At first the tripodal support of the flasks is so turned as to bring them between the lamps and in this way a too rapid reaction is at first avoided. After half an hour the tripods are so turned as to bring each flask directly over the lamp, the flame of which is allowed to impinge directly against the glass. The flame is so regulated that after the evolution of the sulfur dioxid has nearly ceased the contents of the flask are brought into gentle ebullition. The boiling is continued until the contents of the flask are colorless, usually about two hours. As a rule such substances as cottonseed-meal and dried blood will take a longer time for complete combustion than other fertilizing materials. During the combustion the flasks are closed with an oblong loose-fitting unground glass stopper. When the oxidation is finished the contents of the flasks are allowed to cool, the stoppers are removed, and enough water is added to fill the flasks about three-quarters full. The flasks are gently shaken, and the possibility of breaking, from the heat developed, must not be overlooked. To avoid confusion the flasks are all numbered before beginning the work, and the numbers noted by the analyst in connection with the samples. The contents of each one are next poured into the distillation flask and the digestion vessels are washed with 100 cubic centimeters of water in three portions, and the wash-water added to the liquid. Sometimes in washing out the digestion flask yellow basic mercury compounds separate on its walls, but this does not, in any way, influence the accuracy of the results. The distillation flasks should have about 600 cubic centimeters capacity. To avoid the transfer the digestion may take place in the distillation flask in which case the latter must be made of special glass as indicated.

To the liquid thus transferred are added seventy-five cubic centimeters of soda-lye containing one and one-half times as much potassium sulfid as is necessary to combine with the mercuric sulfate present. The lye is of such a strength that sixty cubic centimeters are sufficient to neutralize the acid present. It has a specific gravity of 1.375 and contains thirty-three grams of potassium sulfid in a liter.

In order to avoid the bumping which may take place during the distillation, some granulated zinc should be added.

The distillation flask is closed with a rubber stopper carrying a bulb-tube which ends above in a glass tube about three-quarters of a meter long, bent at an acute angle, and passing obliquely downward on a convenient support. This tube is connected by a rubber, with the end tube bent nearly at a right angle and dipping into the standardized acid in the erlenmeyer receiver. The general arrangement of the distilling apparatus is shown in figure 12. Since the contents of the vessel are warmed by mixing with the soda-lye, the flame can be turned on at full head at once at the commencement of the operation. In about a quarter of an hour the liquid in the receiver will be at the boiling-point, and the boiling should be continued for five minutes more, making twenty minutes in all for the completion of the distillation. By this boiling the contents of the receiver are not charged with carbon dioxid, as might happen if a condenser were used. The receiver contains twenty cubic centimeters of a standardized sulfuric acid solution and about fifty cubic centimeters of water.

The acid used should contain 38.1 grams of sulfuric acid of 1.845 specific gravity in a liter; and it should be set by titration with chemically pure sodium carbonate. For this purpose seven-tenths gram of sodium carbonate is heated in a platinum crucible for two hours over a small flame, weighed, and placed in an erlenmeyer together with twenty cubic centimeters of the sulfuric acid, care being taken to avoid loss from the vigorous evolution of carbon dioxid. After boiling for ten minutes all the carbon dioxid is removed from solution. After cooling, the excess of acid is determined by titration with a standardized barium hydroxid solution, using rosolic acid as indicator.

The solution of barium hydroxid is made as follows: Digest, with warm water, 260 grams of caustic baryta, Ba(OH)₂, until it is nearly all dissolved, filter, and make up to a volume of ten liters and keep in a flask free of carbon dioxid. A solution of barium hydroxid is to be preferred to the corresponding sodium compound for titration. If traces of carbonate be formed in the two liquids, the sodium salt will remain in solution while the barium compound will settle at the bottom of the flask.

The Indicator.—The indicator used to determine the end of the reaction is made by dissolving one gram of rosolic acid in fifty cubic centimeters of alcohol. From one to two drops are enough for each titration. The color reaction is less definite as the quantity of ammonia in the liquid increases. When the titration solutions have been prepared as above described it is found to require about ninety of the barium hydroxid to neutralize twenty cubic centimeters of the sulfuric acid.

By direct titration with sodium carbonate it is ascertained how many grams of nitrogen the twenty cubic centimeters of sulfuric acid represent.

184. The Official Kjeldahl Method. Not Applicable in the Presence of Nitrates.[153]—Reagents.—(1) Acids.—(a) Standard hydrochloric acid the absolute strength of which has been determined by precipitating with silver nitrate, and weighing the silver chlorid as follows:

To any convenient quantity of the acid to be standardized, add solution of silver nitrate in slight excess, and two cubic centimeters of pure nitric acid, of specific gravity 1.2. Heat to boiling-point, and keep at this temperature for some minutes without allowing violent ebullition, constantly stirring until the precipitate assumes the granular form. Allow to cool somewhat, and then pass the fluid through the asbestos. Wash the precipitate by decantation, with 200 cubic centimeters of very hot water, to which have been added eight cubic centimeters of nitric acid and two cubic centimeters of dilute solution of silver nitrate containing one gram of the salt in 100 cubic centimeters of water. The washing by decantation is performed by adding the hot mixture in small quantities at a time, and beating up the precipitate well with a thin glass rod after each addition. The pump is kept in action all the time, but to keep out dust during the washing the cover is only removed from the crucible when the fluid is to be added.

Put the vessels containing the precipitate aside, return the washings once through the asbestos so as to obtain them quite clear, remove them from the receiver, and set aside to recover the excess of silver. Rinse the receiver and complete the washing of the precipitate with about 200 cubic centimeters of cold water. Half of this is used to wash by decantation and the remainder to transfer the precipitate to the crucible with the aid of a trimmed feather. Finish washing in the crucible, the lumps of silver chlorid being broken down with a glass rod. Remove the second filtrate from the receiver and pass about twenty cubic centimeters of ninety-eight per cent alcohol through the precipitate. Dry at from 140° to 150°. Exposure for half an hour is found more than sufficient, at this temperature, to dry the precipitate thoroughly.

(b) Standard sulfuric acid, the absolute strength of which has been determined by precipitation with barium chlorid and weighing the resulting barium sulfate:

For ordinary work half normal acid is recommended, i. e., containing 18.2285 grams of hydrochloric or 24.5185 grams sulfuric acid to the liter; for work in determining very small amounts of nitrogen, one-tenth normal acid is recommended. In titrating mineral acids against ammonia solutions, use cochineal as indicator.

(c) Sulfuric acid, specific gravity 1.84, free of nitrates and also of ammonium sulfate, which is sometimes added in the process of manufacture to destroy nitrogen oxids:

(2) Standard alkali, the strength of which, relative to the acid, has been accurately determined. One-tenth normal ammonia solution, i. e., containing 1.7051 grams of ammonia to the liter, is recommended for accurate work:

(3) Metallic mercury or mercuric oxid, prepared in the wet way: That prepared from mercuric nitrate can not be safely used.

(4) Potassium permanganate finely pulverized:

(5) Granulated zinc, pumice stone, or one-half gram of zinc dust is to be added to the contents of the flasks in distillation, when found necessary, in order to prevent bumping:

(6) Potassium sulfid.—A solution of forty grams of commercial potassium sulfid in one liter of water:

(7) Soda.—A saturated solution of sodium hydroxid free of nitrates:

(8) Indicator.—Solution of cochineal prepared as follows: Tincture of cochineal is prepared by digesting for a day or two, at ordinary temperatures, and frequently agitating, three grams of pulverized cochineal in a mixture of fifty cubic centimeters of strong alcohol with 200 cubic centimeters of distilled water. The solution is decanted or filtered through Swedish paper.

Apparatus.—(1) Kjeldahl digestion flasks of hard, moderately thick, well-annealed glass: These flasks are about twenty-two centimeters long, with a round, pear-shaped bottom, having a maximum diameter of six centimeters and tapering out gradually in a long neck, which is two centimeters in diameter at the narrowest part, and flared a little at the edge. The total capacity is from 225 to 250 cubic centimeters.

(2) Distillation flasks of ordinary shape, of 550 cubic centimeters capacity, or preferably flasks of well-annealed glass, of the same capacity, of pear-shaped bottom, for both digestion and distillation, fitted with a rubber stopper and a bulb-tube above to prevent the possibility of sodium hydroxid being carried over mechanically during distillation: The bulbs are about three centimeters in diameter, the tubes being of the same diameter as the condenser and cut off obliquely at the lower end. This is adjusted to the tube of the condenser by a rubber tube.

Manipulation.—(1) The Digestion.—From seven-tenths to three and five-tenths grams of the substance to be analyzed, according to its proportion of nitrogen, are brought into a digestion flask with approximately seven-tenths gram of mercuric oxid or its equivalent in metallic mercury and twenty cubic centimeters of sulfuric acid. The flask is placed in an inclined position, and heated below the boiling-point of the acid for from five to fifteen minutes or until frothing has ceased. If the mixture froth badly, a small piece of paraffin may be added to prevent it. The heat is then raised until the acid boils briskly. No further attention is required until the contents of the flask have become a clear liquid, which is colorless or at least has only a very pale straw color. The flask is then removed from the frame, held upright, and while still hot, potassium permanganate is dropped in carefully and in small quantities at a time until, after shaking, the liquid remains of a green or purple color.

(2) The distillation.—After cooling, the contents of the flask are transferred to the distilling flask with about 200 cubic centimeters of water, a few pieces of granulated zinc, pumice stone, or one-half gram of zinc dust when found necessary to keep the contents of the flask from bumping, and twenty-five cubic centimeters of potassium sulfid solution are added, shaking the flask to mix its contents. Next add fifty cubic centimeters of the soda solution, or sufficient to make the reaction strongly alkaline, pouring it down the sides of the flask so that it does not mix at once with the acid solution. Connect the flask with the condenser, mix the contents by shaking, and distil until all ammonia has passed over into the standard acid. The first 150 cubic centimeters of the distillate will generally contain all the ammonia. This operation usually requires from forty minutes to one hour and a half. The distillate is then titrated with standard alkali.

The use of mercuric oxid in this operation greatly shortens the time necessary for digestion, which is rarely over an hour and a half in case of substances most difficult to oxidize, and is more commonly less than an hour. In most cases the use of potassium permanganate is quite unnecessary, but it is believed that in exceptional cases it is required for complete oxidation, and in view of the uncertainty it is always used. The potassium sulfid removes all the mercury from the solution, and so prevents the formation of mercurammonium compounds which are not completely decomposed by soda solution. The addition of zinc gives rise to an evolution of hydrogen and prevents violent bumping. Previous to use, the reagents should be tested by a blank experiment with sugar, which will partially reduce any nitrates that are present, which might otherwise escape notice.

185. The Distillation Apparatus in Use in the Laboratory of the Department of Agriculture.—In this laboratory the distilling apparatus is arranged as shown in Figure 13. The flasks are the same as are used in the digestion. They are connected to the block tin condensers by the bulb and rubber tubes, shown hanging on the projecting ends of the block tin condensers on the right and left of the figure. The condensers are contained in a trough through which cold water flows during the distillation. The bulb above the flask carries an emergent tube which extends to near the center of the bulb and is bent laterally to avoid any danger of carrying over any alkali that may be projected into the bulb during boiling. The boiling is continued usually for nearly an hour or until bumping begins. The table on which the apparatus is placed is so arranged as to permit of easy access on all sides. The standard acid is held in erlenmeyers placed on wooden blocks so that the end of the condenser which is a drawn-out glass tube, dips beneath the surface of the acid.

186. Patrick’s Distilling Flask.—To avoid the expense and annoyance attending the breaking of the distilling flasks Patrick has proposed to make them of copper.[154] The size, about half a liter, made for the evolution of oxygen for experimental purposes, may be used. A little excess of potassium sulfid is used to make up for any of it which might be consumed by the copper. About twenty-five cubic centimeters of this solution are recommended. No zinc or pumice stone is required to prevent bumping and the distillation may be finished within thirty minutes, thus securing a saving of time. There will doubtless be a slight corrosion of the flasks by the sulfid employed but where the gunning oxidation process is practiced this danger would be avoided.

187. Modifications of the Kjeldahl Process.—It would be impracticable here to give even a summary of the many unimportant changes which the moist combustion process has undergone since the first papers of its author were published. These changes may be divided into three classes; viz., 1. Those changes which refer solely to the quantities of substance taken for analysis, to the composition of the acid mixture, to the duration of the digestion, to the form and size of the flasks, both for digestion and distillation, and to the manner of distillation and of titration. For references to the papers on these subjects the reader may consult the periodic journals.[155] The most important of these minor changes are the following: Instead of the titration by means of separated iodin most chemists have had recourse to the simpler method of direct titration of the excess of acid by a set solution of an alkali. Barium, sodium, and potassium hydroxids are the alkaline solutions most employed. This process permits of a larger quantity of the sample being taken for combustion and of the use of a larger quantity of acid in the receiver. It also implies the use of a larger digestion flask. In fact it is now quite common to make the digestion in a special glass flask large enough to be used also for the distillation. This saves one transfer of the material with the possible danger of loss attending it.

In the distillation it is a common practice, especially in Germany, to do away with the condensing worm and to carry a long glass tube from the distilling flask directly into the acid on the receiver. The only inconvenience in this method is the heating of the contents of the receiving flask, but this is attended with no danger of loss of ammonia and the distillate, on account of the high temperature it acquires, is left free of carbon dioxid. Many of these minor changes have tended to simplify the process, but without affecting the principle of the method in the least.

2. In the second place a class of changes may be mentioned in which there is a marked difference in the method of effecting the oxidation secured by the introduction of a substance, usually a metal, during the digestion for the purpose of accelerating the oxidation. In the original process the only aid to oxidation was applied at the end of the digestion in the use of potassium permanganate. In the modifications now under consideration a metallic oxid or metal is applied at the beginning of the digestion. Copper and mercury are the metals usually employed. A separate paragraph will be given to the description of this modification known as the process of Wilfarth.

3. The third class of changes is even more radical in its nature, having for its object the adaptation of the moist combustion method to oxidized or mineral nitrogen. The chief feature of this class of changes consists in the introduction of a substance rich in hydrocarbons, and capable of easily forming nitro compounds, for the purpose of holding the oxids of nitrogen which are formed during the combustion and helping finally to reduce them to the form of ammonia. The chief varieties of this class of changes were proposed by Asboth, Jodlbaur, and Scovell, and will be fully set forth in separate paragraphs.

188. Method of Wilfarth.—The basis of this modification as already noted rests on the fact that certain metallic oxids have the power of carrying oxygen, and thus assisting in a katalytic way in the combustion of organic matter.[156] The copper and mercury oxids are best adapted for this purpose and experience has shown that mercuric oxid, or even metallic mercury gives the best results. The manipulation is carried out as follows: From one to three grams of the sample, according to its richness in nitrogen, are heated with a mixture of twenty cubic centimeters of acid containing two-fifths fuming and three-fifths ordinary sulfuric acid. To this is added about seven-tenths gram of mercuric oxid prepared in the wet way from a mercury salt free of nitrogen. The combustion takes place in the usual kjeldahl flask. If the boiling be continued until the liquid is entirely colorless, final oxidation with potassium permanganate is unnecessary. To save time the combustion may be stopped when a light amber color is reached, and then the oxidation finished with permanganate. Before distilling, a sufficient quantity of potassium sulfid is added to precipitate all the mercury as sulfid and thus prevent the formation of mercurammonium compounds which would produce a deficit of ammonia. A convenient strength of the sulfid solution is obtained by dissolving forty grams of potassium sulfid in one liter of water. Bumping at the end of the distillation is not usual, especially if potash-lye be used, but should it occur it may be stopped by the addition of zinc dust.

Only when a large excess of potassium sulfid is used is there an evolution of hydrogen sulfid, the presence of which, however, does not influence the accuracy of the results.

The presence of mercuric sulfid in the solution tends to prevent bumping during the distillation, but it is advisable, nevertheless, to use a little zinc dust. Other minor modifications consist of forming the acid mixture with equal volumes of concentrated and fuming sulfuric acid containing in one liter 100 grams of phosphoric acid anhydrid,[157] and using metallic mercury instead of mercuric oxid; or a mixture of half a gram of copper sulfate and one gram of metallic mercury,[158] or half a gram of copper oxid and a few drops of platinic chlorid solution containing 0.04 gram of platinum in a cubic centimeter.[159]

189. Modification of Asboth.—In order to adapt the moist combustion process to nitric nitrogen Asboth proposed the use of benzoic acid.[160] For half a gram of saltpeter 1.75 grams of benzoic acid should be used. At the end of the combustion the residual benzoic acid is oxidized by means of potassium permanganate with a subsequent reheating. If the nitrogen be present as an oxid or as cyanid, one gram of sugar is added. The metallic element added is half a gram of copper oxid. Asboth also recommends that the soda-lye used in the distillation be mixed with sodium potassium tartrate for the purpose of holding the copper and manganese oxids in solution and thus preventing bumping. The alkaline liquor contains in one liter 350 grams of the double tartrate and 300 grams of sodium hydroxid.

The principle on which the use of benzoic acid rests is found in the fact that it easily yields nitro-compounds and thus prevents the loss of the nitrogen oxids, these readily combining with the benzoic acid. The nitro-compounds can be subsequently converted into ammonia by treatment with potassium permanganate.

The pyridin and chinolin groups of bodies do not yield all their nitrogen as ammonia by the above treatment.

The conclusions drawn by Asboth from the analytical data obtained were:

(1) Sugar should be used in the ordinary kjeldahl process in those cases where the nitrogen in the organic substance is present as oxids or as cyanogen.

(2) In the case of nitrates good results may be secured with benzoic acid but permanganate must be added at the end.

(3) The kjeldahl-wilfarth process can be applied with substances difficultly decomposed, e. g., alkaloidal bodies.

190. Variation of Jodlbaur.—The benzoic acid method, although a step forward, is not entirely satisfactory in the treatment of nitrates by moist combustion. Jodlbaur has proposed to substitute for the benzoic, phenolsulfuric acid.[161]

From two to five-tenths gram of a nitrate are treated with twenty cubic centimeters of concentrated sulfuric and two and a half of phenolsulfuric acid, together with three grams of zinc dust and five drops of a solution of platinic chlorid of the strength mentioned above. The phenolsulfuric acid is prepared by dissolving fifty grams of phenol in 100 cubic centimeters of strong sulfuric acid. The combustion is continued until the solution is colorless, which may take as much as five hours. If phosphoric acid anhydrid be used as recommended above, the time of the combustion may be diminished by one-half, but in such a case the glass of the combustion flask is strongly attacked and is quite likely to break.

With substances very rich in nitrates it is advisable to rub them first with dry gypsum.

The theory of the process rests on the fact that by a careful admixture of a nitrogenous substance diluted with land plaster, with phenolsulfuric acid, it is possible to change the nitric acid into nitro-phenol, and by the reducing action of zinc dust to change the nitro-product formed into amido-phenol. This afterwards is transformed into ammonium sulfate by heating with sulfuric acid, by which process, at the same time, all other nitrogenous compounds present in the substance, as with Kjeldahl’s method, likewise form ammonium sulfate, only with the difference that addition of mercury is here absolutely necessary for the complete transformation of the slowly decomposed amido-phenol which again brings about the necessity of decomposing the nitrogenous mercury compounds formed in the solution by potassium sulfid, which is added after or with the soda-lye.

191. The Dutch Jodlbaur Method.—The Royal Experiment Station of Holland directs that the jodlbaur process be carried out as indicated below.[162]

The reagents necessary are:

1. Phenolsulfuric acid, prepared by dissolving 100 grams of pure crystallized phenol in pure sulfuric acid (1.84) and making up the solution to a liter with the same sulfuric acid:

2. Zinc, carefully washed and thoroughly dried:

3. Sodium hydroxid solution, the same as is used in the kjeldahl method:

4. Potassium sulfid solution, made by dissolving 355 grams of potassium sulfid (K₂S), or sodium sulfid solution, made by dissolving 250 grams of sodium sulfid (Na₂S) in a liter of water.

As apparatus, are necessary oxidation flasks holding about 200 cubic centimeters, and distillation flasks holding about 750 cubic centimeters, both of bohemian glass.

Manipulation.—Weigh one gram of substance, moisten it with water, dry, and introduce into an oxidation flask. Cover with fifteen cubic centimeters of phenolsulfuric acid and, after cooling, thoroughly mix by gently shaking. After five minutes add from two to three grams of zinc in small proportions, keeping the flask cool, then twenty cubic centimeters of sulfuric acid, and finally two drops of mercury. Boil the mixture till the fluid is colorless. Cool and dilute. Wash into a distillation flask and add an excess of sodium hydroxid solution and twenty-five cubic centimeters of the sodium (or potassium) sulfid solution. Distil and titrate as in the kjeldahl method.

192. The Halle-Jodlbaur Method.—At the Halle station it is the uniform practice to mix the nitrate with gypsum before the combustion.[163] In the case of Chile phosphates ten grams are rubbed with an equal amount of gypsum, and two grams of the mixture, equal to one gram of the nitrate, taken for the determination. In the case of saltpeter mixtures which contain over eight per cent of nitrogen, one gram of the mixture with gypsum is taken, of guanos one and a half grams, and of lower forms of nitrates or mixtures thereof, from three to five grams.

The sample, as prepared above, is treated with thirty cubic centimeters of a mixture of phenolsulfuric acid and phosphoric acid anhydrid. The mixture is prepared by dissolving sixty-six grams of phenol and 250 grams of phosphoric anhydrid in strong sulfuric acid, and, after cooling, mixing the two solutions and making the volume up to 1,650 cubic centimeters with pure sulfuric acid. The mixture contains, in thirty cubic centimeters, one and two-tenths grams of phenol and four grams of phosphoric anhydrid. In the use of phenolsulfuric acid the the presence of phosphoric anhydrid is indispensable in keeping the sulfuric acid water-free and in absorbing the water produced by the combustion.

The phenolsulfuric acid used contains only enough phenol to reduce half a gram of saltpeter.

The sample and acid mixture having been put in the combustion flask the latter is shaken, at intervals, for an hour, and the contents cooled.

The conversion of the nitrates into nitro-phenol compounds is finished in this time, and the next step consists in reducing these bodies to the amido-phenol group. This is accomplished in the cold by nascent hydrogen produced by the addition of zinc dust to the mixture. From one to three grams of the dust are to be used in proportion to the quantity of nitrates originally present.

The flask should be placed in a cooling mixture and the zinc dust added in small portions to prevent a too violent evolution of hydrogen. After the reduction is ended the flask is allowed to stand for two hours, after which the combustion, distillation, and titration are accomplished in the usual way. On cooling, after the end of the combustion, the contents of the flask become solid. They may be brought again into liquid state by shaking and gentle warming.

193. The Official Kjeldahl Method for Nitric Nitrogen.—As has already been stated, the presence of certain organic compounds, rich in hydrocarbons, permits the reduction of nitric nitrogen to ammonia by combustion with sulfuric acid. Benzol, phenol, and salicylic acid have all been used for this purpose. The official chemists have adopted for their method the salicylic acid process first proposed by Scovell.[164]

Besides the reagents and apparatus given under the Kjeldahl method there will be needed:

(1) Zinc dust: This should be an impalpable powder; granulated zinc or zinc filings will not answer.

(2) Sodium thiosulfate:

(3) Commercial salicylic acid:

It is found most convenient to prepare a solution of 33.3 grams of salicylic acid in one liter of the strongest sulfuric acid, and keep it for use rather than to mix it for each combustion. We prefer the thiosulfate process first mentioned below. In the zinc dust method there has been noticed a tendency for the distillation flask to break just at the end of the process.

The Manipulation.—Place from seven-tenths to three and five-tenths grams of the substance to be analyzed in a kjeldahl digesting flask, add sixty cubic centimeters of sulfuric acid containing one gram of salicylic acid, and shake until thoroughly mixed, then add five grams of crystallized sodium thiosulfate; or add to the substance thirty cubic centimeters of sulfuric acid containing two grams of salicylic acid, then add gradually two grams of zinc dust, shaking the contents of the flask at the same time. Finally place the flask on the stand for holding the digestion flasks, where it is heated over a low flame until all danger from frothing has passed. The heat is then raised until the acid boils briskly and the boiling continued until white fumes no longer pour out of the flask. This requires about five or ten minutes. Add now approximately seven-tenths gram of mercuric oxid or its equivalent in metallic mercury, and continue the boiling until the liquid in the flask is colorless or nearly so. In case the contents of the flask are likely to become solid before this point is reached add ten cubic centimeters more of sulfuric acid. Complete the oxidation with a little potassium permanganate in the usual way, and proceed with the distillation as described in the kjeldahl method. The reagents should be tested by blank experiments.

194. The Gunning Moist Combustion Process.—The modification proposed by Gunning was based upon the observation that in the ordinary kjeldahl process the excess of sulfur trioxid in the beginning of the operation soon escapes or unites with water in a form not easily decomposed.[165] During the progress of the combustion the acid diminishes in strength until it is below the concentration represented by the formula H₂SO₄, and in this diluted condition the oxidation takes place more slowly. Gunning proposes to avoid this difficulty by mixing potassium sulfate with the sulfuric acid. This salt forms with the sulfuric acid, acid salts which, by heating, lose water easier than acid and as is well known, they not only act as decomposing and oxidizing media as well as sulfuric acid, but even in a higher degree, resembling the action of sulfuric acid at high temperatures and under pressure.

By heating this mixture of sulfuric acid and potassium sulfate with organic matters in an open vessel, not only the water originally present, but that which is formed during the oxidation is driven off without loss of acid. For this reason instead of the oxidizing mixture becoming weaker, the acid becomes stronger, the boiling-point rises and this, combined with the fluidity of the mass favors the decomposition and oxidation of the organic matter in a constantly increasing ratio.

The original mixture used by Gunning had the following composition; viz., one part of potassium sulfate and two parts of strong sulfuric acid. The substances are united by heat and, on cooling, are in a semi-solid state, melting, however, easily on the application of heat and assuming a condition to be easily poured from vessel to vessel. The quantity of the sample taken should vary in proportion to its nitrogenous content from half a gram to a gram. The combustion takes place in flasks entirely similar to those used in the ordinary kjeldahl process. In the case of liquids, they should be previously evaporated to dryness before the addition of the oxidizing mixture. At the beginning of the combustion there is a violent foaming attended with evolution of some acid and much water, and afterwards of stronger acid. This loss of acid should not be allowed to go far enough to produce too great concentration of the material in the flask. One of the best ways to avoid it is to place a funnel in the flask covered with a watch-glass which will permit of the condensation and return of the escaping acid. As soon as the foaming ceases, the flame should be so regulated as to permit of the volatilized acid being condensed upon the sides of the flask. In the end a colorless mass is obtained in which no metallic oxids are present, and this mass can at once be diluted with water, treated with alkali, and distilled. According to the nature of the substance from half an hour to an hour and a half are required for the complete combustion.

Modifications of the Gunning Method.—As in the case of the kjeldahl method, numerous minor modifications of the gunning method have been made, the most important of which relate to its application to substances containing nitrates. In general the same processes are employed in this case as with the kjeldahl method. One of the best modifications consists in the use of the mixture of salicylic and sulfuric acids followed by the addition of sodium thiosulfate or of potassium sulfate or sulfid. These modifications will be given in detail under the official methods.

195. Reactions of the Gunning Process.—The various reactions which take place during the combustion according to the gunning method have been tabulated by Van Slyke.[166]

The first reaction to take place is the union of sulfuric acid and potassium sulfate to form potassium acid sulfate in accordance with the following equation:

(1) K₂SO₄ + H₂SO₄ = 2KHSO₄.

When heated, the potassium acid sulfate decomposes, forming potassium disulfate and water, thus:

(2) 2KHSO₄ = K₂S₂O₇ + H₂O.

The potassium disulfate at a higher temperature decomposes, forming normal potassium sulfate and sulfur trioxid, thus:

(3) K₂S₂O₇ = K₂SO₄ + SO₃.

At a sufficiently high temperature the two preceding reactions may take place in one, thus:

2KHSO₄ = K₂SO₄ + H₂O + SO₃.

At the temperature at which these reactions take place, the water that is set free does not recombine with the sulfur trioxid nor with the sulfuric acid that is present in excess, but is expelled from the mixture; hence the mixture becomes more concentrated during the digestion. The sulfur trioxid set free acts upon the organic matter in the powerful manner peculiar to it, and the potassium sulfate formed in the last reaction above unites with another molecule of sulfuric acid, and the same round of reactions is repeated continuously so long as there is an excess of free sulfuric acid present in the mixture. As the liquid becomes more concentrated with the continuation of the digestion, the boiling-point increases so that the effect is the same as heating under pressure. The danger of too great concentration and risk of consequent loss of nitrogen is avoided by using increased proportions of sulfuric acid.

As compared with the kjeldahl the gunning method presents the following advantages:

(1) The gunning method requires fewer reagents. As no form of mercury is used no potassium sulfid is needed, and there is no risk of loss from the presence of mercurammonium compounds.

(2) The solution to which caustic soda is added is clear, so that in neutralizing, it is an easy matter to avoid great excess of alkali, and so, in most cases, to avoid foaming and bumping in distillation.

(3) In the blank determinations less nitrogen is found in the reagents used in the gunning method. In only one case was more nitrogen reported in a blank by this method; in all the others the amount averaged considerably less.

196. The Official Gunning Method.—In a digestion flask holding from 250 to 500 cubic centimeters place from seven-tenths to two and eight-tenths grams of the substance to be analyzed, according to its proportion of nitrogen. Then add ten grams of powdered potassium sulfate and from fifteen to twenty-five cubic centimeters (ordinarily about twenty cubic centimeters) of concentrated sulfuric acid. Conduct the digestion as in the kjeldahl process, starting with a temperature below boiling-point and increasing the heat gradually until frothing ceases. Digest until colorless or nearly so. Do not add either potassium permanganate or potassium sulfid. Dilute, neutralize, and distil as in the kjeldahl method. In neutralizing, it is convenient to add a few drops of phenolphthalein indicator, by which one can tell when the acid is completely neutralized, remembering that the pink color, which indicates an alkaline reaction, is destroyed by a considerable excess of strong fixed alkali. The distillation and titration are conducted as in the kjeldahl method. In distilling, the use of zinc or of any substance to prevent bumping or foaming is generally unnecessary, if too great an excess of fixed alkali be avoided. The amount of sulfuric acid recommended by Gunning is two grams for each gram of potassium sulfate; but Voorhees has found that this mixture is so viscous as to cause troublesome foaming frequently, and after cooling it cakes in a hard mass, which may be difficult to redissolve.[167] To avoid foaming and caking, he has found it an effective means to increase the amount of sulfuric acid used, taking instead of two grams to one of potassium sulfate three or four grams of acid to one of potassium sulfate. It is, therefore, suggested in carrying out the work, to use from five to twenty-five cubic centimeters (ordinarily about twenty cubic centimeters) of sulfuric acid for ten grams of potassium sulfate. In case the potassium sulfate is not free from nitrogen compounds, one or two recrystallizations will make it pure.

197. Gunning Method Adapted to Nitrates.—The essential features of this modification are due to Winton and Voorhees.[168] The modifications of the kjeldahl method, for similar purposes, furnished the material details for the gunning modified process. Winton reports good results from digesting for two hours, from half a gram to a gram of the sample with thirty cubic centimeters of sulfuric containing two grams of salicylic acid, in a flask of half a liter capacity. Two grams of zinc dust are then slowly added, with constant shaking, and the flask heated, at first gently, until, after a few minutes boiling, dense fumes are no longer emitted. Three grams of potassium sulfate are next added and the boiling continued until the solution is colorless, or if iron be present, until a light straw color is produced. On cooling, when the mixture begins to solidify, water is added with caution, and afterwards sodium hydroxid, and the ammonia is obtained by distillation.

In the process, as conducted by Voorhees, about one gram of the sample is digested with ten grams of potassium sulfate and thirty cubic centimeters of sulfuric containing one gram of salicylic acid, and three grains of zinc sulfid. The heat is kept down until frothing ceases, and then the mass kept in gentle ebullition until clear. The distillation is accomplished with the usual precautions. The voorhees process is superior to that recommended by Winton in adding the potassium sulfate at the beginning of the combustion.

198. Official Gunning Method Modified to Include the Nitrogen of Nitrates.—In a digestion flask holding from 250 to 500 cubic centimeters, place from seven-tenths to three and five-tenths grams of the substance to be analyzed, according to the amount of nitrogen present. Add from thirty to thirty-five cubic centimeters of salicylic acid mixture, namely, thirty cubic centimeters of sulfuric to one gram of salicylic acid, shake until thoroughly mixed, and allow to stand from five to ten minutes, with frequent shaking; then add five grams of sodium thiosulfate and ten grams of potassium sulfate. Heat very gently until frothing ceases, then strongly until nearly colorless. Dilute, neutralize, and distil the same as in the gunning method.

DETERMINATION OF NITROGEN IN
DEFINITE FORMS OF COMBINATION.

199. Introductory Considerations.—In the foregoing pages has been given a summary of the methods most in vogue for the estimation of nitrogen in fertilizers and fertilizing materials. There are many cases in which the analyst may have to deal with a definite chemical compound, and where a modified or shorter method may be used. There are other cases in which the nitrogen may be present in two or three definite forms, as in artificially mixed fertilizers, and where it is desirable to show the proportions in which the various forms are present. For these reasons it is necessary to be able to use methods by which the percentage of nitrogen in its various forms may be relatively as well as absolutely determined. Such a case would be presented for instance, in that of a fertilizer containing dried blood, sodium nitrate, and ammonium sulfate. It is evident here that the total nitrogen could be determined by the volumetric method by combustion with copper oxid, or by the moist combustion process adapted to nitric nitrogen, but the method of determining the percentage of each constituent has not yet been described.

We have to deal here with a case entirely similar to that of phosphoric acid in a superphosphate. There is no doubt whatever of the uneven assimilability of the different forms of nitrogen. A nitrate, for instance, is already in condition for assimilation by plants. An ammoniacal salt is only partly changed to a state suited to plant nutrition while organic nitrogen is forced to undergo a complete transformation before it becomes available to supply the needs of the growing plant. It is important, therefore, equally to the analyst, the merchant, and the agronomist, to know definitely the forms of combination in which the nitrogen exists and the relative proportion of the different combinations.

200. Nitrogen as Ammonia.—The most frequent form in which nitrogen as ammonia is used for fertilizing is as sulfate. The method of determination to be described is, however, equally applicable to all ammonia salts. When no other form of nitrogenous compound is present the ammonia can be easily and directly determined by distillation with soda- or potash-lye, as described in the final part of the moist combustion process.

To one gram of the ammonia salt add from 200 to 300 cubic centimeters of water and thirty grams of the soda-lye used in the moist combustion process; distil, collect the ammonia, and titrate the excess of sulfuric acid exactly as there described.

Fresenius recommends that the ammonia expelled by distillation be taken up by one-fifth normal sulfuric acid, the excess of which is titrated with one-fifth normal soda, using phenolphthalein as an indicator. If the distillate, on examination, be found to contain thiocyanate, soda-lye cannot be used for the expulsion of ammonia, but, in its place, caustic magnesia is applied.

In all cases where organic matter containing nitrogen is present, caustic magnesia must be substituted for the soda solution. The magnesia must be added in sufficient excess and the distillation continued a little longer than is necessary when soda-lye is used. Otherwise the details of the operation are the same.

In a mixed fertilizer containing organic nitrogen and ammonia salts, the total nitrogen can be determined by the moist combustion process, and the ammoniacal nitrogen by distillation with magnesia. The difference between the two results will give the nitrogen due to the organic matter.

To avoid any danger whatever of decomposing organic nitrogenous compounds, the ammonia may be determined in the cold by treatment with soda-lye, under a bell-jar containing some set sulfuric acid. The operation must be allowed to continue for many days. Even at the end of a long time it will be found that some ammonia is still escaping. It may therefore be finally inferred that all the nitrogen as ammonia is not obtained by this process, or that even magnesia may gradually convert other nitrogenous compounds into ammonia. In this connection the methods of determining ammonia in soils 406, 407, and 408 of volume one may be consulted.

201. Method of Boussingault.—The official French method is essentially the original method of Boussingault with slight modifications. It is conducted as follows:[169] In case the sample is ammonium sulfate about half a gram is placed in a flask of half a liter capacity, together with 300 cubic centimeters of distilled water and two grams of caustic magnesia. The flask is connected with a condenser of glass or metal which ends in a tube drawn out to a point and dipping beneath the set acid in the receiver in the usual way. The acid is colored with litmus or lacmoid tincture. The distillation is continued until about 100 cubic centimeters have gone over. The receiver is then removed with the usual precautions and the residual acid titrated. Suppose twenty cubic centimeters of normal acid have been employed and twelve and a half cubic centimeters of normal alkali be necessary to neutralize the excess of the acid. Then the nitrogen is found by the following equations: 20.0 - 12.5 = 7.5 and 7.5 × 0.014 = 0.105 gram = weight of nitrogen found. Then 0.105 × 100 ÷ 5 = 21.00 = per cent of nitrogen found.

The distilling apparatus of Aubin is preferred by the French chemists, an apparatus so arranged with a reflux partial condenser, that nearly all the aqueous vapor is returned in a condensed state to the flask while the ammonia, on account of its great volatility, is carried over into the receiver. To avoid the regurgitation which might be caused by the concentrated ammonia gas coming in contact with the acid the separable part of the condensing tube is expanded into a bulb large enough to hold all the acid which lies above its mouth. By the means of this apparatus the ammonia is all collected in the standard acid without greatly increasing its volume and the titration is thus rendered sharper. The employment of caustic magnesia has the advantage of not decomposing any organic matters or cyanids that may be present.

If the sample under examination hold part of its ammonia as ammonium magnesium phosphate it will be necessary first to treat it with sulfuric acid in order to set the ammonia free and then to use enough of the magnesium oxid to neutralize the excess of the sulfuric acid and still supply the two grams necessary for the distillation. When the sample contains a considerable quantity of organic matter it sometimes tends to become frothy towards the end of the distillation. This trouble can be avoided by introducing into the flask one or two grams of paraffin.

Where carbon dioxid is given off during the distillation the contents of the receiver must be boiled before titration, or else lacmoid must be used as an indicator instead of litmus.

202. Determination of Thiocyanates in Ammoniacal Fertilizers.—The extended use of ammonium sulfate as a fertilizer renders it important to determine the actual constituents which may be present in samples of this material. The following bodies have been found in commercial ammonium sulfates: Sulfuric acid, chlorin, ammonia, thiocyanic acid, potash, soda, lime and iron oxid. These are found in the soluble portions. In the insoluble portions have been found silica, sulfuric acid, lime, magnesia and iron oxid. A sample of commercial ammonium sulfate analyzed by Jumeau contained the following substances:[170]

The determination of the thiocyanic acid in the thiocyanate is generally made by the oxidation of the sulfur to sulfuric acid and its subsequent weighing in the form of barium sulfate. Jumeau has modified the method by determining the amount of the thiocyanate by means of a titrated liquid. The method is practiced as follows:

A solution of ammonium thiocyanate is prepared, containing eight grams of this salt per liter, and its exact content of thiocyanate is rigorously determined by titration with silver nitrate or by the weight of the barium sulfate produced after the oxidation of the sulfur. Ten cubic centimeters of the titrated liquor are taken and diluted with water to about 100 cubic centimeters and ten cubic centimeters of pure sulfuric acid added. Afterward, drop by drop, a solution of potassium permanganate is added, containing about ten grams of that salt per liter. The permanganate is instantly decolorized. There is an evolution of hydrocyanic acid as the thiocyanate passes to the state of sulfuric acid. A single drop in excess gives to the mixture the well-known rose coloration of the permanganate solution which persists for several hours. The number of cubic centimeters necessary to produce the persistent rose tint is noted and the same operation is carried on with from one-half to one gram of the unknown product which is to be assayed. A simple proportion indicates the content of the thiocyanate in the unknown body. The process is of great exactitude and permits the rapid determination of thiocyanic acid in the presence of chlorids, cyanids, etc., which remain without action upon the permanganate. In case chlorids and cyanids are absent the thiocyanate can be determined directly by silver nitrate either by weighing the precipitate or by the process of Volhardt based upon the precipitation of the silver by thiocyanate in the presence of a ferric salt. The end of the reaction is indicated by the red coloration which the liquid shows when the thiocyanate is in excess.

203. Separation of Albuminoid from Amid and Other Forms of Nitrogen in Organic Fertilizers.—It may be of interest to the dealer, farmer, and analyst, to discriminate between the albuminoid and other nitrogen in fertilizers, such as oil-cakes. The final value of the nitrogen for plant nourishment is not greatly different, but the immediate availability for nitrification is a matter of some importance. The most convenient process in such a case is the copper hydroxid separation process as improved by Stutzer.[171] The process is conveniently carried out in accordance with the method prescribed by the official chemists.[172]

Total Crude Protein.—Determine nitrogen as directed for nitrogen in fertilizers and multiply the result by 6.25 for the crude protein.

Determination of Albuminoid Nitrogen.—To from seven-tenths to eight-tenths gram of the substance in a beaker add 100 cubic centimeters of water, heat to boiling, or in the case of substances rich in starch, heat on the water-bath ten minutes, and add a quantity of cupric hydroxid mixture containing from one-half to six-tenths gram of the hydroxid; stir thoroughly, filter when cold, wash with cold water, and put the filter and its contents into the concentrated sulfuric acid for the determination of nitrogen. The filter-papers used must be practically free of nitrogen. Add sufficient potassium sulfid solution to completely precipitate all copper and mercury, and proceed as in the moist combustion process for nitrogen. If the substance examined consist of seed of any kind, or residues of seeds, such as oil-cake or anything else rich in alkaline phosphates, add a few cubic centimeters of a concentrated solution of alum just before adding the cupric hydroxid, and mix well by stirring. This serves to decompose the alkaline phosphates. If this be not done cupric phosphate and free alkali may be formed, and the protein-copper may be partially dissolved in the alkaline liquid.

Cupric Hydroxid.—Prepare the cupric hydroxid as follows: Dissolve 100 grams of pure cupric sulfate in five liters of water, and add twenty-five cubic centimeters of glycerol; add a dilute solution of sodium hydroxid until the liquid is alkaline; filter, rub the precipitate up with water containing five cubic centimeters of glycerol per liter, and then wash by decantation or filtration until the washings are no longer alkaline. Rub the precipitate up again in a mortar with water containing ten per cent of glycerol, thus preparing a uniform gelatinous mass that can be measured out with a pipette. Determine the quantity of cupric hydroxid per cubic centimeter of this mixture.

Amid Nitrogen.—The albuminoid nitrogen determined as above subtracted from the total, gives that part of the organic nitrogen existing in the sample as amids and in other allied forms.

204. Separation of Nitric and Ammoniacal from Organic Nitrogen.—The nitrogen being present in three forms, viz., organic, ammoniacal, and nitric, the separation of the latter two may be accomplished by the following procedure:[173] One gram of the fertilizer is exhausted on a small filter with a two per cent solution of tannin, using from thirty to forty cubic centimeters in small portions. This is sufficient to dissolve all the nitrates and the greater portion of the ammoniacal salts, while the tannin renders insoluble all the organic nitrogenous compounds. The filter and its contents are treated for nitrogen by the kjeldahl process. When the distillation and titration are completed the solution obtained by the aqueous tannin is added to the distilling flask and the operation continued. This represents the ammoniacal nitrogen.

The nitric acid is estimated by the ferrous iron or other appropriate method in another portion of the substance.

This method can be used even when the fertilizer contains ammonium magnesium phosphate. In this case digest one gram for fifteen hours in dilute soda-lye solution, which easily dissolves the ammonium magnesium phosphate. Filter and wash the insoluble part with the tannin solution. The residue is treated as above. The filtered solution distilled with soda-lye furnishes the ammonia. The nitrates are estimated by one of the methods above mentioned.

205. Nitric Nitrogen.—The methods of estimating nitric nitrogen, both when present in weighable quantities and as mere traces have been sufficiently described in the first volume. For convenience, however, the standard methods of procedure will be given here. The moist combustion methods adapted to nitrates and the volumetric copper oxid process have already been described. Of the reduction methods the process of Ulsch is one of the easiest of application and also reliable. As practiced by the official chemists the manipulation is conducted as described in the first volume, page 539.

206. Ulsch Method, Applicable to Mixed Fertilizers.—The method of Ulsch which is found to give good results with pure nitrates or with nitrates in the absence of other forms of nitrogen may also be adapted to mixed fertilizers containing nitrogen in more than one form. Street has developed such a method and shown, by analytical data, that it is applicable in a great number of cases.[174] The process is based on the substitution of magnesia for soda in the distillation and is carried on as follows:

Place one gram of the sample in a half liter flat-bottomed flask. Add about thirty cubic centimeters of water, one gram of reduced iron, and ten cubic centimeters of sulfuric acid diluted with an equal volume of water, shake well, and allow to stand for a short time. This will remove the danger of an explosion caused by the otherwise violent action which takes place. Close the neck of the flask with a rubber stopper through which passes a glass dropping-bulb filled with water. The flask having been stoppered, place it on a slab to which a moderate heat is applied. Allow the solution to come slowly to a boil and then boil for five minutes and cool. Add about 100 cubic centimeters of water, a little paraffin, and about five grams of magnesium oxid. Boil for forty minutes, after which time all the ammonia will be distilled, and collect the ammonia in set acid.

The magnesia causes a slight frothing, which can easily be controlled by adding a little paraffin and by bringing to a boil very gradually. Fully forty minutes are necessary to distil all the ammonia. Tests were made after thirty minutes boiling and traces of ammonia were still found; after forty minutes these traces entirely disappeared.

The method is a quick one. One man can easily do six determinations at a time, and these six determinations can be made in but a little over an hour. Magnesia gives results closely agreeing with theory and causes a very slight frothing, which can be easily controlled. One gram of reduced iron is sufficient in all ordinary complete fertilizers.

Magnesia is preferred to caustic soda in the distillation because it produces less frothing and by reason of the danger of some of the soda-lye being carried over mechanically and thus tending to produce an error of a plus nature. In the use of magnesia, assurance must be had that it is strongly in excess. Being less active in its effects a longer time for the distillation must be taken than when soda-lye is used. The modified ulsch method just described is recommended provisionally and with the expectation that each analyst will ascertain its true merits before allowing it to displace longer approved processes.

207. Method of Schlösing-Wagner.—The Schlösing-Wagner method for estimating nitrogen in the nitrates of fertilizers is carried out at the Halle Experiment Station as follows:[175]

A flask, figure 14, of about 250 cubic centimeters capacity, is provided with a rubber stopper with two holes. Through one of them is passed the stem of a funnel carrying a glass stop-cock. The other carries a delivery-tube leading to the receiving vessel. The end of the delivery tube is bent so as to pass easily under the mouth of the measuring burette and is covered with a piece of rubber tubing.

Fifty cubic centimeters of saturated ferrous chlorid solution and the same quantity of ten per cent hydrochloric acid are placed in the flask. The ferrous chlorid solution is obtained by dissolving nails or other small pieces of iron in hot hydrochloric acid and it is kept in glass stoppered flasks, of about fifty cubic centimeters capacity, entirely filled. The content of one flask is enough for about twelve determinations and by using the whole content of a flask as soon as possible after opening, any danger of oxidation which would take place in a large flask frequently opened is avoided.