540 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Chlorides. 



Chlorides are generally estimated by Volhard's method. The 

 principle of this method consists in precipitating the chlorides by ex- 

 cess of a standard solution of silver nitrate in the presence of nitric 

 acid. The excess of silver is then estimated in an aliquot part of 

 the filtrate with a solution of potassium or ammonium thiocyanate, 

 which has been previously standardised against the silver solution, a 

 ferric salt being used as indicator. 



The following solutions are required : 



(1) *iN silver nitrate solution, or a solution of such a strength that 

 I c.c. corresponds to o-oi gm. NaCl. 



(2) *iN potassium or ammonium thiocyanate solution standardised 

 against the silver nitrate solution. 



(3) Pure nitric acid free from chlorides. 



(4) A saturated solution of iron alum. 



Procedure. 



(1) Standardisation of the Thiocyanate Solution. 10 c.c. of the 

 silver nitrate solution are placed with a pipette in a beaker; 5 c.c. 

 of pure nitric acid, 5 c.c. of iron alum solution and 80 c.c. of water 

 are added. The thiocyanate soluton is run in from a burette until a 

 permanent red tinge is obtained The amount required for the 10 c.c. 

 silver nitrate solution is noted. 



(2) Analysis. 10 c.c. of urine are placed with a pipette in a IOQC.C. 

 measuring flask. About 4 c.c. of pure nitric acid and 20 c.c. with a pipette 

 of the standard silver nitrate solution are added. The flask is filled up to 

 the mark with distilled water, the contents are mixed and filtered into 

 a dry vessel through a dry paper. 



50 c.c. of the filtrate are taken with a pipette, some iron alum is 

 added as indicator and they are titrated with the thiocyanate solution 

 until a permanent red colour is obtained. 



The calculation of the result is : 



50 c.c. filtrate = S c.c. KCNS 

 .. 100 c.c. = 28 c.c. KCNS. 

 Now x c.c. KCNS = 10 c.c. AgNO a 

 10 x 28 



,%2SC.C. = 



This is the excess not utilised to precipitate the chlorides. 



..( 20- J = amount of AgNO, solution used. 

 As -iN AgNO a = -iN NaCl, i c.c. solution = -00585 gm. NaCl. 



Hence NaCl in gm. in the volume passed in 24 hours is calculated, 



