194 EXPERIMENT STATION RECORD. 



The method applied to the determination of phosphoric acid in fertil- 

 izers is as follows: Dissolve 4: to 5 gm. of the substance in 20 to 30 cc. 

 of nitric acid (1.2 sp. gr.). Make solution up to 500 cc. and use 20 to 50 

 cc. for each determination. Dilute to 50 cc. if necessary, and add 1 to 

 2 gm. of silver nitrate in solid form or concentrated solution. Add 

 sloMiy from a pipette a warm normal solution of sodium hydrate until 

 the yellow precipitation no longer disappears on stirring, then add drop 

 by drop 10 per cent solution of ammonia until no further precipitation 

 occurs. After boiling 5 to 10 minutes and standing a short time, filter 

 and wash free from lime. To prevent cloudiness of filtrate, wash finally 

 with a 2 to 3 per cent solution of potassium nitrate. Wash the precipi- 

 tate back ijito the flask in which the precipitation was made with the 

 same solution, add 3 to 6 gm. of sodium chlorid, and warm a few min- 

 utes. Filter the solution of sodium phosphate (]^a3P04) thus obtained 

 into a glass-stoppered flask and wash the precipitate with the potassium- 

 nitrate solution until the washings are neutral. Add to the filtrate 5 

 cc. of half normal sulphuric acid, 3 gm. potassium iodid, and 10 cc. of a 

 5 per cent solution of potassium bromate, dilute to 100 to 120 cc, warm 

 for one-half hour at 40 to 50° C. or allow to stand for a day at ordinary 

 temperature before titrating the separated iodid with thiosulphate 

 solution. 



In case of phosphates rich in iron, such as Thomas slag, precipitate 

 the phosphoric acid by either the molybdic or the citrate method, wash 

 the precipitate with 2i per cent ammonia solution and then with 90 per 

 cent alcohol to remove excess of ammonia. Wash the precipitate into 

 a stoppered flask, add 30 cc, of tenth-normal sulphuric acid for each 

 tenth gram of i^hosphoric acid present. The following reaction occurs : 

 2MgNH4 PO4 + 3H2SO4 = 2MgS04 + ( NH4) 2 SO4 + 2H3PO4. Titrate the 

 free phosphoric acid in the manner prescribed above. 



A study of the Kjeldahl method and its modifications, G. 

 KlVlERE and G. Bailhache {Bui. Soc. Chim. Paris, ser. 3, 15-16 {1S96), 

 No. 12, pp. 806-811). — The use of phosphoric acid or bioxid and phos- 

 phate of manganese did not give satisfactory results. In the latter 

 case a certain amount of nitric nitrogen was driven off. The use of 

 sesquioxid of iron and sulphate of iron furnished more satisfactory 

 results, but the method is complicated and results in a bulky precipi- 

 tate when soda is added which interferes with distillation, Vanadic, 

 molybdic, and arsenic acids were tested and were found too slow in 

 action, but boric acid gave quite satisfactory results. 



On the assumption that the quickness and completeness of reduction 

 would be promoted by raising the boiling point of the sulphuric acid, 

 varying amounts of sulphate of jiotash (1 to 10 gm.) were added to 

 the acid. Since the smaller amounts gave as good results as the 

 larger, it was concluded that the increased eff'ectiveness was due to the 

 character of the salt added and not to the increased temperature. 

 Although the mixture gave good results as a rule, its action on sub- 

 stances like horn was too slow, and pyrophosphate of soda (1 to 2 gm.) 



