ACETONE BODIES 65 



acetate is the most satisfactory means. Six to 7 c.c. of 3 M ace- 

 tate are added to the cooled solution of redissolved precipitate. 

 Then the 0.2 M KI is run in rapidly from a burette with constant 

 stirring. If more than a small amount of mercury is present, a 

 red precipitate of HgI 2 at once forms, and redissolves as soon 

 as 2 or 3 c.c. of KI in excess of the amount required to form the 

 soluble K2Hgl4 have been added. If only a few mg. of mercury 

 are present, the excess of KI may be added before the Hgl2 has 

 had time to precipitate so that the titrated solution remains 

 clear. In this case not less than 5 c.c. of the 0.2 M KI are added, 

 as it has been found that the final titration is not satisfactoiy if 

 less is present. The excess of KI is titrated back by adding 

 0.05 M HgCl2 from another burette until a permanent red pre- 

 cipitate forms. Since the reaction utilized is HgCl2+4KI = 

 K 2 HgI 4 +KCl, 1 c.c. of 0.05 M HgCl 2 is equivalent in the titra- 

 tion to 1 c.c. of the 0.2 M KI. 



In preparing the two standard solutions the 0.05 M HgCk is 

 standardized by the sulphide method, and the iodine is stand- 

 ardized by titration against it. A slight error appears to be 

 introduced if the iodide solution is gravimetrically standardized 

 and used for checking the mercury solution, instead of vice versa. 



In standardizing the mercuric chloride the following procedure 

 has been found convenient: 25 c.c. of 0.05 M HgCk are measured 

 with a calibrated pipette, diluted to about 100 c.c., and H^S is 

 run in until the black precipitate flocculates and leaves a clear 

 solution. The HgS, collected in a Gooch crucible and dried at 

 110, should weigh 0.2908 gm. if the solution is accurate. 



Both by gravimetric analyses of the basic mercuric sulphate- 

 acetone precipitate and by titration, the mercury content of the 

 precipitate has been found to average 76.9 per cent. On this 

 basis, each c.c. of 0.2 M KI solution, being equivalent to 10.0 mg. 

 of Hg, is equivalent to 13.0 mg. of the mercury acetone preci- 

 pitate. 



Titration is not quite so accurate as weighing, but, except when 

 the amounts determined are very small, the titration is satisfac- 

 tory. 



Calculation. 1 mg. of /3-hydroxybutyric acid yields 8.45 

 mg. of precipitate. 1 mg. of acetone yields 20.0 mg. of precipi- 

 tate. 1 c.c. of 0.2 M KI solution is equivalent to 13 mg. of precipi- 

 tate in titration of the latter. 



