170 ESSENTIALS OF CHEMICAL PHYSIOLOGi^ 



Into this mixture allow the mercuric nitrate solution to flow from a burette, 

 stirring the mixture the while. A precipitate forms, which redissolves on 

 stirring ; add the mercuric nitrate solution till a permanent precipitate (not 

 an opalescence) forms ; the reaction is then complete. The strength of the 

 mercurial solution is thus determined, and it is then diluted so that 20 c.c. 

 = 0*2 gramme of sodium chloride = 10 c.c. of the standard sodium chloride 

 solution ; 1 c.c, therefore, corresponds to 0*01 gramme of sodium chloride, 

 or 0*006059 gramme of chlorine. 



ii. Baryta mixture, made by adding two volumes of barium hydrate 

 solution to one of barium nitrate solution, both saturated in the cold. 



iii. Dilute nitric acid (1 in 20). 



Analysis. — Take 40 c.c. of urine. 



Add 20 c.c. of baryta mixture. Filter off the precipitate which forms, 

 which consists of sulphate and phosphate of barium. 



Take 15 c.c. of the filtrate: this corresponds to 10 c.c. of the original 

 urine. 



Render this slightly acid with dilute nitric acid. 



Run in the standard mercuric nitrate solution from a burette, stirring the 

 mixture well imtil a permanent precipitate appears. 



Read off the number of c.c. used ; multiply by 0*01. This gives the 

 amount of chloride as sodium chloride contained in 10 c.c. urine. 



Explanation and Corrections. — This test depends on the fact that when 

 mercuric nitrate and sodium chloride in solution are mixed, sodium nitrate 

 and mercuric chloride, which are both soluble in water, are formed. It is 

 not tUl all the chloride in the urine is so decomposed that mercuric nitrate 

 begins to combine with the urea present to form a permanent white pre- 

 cipitate. Hence the necessity of estimating the chlorides when using 

 Liebig's method for the determination of urea. 



In order to obtain the exact point at which the precipitate becomes a 

 permanent one, the process must be repeated in another specimen. The 

 advantage of this process is its simplicity : its disadvantage is that the end 

 point is rather obscure. 



If the urine used is albuminous, the albumin must be first removed by 

 boiling, after the addition of a few drops of acetic acid, and filtering off the 

 precipitated proteid. 



