CH. XIII.] CHLORIDES. 353 



Standardisation of the thiocyanate. In a beaker place 10 cc. of the 

 silver nitrate, accurately measured : add 5 cc. of pure nitric acid, 5 cc. of iron 

 alum and 80 cc. of distilled water. Titrate the whole with the thiocyanate 

 from a burette until a faint permanent red tinge is obtained. Note the 

 amount required for the 10 cc. of silver nitrate. 



Method. In a 100 cc. cylinder or measuring flask place 10 cc. 

 of urine, accurately measured by a pipette, 20 cc. of the standard 

 silver solution, also accurately measured, about 4 cc. of pure nitric 

 acid, and 5 cc. of the iron alum. Add distilled water to the 100 cc. 

 mark, and mix thoroughly by pouring into a beaker and stirring well. 

 Filter off the precipitated silver chloride through a dry paper into a 

 dry vessel. Of the filtrate take 50 cc., accurately measured, and 

 titrate it with the potassium thiocyanate solution till a faint per- 

 manent red tinge is obtained. 



NOTES. i. It is very important to remember to add the nitric acid. It 

 renders the silver chloride insoluble and prevents the precipitation of the silver 

 compounds of the purine bases in those cases in which the urine is alkaline. 



2. Some workers titrate without filtering off the silver chloride, but the 

 end point is apt to be uncertain owing to the decomposition of the chloride 

 by the thiocyanate. 



Calculation and Example. 



19-6 cc. of the KCNS were required for 10 cc. of the AgNO 3 . 



10 

 So i cc. of the KCNS ~ ^^5 = 0-51 cc. AgNO a . 



50 cc. urinary filtrate required n-6 cc. KCNS, 



So 100 cc. urinary filtrate would require 23*2 cc. KCNS and would 

 therefore contain 23-2 x 0-51 = n-8 cc. of the AgNO 3 . 



So 20 - 11-8=8-2 cc. of the AgNO s have been precipitated. 

 Now i cc of the AgNO 3 = o-oi gm. NaCl, 

 So NaCl in 10 cc. urine = 8-2 x o-oi gram. 

 So NaCl in 100 cc. urine = 0-82 gram. 



413. The estimation of chlorides by Larrson's method.* 



Principle. The pigments, urates and other interfering substances are 

 removed from the urine by adsorption with charcoal. The chlorides are 

 estimated in a measured amount of the filtrate by direct titration with silver 

 nitrate, using potassium chromate as an indicator. 



Reagents required. 



1. Standard silver nitrate (see Ex. 412). 



2. A high quality, pure absorbing charcoal (see p. 390). Ordinary 

 animal charcoal is quite useless. 



3. A 5 per cent, solution of potassium chromate. 



* Biochem. Zeitschrift , xlix, p. 479. 



