Principles and practice of agricultural analysis. Volume 2 (of 3), Fertilizers

by Harvey Washington Wiley

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.