1022 EXPERIMENT STATION RECORD. 



The Wagner method of determining citrate-soluble phosphoric 

 acid in Thomas slag, B. Sjollema (Chem. Ztg., 21 {1897), No. 96, 

 pp. 999, WOO). — The following modification of the citrate method is 

 used: To 50 cc. of the clear filtrate obtained by the Wagner process 

 add 50 cc. of an alkaline citrate solution, prepared by dissolving 500 gm. 

 of citric acid in 4 liters of ammonia of 0.9L» specific gravity and mak- 

 ing up to 5 liters, and 15 cc. of magnesia mixture each 2 liters of 

 which contain 110 gm. of magnesium chlorid, 110 gm. of ammonia 

 chlorid, and 700 cc. of ammonia of specific gravity 0.9(5. Shake for 

 ten minutes in a rotary apparatus, allow to stand for two hours or 

 until the following day, and then filter and determine the phosphoric 

 acid in the usual way. The results by this method agree very satis- 

 factorily with those obtained by the Wagner molybdic method. In a 

 few cases the results were too high on account of the separation of 

 silica. This was noted especially in cases where the filtration was par- 

 ticularly slow. 



The valuation of phosphatic fertilizers on the basis of their 

 content of citrate-soluble phosphoric acid, P. Wagner (Chem. Ztg., 

 21 (1897), No. 87, pp. 905-911). — The history of the method of deter- 

 mining citrate soluble phosphoric acid is traced and the agreement 

 between the results obtained by this method and by experiments with 

 plants is discussed, especial attention being given to the use of the 

 method in determining the fertilizing value of Thomas slag. Experi- 

 ments arc reported which show a uniform and very close agreement 

 between the citrate-solubility of different phosphates, especially Thomas 

 slags, and the actual availability of the phosphoric; acid to plants. The 

 precautions which must be observed in order to get concordant results 

 by this method are explained in detail, strict conformity with the letter 

 of the directions being especially emphasized. In a series of tests of 

 the direct citrate method for determining phosphoric acid in the Wagner 

 extract, it was found that unless the freshly prepared extract was used 

 the results were too high in case of slags rich in silica on account of 

 the separation of the silicic acid in the precipitate. The error thus 

 introduced was considerable in case of extracts only two hours old and 

 increased with the age of the extract. If the alkaline citrate solution 

 was added to the fresh extract and the solution shaken, no separation 

 of silicic acid occurred for eight or more hours. 



The conclusion drawn from these tests is that in the use of the direct 

 citrate method for determining phosphoric acid in the Wagner extract, 

 a mixture 1 of the alkaline citrate solution and magnesia mixture should 

 be added to the fresh extract, but that the solution may be filtered 

 immediately or it may stand an hour or more after precipitation before 

 filtration without fear of separation of silicic acid. The mixture used 

 should be prepared as follows: Dissolve 200 gin. of citric acid in 1 liter 

 of 20 per cent ammonia. Mix this solution with 1 liter of the ordinary 

 magnesia mixture. The determination of phosphoric acid is made as 

 follows: To 50 cc. of the freshly prepared extract add 50 cc. of the 



