172 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



ESTIMATION OF SULPHATES 



The sulphates in the urine are of two kinds: the pre-formed sulphates — 

 viz. those of soda and potash, and the combined or ethereal sulphates. 



(a) For the determination of the total amount of sulphuric acid (SO3) (i.e. 

 pre-formed and combined sulphuric acid together) in the urine, one of two 

 methods is adopted : 



1. Volumetric method. 



2. Gravimetric method. 



Both methods will be given here ; the former is, however, better suited 

 for class experiments. 



1. Volumetric Determination. — This process consists in adding to a 

 given volume of the urine a standard solution of chloride of barium so long 

 as a precipitate of barium sulphate is formed. 



The following solutions are necessary : — 



i. Standard barium chloride solution : 30"5 grammes of crystallised 

 chloride of barium in a litre of distilled water ; 1 c.c. of this solution corre- 

 sponds to O'Ol gramme of sulphuric acid (SO3). 



ii. Solution of sulphate of potash : 20 per cent. 



iii. Pure hydrochloric acid. 



Method. — 100 c.c. of urine are taken in a flask. This is rendered acid by 

 5 c.c. of hydrochloric acid, and boiled. The combined sulphates are thus 

 converted into ordinary sulphates, and give a precipitate like them with 

 barium chloride. The chloride of barium solution is allowed to drop into 

 this mixtvure as long as any precipitate occurs, the mixture being heated before 

 every addition of barium chloride to it. After adding 5 to 8 c.c. of the 

 standard solution, allow the precipitate to settle ; pipette off a few drops of 

 the clear supernatant fluid into a watch-glass ; add to it a few drops of the 

 standard barium chloride solution. If any precipitate occurs, return the 

 whole to the flask and add more barium chloride ; again allow the precipitate 

 to settle, and test as before ; go on in this way until no more barium sulphate 

 is formed on the addition of barium chloride. 



Excess of barium chloride inust also be avoided; when only a trace of 

 excess is present a drop of the clear fluid removed from the precipitate gives 

 a cloudiness with a drop of the potassium sulphate solution placed on a 

 glass plate over a black ground. If more than a cloudiness appears, too 

 large a quantity of barium chloride has been added, and the operation must 

 be repeated. From the quantity of barium chloride solution used, the per- 

 centage of sulphuric acid in the urine is calculated. 



2. Qravimetric Determination (i.e. by weight). — This method consists in 

 weighing the precipitate of barium sulphate obtained by adding barium 

 chloride to a known volume of urine ; 100 parts of sulphate of barium 

 correspond to 84-33 parts of sulphuric acid (SO.;). 



MetJiod (Salkowski). — 100 c.c. of urine are taken in a beaker. This is 

 boiled with 5 c.c. of hydrochloric acid as before. 



Chloride of barium is added till no more precipitate occurs. 



The precipitate is collected on a small filter of known ash, and washed 

 with hot distilled water till no more barium chloride occurs in the filtrate — 



