CH. XHI.J SULPHATES. 355 



Method. In a beaker of about 100 cc. capacity place 50 cc. 

 urine, add 5 cc. of the sodium acetate solution and about I cc. of the 

 cochineal tincture. Have a burette ready containing the stand- 

 ardised uranium acetate solution. Heat the urine to boiling 

 point, remove the flame and run in the uranium acetate as long as 

 a precipitate is formed. Heat the mixture again just to boiling 

 point, and cautiously add uranium acetate, drop by drop, till the 

 red colour is converted to a green. 



Calculation. 



i cc. of the uranium acetate = 0-005 gram. P 2 O 5 . 



Thus if 50 cc. of urine require 15-2 cc. uranium, the percentage of P 2 O 5 is 

 2 x 15-2 x 0-005 = 0-152 gram. 



K. Sulphates. 



Sulphates can be determined gravimetrically as barium sulphate or 

 volumetrically by means of benzidine. The latter is much more convenient, 

 but both methods are given below. The difficulty encountered with the 

 gravimetric method is that of preventing adsorption of other substances. For 

 that reason the barium must be added very slowly and the method is extremely 

 tedious. 



415. The estimation of total sulphates by Folin's method.* 



Place 25 cc. of urine in a 250 cc. Erlenmeyer flask, add 20 cc. 

 of hydrochloric acid (i volume of concentrated HC1 to 4 volumes of 

 water) and boil gently for 30 minutes, covering the mouth of the 

 flask with a small watch glass. Cool the flask under the tap and 

 dilute to about 150 cc. with water. Add 10 cc. of 5 per cent, barium 

 chloride solution slowly, drop by drop, to the cold solution, which 

 must not be stirred or shaken during the addition, nor for at least 

 one hour after. Then shake well, filter through a weighed Gooch 

 crucible (see note to Ex. 410), wash with 250 cc. of cold water, dry 

 in an air bath, or over a very low flame. Ignite, cool and weigh. 



Calculation. Weight of BaSO 4 x i -366 = SO 8 per cent. 



NOTES. Instead of using a Gooch crucible a washed " Barium sulphate " 

 filter paper may be used. 



After washing and drying the ignition may be carried out in a platinum or 

 porcelain crucible, previously weighed. After ignition, the ash should be 

 treated with a drop of 25 per cent, sulphuric acid, cautiously dried and heated 

 again. 



A correction must be made for the weight of the ash of the paper. 



* Journ. Biol. Chem., i., p. 150. 



