390 RECOVERY OF AMMONIUM MOLYBDATE THE RECOVERY OF AMMONIUM MOLYBDATE USED IN PHOSPHATE ESTIMATIONS. BY J. A. PRESCOTT B.Sc. THE extensive use made of ammonium molybdate in agricultural laboratories for the determination of phosphates and its present high price have led to attempts at recovering the molybdate from the residues obtained after the phosphate estimation has been carried out. Practically all the molybdenum salts have been extracted from the Australian ores in Germany and at the-outbreak of war the price of ammonium molybdate went up to 40s. per pound as against the normal price of 4s. 6d. Owing to the enormous excess of reagent required in proportion to the phosphate to be estimated this price would have become almost prohibitive if some method of recovery had not been devised.By the use of the method described below several hundred estimations of phosphate have been carried out at the Rothanisted Laboratories with molybdate which was in hand at the outbreak of war. The method is fairly simple and has the further advantage that it can be carried out quite satisfactorily by the ordinary skilled but untrained laboratory assistant,. The residues to be dealt with are usually of three kinds : 1. Acid liquors from which the yellow phospho-molybdate precipitate has been removed by filtration. This contains ammonium molybdate ammonium nitrate and a large excess of acid usually nitric or sulphuric acid in addition to calcium mag-nesium and other metals derived from the fertiliser or soil which has been analysed. 2. Alkaline or neutral liquors obtained by dissolving the yellow precipitate in ammonia and the subsequent precipitation with magnesium mixture or by dissolving the yellow precipitate in excess of standard alkali as in the Pemberton titration method which is that at present in use at Rothamsted (J.A. Prescott J.Agric. Sci., 1914 6 111 ; ANALYST 1914 39 329). 3. Ignited residues consisting chiefly of the oxides of molybdenum. Method of Recovery. The method is based on the fact that if ammonium molybdate is boiled with excess of acid molybdic acid is precipitated. All residues are thrown into a large evaporating basin of several litres capacity which is kept heated so as to concentra.te the liquors thrown in. Owing to the preponderance of acid residues there is no necessity to add any more acid.As the liquors are concentrated rnolybdic acid is precipitated and eventually a strongly acid syrupy liquid is obtained from which crystals of ammonium sulphate or ammonium nitrate separate on cooling. A certain proportion of phospho-molybdate separates out with the molybdic acid. The whole is filtered and the yellow molybdic acid washed with cold water and then dissolved in excess of ammonia ; to the arnmoniacal solution magnesium mixture is carefully added till no more phosphate precipitates and the solution is then filtered concen-trated to a small bulk keeping ammonia present in excess and allowed to crystallise THE COMPOSITION OF DUTCE CHEESE ETC. 39 1 The mother liquors are worked up for further quantities of ammonium molybdate, and if necessary the molybdate obtained is recrystallised. On some occasions it wag found that the molybdic acid gave a blue solution on the addition of ammonia due to the presence of lower oxides of molybdenum. This was due to the fact that once or twice the recovery liquors had been evaporated to dryness and the molybdic acid in consequence overheated. This would probably occur also with residues which consisted of the ignited yellow precipitate. The addition of hydrogen peroxide to the solution in these cases was found to oxidise the solution rapidly and a clean product was then readily obtained. ROTHAMSTED EXPERIMENTAL STATION, HARPENDEN