Chapter VII: Part 7

=132. The Citrate Method Applied to Samples with Small Content of Phosphoric Acid.=—It is well established that the citrate method does not give satisfactory results when applied to samples containing small percentages of phosphoric acid, especially when these are of an organic nature, as for instance, cottonseed cake-meal. In this laboratory attempts have been made to remedy this defect in the process so as to render the use of the method possible even in such cases.[120] Satisfactory results have been obtained by adding to the solution of the cake-meal a definite volume of a phosphate solution of known strength. Solutions of ordinary mineral phosphates are preferred for this purpose. The following example will show the application of the modified method:

In a sample of cake-meal, (cottonseed cake and castor pomace) the content of phosphoric acid obtained by the molybdate method, was 2.52 per cent.

Determined directly by the citrate method, the following data were obtained:

Allowing to stand thirty hours after adding magnesia mixture, 1.08 and 1.53 per cent in duplicates.

Allowing to stand seventy-two hours after adding magnesia mixture, 2.17 and 2.30 per cent in duplicates.

In each case fifty cubic centimeters of the solution were taken, representing half a gram of the sample.

In another series of determinations twenty-five cubic centimeters of the sample were mixed with an equal volume of a mineral phosphate solution, the value of which had been previously determined by both the molybdic and citrate methods. The fifty cubic centimeters thus obtained represented a quarter of a gram each of the cake-meal and mineral phosphates. The filtration followed eighteen hours after adding the magnesia mixture. The following data show the results of the determinations:

Per cent Per cent Per cent Per cent
P₂O₅ in P₂O₅ in P₂O₅ P₂O₅ in
mineral organic found in organic
phosphate. sample. mixture × 2. sample.

1 15.37 2.52 17.90 2.53
2 29.16 2.52 31.68 2.52
3 31.37 2.52 33.83 2.45
4 31.58 2.52 34.20 2.62
----
Mean content of P₂O₅ in organic sample 2.53

It is thus demonstrated that the citrate method can be applied with safety even to the determination of the phosphoric acid in organic compounds where the quantity present is less than three per cent. It is further shown that solutions of mineral phosphates varying in content of phosphoric acid from fifteen to thirty-two per cent may be safely used for increasing the content of that acid to the proper degree for complete precipitation. In cases where organic matters are present they should be destroyed by moist combustion with sulfuric acid as in the determination of nitrogen to be described in the next part.

=133. Direct Precipitation of the Citrate-Soluble Phosphoric Acid.=—The direct determination of citrate-soluble phosphoric acid by effecting the precipitation by means of magnesia mixture in the solution obtained from the ammonium citrate digestion, has been practiced for many years by numbers of European chemists, and the process has even obtained a place in the official methods of some European countries. Various objections have been urged, however, against the general employment of this method in fertilizer analysis on account of the inaccuracies in the results obtained in certain cases, and it has, therefore, been used to but a very limited extent in this country. Since it is impracticable to effect the precipitation with ammonium molybdate in the presence of citric acid the previous elimination or destruction of this substance has been recognized as essential to the execution of a process involving the separation of the phosphoric acid as phosphomolybdate.

It is evident from the data cited in the preceding paragraph, that great accuracy may be secured in this process by adding a sufficient quantity of a solution of a mineral phosphate and proceeding by the citrate method.

Ross has also proposed to estimate the acid soluble in ammonium citrate directly by first destroying the organic matter by moist combustion with sulfuric acid.[121] He recommends the following process:

After completion of the thirty minutes’ digestion of the sample with citrate solution, twenty-five cubic centimeters are filtered at once into a dry vessel. If the liquid be filtered directly into a dry burette, twenty-five cubic centimeters can be readily transferred to another vessel without dilution. After cooling, run twenty-five cubic centimeters of the solution into a digestion flask of 250-300 cubic centimeters capacity, add about fifteen cubic centimeters of concentrated sulfuric acid and place the flask on a piece of wire gauze over a moderately brisk flame; in about eight minutes the contents of the flask commence to darken and foaming begins, but this will occasion no trouble, if an extremely high, or a very low flame be avoided. In about twelve minutes the foaming ceases and the liquid in the flask appears quite black; about one grain of mercuric oxid is now added and the digestion is continued over a brisk flame. The operation can be completed in less than half an hour with ease, and in many cases, twenty-five minutes. After cooling, the contents of the flask are washed into a beaker, ammonia is added in slight excess, the solution is acidified with nitric, and after the addition of fifteen grams of ammonium nitrate, the process is conducted as usual.

In case as large an aliquot as fifty cubic centimeters of the original filtrate be used, ten cubic centimeters of sulfuric acid are added, and the digestion is conducted in a flask of 300-500 cubic centimeters capacity; after the liquid has blackened and foaming has progressed to a considerable extent, the flask is removed from the flame, fifteen cubic centimeters more of sulfuric acid are added, and the flask and contents are heated at a moderate temperature for two or three minutes; the mercuric oxid is then added and the operation completed as before described.

Following are some of the advantages offered by the method described:

(1) It dispenses with the necessity of the execution of the frequently tedious operation of bringing upon the filter and washing the residue from the ammonium citrate digestion, while the ignition of this residue together with the subsequent digestion with acid and filtration are also avoided.

(2) It affords a means for the direct estimation of that form of phosphoric acid which, together with the water-soluble, constitutes the available phosphoric acid, thus enabling the latter to be determined by making only two estimations.

(3) In connection with the advantages above mentioned it permits of a considerable saving of time, as well as of labor required in manipulation.

In addition to the tests with mercuric oxid, both potassium nitrate and potassium sulfate were used in the digestion to facilitate oxidation. With the former, several additions of the salt were necessary to secure a satisfactory digestion, and even then the time required was longer than with the mercury or mercuric oxid digestion. With potassium sulfate, the excessive foaming which took place interfered greatly with the execution of the digestion process.

=134. Availability of Phosphatic Fertilizers.=—There is perhaps no one question more frequently put to analysts by practical farmers than the one relating to the availability of fertilizing materials. The object of the manufacturer should be to secure each of the valuable ingredients of his goods in the most useful form. The ideal form in which phosphoric acid should come to the soil is one soluble in water. Even in localities where heavy rains may abound, there is not much danger of loss of soluble acid by percolation. As has before been indicated, the soluble acid tends to become fixed in all normal soils, and to remain in a state accessible to the rootlets of plants, and yet free from danger of leaching. For this reason, by most agronomists, the water-soluble acid is not regarded as more available than that portion insoluble in water, yet soluble in ammonium citrate.

In many of the States the statutes, or custom, prescribe that only the water and citrate-soluble acid shall be reckoned as available, the insoluble residue being allowed no place in the estimates of value. In many instances such a custom may lead to considerable error, as in the case of finely ground bones and some forms of soft and easily decomposable tricalcium phosphates. There are also, on the markets, phosphates composed largely of iron and aluminum salts, and these appear to have an available value often in excess of the quantities thereof soluble in ammonium citrate.

As a rule the apatites, when reduced to a fine powder and applied to the soil, are the least available of the natural phosphates. Next in order come the land rock and pebble phosphates which, in most soils, have only a limited availability. The soft fine-ground phosphates, especially in soils rich in humus, have an agricultural value, almost, if not quite equal to a similar amount of acid in the acid phosphates. Fine-ground bones also tend to give up their phosphoric acid with a considerable degree of readiness in most soils. Natural iron and aluminum phosphates, have also, as a rule, a high degree of availability. In each case the analyst must consider all the factors of the case before rendering a decision. Not only the relative solubility of the different components of the offered fertilizer in different menstrua must be taken into consideration, but also the character of the soil to which it is to be applied, the time of application, and the crop to be grown. By a diligent study of these conditions the analyst may, in the end, reach an accurate judgment of the merits of the sample.

=135. Direct Weighing of the Molybdenum Precipitate.=—It has already been stated that many attempts have, been made to determine the phosphoric acid by direct weighing as well as by titration, as in the Pemberton method. The point of prime importance in such a direct determination is to secure an ammonium phosphomolybdate mixture of constant composition. Unless this can be done no direct method, either volumetric or gravimetric, can give reliable results. Hanamann[122] proposes to secure this constant composition by varying somewhat the composition of the molybdate mixture and precipitating the phosphoric acid under definite conditions. The molybdate solution employed is prepared as follows:

Molybdic acid 100 grams.
Ten per cent ammonia 1.0 liter.
Nitric acid (1.246 sp. gr.) 1.5 liters.

The precipitation of the phosphoric acid is conducted in the cold with constant stirring. It is complete in half an hour. The ammonium phosphomolybdate is washed with a solution of ammonium nitrate and then with dilute nitric acid, dried, and ignited at less than a red heat. It should then have a bluish-black color throughout. Such a body contains 4.018 per cent of phosphoric anhydrid.

Twenty-five cubic centimeters of a sodium phosphate solution containing fifty milligrams of phosphoric acid, treated as above, gave a bluish-black precipitate weighing 1.249 grams, which, multiplied by 0.041018, equaled 50.018 milligrams of phosphorus pentoxid. The method should be tried on phosphates of various kinds and contents of phosphorus pentoxid before a definite judgment of its merits is formed.

CHEMISTRY OF THE MANUFACTURE OF SUPERPHOSPHATES.

=136. Reactions with Phosphates.=—In this country the expressions “acid” and “super” phosphates are used interchangeably. A more correct use of the terms would designate by “acid” the phosphate formed directly from tricalcium phosphate by the action of sulfuric acid, while by “super” would be indicated a similar product formed by the action of free phosphoric acid on the same materials. In Germany the latter compound is called double phosphate.

The reaction which takes place in the first instance is represented by the following formula:

3Ca₃(PO₄)₂ + 6H₂SO₄ + 12H₂O = 4H₃PO₄ + Ca₃(PO₄)₂ + 6(CaSO₄·2H₂O);

and 4H₃PO₄ + Ca₃(PO₄)₂ + 3H₂O = 3[CaH₄(PO₄)₂·H₂O].

A simpler form of the reaction is expressed as follows:

Ca₃(PO₄)₂ + 2H₂SO₄ + 5H₂O = CaH₄(PO₄)₂·H₂O + 2[CaSO₄·2H₂O].

If 310 parts, by weight, of fine-ground tricalcium phosphate be mixed with 196 parts of sulfuric acid and ninety parts of water, and the resulting jelly be quickly diluted with a large quantity of water, and filtered, there will be found in the filtrate about three-quarters of the total phosphoric as free acid. If, however, the jelly, at first, formed as above, be left to become dry and hard, the filtrate, when the mass is beaten up with water and filtered, will contain monocalcium phosphate, CaH₄(PO₄)₂.

If the quantity of sulfuric acid used be not sufficient for complete decomposition, the dicalcium salt is formed directly according to the following reaction:

Ca₃(PO₄)₂ + H₂SO₄ + 6H₂O = Ca₂H₂(PO₄)₂·4H₂O + CaSO₄·2H₂O.

This arises, doubtless, by the formation, at first, of the regular monocalcium salt and the further reaction of this with the tricalcium compound, as follows:

CaH₄(PO₄)₂ + H₂O + Ca₃(PO₄)₂ + 7H₂O = 2[Ca₂H₂(PO₄)₂·4H₂O].

This reaction represents, theoretically, the so-called reversion of the phosphoric acid. When there is an excess of sulfuric acid there is a complete decomposition of the calcium salts with the production of free phosphoric acid and gypsum. The reaction is represented by the following formula:

Ca₃(PO₄)₂ + 3H₂SO₄ + 6H₂O = 2H₃PO₄ + 3[CaSO₄·2H₂O].

The crystallized gypsum absorbs the six molecules of water in its molecular structure.

=137. Reactions with Fluorids.=—Since calcium fluorid is present in nearly all mineral phosphates, the reactions of this compound must be taken into consideration in a chemical study of the manufacture of acid phosphates. When treated with sulfuric acid the first reaction which takes place consists in the formation of hydrofluoric acid: CaF₂ + H₂SO₄ = 2HF + CaSO₄. Since, however, there is generally some silica in reach of the nascent acid, all, or a portion of it, combines at once with this silica, forming silicon tetrafluorid: 4HF + SiO₂ = 2H₂O + SiF₄. This compound, however, is decomposed at once in the presence of water, forming hydrofluosilicic acid: 3SiF₄ + 2H₂O = SiO₂ + 2H₂SiF₆. The presence of calcium fluorid in natural phosphates is extremely objectionable from a technical point of view, both on account of the increased consumption of oil of vitriol which it causes, but also by reason of the injurious nature of gaseous fluorin compounds produced. Each 100 pounds of calcium fluorid entails the consumption of 125.6 pounds of sulfuric acid.

=138. Reaction with Carbonates.=—Most mineral phosphates contain calcium carbonate in varying quantities. This compound is decomposed on treatment with sulfuric acid according to the reaction: CaCO₃ + H₂SO₄ = CaSO₄ + H₂O + CO₂. When present in moderate amounts, calcium carbonate is not an objectionable impurity in natural phosphates intended for acid phosphate manufacture. The reaction with sulfuric acid which takes place produces a proper rise in temperature throughout the mass, while the escaping carbon dioxid permeates and lightens the whole mass, assisting thus in completing the chemical reaction by leaving the residual mass porous, and capable of being easily dried and pulverized. Where large quantities of carbonate in proportion to the phosphate are present the sulfuric acid used should be dilute enough to furnish the necessary water of crystallization to the gypsum formed. For each 100 parts, by weight, of calcium carbonate, eighty parts of sulfuric anhydrid are necessary, or 125 parts of acid of 1.710 specific gravity = 60° Beaumé.

In some guanos a part of the calcium is found as pyrophosphate, and this is acted upon by the sulfuric acid in the following way: Ca₂P₂O₇ + H₂SO₄ = CaH₂P₂O₇ + CaSO₄.

=139. Solution of the Iron and Alumina Compounds.=—Iron may occur in natural phosphates in many forms. It probably is most frequently met with as ferric or ferrous phosphate, seldom as ferric oxid, and often as pyrite, FeS₂. The iron also may sometimes exist as a silicate. The alumina is found chiefly in combination with phosphoric acid, and as silicate.

Where a little less sulfuric acid is employed, as is generally the case, than is necessary for complete solution, the iron phosphate is attacked as represented below:

3FePO₄ + 3H₂SO₄ = FePO₄·2H₂PO₄ + Fe₂(SO₄)₃.

When an excess of sulfuric acid is employed, the formula is reduced to the simple one:

2FePO₄ + 3H₂SO₄ = 2H₃PO₄ + Fe₂(SO₄)₃.

A part of the iron sulfate formed reacts with the acid calcium phosphate present to produce a permanent jelly-like compound, difficult to dry and handle. As much as two per cent of iron phosphate, however, may be present without serious interference with the commercial handling of the product. By using more sulfuric acid as much as four or five per cent of the iron phosphate can be held in solution. Larger quantities are very troublesome from a commercial point of view. The reaction of the ferric sulfate with monocalcium phosphate, is as follows:

3CaH(PO₄)₂ + Fe₂(SO₄)₃ + 4H₂O = 2(FePO₄·2H₃PO₄·2H₂O) + 3CaSO₄.

Pyrite and the silicates containing iron are not attacked by sulfuric acid, and these compounds are therefore left, in the final product, in a harmless state. If the pyritic iron is to be brought into solution aqua regia should be employed.

With sufficient acid the aluminum phosphate is decomposed with the formation of aluminum sulfate and free phosphoric acid:

AlPO₄ + 3H₂SO₄ = Al₂(SO₄)₃ + 2H₃PO₄.

=140. Reaction with Magnesium Compounds.=—The mineral phosphates, as a rule, contain but little magnesia. When present it is probably as an acid salt, MgHPO₄. Its decomposition takes place in slight deficiency or excess of sulfuric acid respectively as follows:

2MgHP₄ + H₂SO₄ + 2H₂O = [MgH₄(PO₄)₂·2H₂O] + MgSO₄

and MgHPO₄ + H₂SO₄ = H₃PO₄ + MgSO₄.

The magnesia, when in the form of oxid, is capable of producing a reversion of the monocalcium phosphate, as is shown below:

CaH₄(PO₄)₂ + MgO = CaMgH₂(PO₄)₂ + H₂O.

One part by weight of magnesia can render three and one-half parts of soluble monocalcium phosphate insoluble.

=141. Determination of Quantity of Sulfuric Acid Necessary for Solution of a Mineral Phosphate.=—The theoretical quantity of sulfuric acid required for the proper treatment of any phosphate may be calculated from its chemical analysis and by the formulas and reactions already given. For the experimental determination the method of Rümpler may be followed.[123]

Twenty grams of the fine phosphate are placed in a liter flask with a greater quantity of accurately measured sulfuric acid than is necessary for complete solution. The acid should have a specific gravity of 1.455 or 45° B. The mixture is allowed to stand for two hours at 50°. It is then cooled, the flask filled with water to the mark, well shaken, and the contents filtered. Fifty cubic centimeters of the filtrate are treated with tenth normal soda-lye until basic phosphate begins to separate. The excess of acid used is then calculated. Example: Twenty grams of phosphate containing 28.3 per cent of phosphoric acid, 10.0 per cent of calcium carbonate, 5.5 per cent of calcium fluorid, and 2.4 per cent of calcium chlorid were treated as above with sixteen cubic centimeters of sulfuric acid containing 10.24 grams of sulfur trioxid. In titrating fifty cubic centimeters of the filtrate obtained as described above, 10.4 cubic centimeters of tenth normal soda-lye were used, equivalent to 0.0416 gram of sulfur trioxid. Then 10.24 × 50 ÷ 1000 = 0.5120 = total sulfur trioxid in fifty cubic centimeters of the filtrate, and 0.5120 - 0.0416 = 0.4704 gram, the amount of sulfur trioxid consumed in the decomposition.

Therefore the sulfur trioxid required for decomposition is 47.04 per cent of the weight of the phosphate employed. One hundred parts of the phosphate would therefore require 47.04 parts of sulfur trioxid = to 73.6 parts of sulfuric acid of 1.710 specific gravity or 92.1 parts of 1.530 specific gravity.

A more convenient method than the one mentioned above consists in treating a small quantity of the phosphate, from one-half to one kilogram, in the laboratory, or fifty kilograms in a lead box, just as would be practiced on a large scale. A few tests with these small quantities, followed by drying and grinding will reveal to the skilled operator the approximate quantity and strength of sulfuric acid to be used in each case. The quantities of sulfuric acid as determined by calculation from analyses and by actual laboratory tests agree fairly well in most instances. There is, however, sometimes a marked disagreement. The general rule of practice is to use always an amount of sulfuric acid sufficient to produce and maintain water-soluble phosphoric acid in the fertilizer, but the sulfuric acid must not be used in such quantity as to interfere with the subsequent drying, grinding, and marketing of the acid phosphate.

For convenience the following table may be used for calculating the quantity of oil of vitriol needed for each unit of weight of material noted:

ONE PART BY WEIGHT OF EACH SUBSTANCE BELOW REQUIRES:

SULFURIC ACID BY SAME UNIT OF WEIGHT.

At 48° At 50° At 52° At 54° At 55°
B. B. B. B. B.
Tricalcium phosphate 1.590 1.517 1.446 1.382 1.352
Iron phosphate 1.630 1.558 1.485 1.420 1.390
Aluminum phosphate 2.025 1.930 1.839 1.756 1.721
Calcium carbonate 1.640 1.565 1.495 1.428 1.411
Calcium fluorid 2.006 2.010 1.916 1.830 1.794
Magnesium carbonate 1.940 1.860 1.775 1.690 1.660

_Example._—Suppose for example a phosphate of the following
composition is to be treated with sulfuric acid; _viz._,[124]

Moisture and organic 4.00 per cent.
Calcium phosphate 55.00 “
Calcium carbonate 3.00 “
Iron and aluminum phosphate
nearly all alumina 6.50 “
Magnesium carbonate 0.75 “
Calcium fluorid 2.25 “
Insoluble 28.00 “

Using sulfuric acid of 50° B., the following quantities will be
required for each 100 kilograms.

Kilos of
acid required.
Calcium phosphate, fifty-five kilos 83.44
“ carbonate three and a half kilos 5.48
“ fluorid, two and a quarter “ 4.52
Aluminum and iron phosphate, six and a half kilos 12.55
Magnesium carbonate, three-quarters of a kilo 1.40
-----
Total 107.39

=142. Phosphoric Acid Superphosphates.=—If a mineral phosphate be decomposed by free phosphoric in place of sulfuric acid the resulting compound will contain about three times as much available phosphoric acid as is found in the ordinary acid phosphate. The reaction takes place according to the following formulas:

(1) Ca₃(PO₄)₂ + 4H₃PO₄ + 3H₂O = 3[CAH₄(PO₄)₂·H₂O].

(2) Ca₃(PO₄)₃ + 2H₃PO₄ + 12H₂O = 3[Ca₂H₂(PO₄)₂·4H₂O].

In each case the water in the final product is probably united as crystal water with the calcium salts produced. The monocalcium salt formed in the first reaction is soluble in water and the dicalcium salt in the second reaction in ammonium citrate. Where fertilizers are to be transported to great distances there is a considerable saving of freight by the use of such a high-grade phosphate, which may, at times, contain over forty per cent of available acid. The phosphoric acid used is made directly from the mineral phosphate by treating it with an excess of sulfuric acid.

AUTHORITIES CITED IN PART FIRST.

[1] Day, Mineral Resources of the United States 193, pp. 703, et seq.

[2] Massachusetts Agricultural Experiment Station, Bulletin 51, March, 1894.

[3] Brown, Manual of Assaying, p. 24.

[4] Bulletin de l’Association des Chimistes de Sucrèrie, No. 2, pp. 7, et seq.

[5] Proceedings of the Twelfth and Thirteenth Meetings of the Society for the Promotion of Agricultural Science, p. 140.

[6] Chemical Division, U. S. Department of Agriculture, Bulletin 43, p. 341.

[7] Rapport adressé par le Comité des Stations agronomiques au sujet des Methodes à suivre dans l’Analyse des Matières fertilisantes.

[8] Die Landwirtschaftlichen Versuchs-Stationen, Band 38, S. 303.

[9] Vid. op. cit. 6, p. 341.

[10] Zeitschrift für analytische Chemie, 1890, S. 390.

[11] Vid. op. cit. 6, p. 342.

[12] Chemisches Centralblatt, Band 2, S. 813.

[13] Transactions of the American Institute of Mining Engineers, Vol. 21, p. 165.

[14] Phosphates of America, p. 144.

[15] Vid. op. et loc. cit. 13.

[16] U. S. Geological Survey, Bulletin No. 47.

[17] Vid. op. et loc. cit. 14.

[18] Vid. op. et loc. cit. 13.

[19] Vid. op. cit. 14, p. 147.

[20] Transactions of the American Institute of Mining Engineers, Vol. 21, p. 168.

[21] Phosphates of America, p. 153.

[22] Die Landwirtschaftlichen Versuchs-Stationen, Band 34, S. 379.

[23] Zeitschrift für analytische Chemie, 1892, S. 383.

[24] Zeitschrift für angewandte Chemie, 1894, Ss. 679 und 701.

[25] Vid. op. cit. supra, 1889, p. 636.

[26] Vid. op. cit. 24, 1891, p. 3.

[27] Rapports presentèes au Congrès International de Chimie Appliqué, Bruxelles, Août, 1894, p. 26.

[28] Vid. op. et loc. cit. 20.

[29] Le Stazioni Sperimentali Agrarie Italiane, Vol. 23, p. 31.

[30] Crookes’ Select Methods, p. 538.

[31] Journal of Analytical and Applied Chemistry, Vol. 5, p. 671. For additional authorities on these methods consult Meyer and Wohlrab, Zeitschrift für angewandte Chemie, 1891, Ss. 170 und 243. Gruber, Zeitschrift für analytische Chemie, Band 30, S. 206. Shephard, Chemical News, May 29, 1891, p. 251. Vögel, Zeitschrift für angewandte Chemie, 1891, Band 12, S. 357.

[32] Journal of the American Chemical Society, April, 1895.

[33] Vid. op. cit. 21, p. 150.

[34] Transactions of the American Institute of Mining Engineers, Vol. 21, p. 170.

[35] Vid. op. cit. supra, p. 173.

[36] Comptes rendus, Tome 54, p. 468.

[37] Crookes’ Select Methods, p. 500.

[38] For details of method see Fresenius quantitative Analysis.

[39] U. S. Department of Agriculture, Chemical Division, Bulletin 43, p. 341.

[40] Letter to B. W. Kilgore, Reporter for Phosphoric Acid to the Association of Official Agricultural Chemists.

[41] Die Landwirtschaftlichen Versuchs-Stationen, Band 38, S. 304.

[42] Communicated by Dr. Solberg.

[43] From the Official Swedish Methods; translated for the author by F. W. Woll.

[44] Methoden van Onderzock aan de Rijkslandbouw-proefstations, 1893, p. 4.

[45] Zeitschrift für analytische Chemie, 1893, S. 64.

[46] Journal of the American Chemical Society, Vol. 16.

[47] Zeitschrift für angewandte Chemie, 1894, S. 678.

[48] Journal für Landwirtschaft, Band 30, S. 23.

[49] Vid. op. cit. 47, p. 544.

[50] Vid. op. cit. 46, Vol. 16, p. 462.

[51] Vid. op. et loc. cit. supra.

[52] Die Agricultur-Chemische Versuchs-Station, Halle a/S., Ss. 56, et seq.

[53] Chemische Industrie, 1890.

[54] Vid. op. et loc. cit. 52.

[55] Chemiker Zeitung, 1890, No. 75, S. 1246.

[56] Vid. op. cit. 43.

[57] Glaser, Zeitschrift für analytische Chemie, 24, 178 (1885). Laubheimer, Ibid, 25, 416 (1886). Müller, Tagebl. d. Naturforscher-Vers. zu Wiesbaden, 1886, 365. Vögel, Chemiker Zeitung, 1888, 85. Stutzer, Ibid, 492. Seifert, Ibid, 1390. v. Reis, Zeitschrift für angewandte Chemie, 1888, 354. Loges, Reportorium für analytische Chemie, 7, 85 (1887). Kassuer, Zeitschrift für Nahrungsmitteluntersuchung und Hygiene, 2, 22 (1888). C. Müller, Die Landwirtschaftlichen Versuchs-Stationen, 35, 438 (1888).

[58] L’Engrais, Tome 9, p. 928.

[59] Vid. op. et loc. cit. 44.

[60] Journal of the American Chemical Society, Vol. 16, p. 462.

[61] Die Landwirtschaftlichen Versuchs-Stationen, Band 41, S. 329.

[62] Journal of Analytical and Applied Chemistry, Vol. 5, p. 685.

[63] Vid. op. cit. supra, Vol. 3, p. 413.

[64] Zeitschrift für angewandte Chemie, 1886, S. 354.

[65] Vid. op. cit. 52, p. 61.

[66] Vid. op. cit. 55, Vol. 18, p. 1153.

[67] Chemiker Zeitung, 1894, No. 88, p. 1934.

[68] Op. cit. supra, 1892, p. 1471.

[69] Vid. op. cit. 60, p. 721.

[70] Zeitschrift für analytische Chemie, Band 29, S. 408.

[71] Mitteilungen der deutschen Landwirtschafts Gesellschaft, 1890-’91, No. 11, S. 131.

[72] Zeitschrift für angewandte Chemie, 1888, S. 299.

[73] Vid. op. cit. 70, p. 409.

[74] Vid. op. cit. 72, 1890, p. 595.

[75] Vid. op. cit. 61, Tome 43, p. 183.

[76] Chemiker Zeitung, Band 18, S. 565.

[77] Chemical News, Vol. 1, p. 97.

[78] Archive für Wissenschaftliche Heilkunde, Band 4, S. 228.

[79] Journal für praktische Chemie, Band 70, S. 104.

[80] Sutton’s Volumetric Analysis, p. 237.

[81] Bulletin de la Société des Agriculteurs de France, 1876, p. 53.

[82] Manual Agenda des Fabricants de Sucre, 1889, p. 307.

[83] Journal of the American Chemical Society, Vol. 15, p. 382, and Vol. 16, p. 278.

[84] Chemical News, Vol. 47, p. 127.

[85] American Chemical Journal, Vol. 11, p. 84.

[86] Vid. op. cit. 83, Vol. 16, p. 282.

[87] Bulletin 43, Chemical Division, U. S. Department of Agriculture, p. 88.

[88] Vid. op. cit. supra, p. 91.

[89] Repertoire de Pharmacie, 1893, p. 153.

[90] Revue de Chimie Analytique Appliqué, 1893, p. 113.

[91] Chemiker Zeitung, 1894, S. 1533.

[92] Blair, Analysis of Iron and Steel, p. 95.

[93] Journal of Analytical and Applied Chemistry, Vol. 7, p. 108.

[94] Journal of the American Chemical Society, Vol. 17, p. 129.

[95] Vid. op. cit. 92, p. 99.

[96] Eighth Annual Report of Purdue University, p. 238.

[97] Receuil des Travaux Chimiques, Tome 12, pp. 1, et seq. Journal of the Chemical Society (Abstracts), Vol. 64, p. 496.

[98] Zeitschrift für angewandte Chemie, 1891, Ss. 279, et seq.

[99] Le Stazioni Sperimentali Agrarie Italiane, February, 1891.

[100] Journal of the American Chemical Society, Vol. 17, p. 43.

[101] Vid. op. et loc. cit. supra.

[102] L’Engrais, Tome 10, p. 65.

[103] Journal of Analytical and Applied Chemistry, Vol. 5, p. 694. Zeitschrift für analytische Chemie, Band 18, S. 99.

[104] Vid. op. cit. 92, p. 103.

[105] Journal of the Chemical Society (Abstracts), Vol. 58, p. 1343.

[106] Comptes rendus, Tome 114, p. 1189.

[107] Vid. op. cit. 24 and 25.

[108] Report communicated to author by W. G. Brown.

[109] Chemisches Centralblatt, 1895, p. 562.

[110] Wiley, Report on Fertilizers to Indiana State Board of Agriculture, 1882.

[111] Proceedings of the Association of Official Agricultural Chemists, Atlanta, 1884, p. 19. Report of Indiana State Board of Agriculture, 1882, p. 230, and Proceedings of the Association of Official Agricultural Chemists, Atlanta, 1884, p. 30. Huston and Jones. (These gentlemen are now investigating all materials used as sources of phosphoric acid in fertilizers; their results here quoted are from unpublished work, and include but a small part of the work so far done.) American Chemical Journal, March, 1884, p. 1. Proceedings of the Association of Official Agricultural Chemists, Atlanta, 1884, p. 23. Ibid, p. 28. Ibid, p. 38. Ibid, p. 45. U. S. Department of Agriculture, Chemical Division, Bulletin No. 7, p. 18. Ibid, Bulletin No. 28, p. 171. Ibid, Bulletin No. 31, p. 100. Ibid, Bulletin No. 31, p. 99.

[112] Manuscript communication to author.

[113] Pamunky phosphate is the so-called “olive earth” found along the Pamunky river, in Virginia. It is almost all precipitated iron and aluminum phosphates, and the product is peculiar in that the iron is almost all in the ferrous condition.

[114] In the work of T. S. Gladding only fifty cubic centimeters of citrate were used.

[115] In the work of T. S. Gladding only fifty cubic centimeters of citrate were used.

[116] Zeitschrift für analytische Chemie, Band 10, S. 133.

[117] Lehrbuch der Düngerfabrication.

[118] Bulletin 54, Purdue Agricultural Experiment Station, p. 4.

[119] Vid. op. cit. supra, p. 7.

[120] Runyan and Wiley; Paper presented to Washington Section of the American Chemical Society, April 11, 1895.

[121] Bulletin 38, Chemical Division, U. S. Department of Agriculture, p. 16.

[122] Chemiker Zeitung, 1895, S. 553.

[123] Die Käuflichen Dungermittel Stoffe, dritte Auflage, 1889.

[124] Wyatt, Phosphates of America, p. 128.

PART SECOND.

NITROGEN IN FERTILIZERS.

=143. Kinds of Nitrogen in Fertilizers.=—Nitrogen is the most costly of the essential plant foods. It has been shown in the first volume, paragraph =23=, that the popular notion regarding the relatively great abundance of nitrogen is erroneous. It forms only 0.02 per cent of the matter forming and pertaining to the earth’s crust. The great mass of nitrogen forming the bulk of the atmosphere is inert and useless in respect of its adaptation to plant food. It is not until it becomes oxidized by combustion, electrical discharges, or the action of certain microorganisms that it assumes an agricultural value.

Having already, in the first volume, described the relation of nitrogen to the soil it remains the sole province of the present part to study it as aggregated in a form suited to plant fertilization. In this function nitrogen may claim the attention of the analyst in the following forms:

1. In organic combination in animal or vegetable substances, forming a large class of bodies, of which protein may be taken as the type. Dried blood or cottonseed-meal illustrates this form of combination.

2. In the form of ammonia or combinations thereof, especially as ammonium sulfate, or as amid nitrogen.

3. In a more highly oxidized form as nitrous or nitric acid usually united with a base of which Chile saltpeter may be taken as a type.

The analyst has often to deal with single forms of nitrogenous compounds, but in many instances may also find all the typical forms in a single sample. Among the possible cases which may arise the following are types:

_a._ The sample under examination may contain nitrogen in all three forms mentioned above.

_b._ There may be present nitrogen in the organic form mixed with nitric nitrogen.

_c._ Ammoniacal nitrogen may replace the nitric in the above combination.

_d._ The sample may contain no organic but only nitric and ammoniacal nitrogen.

_e._ Only nitric or ammoniacal nitrogen may be present.

=144. Determination of the State of Combination.=—Some of the sample is mixed with a little powdered soda-lime. If ammoniacal nitrogen be present free ammonia is evolved even in the cold and may be detected either by its odor or by testing the escaping gas with litmus or turmeric paper. A glass rod moistened with strong hydrochloric acid will produce white fumes of ammonium chlorid when brought near the escaping ammonia.

If the sample contain any notable amount of nitric acid it will be revealed by treating an aqueous solution of it with a crystal of ferrous sulfate and strong sulfuric acid. The iron salt should be placed in a test-tube with a few drops of the solution of the fertilizer and the sulfuric acid poured down the sides of the tube in such a way as not to mix with the other liquids. The tube must be kept cold. A dark brown ring will mark the disk of separation between the sulfuric acid and the aqueous solution in case nitric acid be present. If water produce a solution of the sample too highly colored to be used as above, alcohol of eighty per cent strength may be substituted. The coloration produced in this case is of a rose or purple tint.

Nitric nitrogen may also be detected by means of brucin. If a few drops of an aqueous solution of brucin be mixed with the same quantity of an aqueous extract of the sample under examination and strong sulfuric acid be added, as described above, there will be developed at the disk of contact between the acid and the mixed solutions a persistent rose tint varying to yellow.

To detect the presence of organic albuminoid nitrogen the residue insoluble in water, when heated with soda-lime, will give rise to ammonia which may be detected as described above.

=145. Microscopic Examination.=—If the chemical test reveal the presence of organic nitrogen the next point to be determined is the nature of the substance containing it. Often this is revealed by simple inspection, as in the case of cottonseed-meal. Frequently, however, especially in cases of fine-ground mixed goods, the microscope must be employed to determine the character of the organic matter. It is important to know whether hair, horn, hoof, and other less valuable forms of nitrogenous compounds have been substituted for dried blood, tankage, and more valuable forms. In most cases the qualitative chemical, and microscopic examination will be sufficient. There may be cases, however, where the analyst will be under the necessity of using other means of identification suggested by his skill and experience or the circumstances connected with any particular instance. In such cases the general appearance, odor, and consistence of the sample may afford valuable indications which will aid in discovering the origin of the nitrogenous materials.

SOURCES OF NITROGENOUS FERTILIZERS.

=146. Seeds and Seed Residues.=—The proteid matters in seeds and seed residues, after the extraction of the oil, are highly prized as sources of nitrogenous fertilizers either for direct application or for mixing. Typical of this class of substances is cottonseed-meal, the residue left after the extraction of the oil which is accomplished at the present time mostly by hydraulic pressure. The residual cakes contain still some oil but nearly half their weight consists of nitrogenous compounds. The following table gives the composition of a sample of cottonseed-meal:

Ash 7.60 per cent.
Fiber 4.90 “
Oil 10.01 “
Protein 51.12 “
Digestible carbohydrates, etc. 26.37 “

While the above shows the composition of a single sample of the meal it should be remembered that there may be wide variations from this standard due either to natural composition or to different degrees of the extraction of the oil.

The composition of the ash is given below:

Phosphoric acid, P₂O₅ 31.01 per cent.
Potash, K₂O 35.50 “
Soda, Na₂O 0.57 “
Lime, CaO 5.68 “
Magnesia, MgO 15.19 “
Sulfuric acid, SO₃ 3.90 “
Insoluble, 0.69 “
Carbon dioxid and undetermined, 7.46 “

The cakes left after the expression of the oil from flaxseed and other oily seeds are also very rich in nitrogenous matters; but these residues are chiefly used for cattle-feeding and only the undigested portions of them pass into the manure. Cottonseed cake-meal is not so well suited for cattle-feeding as the others mentioned, because of the cholin and betaïn which it contains; often in sufficient quantities to render its use dangerous to young animals. The danger in feeding increases as the total quantity of the two bases and also as the relative quantity of cholin to betaïn, the former base being more poisonous than the latter. In a sample of the mixed bases prepared in this laboratory from cottonseed cake-meal the cholin amounted to 17.5 and the betaïn to 82.5 per cent of the whole.[125]

The nitrogen contained in these bases is also included in the total nitrogen found in the meal. The actual proteid value of the numbers obtained for nitrogen is therefore less than that obtained for the whole of the nitrogen by the quantity present as nitrogenous bases.

In the United States cottonseed cake-meal is used in large quantities as a direct fertilizer but not so extensively for mixing as some of the other sources of nitrogen. Its delicate yellow color serves to distinguish it at once from the other bodies used for similar purposes. No special mention need be made of other oil-cake residues. They are quite similar in their composition and uses, and manner of treatment and analysis to the cottonseed product.

=147. Fish Scrap.=—Certain species of fish, such as the menhaden, are valued more highly for their oil and refuse than for food purposes. But even where fish in large quantities are prepared for human food, there is a considerable quantity of waste matter which is valuable for fertilizing purposes. The residue of fish from which the fat and oil have been extracted, is dried and ground for fertilizing uses. The fish scrap thus obtained is used extensively, especially on the Atlantic border of the United States, for furnishing the nitrogenous ingredient in mixed fertilizers, and also for direct application to the fields. In fish flesh deprived of oil and water, the content of phosphoric acid is about two and one-half per cent, while the proteid matter may amount to three-quarters of the whole.[126]

The use of fish for fertilizing purposes is not new. As early as 1621 the settlers at Plymouth were taught to fertilize their maize fields by Squanto, an Indian. According to Goode, the value of nitrogen derived from the menhaden alone was two million dollars in 1875.[127] In 1878 it is estimated that 200,000 tons of these fish were captured between Cape Henry and the Bay of Fundy. The use of fish scrap for nitrogenous fertilizing has, since then, become an established industry, and the analyst may well examine his samples for this source of nitrogen when they are manufactured at points on the Atlantic coast, in proximity to great fishing centers.

=148. Dried Blood and Tankage.=—The blood and débris from abattoirs afford abundant sources of nitrogen in a form easily oxidized by the microorganisms of the soil. Blood is prepared for use by simple drying and grinding. The intestines, scraps, and fragments of flesh resulting from trimming and cutting, are placed in tanks and steamed under pressure to remove the fat. The residue is dried and ground, forming the tankage of commerce. Dried blood is richer in proteid matter than any other substance in common use for fertilizing purposes. When in a perfectly dry state, it may contain as much as fourteen per cent of nitrogen, equivalent to nearly eighty-eight per cent of proteid or albuminoid matter. Tankage is less rich in nitrogen than dried blood, but still contains enough to make it a highly desirable constituent of manures. Naturally, it would vary more in its nitrogen content than dried blood.

=149. Horn, Hoof, and Hair.=—These bodies, although quite rich in nitrogen, are not well suited to fertilizing purposes on account of the extreme slowness of their decomposition. Their presence, therefore, should be regarded in the nature of a fraud, because by the usual methods of analysis they show a high percentage of nitrogen, and therefore acquire a fictitious value. The relative value of the nitrogen in these bodies as compared with the more desirable forms, is given in paragraph =5=.

=150. Ammoniacal Nitrogen.=—In ammonia compounds, nitrogen is used chiefly for fertilizing purposes as sulfate. The ideal nitrogenous fertilizer would be a combination of the ammoniacal and nitric nitrogen found in ammonium nitrate. The high cost of this substance excludes its use except for experimental purposes.

=151. Nitrogen in Guanos.=—The nitrogen in guanos may be found partly as organic, partly as ammoniacal, and partly as nitric nitrogen. The high manurial value of guanos and bat deposits in caves, is due not only to their phosphoric acid, but also to the fact that part of the nitrogen is immediately available, while a part becomes assimilable by nitrification during the growing season. The content of nitrogen in guanos is extremely variable, and depends largely on the climatic conditions to which the deposit has been subjected. The state in which it exists is also a variable one, but with a constant tendency to assume finally the nitric condition.

The well-known habits of birds in congregating in rookeries during the nights, and at certain seasons of the year, tend to bring into a common receptacle the nitrogenous matters which they have gathered and which are deposited in their excrement and in the decay of their bodies. The feathers of birds are particularly rich in nitrogen, and the nitrogenous content of the flesh of fowls is also high. The decay therefore, of remains of birds, especially if it take place largely excluded from the leaching of water, tends to accumulate vast deposits of nitrogenous matter. If the conditions in such deposits be favorable to the processes of nitrification, the whole of the nitrogen, or at least the larger part of it, which has been collected in this débris, becomes finally converted into nitric acid, and is found combined with appropriate bases as deposits of nitrates. The nitrates of the guano deposits, and of the deposits in caves, arise in this way. If these deposits be subject to moderate leaching, the nitrate may become infiltered into the surrounding soil, making it very rich in this form of nitrogen. The beds and surrounding soils of caves are often found highly impregnated with nitrates.

While for our purpose, deposits of nitrates only are to be considered which are of sufficient value to bear transportation, yet much interest attaches to the formation of nitrates in the soil even when they are not of commercial importance.

In many soils of tropical regions not subject to heavy rainfalls, the accumulation of these nitrates is very great. Müntz and Marcano[128] have investigated many of these soils, to which attention was called first by Humboldt and Boussingault. They state that these soils are incomparably more rich in nitrates than the most fertile soils of Europe. The samples which they examined were collected from different parts of Venezuela and from the valleys of the Orinoco, as well as on the shore of the Sea of Antilles. The nitrated soils are very abundant in this region of South America, where they cover large surfaces. Their composition is variable, but in all of them calcium carbonate and phosphate are met with, and organic nitrogenous material. The nitric acid is found always combined with lime. In some of the soils as high as thirty per cent of calcium nitrate have been found. Nitrification of organic material takes place very rapidly the year round in this tropical region. These nitrated soils are everywhere abundant around caves, as described by Humboldt, which serve as the refuge of birds and bats. The nitrogenous matters, which come from the decay of the remains of these animals, form true deposits of guano, which are gradually spread around, and which, in contact with the limestone and with access of air, suffer complete nitrification with the fixation of the nitric acid by the lime.

Large quantities of this guano are also due to the débris of insects, fragments of elytra, scales of the wings of butterflies, etc., which are brought together in those places by the millions of cubic meters. The nitrification, which takes place in these deposits, has been found to extend its products to a distance of several kilometers through the soil. In some places the quantity of calcium nitrate is so great in the soils that they are converted into a plastic paste by this deliquescent salt.

=152. Nitric Nitrogen.=—In its purer forms, and suited to manurial purposes, nitric acid exists in combination with sodium as a compound commonly known as Chile saltpeter.

The existence of these nitrate deposits has long been known.[129] The old Indian laws originally prohibited the collection of the salt, but nevertheless it was secretly collected and sold. Up to the year 1821, soda saltpeter was not known in Europe except as a laboratory product. About this time the naturalist, Mariano de Rivero, found on the Pacific coast, in the Province of Tarapacà, immense new deposits of the salt. Later the salt was found in equal abundance in the Territory of Antofagasta, and further to the south in the desert of Atacama, which forms the Department of Taltal.

At the present time the collection and export of saltpeter from Chile is a business of great importance. The largest export which has ever taken place in one year was in 1890, when the amount exported was 927,290,430 kilograms; of this quantity 642,506,985 kilograms were sent to England and 86,124,870 kilograms to the United States. Since that time the imports of this salt into the United States have largely increased.

According to Pissis[130] these deposits are of very ancient origin. This geologist is of the opinion that the nitrate deposits are the result of the decomposition of feldspathic rocks, the bases thus produced gradually becoming united with the nitric acid provided from the air.

According to the theory of Nöllner[131] the deposits are of more modern origin, and due to the decomposition of marine vegetation. Continuous solution of soils beneath the sea gives rise to the formation of great lakes of saturated water, in which occurs the development of much marine vegetation. On the evaporation of this water, due to geologic isolation, the decomposition of nitrogenous organic matter causes generation of nitric acid, which, coming in contact with the calcareous rocks, attacks them, forming calcium nitrate, which, in presence of sodium sulfate, gives rise to a double decomposition into sodium nitrate and calcium sulfate.

The fact that iodin is found in greater or less quantity in Chile saltpeter is one of the chief supports of this hypothesis of marine origin, inasmuch as iodin is always found in sea plants, and not in terrestrial plants. Further than this, it must be taken into consideration that these deposits of sodium nitrate contain neither shells nor fossils, nor do they contain any calcium phosphate. The theory, therefore, that they are due to animal origin is scarcely tenable.

Lately extensive nitrate deposits have been discovered in the U. S. of Columbia.[132] These deposits have been found extending over thirty square miles and vary in thickness from one to ten feet. The visible supply is estimated at 7,372,800,000 tons, containing from 1.0 to 13.5 percent of nitrate. The deposits consist of a mixture of sodium nitrate, sodium chlorid, calcium sulfate, aluminum sulfate, and insoluble silica. It is thought that the amount of these deposits will almost equal those in Chile and Peru.

METHODS OF ANALYSIS.

=153. Classification of Methods.=—In general there are three direct methods of determining the nitrogen content of fertilizers. First the nitrogen may be secured in a gaseous form and the volume thereof, under standard conditions, measured and the weight of nitrogen computed. This process is commonly known as the absolute method. Practically it has passed out of use in fertilizer work, or is practiced only as a check against new and untried methods, or on certain nitrogenous compounds which do not readily yield all their nitrogen by the other methods. The process, first perfected by Dumas, who has also given it his name, consists in the combustion of the nitrogenous body in an environment of copper oxid by which the nitrogen, by reason of its inertness, is left in a gaseous state after the oxidation of the other constituents; _viz._, carbon and hydrogen, originally present.

In the second class of methods the nitrogen is converted into ammonia which is absorbed by an excess of standard acid, the residue of which is determined by subsequent titration with a standard alkali. There are two distinct processes belonging to this class, in one of which ammonia is directly produced by dry combustion of an organic nitrogenous compound with an alkali, and in the other ammonium sulfate is produced by moist combustion with sulfuric acid, and the salt thus formed is subsequently distilled with an alkali, and the free ammonia thus formed estimated as above described. Nitric nitrogen may also be reduced to ammonia by nascent hydrogen either in an acid or alkaline solution as described in volume first.

In the third class of determinations is included the estimation of nitric nitrogen by colorimetric methods as described in the first volume. These processes have little practical value in connection with the analyses of commercial fertilizers, but find their chief use in the detection and estimation of extremely minute quantities of nitrites and nitrates. In the following paragraphs will be given the standard methods for the determination of nitrogen in practical work with fertilizing materials and fertilizers.

=154. Official Methods.=—The methods adopted by the Association of Official Agricultural Chemists have been developed by more than ten years of co-operative work on the part of the leading agricultural chemists of the United States. These methods should be strictly followed in all essential points by all analysts in cases where comparison with other data are concerned. Future experience will doubtless improve the processes both in respect of accuracy and simplicity, but it must be granted that, as at present practiced, they give essentially accurate results.

=155. Volumetric Estimation by Combustion with Copper Oxid.=—This classical method of analysis is based on the supposition that by the combustion of a substance containing nitrogen in copper oxid and conducting the products of the oxidation over red-hot copper oxid and metallic copper, all of the nitrogen present in whatever form will be obtained in a free state and can subsequently be measured as a gas. The air originally present in all parts of the apparatus must first be removed either by a mercury pump or by carbon dioxid or by both together, the residual carbon dioxid being absorbed by a solution of caustic alkali. Great delicacy of manipulation is necessary to secure a perfect vacuum and as a rule a small quantity of gas may be measured other than nitrogen so that the results of the analyses are often a trifle too high. The presence of another element associated with nitrogen, or the possible allotropic existence of that element, may also prove to be a disturbing factor in this long-practiced analytical process. For instance, if nitrogen be contaminated with another element, _e. g._, argon, of a greater density the commonly accepted weight of a liter of nitrogen is too great and tables of calculation based on that weight would give results too high.

First will be given the official method for this process, followed by a few simple variations thereof, as practiced in this laboratory.

=156. The Official Volumetric Method.=—This process may be used for nitrogen in any form of combination.[133]

The apparatus and reagents needed are as follows:

_Combustion tube_ of best hard Bohemian glass, about sixty-six centimeters long and 12.7 millimeters internal diameter.

_Azotometer_ of at least 100 cubic centimeters capacity, accurately calibrated.

_Sprengel mercury air-pump._

_Small paper scoop_, easily made from stiff writing paper.

_Coarse cupric oxid._—To be ignited and cooled before using.

_Fine cupric oxid._—Prepared by pounding ordinary cupric oxid in a mortar.

_Metallic copper._—Granulated copper, or fine copper gauze, reduced and cooled in a current of hydrogen.

_Sodium bicarbonate._—Free from organic matter.

_Caustic potash solution._—Make a supersaturated solution of caustic potash in hot water. When absorption of carbon dioxid, during the combustion, ceases to be prompt, the solution must be discarded.