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

by Harvey Washington Wiley

Two and One-Half Per Cent Ammonia.—One hundred cubic centimeters of ammonia of nine-tenths specific gravity are diluted with water to one liter.

Joulie’s Citrate Solution.—Four hundred grams of citric acid are dissolved in ammonia of nine-tenths specific gravity and diluted to one liter with ammonia of the same strength.

Wagner’s Citrate Solution.—One hundred and fifty grams of citric acid are exactly neutralized with ammonia, then ten grams of citric acid added and diluted to one liter with water.

Sodium Acetate Solution.—One hundred grams of sodium acetate, crystallized, are dissolved in water, treated with 100 cubic centimeters of acetic acid, and diluted to one liter with water.

Calcium Phosphate Solution.—About ten grams of dry, pure tribasic calcium phosphate are dissolved in nitric acid and diluted with water to one liter. In this solution the phosphoric acid is determined gravimetrically by the molybdate or citrate method, and the value of the solution marked on the flask containing it.

Titrated Uranium Solution.—Two hundred and fifty grams of uranium nitrate are dissolved in water, twenty-five grams of sodium acetate added, and the whole diluted to seven liters. One cubic centimeter of this solution corresponds to about 0.005 gram of phosphorus pentoxid. In order to determine its exact value proceed as follows: Twenty-five cubic centimeters of the calcium phosphate solution which, for example, has been found to contain 0.10317 gram of phosphorus pentoxid, are neutralized in a porcelain dish with ammonia, acidified with acetic, treated with ten cubic centimeters of sodium acetate solution, and warmed. Through a burette as much uranium solution is allowed to flow as is necessary to show in a drop of the solution taken out of the dish, when treated with a drop of pure potassium ferrocyanid, a slight brown color. In order to be certain, this operation is repeated two or three times with new quantities of twenty-five cubic centimeters of calcium phosphate solution. Example:

Twenty-five cubic centimeters of the calcium phosphate solution containing 0.10317 gram of phosphorus pentoxid, gave as a mean of three determinations 23.2 cubic centimeters of the uranium solution necessary to produce the brown color with potassium ferrocyanid. Consequently 0.10317 ÷ 23.2 = 0.00445 gram of phosphorus pentoxid equivalent to one cubic centimeter of uranium solution. If, for instance, a quantity of fertilizer weighing exactly five grams, require ten cubic centimeters of the uranium solution for the complete precipitation of its phosphoric acid, then the quantity of phosphoric acid contained in the fertilizer would be equivalent to 10 × 0.0045, equivalent to 0.0445 gram of phosphorus pentoxid. The fertilizer, therefore, contains eight and nine-tenths per cent of phosphorus pentoxid.

Conduct of the Molybdenum Method.—This method rests upon the precipitation of the phosphorus pentoxid by a solution of ammonium molybdate in nitric acid, solution of the precipitate in ammonia, and subsequent precipitation with magnesia.

Manipulation.—Twenty-five or fifty cubic centimeters of a solution of the phosphate which has been made up to a standard volume and containing about one-tenth gram of phosphorus pentoxid, are placed in a beaker together with 100 cubic centimeters of the molybdate solution and treated with as much ammonium nitrate solution as will be sufficient to give the liquid a content of fifteen per cent of ammonium nitrate. The contents of the beaker are well mixed and warmed for about twenty minutes at from 60° to 80°. After cooling, they are filtered and the precipitate washed on the filter with cold water until a drop of the filtrate saturated with ammonia does not become opaque on treatment with ammonium oxalate. The filtrate is then washed from the filter with two and one-half per cent ammonia solution and precipitated slowly and with constant stirring by the magnesia mixture. After standing for two hours the ammonium magnesium phosphate is separated by filtration, washed with two and one-half per cent ammonia until the filtrate contains no more chlorin, and ignited.

Conduct of the Citrate Method.—The principle of this method depends upon the fact that when a sufficient quantity of ammonium citrate is added to phosphate solutions, iron, alumina, and lime are retained in solution when, on the addition of the magnesia mixture in the presence of free ammonia, the phosphoric acid is completely precipitated as ammonium magnesium phosphate.