114. Volumetric Silver Method.—Holleman has proposed a modification of the silver method for the volumetric determination of phosphoric acid, which is conducted in the following manner:[97]
In a flask of 200 cubic centimeters capacity, are placed fifty cubic centimeters of the liquid to be analyzed, which should not contain more than two-tenths gram of phosphoric acid. The solution is treated with ten cubic centimeters of a normal solution of sodium acetate and afterwards with a slight excess of decinormal silver solution, four and five-tenths cubic centimeters for each 0.01 gram of phosphoric acid. The solution is then neutralized with tenth normal sodium hydroxid, the amount required having been previously determined by titrating ten cubic centimeters of the liquid to be analyzed, using phenolphthalein as an indicator. Five times the quantity required for the neutralization of the ten cubic centimeters is added, less one-half cubic centimeter. By this treatment the phosphoric acid in the presence of sodium acetate is completely precipitated as silver phosphate. The excess of silver is determined by diluting the mixture to 200 cubic centimeters, filtering, and titrating 100 cubic centimeters of the filtrate with ammonium thiocyanate. The presence of sulfuric and nitric acids does not interfere with the reaction, but of course hydrochloric acid must be absent. Alkalies and alkaline earth metals may be present, but not the heavy metals.
When iron and aluminum are present 100 cubic centimeters of the solution are precipitated with thirty cubic centimeters of normal sodium acetate, the phosphoric acid is determined in fifty cubic centimeters of the filtrate, and the precipitate of iron and aluminum phosphates is ignited and weighed, and its weight multiplied by 2.225 is added to the phosphoric anhydrid found volumetrically. If ammonia be present it must be removed by boiling, as otherwise it affects the titration with phenolphthalein.
For agricultural purposes this method can have but little value inasmuch as the phosphates to be examined almost always have a certain proportion of iron and aluminum. Inasmuch as the amount of these bases has to be determined gravimetrically, there would be no gain in time and no simplification of the processes by the use of the volumetric method as proposed.
TECHNICAL DETERMINATION
OF PHOSPHORIC ACID.
115. Desirability Of Methods.—In the preceding paragraphs, has been given a statement of the principal methods now in use by chemists and others connected with fertilizer control for the scientific and agronomic determinations of phosphoric acid, and its agricultural value.
A résumé of the important methods, in a form suited to use in a factory for preparing phosphatic fertilizers for the market, seems desirable. In these factories the chemists have been accustomed to use their own, or private methods, and there has not been a general disposition among them to publish their methods and experience for the common benefit. For factory processes, a method should be not only reasonably accurate, but also simple and rapid. It is evident, therefore, that the general principles already indicated must underlie any method which would prove useful to factory work. Albert has made a résumé of such methods applicable for factory control, and these are given here for convenience, although they are, in many respects, but condensed statements of methods already described.[98]
116. Reagents.—Molybdate Solution.—One hundred and ten grams of pure molybdic acid are dissolved in ammonia of nine-tenths specific gravity and diluted with water to one liter. The solution is poured into one liter of nitric acid, of one and two-tenths specific gravity, and, after standing a few days, filtered.
Concentrated Ammonium Nitrate Solution.—Seven hundred and fifty grams of pure ammonium nitrate are dissolved in water and made up to one liter.
Magnesia Mixture.—Fifty-five grams of magnesium chlorid; seventy grams of ammonium chlorid; 130 cubic centimeters of ammonia of nine-tenths specific gravity are dissolved and diluted with water to one liter.