties of the muck can thus be determined and compared with the portions which have been changed under cultivation. If the vegetation in different parts of the field vary it is an indication that the muck is not homogeneous, and in such cases all the different kinds must be separately sampled. Any alluvial deposit should be carefully separated from the muck found in situ, for the two layers are radically different in nature.

The sampling should be made by digging a pit, if possible to the bottom of the muck formation, and taking the samples at depths of one foot from one or all of the sides. The samples from sections of even depth are to be mixed and about five kilograms of the well-mixed sample preserved. Blocks of the unbroken and unshattered material should also be taken from each section for the purpose of determining permeability to water and air. All living vegetable matter should be as fully as possible removed before the sampling begins. The nature of the subsoil must be observed, and it should be stated whether it be sand, clay, limestone, etc. Fresh samples should be taken at various depths for the purpose of determining the content of moisture in the manner described in paragraph 65. The tubes used are made sharp at the end to be inserted in the soil, and so arranged as to cut cylinders of soil a trifle smaller than their interior diameter. By this means the sample slips easily into its place. The same care and judgment must be used in taking these samples as are required in the case of common soils.

Illustration.—Samples of muck soil taken at Runnymede, Florida.

(a) Formation. Littoral fresh-water lacustrine deposits, varying from a few inches to four feet in depth, and from a few feet to half a mile in width.

(b) Vegetation before drainage. Saw grass (Cladium Mariscus or effusum).

(c) Principal present vegetation (see pages 59–60).

(d) Kinds of Soil.—The muck shows two distinct colors, black and brown. The vegetation, however, seems to be the same in both cases. The black muck has the appearance of being more thoroughly decomposed.

(e) Geologic Formation.—This portion of the Florida peninsula is covered generally with sand due to marine submergence during recent geologic periods. The forest growth is pine. The drainage from the pine land is towards the muck deposits. The pine land lies from four to ten feet higher than the surface of the muck and is much less subject to frost.

522. Water Content.—The capacity of a muck soil for retaining water is very great. In a moist state these soils are heavy and apparently quite firm. When dry they are light and fluffy and unsuited to hold the rootlets of plants. Saturated to their greatest capacity they hold considerably more than their own weight of water. Attention has already been called to the danger of drying such samples at a high temperature. As in most cases of drying exposure at the temperature of boiling water until a constant weight is obtained is a perfectly safe way. It is hard to say what comes off in addition to water at a higher temperature. All that comes off even at the temperature of boiling water is not water.

The method of determination usually employed in this laboratory is the following:

From four to five grams of the material are spread as evenly as possible over the flat bottom of a circular aluminum dish, about seven centimeters in diameter. The dish is exposed for three hours at the temperature of boiling water and then kept for two hours in an air-bath at 110°. At the end of this time constant weight is obtained. Additional drying at 110° for five hours, usually gives no further loss of volatile matter. The dish should be covered during weighing on account of the hygroscopicity of the residue. When well sampled the dry matter thus obtained serves as the basis of calculation for the general analytical data.

Results.—Samples of muck soil taken in brass tubes in March during the dry season had the following contents of moisture:

It is thus seen that the normal content of moisture in such a soil during the dry season, exclusive of the top layer, is about eighty per cent.

523. Organic Carbon and Hydrogen.—The organic carbon and hydrogen in muck soils are determined on the carefully dried sample by combustion with copper oxid. This process gives not only the quantities of these bodies combined as humus, but also those in a less advanced state of decomposition and present as fatty bodies or resins. The method employed is given on pages 319–20.

Results.—The data obtained on a sample of muck soil from Florida are as follow:

The last sample was largely mixed with sand, the muck at the point when it was taken not being quite three feet deep.

524. Total Volatile and Organic Matter and Ash.—The ignition of the sample should be very carefully conducted at the lowest possible temperature. About five grams of the air-dried sample or double that amount of the moist sample should be taken. In the latter case the calculations should be made on the basis of the dry material. The ignition should be continued with frequent stirring with a platinum wire until all organic matter is destroyed. At the same time in a large dish one or more kilograms of the sample should be ignited in order to secure an ash for analysis. The ash should be quickly weighed to avoid absorption of moisture.

525. Sulfur.—The sulfur present in muck is combined either in an organic form or with iron. It may be estimated by the method of Fleischer.^[346]

From five to ten grams of the sample are ignited carefully in a hard glass tube in a stream of air or better of oxygen. The sulfur compounds escape as sulfuric or sulfurous acid.

The combustion is carried on in the apparatus shown in Fig. 93.

The end of the tube A, next to B, is lightly stopped with a plug of glass wool, the substance introduced and held in place by a second plug of glass wool next to C. A is connected to the working flask C, containing water, as is shown in the illustration. The chief object of the flask is to control the rate of aspiration of the air or oxygen. A is also connected with the bulb-tube B, as shown in the figure. B contains potash-lye, free of sulfur. On the top of B is placed a drying tube filled with glass pearls, moistened with potash-lye. This is connected with the aspirator by a small bulb-tube bent at right angles, as indicated. The bulb of this tube contains a little neutral litmus solution, which must suffer no change of color during the progress of this analysis. The tube, thus arranged, is placed in a combustion furnace and gradually heated to redness, beginning with the part next to B. A moderate stream of air or oxygen is passed through the tube during the operation. Any product of the combustion collecting in the tube before reaching B, is driven into B by careful heating. At the end of the combustion the contents of B are acidified with hydrochloric acid, and treated with bromin to convert the sulfurous into sulfuric acid. The excess of bromin is afterwards removed by boiling, and the sulfuric acid precipitated by barium chlorid and estimated in the usual way.

The total sulfuric acid having thus been determined, the sample is extracted with water and the sulfuric acid estimated in the residue.

The sulfuric acid in a muck which is injurious to vegetation is classified by Fleischer, as follows:

(1) Free sulfuric acid. (The residue which is obtained by calculation as sulfates of the bases in the water extract.)

(2) The sulfuric acid contained as copperas (calculated from the iron oxid content of the aqueous extract).

(3) Sulfuric acid arising from the oxidation of pyrites (calculated from the sulfuric acid obtained by treatment of the water-extracted sample).

A better idea of the distribution of the sulfur in the sample can be obtained by estimating it according to the method given in paragraph 385.

526. Phosphoric Acid.—The method for determining the phosphorus in muck is given in paragraph 382. The process given in 378 may also be used.

The method of extraction with hydrochloric acid is wholly unreliable as a means of determining the available phosphoric acid in muck.

There are some vegetable soils which contain so much iron and lime that the whole of the acid ordinarily used would be consumed thereby. This fact has been clearly pointed out by Wiklund in determining the phosphoric acid in a large number of peaty soils.^[347] His experiments, were made with acid of only four per cent strength. In some cases, however, it may be found useful to determine the quantity of phosphoric acid which can be extracted with hydrochloric acid, and afterwards to separate the humus and determine the content of phosphoric acid therein.

527. Humus.—In this laboratory the humus is estimated by the method of Huston and McBride, as given in paragraph 312. In samples so rich in organic matter the method of Grandeau does not give as good results.

Often more than half the weight of the dry substance is soluble in ammonia after treatment with acid. The nitrogen in the original sample and the separated humus should be estimated by moist combustion with sulfuric acid in the usual manner.

528. The Mineral Contents of Humus.—The material obtained by precipitating the alkaline extract of a vegetable earth with an acid does not consist alone of oxygen, carbon, hydrogen, and nitrogen. The complex molecules which make up this mixture contain certain quantities of iron, sulfur, and phosphorus in an organic state. These bodies are left as inorganic compounds on ignition, provided there is enough of base present to combine with all the acid elements. Much of the sulfur and phosphorus, however, in these compounds might be lost by simple ignition. In such cases moist oxidation of these bodies must be practiced, or the gases of combustion passed over bodies capable of absorbing the oxidized materials in order to detect and determine them. The proper methods of accomplishing this have already been pointed out for vegetable soils, and the same processes are applicable in the case of extracted and precipitated humus.

Another proof that both phosphorus and sulfur are present in humus in an organic state is found in the fact pointed out by Eggertz and Nilson, that the ash of muck soils is always richer in sulfuric and phosphoric acids than the solution obtained therefrom by hydrochloric acid.^[348]

In a sample of muck examined by them there was found in the ash 1.46 per cent SO₃, and in the acid extract only 0.05 per cent SO₃; and in the ash 0.3 per cent P₂O₅, while in the extract only 0.04 per cent P₂O₅.

529. Combustion of the Humus.—The percentage composition of the extracted humus can be determined, after drying to constant weight, by combustion with copper oxid. There is little use in trying to assign definite chemical formulas to any of the components of the complex which we call humus. Some of the supposed formulas have been given on pages 61 and 62.

530. Ether Extract.—Most peaty soils, when very dry, are not easily moistened with water. This is due to a superficial coating of fatty or resinous bodies which prevents the water from reaching the muck particles. In such cases water will pass between the particles and percolate to a considerable depth, but without wetting. This oily matter can be removed by treating the dry material with ether in any approved extraction apparatus. For the separation of the more purely fatty bodies, light petroleum may be used, while the total of such matters is extractable with sulfuric ether. The extracted bodies should be examined to determine their nature, whether fatty, resinous, or of other materials soluble in ether. The quantity of this material in some muck soils is remarkably high. In a Florida muck, examined in this laboratory, 18.95 per cent in the air-dried substance, which contained still 41.83 per cent of water, or about 32.5 per cent of the water-free material were found to be soluble in ether.

The color of the ether extract may be almost black, showing the extraction of a part of the humus or coloring matter in the muck. This extractive coloring matter may also be a partial oxidation product of the original chlorophyl of the plant.

531. Further Examination of the Ether Extract.—The ether extract should be first treated with petroleum ether, unless this substance be used first in extraction. Afterwards, it is to be exhausted with strong alcohol, and the quantities of material soluble in the three reagents separately determined.

The nitrogen is further to be determined in the several extracts, and, for control, in the residue of the muck.

The method of procedure practiced in this laboratory is to first extract the sample with petroleum ether, which will yield any free fat acids, fats, or oils, waxes, and possibly some resinous matter. A weighed portion of the sample, about two grams, is extracted quantitively by one of the methods which will be described in the second volume of this manual.

From two to five kilograms of the sample are then extracted in bulk for the purpose of securing a sufficient quantity of the material to use for further analysis.^[349]

In each case the petroleum is followed by pure ether, and in this way the chlorophyl, resins, etc., are obtained. This extract is examined also for its several proximate constituents.^[350]

The treatment with ether is followed by extraction with absolute alcohol for the removal of tannins and other glucosides, resins insoluble in ether, etc., and the extract subjected to the usual examination.^[351] Instead of absolute alcohol a spirit of ninety-five per cent strength, or even of eighty per cent, may be used. The final residue should be subjected to the usual determination for nitrogen, volatile matter, ash, etc., in the manner already described. The large amount of resinous and other matters soluble in petroleum and ether, which is found in the Florida muck soils, is probably due to the proximity of pine forests, the débris of which, sooner or later, find their way to these littoral deposits. Considerable portions of organic humic acids and even humus itself, may also be removed by ether and alcohol and in every case nitrogen should be determined in these extracts.

RARE CONSTITUENTS OF SOILS.

532. Estimation of Copper.—The natural occurrence of copper in many vegetables has acquired additional significance by reason of its relation to added copper in canned peas and other preserves. Formerly, copper was not regarded, in any sense, as a plant food. Even now it can scarcely be considered as more than an accidental and non-essential constituent of vegetable matter. It is by no means certain, however, that copper may not be, in some sense, in organic combination, as phosphorus and sulfur often are. It is said, also, to be found in certain animal organisms, notably in the oyster. In the estimation of copper in soils, there is first made a hydrochloric acid solution of the sample. The solution is treated with well-washed hydrogen sulfid until well saturated. The precipitate is collected at once on a gooch, and washed with water containing the precipitating reagent. The filtrate is dried, gently ignited or roasted, and dissolved in aqua regia. After evaporating to dryness on a steam-bath, water and hydrochloric acid are added, and the copper reprecipitated in the manner described above.

If zinc be present in the sample the solution should be made very strongly acid with hydrochloric before the treatment with hydrogen sulfid, otherwise some zinc may be carried down with the copper.^[352] If lead be present it is also precipitated with the copper and can be separated as described below. In the filtrate from the solution in nitric acid after the second precipitation the copper is precipitated as hydroxid by potash, collected in a porcelain gooch, dried, ignited, and weighed as CuO. Or the copper may be secured as sulfate and estimated electrolytically in the manner described in volume second for the gravimetric estimation of sugar.

533. Estimation of Lead.—If the soil contain lead this metal will be thrown down with the copper as sulfid in the manner described above. In this case the mixed sulfids are dissolved in nitric acid, diluted with water, filtered, and washed. The filtrate is treated with sulfuric acid in considerable excess, and evaporated until all the nitric acid has passed off and the sulfuric acid begins to escape. After cooling, water is added and the lead sulfate collected on a porcelain gooch and washed with water containing sulfuric acid. Finally it is washed with alcohol, dried, ignited, and the lead weighed as PbSO₄.

534. Estimation of Zinc.—If zinc be present in the hydrochloric acid extract of a soil it may be estimated as carbonate after freeing it carefully of iron. The principal part of the iron should first be separated in the usual way by sodium acetate. In the warm solution (acid with acetic) the zinc is precipitated by hydrogen sulfid in excess. The beaker in which the precipitation takes place should be left covered in a warm place at least twelve hours. After collecting the zinc sulfid on a filter it is washed with water saturated with hydrogen sulfid. In order to free the zinc from every trace of iron it is better to dissolve the precipitate in hot dilute hydrochloric acid and reprecipitate as above, and, after boiling with some potassium chlorate, saturate it with ammonia. Any remaining trace of iron is precipitated as ferric hydroxid while the zinc remains in solution. The ferric hydroxid is separated by filtration and the filtrate, after acidifying with acetic, is treated with hydrogen sulfid as above. The zinc sulfid is dissolved again in hot hydrochloric acid, oxidized with potassium chlorate, the acid almost neutralized with soda and the zinc precipitated as carbonate with the sodium salt. After precipitation, the contents of the beaker are boiled until all free carbon dioxid is expelled, the carbonate collected on a filter, washed with hot water, dried, ignited, and weighed as ZnO.

535. Estimation of Boron.—Boron has been found in the ashes of many plants and agricultural products. Whether or not it be an essential or only accidental constituent of plants has not been determined. Its occurrence in the soil, nevertheless, is a matter which the agricultural chemist can not overlook. The boron should be dissolved from the soil by gently heating with dilute nitric acid followed by washing with hot water. Boiling should be avoided on account of the volatility of boric acid. In the solution thus obtained, concentrated on a bath at a moderate temperature to a convenient volume, the boron is to be estimated by the method given in paragraphs 518 and 519.

AUTHORITIES CITED IN PART EIGHTH.

Note.—On page 557, paragraph 500, ninth line, read “red-yellow” instead of “blue.”