Chapter VI: Part 6

_Manipulation._—Twenty-five or fifty cubic centimeters of a solution of the phosphate which has been made up to a standard volume and containing about one-tenth gram of phosphorus pentoxid, are placed in a beaker together with 100 cubic centimeters of the molybdate solution and treated with as much ammonium nitrate solution as will be sufficient to give the liquid a content of fifteen per cent of ammonium nitrate. The contents of the beaker are well mixed and warmed for about twenty minutes at from 60° to 80°. After cooling, they are filtered and the precipitate washed on the filter with cold water until a drop of the filtrate saturated with ammonia does not become opaque on treatment with ammonium oxalate. The filtrate is then washed from the filter with two and one-half per cent ammonia solution and precipitated slowly and with constant stirring by the magnesia mixture. After standing for two hours the ammonium magnesium phosphate is separated by filtration, washed with two and one-half per cent ammonia until the filtrate contains no more chlorin, and ignited.

_Conduct of the Citrate Method._—The principle of this method depends upon the fact that when a sufficient quantity of ammonium citrate is added to phosphate solutions, iron, alumina, and lime are retained in solution when, on the addition of the magnesia mixture in the presence of free ammonia, the phosphoric acid is completely precipitated as ammonium magnesium phosphate.

_Manipulation._—From ten to fifty cubic centimeters of the solution of the phosphate to be determined are treated with fifteen cubic centimeters of the Joulie citrate solution avoiding warming. A few pieces of filter-paper, the ash content of which is known, are thrown in and, with stirring, fifteen cubic centimeters of magnesia mixture slowly added and if necessary also some free ammonia. By the small pieces of filter-paper the collection of the precipitate against the sides of the vessel and on the stirring rod is prevented and in this way the production of the precipitate hastened. After standing from one-half an hour to two hours the mixture is filtered, ignited, and weighed. If it be preferred to estimate the phosphoric acid by titration, the precipitate is dissolved in a little nitric acid, made slightly alkaline with ammonia, and then acid with acetic and then afterwards titrated with the standard uranium solution.

_Conduct of the Uranium Method._—The principle upon which this method rests depends upon the fact that uranium nitrate or acetate precipitates uranium phosphate from solutions containing phosphoric acid and which contain no other free acid except acetic. In the presence of ammonium salts the precipitate is uranium ammonium phosphate having the formula PO₄NH₄UrO₂. The smallest excess of soluble uranium salt is at once detected by the ordinary treatment with potassium ferrocyanid.

_Manipulation._—In all cases the solution is first made slightly alkaline with ammonia and then acid by a few drops of acetic, so that no free mineral acid may be present.

(1) _With liquids free from iron_:

If, on the addition of ammonium or sodium acetate, no turbidity be produced, the liquid is free from iron and alumina. In this case from ten to fifty cubic centimeters of the solution containing about one-tenth gram of phosphorus pentoxid are treated with ten cubic centimeters of sodium acetate, and afterwards with a quantity of uranium solution corresponding, as nearly as possible, to its supposed content of phosphorus pentoxid, and heated to boiling. From the heated liquid by means of a glass rod, one or two drops are taken and placed upon a porcelain plate and one drop of a freshly prepared solution of potassium ferrocyanid allowed to flow on it. If no brown color be seen at the point of contact of the two drops, additional quantities of the uranium solution are added and, after boiling, again tested with potassium ferrocyanid until a brown color is distinctly visible. The quantity of the uranium solution thus having been determined, duplicate analyses can be made and the whole quantity of the uranium solution added at once with the exception of the last drops, which are added as before.

(2) _Solutions containing iron and alumina._

The solution is treated with the ammonium citrate solution of Joulie, the magnesia mixture added slowly, and the precipitate collected on a filter and washed with two and one-half per cent ammonia. The precipitate is then dissolved in nitric acid, made alkaline with ammonia, and then acid with acetic. This solution is then treated with ten cubic centimeters of sodium acetate and titrated with uranium, as described in (1). As an alternative method, 200 cubic centimeters of the superphosphate solution may be treated with fifty cubic centimeters of sodium acetate, allowed to stand for some time, and filtered through a filter of known ash content. In fifty cubic centimeters of the filtrate, which correspond to forty cubic centimeters of the original solution, phosphoric acid may be determined as described above. The precipitate, consisting of iron and aluminum phosphates, is washed three times on the filter with boiling water, dried, and ignited in a platinum dish. The weight of ignited precipitate, diminished by the weight of the ash contained in the filter and divided by two, gives the quantity of phosphorus pentoxid which it is necessary to add to that obtained by titration.

=117. Determination of the Phosphoric Acid in all Phosphates and Basic Slags.=—

(1) _Total phosphoric Acid_:

Five grams of the fine phosphate meal, or slag meal, are moistened in a flask of 500 cubic centimeters content with some water and boiled on a sand-bath with forty cubic centimeters of hydrochloric acid of from 16° to 20° Beaumé. The boiling is continued until only a few cubic centimeters of a thick jelly of silicic acid remain. After cooling, some water is added and the phosphate shaken until the thick lumps of silica are finely divided. The flask is then filled to 500 cubic centimeters and its contents filtered. Fifty cubic centimeters of the filtrate are treated with fifteen cubic centimeters of the Joulie solution and treated in the manner described with magnesia mixture, precipitated, ignited, and weighed. The precipitate can also be dissolved and treated with uranium solution as described.

The method used by Oliveri may also be employed and it is carried out as indicated in the following description:[99]

A weighed quantity of the slag is reduced to a fine powder. To five grams of the sample is added three times its weight of potassium chlorate and the whole is intimately mixed. The mixture is then placed in a porcelain dish and hydrochloric acid is added, little by little, until the potash salt is completely decomposed. It is evaporated until the mass is dry. The material is then treated with fuming nitric acid, and the determination of the phosphorus is made by the ordinary gravimetric method.

By carrying on the operation as described above, a reduction of phosphoric acid is avoided, and the presence of an abundant quantity of potash prevents the formation of basic iron phosphate which is insoluble in nitric acid.

(2) _Citrate-Soluble Phosphoric Acid._—One gram of the basic slag or phosphate is placed in a 100 cubic centimeter flask and covered with Wagner’s acid citrate solution making the total volume up to 100 cubic centimeters. With frequent shaking the flask is kept at 40° for an hour, or it may be allowed to stand for twelve hours at room temperature with frequent shaking. In fifty cubic centimeters of the filtrate from this flask the phosphoric acid is determined by the magnesia mixture as described. Since, in the present case, the precipitate of ammonium magnesium phosphate contains some silicic acid it cannot be directly ignited but must be treated in the following manner: The precipitate and the filter are thrown into a porcelain dish, the filter-paper torn up into shreds with a glass rod, the precipitate dissolved in nitric acid, neutralized with ammonia, acidified with acetic, and treated with uranium solution. The phosphoric acid may also be estimated by the gravimetric method by dissolving the precipitate again in hydrochloric or nitric acid, evaporating to dryness, and drying for one hour at from 110° to 120°, dissolving again in hydrochloric acid, filtering, and washing the precipitate well. The filtrate, which is now free from silica, can be treated with Joulie’s solution, precipitated with magnesia mixture, the precipitate washed, ignited, and weighed as described. The molybdenum method is preferred in the estimation of citrate-soluble phosphoric acid, especially in slags. For this purpose fifty cubic centimeters of the filtrate from the solution of one gram of slag in 100 cubic centimeters of Wagner’s citrate liquid are treated with 100 cubic centimeters of molybdenum solution and thirty cubic centimeters of ammonium nitrate solution, warmed for twenty minutes at 80°, filtered after cooling, and the yellow precipitate washed with cold water. The water will gradually dissolve all the silicic acid from the yellow precipitate and carry it into the filtrate. The yellow precipitate is then dissolved in two and one-half per cent liquid ammonia and precipitated with magnesia mixture and the precipitate washed, ignited, and weighed in the way described.

=118. Determination of Phosphoric Acid in Superphosphates.=—(1) _Citrate-Soluble Phosphoric Acid._—Five grams of the superphosphate are rubbed with 100 cubic centimeters of Wagner’s acid citrate solution in a mortar and washed into a flask of 500 cubic centimeters content and diluted to 500 cubic centimeters with water. With frequent shaking the flask is allowed to stand for twelve hours, after which its contents are filtered. Fifty cubic centimeters of the filtrate are treated with ten cubic centimeters of the Joulie solution and fifteen cubic centimeters of the magnesia mixture and, if necessary, made distinctly alkaline with ammonia, vigorously stirred, and, after two hours, filtered. The precipitate is washed, ignited, and weighed as described, or titrated, after solution in nitric acid and the addition of sodium acetate, with uranium solution.

Example:
The weighed precipitate has 0.1272 gram Mg₂P₂O₇ then the phosphate
contains 12.72 × 2 × 0.64 = 16.28 per cent of citrate-soluble P₂O₅.

(2) _Water-Soluble Phosphoric Acid._—Twenty grams of superphosphate are rubbed in a mortar and washed into a flask of one liter content and made up to the mark with water. After two hours’ digestion with frequent shaking, the contents of the flask are filtered through a folded filter. Twenty-five cubic centimeters of the filtrate equivalent to five-tenths gram of the substance are precipitated with magnesia mixture, the precipitate filtered, washed, ignited, and weighed, or the moist filtrate may be dissolved upon the filter with a little nitric acid, treated with sodium acetate, and titrated, as described, with uranium solution.

Example: 14.5 cubic centimeters of the uranium solution are required for the precipitate from twenty-five cubic centimeters of the original solution = 0.5 gram superphosphate; it contains then 14.5 × 0.00445 = 0.0645 gram P₂O₅. Consequently the superphosphate contains 12.90 per cent of water-soluble P₂O₅.

_Total Phosphoric Add._—Twenty grams of the superphosphate are boiled with fifty cubic centimeters of hydrochloric acid of from 16° to 18° Beaumé for about ten minutes and, after cooling, made up to one liter with water and filtered. Twenty-five cubic centimeters of the filtrate are treated with ten cubic centimeters of Joulie’s citrate solution, a few pieces of filter-paper thrown in, fifteen cubic centimeters of magnesia mixture added, and the whole thoroughly stirred. After standing two hours the contents of the flask are filtered and the precipitate is washed with dilute ammonia and the filter and the precipitate are placed in a platinum crucible. The crucible is heated slowly until the moisture is driven off and the filter burned. Then the temperature is gradually raised to a white heat. The residue is cooled and weighed. Example:

The precipitate weighs, after the subtraction of the filter ash, 0.1390 gram; then the superphosphate contains 13.90 × 2 × 0.64 = 17.79 per cent phosphoric acid.

MISCELLANEOUS NOTES ON PHOSPHATES AND PHOSPHATIC FERTILIZERS.

=119. Time Required for Precipitation of Phosphoric Acid.=—The length of time required for the complete precipitation of the phosphoric acid by molybdate mixture is perhaps much less than generally supposed. At 65° the precipitation, as shown by de Roode, is complete in five minutes.[100] In a given case the weight of pyrophosphate obtained after five minutes was 0.0676 gram, and exactly the same weight was found after twenty-four hours. In view of these facts analysts would often be able to save time by omitting the delay usually demanded by the setting aside of the yellow precipitate for a few hours in order to secure a complete separation of the phosphoric acid. In the method of the official chemists it is directed that the digestion at 65° be continued for one hour, and this time may possibly be shortened with advantage. In all cases, however, where there is any doubt in regard to the complete separation, some of the molybdate solution should be added to the filtrate and, with renewed digestion, it should be noted whether any additional precipitate be formed.

=120. Examination of the Pyrophosphate.=—In fertilizer control it is not usually thought necessary to examine the magnesium pyrophosphate for impurities. Among those most likely to be found is silica. It is proper, in all cases where accuracy is required, to dissolve the precipitate in nitric acid, boil for some time to convert the pyro- into orthophosphate, and reprecipitate with molybdate and magnesia mixture. This treatment will separate the silica which remains practically insoluble after the first ignition. It has been observed by some analysts that the results obtained by the official method are a trifle too high and also that on re-solution the second precipitate of pyrophosphate weighs less than the first.[101] The difference in most cases is very little but it may become a quantity of considerable magnitude in samples where soluble silica is found in notable quantities. The danger of contamination with iron, alumina, and arsenic has already been mentioned but it is not of sufficient importance to warrant further attention.

=121. Iodin in Phosphates.=—The presence of iodin has been detected in many natural phosphates and is of interest in the discussion of the problem of their origin.[102] A qualitative test for the detection of iodin may be applied in the following manner: Some finely ground phosphate is mixed with strong sulfuric acid and the gases arising from the reaction are aspired into some carbon disulfid or chloroform. The violet coloration arising indicates the presence of iodin. The gases carrying the iodin may also be brought into contact with starch-paste producing the well-known blue color.

The quantity of iodin present in a phosphate is rarely more than one or two-tenths of one per cent. It can be determined as a silver salt, in the absence of chlorin or by any of the standard methods found in works on qualitative analysis.

Iodin is quite a constant constituent of Florida phosphates.

For a quantitative determination, the sample is treated with an excess of strong sulfuric acid in a closed flask and during the decomposition a stream of air is aspired through the flask and caused to bubble through absorption bulbs containing sodium hydroxid in solution.

The temperature of the decomposition may be raised to about 200°. After the solution of the sample the sodium iodid formed is oxidized by heating with potassium permanganate, acidulated and mixed with a solution of potassium iodid to hold the free iodin in solution. The free iodin is determined in the usual way by titration with standard sodium thiosulfate solution. The reactions preparatory to the titration are represented by the following formulas:

2KI + H₂SO₄ = K₂SO₄ + 2HI.
2HI + H₂SO₄ = 2H₂O + SO₂ + 2I.
6I + 6NaOH = NaIO₃ + 5NaI + 3H₂O.
NaI + 2KMnO₄ + H₂O = NaIO₃ + 2KOH + 2MnO₂.
HIO₃ + 5HI = 6I + 3H₂O.

The titration is represented by the following reaction:

2Na₂S₂O₂ + 2I = 2NaI + 2NaI + Na₂S₄O₄.

The decinormal solution of sodium thiosulfate may be used. Grind the crystals of the salt to a fine powder, dry between blotting papers, and use 24.8 grams of the dried salt per liter. The quantity of iodin found in phosphates is so minute that it is hardly worth while to make a quantitative determination of it.

=122. Occurrence of Chromium in Phosphates.=—In some phosphates a small quantity of chromium has been found. In a sample of phosphate from the Island of Los Roques in the Caribbean Sea, Gilbert found three-fourths per cent of chromium oxid (Cr₂O₃). The phosphates containing chromium have a greenish color and are characterized by great insolubility in solutions containing organic acids. The chromium is to be determined by the usual methods described in mineral analysis.

=123. Estimation of Vanadium.=—In the complete analysis of basic slags it becomes necessary to determine the presence of vanadium and its quantity. The method used in this laboratory for the purpose is the volumetric process of Lindemann.[103] It is conducted as follows: Dissolve four grams of the finely powdered slag in sixty cubic centimeters of dilute sulfuric acid (1: 4), boil for a few minutes, cool, make the volume up to 100 cubic centimeters, filter, and take an aliquot part for the determination. Add decinormal potassium permanganate solution in slight excess to secure the oxidation of the vanadium to vanadium pentoxid. Add, drop by drop, a weak solution of ferrous sulfate until the pink color just disappears. Prepare a ferrous sulfate solution by dissolving 2.183 grams of piano wire in sulfuric acid and making the volume to one liter. Titrate the vanadic mixture with this solution until a drop of the clear liquor removed and brought in contact with potassium ferricyanid shows a distinctive blue-green color.

One cubic centimeter of the ferrous sulfate solution is equivalent to 0.002 gram of vanadium, 0.002888 gram of vanadium dioxid, and 0.003648 gram of vanadium pentoxid. The ferrous sulfate solution may also be made and standardized by any of the approved methods in common use.

The method described by Blair, designed especially for the estimation of vanadium in iron and steel, is conducted in the following manner:[104] Five grams of the drillings are dissolved in fifty cubic centimeters of nitric acid of 1.24 specific gravity. The solution is evaporated to dryness in a porcelain dish and heated thereafter until the nitrates are nearly decomposed. After cooling, the dried mass is transferred to a mortar and finely ground with thirty grams of dry sodium carbonate and three grams of sodium nitrate. The finely ground materials are placed in a platinum dish and fused for an hour at a high temperature. Spread the fused mass over the sides of the dish while cooling, and afterwards dissolve in hot water, filter, and wash until the volume is a little over half a liter. Add nitric acid to decompose carbonates, but not completely, and boil to get rid of carbon dioxid, being careful to keep the mass always slightly alkaline. Add nitric acid, drop by drop, until slightly in excess, and then sodium carbonate to marked alkalinity, boil, and filter. Add a slight excess of nitric acid to the filtrate, and the development of a yellow color will indicate the presence of vanadic acid. Add to the solution a small quantity of mercurous nitrate and then an excess of mercuric oxid, suspended in water to render the solution neutral and insure the complete precipitation of mercurous vanadate. The mercurous salt also precipitates phosphoric, chromic, tungstic, and molybdic acids which may be present. Boil, filter, and wash the precipitate with hot water, dry, and ignite. Fuse the residue with sodium carbonate and a little nitrate. Dissolve the fused mass, after cooling, in a little water and filter. Add to the filtrate, ammonium chlorid in excess, from three to five grams for each 100 cubic centimeters of the solution, and allow to stand, with occasional stirring, for some time. Ammonium vanadate, insoluble in a saturated solution of ammonium chlorid, separates as a white powder. It is necessary to keep the solution alkaline, and a drop of ammonia should be added from time to time for this purpose. The appearance of a yellowish tint at any time indicates that the solution has become acid, and this acidity must be corrected, or else the results will be too low. Separate the ammonium vanadate by filtration; wash first with a saturated solution of ammonium chlorid containing a little free ammonia, and then with alcohol. Dry, ignite, and moisten with a few drops of nitric acid; again ignite to obtain the compound as vanadium pentoxid. This compound contains 56.22 per cent of vanadium. The method of Rosenheim and Holversheet may also be used.[105] It is based on the preliminary precipitation of the vanadic acid as a barium or lead salt. The substance supposed to contain vanadium is first brought into solution in such a manner as to secure it as vanadic acid, which is then precipitated with barium chlorid or lead acetate. The precipitate is boiled with hydrochloric acid and potassium bromid, and the liberated bromin determined by the quantity of iodin set free from potassium iodid. In the absence of bodies, such as molybdic acid, which are reduced by sulfurous acid or hydrogen sulfid, the vanadic acid may also be determined by reducing it with one of these reagents and, after removing the excess by boiling, titrating the vanadium tetroxid with potassium permanganate. When vanadic and phosphoric acids occur together the former may be first reduced to tetroxid with sulfurous acid, and after expelling excess of this reagent, the phosphoric acid may be separated with molybdate solution and removed by filtration. When the amount of vanadic acid is large the phosphoric acid should be separated rapidly at 55°-60°, using a considerable excess of the molybdate; or the vanadic acid may first be determined in the solution volumetrically by the bromin process above described, and afterwards the phosphoric acid obtained by evaporating to dryness with a little sulfuric acid, taking the residue up with water, reducing the vanadic with sulfurous acid and precipitating the phosphoric acid with molybdate solution as described above.

=124. Fluorin in Bones.=—Fluorin is not only a constituent of mineral phosphates but also of bones. According to the researches of Carnot there is often as much as one-half per cent of calcium fluorid in bones and teeth.[106] Gabriel has suggested a means of determining a minimum limit of fluorin in bones and teeth by the development of etchings in comparison with known quantities of pure calcium fluorid. The minimum quantity of calcium fluorid necessary to produce a distinct etching, in known conditions, having been determined, the test is applied to known weights of ignited bone or teeth. He concludes from his results, that the ash of bones and teeth often contains less than one-tenth per cent of fluorin. Since, however, there is a loss of fluorin from calcium fluorid, on ignition, the whole of the fluorin may not have been available in the tests described.

=125. Note on the Separation of Iron and Aluminum Phosphates from the Calcium Compound.=—There are many points of difference noted in the descriptions given by authors of the deportment of the iron, and aluminum phosphates in presence of a large excess of the calcium salt. Especially is this true of the statements made by Hess and Glaser[107] in paragraphs =34= and =35=. The subject is of such importance, from an analytical point of view, as to merit a careful study.

In this laboratory a thorough investigation of the mutual deportment of these three phosphates has been made by Brown with the following results:[108] When a mixture containing a known weight of the salts was treated exactly as Hess directs, in no case was there a complete separation of the iron aluminum phosphate from the calcium salt. In order to discover the cause of the failure, pure solutions of calcium and iron aluminum phosphates were treated under identical conditions by the necessary reagents. Fifty cubic centimeters of a solution of calcium phosphate, containing about one gram of the salt, were treated with 100 cubic centimeters of water and fifty cubic centimeters of the commercial ammonium acetate containing 150 grams of the salt in a liter. An immediate precipitate was produced at ordinary temperature, and on heating to 60° it became abundant. The addition of ammonium chlorid, phosphate, and nitrate in successive portions, does not prevent the precipitation. Making the solution more dilute lessens the difficulty when twenty cubic centimeters of a ten per cent solution of ammonium phosphate are first added, followed by the usual quantity of ammonium acetate; a clear crystalline precipitate is sometimes observed. Experience also shows that the trouble is not due to an excess of the ammonium acetate.

In treating a solution of iron aluminum phosphate, in similar circumstances, with the ammonium acetate, it is found that a complete precipitation takes place.

Since diluting the solution of the calcium salt diminishes its tendency to form a precipitate with the ammonium acetate the true method of separation seems to lie in that direction. The calcium salt is held completely in solution when the separation is made in the following way.

The solution containing the mixed phosphates is diluted so as to contain not more than one gram thereof in half a liter. To this is added one drop of dimethylanilin orange, and afterwards ammonium hydroxid, until a very slight precipitate is formed. The mixture is heated to 70° and from twenty to twenty-five cubic centimeters of a twenty-five per cent solution of acid ammonium acetate are added, enough to change the rose color of the indicator to orange. The iron aluminum phosphate is separated by filtration and washed with a hot five per cent solution of ammonium nitrate.

The washed precipitate shows no impurity due to calcium, as proved by dissolving it, reprecipitating and filtering, adding ammonium hydroxid to the filtrate, and heating for a long time. Sometimes a slight troubling of the clear liquid may be observed which may be due to a slight solubility of the iron aluminum phosphate in washing, an accident that may occur if the temperature be allowed to fall below 70°, but no weighable amount of material is obtained. If due to calcium phosphate, a greater dilution in the first precipitation will remove even this mere trace of that salt. In the above conditions the contamination of the iron aluminum precipitate with calcium phosphate may be entirely avoided. We had also undertaken here the problem of separating the phosphoric acid by the citrate method, followed by a destruction of the citric acid in the filtrate by combustion with sulfuric acid according to the kjeldahl process, and final separation of the iron and alumina in the residues when our attention was called to substantially the same process as described by Jean.[109] The method merits a further critical examination.

=126. Phosphoric Acid Soluble in Ammonium Citrate.=—There is no other point connected with the determination of phosphoric acid which has excited so much discussion and about which there is such difference of opinion as the solubility of phosphates in ammonium citrate. It was clearly established by Huston, in 1882, that the ammonium citrate, as used in fertilizer analysis, would attack normal tricalcium phosphate as it exists in bones.[110]

In a raw bone, finely ground, containing 20.28 per cent of phosphoric acid, the following quantities were found to be soluble in a neutral ammonium citrate solution of 1.09 specific gravity.

Time of digestion, thirty minutes.
Temperature 30° 40° 50° 60°
Per cent P₂O₅ dissolved 2.76 4.01 3.39 5.88

From this it appears that the quantity of acid dissolved increases with the temperature of digestion with the exception of the number obtained at 50°. When the time of digestion was increased there was also found a progressive increase in the amount of acid passing into solution. At 40° for forty-five minutes the per cent dissolved was 4.97, and at 40° for one hour it was 5.92. These early determinations had the effect of calling attention to the thoroughly empirical process which was in use, in many modified forms, by agricultural chemists, the world over for determining so-called reverted phosphoric acid in fertilizers. Since the publication of the paper above named many investigations have been undertaken by Huston and others relating to this matter.[111] The general results of these studies, tabulated by Huston, are given below.[112]

INFLUENCE OF THE TIME OF DIGESTION.

(A) = Temperature, degrees C.
(B) = Citrate-soluble phosphoric acid.
(C) = Total phosphoric acid.

Material. Authority. (A) Time of (B) (C)
digestion. Per cent. Per cent.

Bone meal, F. B. Dancy, { 65 ½ hour 10.60 19.75
{ 65 1 “ 11.28 19.75

Orchilla F. B. Dancy, { 65 ½ “ 6.62 21.68
guano, { 65 1 “ 6.85

Navassa F. B. Dancy, { 65 ½ “ 4.64 31.27
rock, { 65 1 “ 4.81 31.27

Navassa F. B. Dancy, { 65 ½ “ 9.00 11.47
superphos., { 65 1 “ 9.21 11.47

Bone meal, H. A. Huston, { 40 ½ “ 4.01 20.28
{ 40 1 “ 5.92 20.28

Bone meal, H. A. Huston and { 65 ½ “ 6.17 23.58
raw, W. J. Jones, Jr., { 65 1 “ 6.49 23.58
{ 65 2 hours 8.22 23.58
{ 65 5 “ 9.31 23.58

Steamed H. A. Huston and { 65 ½ hour 10.59 27.67
bone, W. J. Jones, Jr., { 65 1 “ 12.21
{ 65 2 hours 14.61
{ 65 5 “ 17.94
{ 65 10 “ 19.73

Florida H. A. Huston and { 65 ½ hour 0.56 19.75
soft rock, W. J. Jones, Jr., { 65 2 hours 1.69
{ 65 5 “ 1.47

Precipitated H. A. Huston and { 65 ¼ hour 26.72 33.34
calcium W. J. Jones, Jr., { 65 ½ “ 27.26
phosphate { 65 1 “ 27.28
from glue { 65 2 hours 27.29
works,

Pamunky H. A. Huston and { 65 ½ hour 4.43 13.84
phosphate,[113] W. J. Jones, Jr.,{ 65 1 “ 8.28
{ 65 2 hours 10.34
{ 65 5 “ 11.80
{ 65 10 “ 12.58

Calcined H. A. Huston and { 65 ½ hour 21.24 45.15
Redonda, W. J. Jones, Jr.,{ 1 “ 31.70
{ 2 hours 36.92
{ 5 “ 41.00
{ 10 “ 42.70

South H. A. Huston and { 65 ½ hour 2.82 25.51
Carolina W. J. Jones, Jr.,{ 1 “ 3.13
rock, { 2 hours 3.57
{ 5 “ 3.88

INFLUENCE OF TEMPERATURE.

(A) = Temperature, degrees C.
(B) = Citrate-soluble phosphoric acid.
(C) = Total phosphoric acid.

Material. Authority. Time of (A) (B) (C)
Digestion Per cent. Per cent.

Apatite T. S. Gladding[114]{ ½ hour 40 0.30
Canadian, { ½ “ 65 0.56

Orchilla T. S. Gladding, { ½ “ 40 4.63
guano, { ½ “ 65 4.81

South T. S. Gladding, { ½ “ 40 1.09
Carolina { ½ “ 65 1.35
river rock,

Navassa T. S. Gladding, { ½ “ 40 2.73
rock, { ½ “ 65 2.53

Grand T. S. Gladding, { ½ “ 40 1.16
Connetable, { ½ “ 65 1.96

Redonda, S. W. Johnson and { ½ “ 40 1.70 36.68
E. H. Farrington, { ½ “ 65 1.85

South S. W. Johnson and { ½ “ 40 1.32 25.48
Carolina E. H. Farrington, { ½ “ 65 1.65
rock,

Orchilla S. W. Johnson and { ½ “ 40 4.92 21.05
guano, E. H. Farrington, { ½ “ 65 5.85

Navassa S. W. Johnson and { ½ “ 40 4.10 29.90
rock, E. H. Farrington, { ½ “ 65 4.22

Acid S. W. Johnson and { ½ “ 40 11.95 16.50
Navassa, E. H. Farrington, { ½ “ 65 13.53

Fine-ground S. W. Johnson and { ½ “ 40 9.40 23.50
bone, E. H. Farrington, { ½ “ 65 12.90

South C. V. Sheppard, Jr.{ ½ “ 40 1.72 24.50
Carolina also H. C. White, { ½ “ 65 2.11
land rock,

Orchilla C. V. Sheppard, Jr.{ ½ “ 40 6.48 15.85
guano, also H. C. White, { ½ “ 65 6.75

Calcined C. V. Sheppard, Jr.{ ½ “ 40 5.70 44.85
Redonda, also H. C. White, { ½ “ 65 10.20

Raw C. V. Sheppard, Jr.{ ½ “ 40 4.49 43.79
Redonda, also H. C. White, { ½ “ 65 7.92

Acid C. V. Sheppard, Jr.{ ½ “ 40 3.55 18.25
phosphate, also H. C. White, { ½ “ 65 4.05
S. C. 10.35
per cent
water-soluble,

Acid C. V. Sheppard, Jr.{ ½ “ 40 10.85 16.20
Navassa, also H. C. White,{ ½ “ 65 11.00
2.85 per cent {
water-soluble,

Bone, H. A. Huston, { ½ “ 30 2.76 20.28
{ ½ “ 40 4.01
{ ½ “ 50 3.39
{ ½ “ 60 5.88

Acid Sheppard and { ½ “ 40 3.46 15.95
phosphate, Robertson, { ½ “ 60 3.82
11.41 per cent
water-soluble,

Calcined H. A. Huston, { ½ “ 40 2.18 45.46
Redonda, { ½ “ 50 5.52
{ ½ “ 65 21.24
{ ½ “ 75 32.90
{ ½ “ 85 39.52

Calcined H. A. Huston and { 5 hours 40 26.78 42.90
Redonda, W. J. Jones, Jr., { 5 “ 65 38.19
{ 5 “ 85 41.57

Pamunky H. A. Huston and { 5 “ 40 3.10 13.84
phosphate, W. J. Jones, Jr., { 5 “ 65 11.80
{ 5 “ 85 12.82

Raw bone, H. A. Huston and { 2 “ 40 5.96 23.58
W. J. Jones, Jr., { 2 “ 65 8.22
{ 2 “ 85 8.71

Steamed H. A. Huston and { 5 “ 40 16.02 27.67
bone, W. J. Jones, Jr., { 5 “ 65 20.22
{ 5 “ 85 20.66

Precipitated H. A. Huston and { 2 “ 40 24.14 33.34
calcium W. J. Jones, Jr., { 2 “ 65 23.45
phosphate { 2 “ 85 22.46
from glue works,

Florida H. A. Huston and { 2 “ 40 0.00 19.75
soft rock, W. J. Jones, Jr., { 2 “ 65 1.69
{ 2 “ 85 1.99

INFLUENCE OF QUANTITY OF MATERIAL USED.

(A) = Temperature, degrees C.
(B) = Quantity of material used.
(c) = Citrate-soluble phosphoric acid.
(D) = Total phosphoric acid.

Material. Authority. Time (A) (B) (C) (D)
Grams. Per cent. Per cent.

Orchilla F. B. Dancy, { ⅔ hour 40 2.0 9.94 21.68
guano, { ⅔ “ 40 1.0 12.14
{ ⅔ “ 40 0.5 13.51
{ ½ “ 65 2.0 6.62
{ ½ “ 65 1.0 9.33

Redonda, S. W. Johnson { ½ “ 40 2.0 1.70 36.68
and E. H. { ½ “ 40 0.4 3.46
Farrington, { ½ “ 65 2.0 1.85
{ ½ “ 65 0.4 5.26

South S. W. Johnson { ½ “ 40 2.0 1.32 25.48
Carolina and E. H. { ½ “ 40 0.4 1.33
rock, Farrington, { ½ “ 65 2.0 1.65
{ ½ “ 65 0.4 3.36

Orchilla T. S. Gladding,{ ½ “ 65 2.0 5.87
guano, { ½ “ 65 0.4 13.05

Calcined H. A. Huston, { ½ “ 65 0.5 16.80 45.46
Redonda, { ½ “ 65 1.0 18.26
{ ½ “ 65 2.0 21.24
{ ½ “ 65 3.0 23.22
{ ½ “ 65 5.0 24.66
{ ½ “ 65 10.0 28.64

Calcined H. A. Huston, { 5 hours 65 0.5 41.77 45.46
Redonda, { 5 “ 65 2.0 41.53
{ 5 “ 65 8.0 39.86

Pamunky H. A. Huston { 5 “ 65 0.5 11.81 13.84
phosphate, and W. J. { 5 “ 65 2.0 11.80
Jones, Jr., { 5 “ 65 4.0 11.44

Raw bone, H. A. Huston { 2 “ 65 0.5 16.49 23.58
and W. J. { 2 “ 65 2.0 8.22
Jones, Jr., { 2 “ 65 4.0 7.22

Steamed H. A. Huston { 5 “ 65 0.5 26.40 27.67
bone, and W. J. { 5 “ 65 2.0 17.94
Jones, Jr., { 5 “ 65 4.0 12.12

Precipitated H. A. Huston { 2 “ 65 0.5 33.34 33.34
calcium and W. J. { 2 “ 65 2.0 27.29
phosphate Jones, Jr., { 2 “ 65 4.0 19.49
from glue
works,

Florida H. A. Huston { 2 “ 65 0.5 5.50 19.75
soft rock, and W. J. { 2 “ 65 2.0 1.69
Jones, Jr., { 2 “ 65 4.0 1.27

Influence of Acidity and Alkalinity.

(A) = Temperature, degrees C.
(B) = 100 cc neutral citrate + citric acid.
(C) = 100 cc neutral citrate + ammonia equivalent to citric acid.
(D) = Per cent of phosphoric acid dissolved.
(E) = Per cent total phosphoric acid.

Material. Authority. Time (A) (B) (C) (D) (E)

Navassa T. S. Gladding[115]{ ½ hour 65 0.00 0.00 2.53
rock, { ½ “ 65 0.733 gm 4.87
{ ½ “ 65 0.733 gm 1.22

South T. S. Gladding, { ½ “ 65 0.00 0.00 1.35
Carolina { ½ “ 65 0.733 “ 2.89
rock, { ½ “ 65 0.733 “ 1.06

Grand T. S. Gladding, { ½ “ 65 0.00 0.00 1.97
connetable, { ½ “ 65 0.733 “ 1.12
{ ½ “ 65 0.733 “ 11.44

Dissolved H. B. McDonnell, { ½ “ 65 0.00 0.00 2.49 11.51
bone-black { ½ “ 65 0.01 “ 2.42
and { ½ “ 65 0.01 “ 2.37
cottonseed-meal,

Ground H. B. McDonnell, { ½ “ 65 0.00 0.00 8.66 26.62
bone, { ½ “ 65 0.01 “ 9.18
{ ½ “ 65 0.01 “ 8.00

Calcined H. B. McDonnell, { ½ “ 65 0.00 0.00 30.61 45.11
Redonda, { ½ “ 65 0.01 “ 29.42
{ ½ “ 65 0.01 “ 32.47

Dissolved H. A. Huston, { ½ “ 65 0.00 0.00 2.24 11.32
bone-black { ½ “ 65 1.00 “ 2.24
and { ½ “ 65 1.00 “ 2.21
cottonseed-meal,

Ground H. A. Huston { ½ “ 65 0.00 0.00 8.78 26.35
bone, and W. J. { ½ “ 65 1.00 “ 13.48
Jones, Jr., { ½ “ 65 1.00 “ 5.35

Calcined H. A. Huston { ½ “ 65 0.00 0.00. 25.54 45.15
Redonda, and W. J. { ½ “ 65 1.00 “ 18.84
Jones, Jr., { ½ “ 65 1.00 “ 35.20

South H. A. Huston { ½ “ 65 0.00 0.00 1.81 27.67
Carolina and W. J. { ½ “ 65 1.00 “ 4.59
rock, Jones, Jr., { ½ “ 65 1.00 “ 0.74

Basic H. A. Huston { ½ “ 65 0.00 0.00 6.98 19.42
slag, and W. J. { ½ “ 65 1.0 “ 10.12
Jones, Jr., { ½ “ 65 1.0 “ 5.49

Pamunky H. A. Huston { 5 h’rs 65 0.00 0.00 11.80 13.84
phosphate, and W. J. { 5 “ 65 1.0 11.79
Jones, Jr., { 5 “ 65 1.0 “ 12.28

Raw bone, H. A. Huston { 2 “ 65 0.00 0.00 8.22 23.58
and W. J. { 2 “ 65 1.0 “ 11.20
Jones, Jr., { 2 “ 65 1.0 “ 4.02

Steamed H. A. Huston { 5 “ 65 0.00 0.00 17.94 27.67
bone, and W. J. { 5 “ 65 1.0 “ 22.55
Jones, Jr., { 5 “ 65 1.0 “ 9.64

Precipitated H. A. Huston { 2 “ 65 0.00 0.00 24.20 33.34
calcium and W. J. { 2 “ 65 1.0 “ 30.70
phosphate Jones, Jr., { 2 “ 65 1.0 “ 20.67
from glue works,

Florida H. A. Huston, { 2 “ 65 0.00 0.00 1.69 19.75
soft rock, and W. J. { 2 “ 65 1.0 “ 3.37
Jones, Jr., { 2 “ 65 1.0 “ 0.72

Calcined H. A. Huston { 5 “ 65 0.00 0.00 40.64 44.30
Redonda, and W. J. { 5 “ 65 1.0 “ 40.05
Jones, Jr., { 5 “ 65 1.0 “ 41.01

In the above tabulations no mention is made of the work of Fresenius, Neubauer, and Luck, on whose researches the citrate method is based, but an examination of their original paper shows that the temperature conditions are not carefully enough controlled to justify us in tabulating their results.[116] An attempt has been made to include in the above tables, work made under well-defined conditions, which will illustrate the various points under consideration. While each authority of value upon the subject is represented, no attempt has been made to include all the work done by any of them. One element that seems to have been generally overlooked in discussing the problem is that nearly all results have been obtained from one-half hour’s treatment of the material. This means simply the study of an incomplete reaction, and one which is interrupted while the solution is very rapidly going on. This, of course, is only clearly brought out by a comparison of long-time and short-time work in the various tables. In the opinion of Huston very much more work will have to be done before it can be assumed that we have any very clear knowledge of this subject, and very likely the final result will be that all kinds of goods cannot be examined by the same method. The fact that half a gram of dicalcium phosphate is instantly soluble in 100 cubic centimeters of citrate solution, at ordinary temperatures, while an equal amount of iron and aluminum phosphate is acted upon very slowly at ordinary temperatures will probably have to be taken into consideration, as well as the fact that dicalcium phosphate is less soluble in hot solutions of ammonium citrate than it is in cold solutions, while the reverse is true of the precipitated iron and aluminum phosphate.

At present, the only conclusion that can be safely drawn from the work, is that it would be unsafe to make any generalization upon the subject until more facts are at hand, except that the present methods are unscientific and, unsatisfactory. As the work progresses, new features present themselves, and in such a way as to show that they must be given careful consideration before drawing any final conclusions in the matter.

=127. Arbitrary Determination of Reverted Phosphoric Acid.=—The so-called reverted phosphoric acid, that is, the acid insoluble in water and soluble in a solution of ammonium citrate, is the most difficult constituent of commercial fertilizers from the point of view of the scientific analyst. A review of all the standard methods which have been given in the preceding pages for its determination must convince every careful observer that, as a rule, each process is based on arbitrary standards, and can give only concordant results when carried out under strictly unvarying conditions. For this reason there can be no just comparison between the results obtained by different methods, which vary from each other only in slight particulars. When, on the other hand, the processes are radically different, the deviations in data become more pronounced.

In such a condition of affairs the analyst is left to choose between methods. He must be guided in his choice not only by what seems to be the most scientific and accurate process, but also, to a certain extent, by the general practice of his professional brethren. For this country, therefore, it is strongly urged that the methods adopted by the Association of Official Agricultural Chemists, be followed in every detail.

By the phrase “reverted phosphoric acid,” was originally meant an acid once soluble in water, as CaH₄(PO₄)₂, and afterwards changed to a form insoluble in water, but soluble in ammonium citrate as Ca₂H₂(PO₄)₂. But in practice this has never been the true signification of the term. In the manufacture of acid and superphosphates there is formed, more or less of the dicalcium phosphate, either directly or after a time, and this salt which, in no sense can be called reverted, is entirely soluble in ammonium citrate. The iron and aluminum phosphates are also, to a certain degree, soluble in the same reagent. When an acid phosphate, containing various forms of calcium phosphate, is applied to a soil containing iron and alumina, the soluble parts of the compound tend to become fixed by union with those bases, or by precipitation as Ca₂H₂(PO₄)₂. But it is not alone reverted phosphate formed in this way, which the analyst is called on to determine in a fertilizer, although he may have occasion to treat it in soil analysis.

The expression “reverted phosphoric acid,” therefore, in practice not only includes a dicalcium phosphate, which once may have been the monocalcium salt, but also all of that salt originally existing in the superphosphate, and formed directly during its manufacture, as well as any iron and aluminum phosphates present which are soluble in ammonium citrate. The expression “citrate-soluble” is, therefore, to be preferred to “reverted” phosphoric acid.

In the reversion of the phosphoric acid in superphosphates the iron plays a far more important role than the aluminum sulfate. It was formerly supposed that the reversion took place as indicated in the following formula: 2CaH₄(PO₄)₂ + Fe₂O₃ = 2(CaHPO₄, FePO₄) + 3H₂O, while Wagner affirms that the reverted acid compounds consist of varying quantities of ferric oxid, aluminum oxid, phosphorus pentoxid, and calcium oxid, in various states of combination.[117] The more probable reaction is the following: 3CaH₄(PO₄)₂ + Fe₂(SO₄)₃ + 4H₂O = 2(FePO₄, 2H₃PO₄, 2H₂O) + 3CaSO₄. This reaction can be demonstrated by adding to a superphosphate solution one of a ferric salt. In addition to free phosphoric acid, iron phosphate is separated, which gradually passes into an insoluble form by the abstraction of water due to the crystallization of the gypsum. The alumina present in a superphosphate seems to have no direct influence on the process of reversion. Its phosphate salt is not acted on by the acid calcium phosphate. Even when a superphosphate solution is treated with alum no precipitation is produced, except on warming, and this disappears when the mass is again cold.

It is therefore not necessary in the process of manufacture to separate the alumina by digestion with a hot soda-lye before treating the mass with sulfuric acid.

In order to avoid the reversion of the phosphoric acid several plans have been proposed. One of the best is to use a little excess of sulfuric acid in the manufacture. This tends to hold the phosphoric acid in soluble form but is objectionable on account of drying, handling, and shipping the fertilizer. During the digestion, moreover, it is important that the temperature does not rise above 120°. Another method consists in adding to the dissolved rock a quantity of common salt chemically equivalent to its iron content. Ammonium sulfate also helps to hold the phosphoric acid water-soluble.

=128. Influence of Movement.=—The influence of time and temperature of digestion, and of variations in the composition of the ammonium citrate on the quantity of phosphoric acid dissolved by that reagent has been pointed out. Of great importance also in the process is the character of the movement to which the materials are subjected during the digestion. For this reason various mechanical devices have been constructed to secure uniformity of solution. Inasmuch as the temperature factor must also be faithfully observed, the best of these devices are so arranged as to admit of a uniform motion within a bath of water kept at the desired temperature which, by the Association method, is 65°.

=129. Digestion Apparatus for Reverted Phosphates.=—The digestion apparatus used by Huston consists of two wheels twenty-five centimeters in diameter, mounted on the same axis, having a clear space of four and one-half centimeters between them.[118] In the periphery of each wheel are cut twelve notches, which are to receive the posts bearing the rings through which the necks of the flasks pass. The posts are held in place by nuts which are screwed down on the faces of the wheel. Should it become necessary to take the apparatus apart, it is only necessary to loosen the nuts and the set screw holding one wheel to the shaft and all the parts can at once be removed. The posts extend ten centimeters beyond the face of the wheels, and the rings are four centimeters in internal diameter. Perforated plates, bearing a cross-bar, and held in place by strong spiral springs attached to the plate and the base of the posts, serve to hold the flasks in place. Each plate has a number stenciled through it for convenience in identifying the flasks when it is time to remove them. Attached to the outside of each post, close to the outer end, is a heavy wire which passes entirely around the apparatus, serving to keep the plates in place after they are removed from the flasks.

The apparatus is mounted on a substantial framework, thirty-six centimeters high and thirty centimeters wide at the base. The space in which the wheel revolves is fourteen centimeters wide. The base bars connecting the two sides are extended seven centimeters beyond one side, and serve for the attachment of lateral bracing. At the top of the framework, at one side, is attached a heavy bar forty-five centimeters long, which serves to carry the cog gearing which transmits the power. The upright shaft carries a cone pulley to provide for varying the speed. The usual speed is two revolutions a minute for the wheel carrying the flasks. The entire apparatus is made of brass. The details of construction are shown in Fig. 8. Round-bottomed flasks are used, and rubber stoppers are held in place by tying or by a special clamp shown at the lower right-hand of the figure.

When high temperatures are used, the plates and flasks are handled by the hooks shown at the left and right-hand upper corners of the figure.

When any other than room temperature is desired, the whole apparatus is immersed in water contained in the large galvanized tank forming the back-ground of the figures. The tank is seventy-five centimeters long, seventy-five centimeters high, and thirty centimeters wide. At one end, near the top, is an extension to provide space for heating the fluid in the flasks before introducing the solid in such cases as may be desired.

The apparatus is held in place by angle irons soldered to the bottom of the tank and a brace resting against the upright bar bearing the gear-wheels.

The water in the tank is heated by injecting steam, or by burners under the tank. As the tank holds about 300 pounds of water the work is not subject to sudden changes of temperature, and little trouble has been experienced in raising and maintaining the temperature of the water, especially when steam is used.

An electric motor, or a small water-motor with only a very moderate head of water, will furnish ample power.

=130. Comparison of Results.=—The following data show the results obtained by the digester as compared with those furnished by the official method, temperature and time of digestion being the same in each instance.

AMMONIUM CITRATE SOLUTION ON PHOSPHATES.

-------------------------------------------------------------------
Time Total Removed Removed
of phos. by official by
Substance. treatment. acid. method. digester.
Per cent. Per cent. Per cent.
-------------------------------------------------------------------
½ hour 27.67 10.59 14.52
1 “ 12.21 14.82
2 hours 14.61 17.56
Steamed bone, 3½ “ 16.48 18.53
5 “ 17.94 20.22
7½ “ 18.99 20.25
10 “ 19.73 21.18
-------------------------------------------------------------------
½ hour 13.86 4.43 4.11
1 “ 8.28 6.82
2 hours 10.34 9.76
Marl, 3½ “ 11.00 11.31
5 “ 11.80 11.83
8 “ 12.51 12.64
10 “ 12.58 13.00
-------------------------------------------------------------------
½ hour 19.38 12.09 12.28
1 “ 12.47 12.40
Acidulated bone, 2 hours 12.20 12.43
3½ “ 12.40 12.24
5 “ 12.43 12.26
-------------------------------------------------------------------
Bone, ½ hour 21.40 6.97 8.48
Ammoniated dissolved bone, ½ “ 18.22 9.28 10.63
Cottonseed-meal and
castor pomace, ½ “ 2.52 0.23 0.25
Phospho bone, ¼ “ 16.55 7.06 7.21
-------------------------------------------------------------------

In comparing duplicates, the results from the use of the digester are found to be subject to less variation than those from the usual method.

=131. Huston’s Mechanical Stirrer.=—The stirring apparatus shown in Fig. 9 differs from those which have heretofore come into use, in requiring but a single belt to drive all the stirring rods, and in having all the parts protected from the laboratory fumes.[119] The details of the belt system are shown in the small diagram in the lower central part of the figure. The apparatus is mounted on a substantial wooden box, 200 centimeters long, thirty centimeters high, and eighteen centimeters wide. The driving pulleys, ten centimeters in diameter, are enclosed in the upper part of the case. The shafts on which these pulleys are mounted extend through the bottom of the enclosing box and carry a wooden disk, eleven centimeters in diameter, to prevent particles of foreign matter from falling into the beakers. The shafts extend two centimeters below these disks, and to the end of the shafts the bent stirring rods are attached by rubber tubing.

The board forming the support of the driving pulleys is extended two centimeters in front of the apparatus, and in this extension twelve notches are cut, in which are held the corks carrying the tubes which contain the solution to be used in precipitating the material in the beakers.

The ends of these tubes are drawn out to a fine point so as to deliver the liquid at the rate of about one drop per second.

The front of the apparatus is hinged and permits the whole to be closed when not in use, or during the precipitation.

The apparatus has proven extremely satisfactory in the precipitation of ammonium magnesium phosphate. The precipitate is very crystalline, and where the stirring is continued for some minutes, after the magnesia solution has all been added, no amorphous precipitate is observed on longer standing.