by Sodium Thiosulphate. 49 



made to ascertain whether the careful regulation of tempera- 

 ture, dilution and amount of acid present might not produce 

 beneficial results. 



That portion of the process dealing with mercuric chloride 

 was first taken up. The mercuric chloride used was pulver- 

 ized, dried at 100° and its purity proved by several determina- 

 tions as mercuric sulphide. The sodium thiosulphate was made 

 up of approximately -^th. n o rma l strength and standardized on 

 decinormal iodine, which in turn was titrated against deci- 

 normal arsenious acid made from pure resublimed arsenious 

 oxide. 



For the action of sodium thiosulphate upon the mercuric 

 chloride Scherer gives the equation, 



3HgCl 2 + 2Na 2 S 2 3 + 3H 2 = 2HgS.HgCl 2 + 2Na 2 S0 4 + 4HCl. 



According to my experience, the action results in the forma- 

 tion of a dense white precipitate which refuses to settle either 

 by shaking or standing, thus making it impossible to fix the 

 end reaction by reading the first drop of thiosulphate which 

 produces no further white precipitate in the solution contain- 

 ing the mercuric chloride. Recourse must be had therefore to 

 filtering. By far the quickest and neatest method is to use the 

 asbestos filter deposited on a large perforated platinum cone.* 

 This cone is set in a glass funnel by means of a rubber con- 

 nector and the funnel is passed through the stopper of a large 

 side-necked Erlenmeyer connected with an exhaust pump. A 

 little asbestos fiber shaken in the liquid to be filtered was 

 found to be very beneficial in preventing the precipitate from 

 running through the filter. In all the following experiments 

 the thiosulphate was run into the solution containing the mer- 

 curic chloride in excess, the whole shaken up with asbestos 

 fiber, filtered and the excess of thiosulphate determined by 

 ^-th normal iodine. This method of procedure seems to be 

 far preferable to attempting to catch the end of the reaction 

 by running in the thiosulphate until the last drop produces no 

 precipitate. In the experiments shown in Table I no atten- 

 tion was paid to the temperature of the solution and the thio- 

 sulphate was run in until the liquid turned brown. In every 

 case the solution was allowed to stand until there was no 

 further visible change of color. 



A glance at the table shows that the results are most irregu- 

 lar. In Table II is seen the result of regulating the tem- 

 perature and the length of standing after the addition of the 

 sodium thiosulphate. 



*Amer. Chem. Jour., i, 321. 



Am. Jour. Sci. — Fourth Series, Vol. X, No. 55. — July, 1900. 

 4 



