PRACTICE XXIX 



DETERMINATION OF CITRATE INSOLUBLE PHOSPHORUS 



Use the same materials and amounts as in Practice XXV. If the 

 material is acid, wash the weighed sample on a 9-cm. filter with 

 water until free of acid. 



Heat in a water bath 100 cc. of the neutral ammonium citrate 

 solution to 65 in a 200-cc. Erlenmeyer flask, loosely stopped with a 

 stopper holding a 100 thermometer. When 65 is reached put in the 

 sample and shake thoroughly. Place in the bath at 65 and let stand 

 for thirty minutes, shaking every five minutes. At the end of thirty 

 minutes filter and wash thoroughly with water at 65 until all soluble 

 phosphorus is removed (test for soluble phosphorus with 1 cc. of 

 ammonium molybdate solution). Transfer the filter and its contents 

 to a crucible, dry, and burn off all organic matter. Transfer to a 

 beaker, add about 15 cc. of nitric acid, and heat until all phosphorus 

 is dissolved. Make up to 250 cc., mix well, transfer to a dry bottle, 

 and let settle. Determine phosphorus in 25-cc. portions, according to 

 the directions given in Practice XX. 



cc. HXO 8 cc. KOH to mg. P % P 



used dissolve 



ppt. 



Av. 



In making any calculations, value citrate soluble phosphorus at 

 12 cents and citrate insoluble phosphorus at 3 cents per pound. 



65 



