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 digestion. | (B) Per cent. | (C) Per cent. | ||
|---|---|---|---|---|---|---|---|
| Bone meal, | F. B. Dancy, |  | 65 | ½ | hour | 10.60 | 19.75 |
| 65 | 1 | “ | 11.28 | 19.75 | |||
| Orchilla guano, | F. B. Dancy, | | 65 | ½ | “ | 6.62 | 21.68 |
| 65 | 1 | “ | 6.85 | ||||
| Navassa rock, | F. B. Dancy | | 65 | ½ | “ | 4.64 | 31.27 |
| 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 bone, | H. A. Huston and |  | 65 | ½ | hour | 10.59 | 27.67 |
| 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 from | 65 | 1 | “ | 27.28 | |||
| glue works | 65 | 2 | hours | 27.29 | |||
| 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 | |||||