COMPOSITION AND CHARACTERS OF URINE 1263 



ESTIMATION OF PHOSPHATES. The method depends upon the precipitation 

 of all the phosphates by a standard solution of uranium acetate or uranium nitrate 

 in the presence of sodium acetate and acetic acid as (Ur0 2 )HP0 4 . The deter- 

 mination of the end-point, when soluble uranium salt is in solution, is shown 

 by means of potassium ferrocyanide, or by cochineal tincture, which becomes 

 green. 



The following reagents are required : 



(1) Acid sodium acetate solution (100 grm. NaAc, 30 grm. HAc, 1000 c.c. H 2 O). 



(2) Cochineal tincture (5 grm. cochineal extracted for several days with 150 c.c. 



alcohol and 100 c.c. water and then filtered). 



(3) Standard uranium solution (1 c.c. = -005 grm. P 2 5 or 5 mg.). 



This must be prepared by standardising against a standard phosphate solution* 

 Generally sodium phosphate is employed ; about 12 grm. are weighed out and 

 dissolved in 1000 c.c. water ; 50 c.c. of this solution are evaporated to dryness, 

 incinerated, and weighed as pyrophosphate. From the weight of this the amount 

 of P 2 5 in 50 c.c. can be calculated and the remainder of the solution can be 

 diluted, so that 50 c.c. contain 0-1 grm. P 2 O 5 . It is simpler to use acid potassium 

 phosphate, KH 2 P0 4 , which can be weighed directly and dissolved in water, so 

 that 50 c.c. contain 0-1 grm. P 2 6 . Fifty cubic centimetres of this solution 

 are titrated with the uranium solution (36 grm. in one litre) in the manner 

 described below, and the uranium solution is then diluted so that 1 c.c. = 

 5 mg. P 2 O 5 . 



The method of analysis is carried out as follows : Place 50 c.c. urine with a 

 pipette in a 100 c.c. beaker, add 5 c.c. acid sodium acetate solution and a few 

 drops of cochineal tincture. Heat the urine to boiling and run in slowly the 

 standard uranium acetate solution from a burette as long as a precipitate is 

 formed. Again heat to boiling and add the uranium solution drop by drop, 

 until the red colour is changed to green. This end-point can also be tested by 

 taking out a drop and placing it in contact with a drop of potassium ferrocyanide 

 solution or a little heap of finely powdered substance 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 may be required to reach this point 

 than to turn the cochineal green.) 



The principle of the estimation of sulphates has already been described. It 

 is not advisable to attempt these volumetrically. 



