ANALYSIS OF URINE 



545 



FIG. 74. 



Estimation of Total Phosphates. 



The above method gives the amount of inorganic phosphates. In order 

 to estimate the total phosphates, i.e. inorganic phosphate + phosphoric acid 

 in organic combination, the latter must be oxidised and converted into in- 

 organic phosphate. This is rapidly effected and estimated by- Neumann's 

 method with certain improvements : 



10 c.c. of urine are placed in a 500 c.c. round-bottom Jena glass 

 flask; 10 c.c. of concentrated sulphuric acid and about 5 c.c. 

 of concentrated nitric acid are added. The mixture is care- 

 fully heated in a fume cupboard until oxides of nitrogen are no 

 longer evolved. The residue should be white or nearly white ; if 

 not, the flask is allowed to cool, 3 or 4 c.c. of nitric acid are 

 added and it is again heated. When the oxidation is complete, 

 as shown by the white appearance of the acid, the flask is allowed 

 to cool ; 150 c.c. of water and 20 c.c. of 80 per cent, ammonium 

 nitrate are added and the solution heated to about 85; about 20 

 c.c. of 10 per cent, ammonium molybdate are added. The solu- 

 tion, which becomes yellow, is shaken ; ammonium phospho- 

 molybdate separates out. After 10-15 minutes, or when the 

 solution is cold, the precipitate is filtered off by suction through 

 a straight glass funnel in which a perforated platinum plate is fused, 

 as in Fig. 74, and upon which there is an accurately fitting filter 

 paper. The flask and precipitate are washed free from acid 

 with water; the precipitate and filter paper are returned to the 

 flask by a stream of water. A known excess of ^N sodium hydrate 

 is added to dissolve the precipitate and about 300 c.c. of water. 

 The solution is boiled to remove ammonia, cooled and the excess of alkali is 

 titrated with ^N sulphuric acid, using 6-8 drops of phenolphthalein as indicator. 



Each c.c. of *5N alkali used to dissolve the precipitate corresponds to 

 1*268 mgm. of P 2 O 5 or '5536 mgm. of P. 



Gravimetric Estimation of Inorganic Phosphates. 



10 c.c. of urine are diluted with 20-30 c.c. of water; an excess of 

 magnesium citrate mixture 1 (5-10 c.c.) is added and the solution is made 

 distinctly alkaline with ammonia. It is stirred well, covered with paper 

 and allowed to stand at least 12 hours. The precipitate of ammonium 

 magnesium phosphate is collected on a small filter paper, washed thoroughly 

 with dilute ammonia and dried at 100. The precipitate and filter paper 

 are incinerated in a weighed crucible and heated to bright redness for 15-30 

 minutes so as to convert the ammonium magnesium phosphate into magnesium 

 pyrophosphate. The crucible is cooled in a desiccator and weighed. The 

 increase in weight gives the amount of magnesium pyrophosphate. This 

 amount multiplied by "638 gives the amount of P 2 O 5 . 



Estimation of Organic Phosphorus, 



The difference between the amounts of total phosphates and inor- 

 ganic phosphates by the gravimetric method is the amount of organic 

 phosphate in terms of P 2 O 5 . 



1 This mixture should be used instead of magnesia mixture as it prevents the precipita- 

 tion of calcium salts and organic matter. 



It is prepared by dissolving 400 gm. of citric acid in 500 c.c. of water. To the hot 

 solution 20 gm. of magnesium oxide are added. The solution is cooled and treated with 

 400 c.c. of *88o ammonia. The volume is made up to 1500 c.c. and after standing for 24 

 hours the solution is filtered. 



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