(b) Alternate Method.—To 370 grams of commercial citric acid add commercial ammonia, of 0.96 specific gravity, until nearly neutral; reduce the specific gravity to nearly 1.09 and proceed as follows: Prepare a solution of fused calcium chlorid 200 grams to the liter, and add four volumes of strong alcohol. Make the mixture exactly neutral, using a small amount of freshly prepared corallin solution as a preliminary indicator, and test finally by withdrawing a portion, diluting with an equal volume of water, and testing with cochineal solution. Fifty cubic centimeters of this solution will precipitate the citric acid from ten cubic centimeters of the citrate solution. To ten cubic centimeters of the nearly neutral citrate solution add fifty cubic centimeters of the alcoholic calcium chlorid solution, stir well, filter at once through a folded filter, dilute with an equal volume of water, and test the reaction with neutral solution of cochineal. If acid or alkaline, add ammonia or citric acid, as the case may be, to the citrate solution, mix, and test again as before. Repeat this process until a neutral reaction of the citrate solution is obtained. At the end the specific gravity must be 1.09 at 20°.
(2) Molybdic Solution.—[See paragraph 22].
(3) Ammonium Nitrate Solution.—Dissolve 200 grams of commercial ammonium nitrate in water and bring to a volume of two liters.
(4) Magnesia Mixture.—[See paragraph 22].
(5) Dilute Ammonia for Washing.—[See paragraph 22].
(6) Magnesium Nitrate.—Dissolve 320 grams of calcined magnesia in nitric acid, avoiding an excess of the latter; then add a little calcined magnesia in excess, boil, filter from the excess of magnesia, ferric oxid, etc., and bring to volume of two liters.
54. Official Methods with Norwegian Fertilizers.—The Director of the Chemical Control Station of Norway, expresses the opinion, that for Norwegian, Swedish, Danish, and German conditions, the American methods for the determination of phosphoric acid, notwithstanding their analytical exactness, are quite inapplicable.[40] In those countries are found many, in part, poorly pulverized and badly mixed manures, such as ammonium superphosphate, potassium superphosphate, and potassium ammonium superphosphate, and these can not usually be so well pulverized and mixed that one can take out a true average sample of from two to two and five-tenths grams. Care in the analysis is useless when the material employed does not represent the average condition of the materials investigated. Therefore, in the countries named, often from ten to twenty grams, and almost never less than five grams of substance are taken in the preparation of the solutions, except for instance, in the determination of nitrogen and reverted phosphoric acid.
55. The Molybdic Acid Method, as Practiced by Direction of the Union of the German Experiment Stations.—The method adopted by the German Experiment Stations is essentially that used at Halle.[41] The samples are brought into solution in the following way: For the estimation of phosphoric acid in bone-meal, fish-guano and raw phosphates, and the total phosphoric acid in superphosphates, five grams of the sample are dissolved in fifty cubic centimeters of aqua regia, made of three parts of hydrochloric acid of 1.12 specific gravity and one part of nitric acid of 1.25 specific gravity, or the solution may be made of a mixture of twenty cubic centimeters of nitric acid of 1.42 specific gravity and fifty cubic centimeters of sulfuric acid of 1.8 specific gravity. The boiling should continue for half an hour. The solution is made up to half a liter and filtered. Fifty cubic centimeters of the filtrate containing the phosphoric acid, with double superphosphates twenty-five cubic centimeters, are digested with 200 cubic centimeters of ammonium molybdate solution for three hours at 50° in a water-bath and, after cooling, filtered, so that as little as possible of the precipitate is collected upon the filter, which is made of strong paper.
The yellow precipitate is washed by decantation in the flask nine times with twenty cubic centimeters of molybdic solution diluted with one volume of water and the filter washed out once with the same quantity of liquid. The funnel, with the filter, is immediately placed upon the flask and the portion of the precipitate collected in the filter dissolved in five per cent ammonia, which is easily accomplished by throwing ammonia upon it from a wash-bottle. Afterwards the filter is washed with a sufficient quantity of hot water and finally removed. The contents of the flask are neutralized warm, with hydrochloric acid, the acid being added until the precipitate first formed, after continued shaking, is again dissolved in the liquid. The solution is then cooled and treated, drop by drop, with constant stirring, with twenty cubic centimeters of magnesia mixture. Finally twenty-five cubic centimeters of dilute ammonia solution are added, the precipitate is not shaken, and, after two hours, is filtered through a gooch.
For the filtering of the ammonium magnesium phosphate by the molybdic method, freshly prepared felts are always employed since the remarkably fine crystalline precipitates will pass through a filter which has once been used. It is necessary also that special precautions be taken in the ignition. The crucible should be heated in a platinum cap, which has the purpose of protecting the contents of the crucible from the access of reducing gases during the ignition. After redness has been reached the cap can be removed and the crucible transferred to a blast where it is strongly ignited for ten minutes before weighing. The precipitate should be pure white.