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°.
