48 J. T. Norton, Jr. — Titration of Mercury 



Art. V. — The Titration of Mercury by Sodium Thio- 

 sulphate ; by John T. .Norton, Jr. 



[Contributions from the Kent Chemical Laboratory of Yale University— XCV\] 



According to J. J. Scherer* mercurous nitrate, mercuric 

 nitrate and mercuric chloride may be estimated by direct titra- 

 tion with sodium thiosulphate, Hg 2 S, 2HgS . Hg(N0 3 ) 2 , and 

 2HgS.HgCl 2 being the precipitates obtained in each case. I 

 have been unable to obtain access to Scherer's original publica- 

 tion, but Sutton f gives the following very general directions 

 for this process : 



(a) "Mercurous salts. — The solution containing the metal 

 as a protosalt only is diluted, gently heated and the thiosul- 

 phate delivered in from the burette at intervals, meanwhile 

 well shaking until the last drop produces no brown color. The 

 sulphide settles freely and allows the end of the reaction to be 

 easily seen. One cm 3 of the -^ normal solution of thiosul- 

 phate = 0-02 grams Hg or 0-0208 HgO. 



(b) Mercuric nitrate. — The solution is considerably diluted, 

 put into a stoppered flask, nitric acid added and the thiosul- 

 phate cautiously added from the burette, vigorously shaken 

 meanwhile, until the last drop produces no further precipitate. 

 Scherer recommends that when the greater part of the metal is 

 precipitated the mixture should be diluted to a definite vol 

 ume, the precipitate allowed to settle and a measured quantity 

 of the clear liquid taken for titration ; the analysis may then 

 be checked by a second titration of the clear liquid if needful. 

 One cm 3 of J ¥ normal thiosulphate =-015 of Hg. or -0162 of 

 HgO. 



(c) Mercuric chloride. — With mercuric chloride the end of 

 the process is not so easily seen. The very dilute solution is 

 acidified with hydrochloric acid, heated nearly to boiling, and 

 the thiosulphate cautiously added so long as a white precipi- 

 tate is seen to form ; any great excess of the precipitant pro- 

 duces a dirty-looking color. Filtration is necessary to dis- 

 tinguish the exact ending of the reaction. One cm 3 of ^th 

 normal thiosulphate =-015 Hg or -0162 HgO." 



Fresenius:}; gives practically the same directions, but omits 

 all mention of that portion of the process dealing with mer- 

 curous nitrate. 



In view, therefore, of the scant information available on the 

 subject and of the apparent difficulty of working the process 

 accurately according to the directions given, an attempt was 



* His Lehrbuch der Chemie, i, 513. f Volumetric Analysis, p. 220. 



% Quantitative Analysis. 



