Thomson has shown[84] that of the three hydrogen atoms in phosphoric acid two must be saturated with alkali before the reaction with phenolphthalein is neutral. Therefore, when the yellow precipitate is broken up by an alkali, according to the reaction to follow, only four of the six molecules of ammonium are required to form a neutral ammonium phosphate as determined by the indicator employed. The remaining two molecules of ammonium unite with the molybdenum forming also a salt neutral to the indicator.
Phenolphthalein is preferred because, as has been shown by Long, its results are reliable in the presence of ammonium salts unless they be present in large quantity, and if the solution be cold and the indicator be used in sufficient quantity.[85] To prepare the indicator for this work, one gram of phenolphthalein is dissolved in 100 cubic centimeters of sixty percent alcohol. At least one-half of a cubic centimeter of the solution is used for each titration.
The advantages claimed for the method are its speed and accuracy. Much time is saved by avoiding the necessity for the removal of the silica by evaporation. The results of analyses with and without the removal of the silica are practically identical. When the silica is not removed it is noticed that the filtrate from the yellow precipitate has a yellow tint.
The reaction is represented by the following formula:
(NH₄)₆(PO₄)₂(MoO₃)₂₄ + 46KOH = (NH₄)₄(HPO₄)₂ + (NH₄)₂MoO₄
+ 23K₂MoO₄ + 22H₂O.
From this reaction it is seen that the total available acidity of one molecule of the yellow precipitate titrated against phenolphthalein is equivalent to twenty-three molecules of potassium hydroxid.
Calculation of Results.—The standard alkali is of such strength that one cubic centimeter is equal to one per cent of phosphoric acid when one gram of material is employed and one-tenth of it taken for each determination. In a given case one gram of a sample was taken and one-tenth of the solution used. Fifty cubic centimeters of alkali were added to the yellow precipitate. It required thirty-two cubic centimeters of standard alkali to neutralize the excess.
The alkali consumed by the yellow precipitate was 50 - 32 = 18. The sample therefore contained eighteen per cent of phosphoric acid.
Comparison with Official Method.—A comparison of the Pemberton volumetric with the official method of the Association of Agricultural Chemists has been made by Day and Bryant.[86] The comparisons were made on samples containing from 1.45 to 37.28 per cent of phosphoric acid and resulted as follows:
| Substance. | Per cent P₂O₅, Official. | Per cent P₂O₅, Pemberton. | ||
|---|---|---|---|---|
| No. 1. | Florida | rock | 1.45 | 1.32 |
| “ 2. | “ | “ | 4.40 | 4.53 |
| “ 3. | Sodium | phosphate | 19.78 | 19.99 |
| “ 4. | “ | “ | 19.72 | 19.73 |
| “ 5. | Florida | rock | 37.28 | 37.22 |