544 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Phosphates. 



The amount of phosphates in urine bears a direct relation to the 

 amount of food ingested. Phosphorus compounds phosphates, nucleic 

 acid, lecithin, phosphoprotein are present in all foods. The animal 

 converts the organic compounds into inorganic phosphate. Only a 

 small quantity of phosphorus is excreted in the urine in organic com- 

 bination. The average daily excretion of P 2 O 5 is about 2*5 gm. 



Volumetric Estimation of Inorganic Phosphates. 



The usual method of estimating phosphates in urine depends upon 

 their precipitation as uranium phosphate (UrO 2 )HPO 4 by a standard 

 solution of uranium acetate, or uranium nitrate in the presence of 

 sodium acetate and acetic acid : 



UrO 2 (C 2 H 3 O 2 ) 2 + KH 2 PO 4 = UrO 2 HPO 4 -f KC 2 H 3 O a + C 2 H 4 O 2 



The determination of the end point, at which excess of soluble 

 uranium salt is first in solution, is shown by means of potassium ferro- 

 cyanide, or by cochineal tincture, which becomes green at this point. 



The following reagents are required : 



(i) Acid sodium acetate solution. (2) Cochineal tincture. 



(3) p iN uranium solution of which I c.c. = 0*00355 gm. P 2 O 5 , or 

 a solution of such a strength that I c.c. = 0*005 g m - or 5 mgm. P 2 O & . 



The uranium solution cannot be prepared directly but requires to be 

 standardised against a standard phosphate solution. Generally stdium phos- 

 phate is employed; 12 gm. are weighed out and dissolved in 1000 c.c. of 

 water ; 50 c.c. of this solution are evaporated to dry ness, incinerated and 

 weighed as pyrophosphate. From this weight the amount of P 2 O 6 in 50 c.c. 

 can be calculated and the remainder of the solution can be diluted, so 

 that 50 c.c. contain o - i gm. P 2 O 5 . It is simpler to use acid potassium phos- 

 phate, KH 2 PO 4 , which can be weighed out directly and dissolved in water, so 

 that 50 c.c. contain o'i gm. P 2 O 6 . 50 c.c of this solution are titrated with 

 the uranium solution (36 gm. in i litre) in the manner described below ; the 

 uranium solution is then diluted so that i c.c. = 5 mgm. or 3-55 mgm. of P 2 O 5 . 



Procedure. 



50 c.c. of urine are placed with a pipette in a 100 c.c. 

 beaker, 5 c.c. of acid sodium acetate solution and a few drops of 

 cochineal tincture are added. The urine is heated to boiling and the 

 standard uranium acetate solution is run in slowly from a burette 

 as long as a precipitate is formed. The solution is kept boiling 

 and uranium solution is added, drop by drop, until the red colour is 

 changed to green. The end point is best tested by taking out a drop 

 of the solution and placing it in contact with a drop of potassium 

 ferrocyanide solution, or a little heap of finely powdered ferrocyanide, 

 on a white piece of porcelain. A brown colour or precipitate is formed 

 when excess of soluble uranium salt is present in the solution. (A 

 few more drops are generally required to reach this point than to turn 

 the cochineal green.) The calculation of the result is : 



50 c.c. of urine = n c.c. of uranium solution = n x 0-005 g m - 

 or n x -00355 gm. P 2 O 5 . Hence the quantity of P 2 O 5 in the 24 hours' 

 quantity of urine is calculated. 



