Mercuric Chloride by Phosphorous Acid. 355 



mal behavior of the mercuric chloride when sufficiently diluted 

 (0*005 mols per liter). Since equation (1) is a correct repre- 

 sentation of the final result, the fact that the reaction is of the 

 first order with respect to mercuric chloride indicates that it 

 must take place in stages. "No intermediate stage is known in 

 the change of HgCl., to HgCl, but between phosjmorous and 

 phosphoric acid we have the known compound hypophosphoric 

 acid H 2 P0 3 . 



It will be shown in this article that the first stage of the 

 reaction is probably the oxidation of phosphorous to hypophos- 

 phoric acid. 



(a) HgCl, + H 3 P0 3 - HgCl + H 2 P0 3 + HC1, the hypophos- 

 phoric acid then reacting with the water, especially in the pres- 

 ence of hydrochloric acid,* as follows : 



(b) 2H,P0 3 + H 3 = H.PO, + H 3 P0 4 . 



If we assume that reaction (b) is fairly rapid as compared with 

 reaction (a), the concentration of the phosphorous acid at any 

 time t may be represented approximately by b — \x, since for 

 every two molecules of phosphorous acid decomposed one 

 molecule of phosphorous acid is regenerated. The rate of the 

 reaction between mercuric chloride and phosphorous acid may 

 therefore be represented by the differential equation : 



dx 



—- - = Jc(a-x) (b—ix), (2) where a and b denote the initial 



Civ 



concentrations of the mercuric chloride and phosphorous acid 

 respectively, and x the amount of the reacting substances decom- 

 posed or of the mercurous chloride f or of the hydrochloric acid 

 formed, all expressed in s;ram mols per liter. Setting a = A ; 

 2b = B 



^ t =K(A-x)(B-x) (3) 



K in the table below is obtained by substituting the experi- 

 mental results in the integrated form of the above equation, 



K ?g— log ( A -*) B . (4) 



~ (A-B)t 8 (B-a)A V ; 



* For the decomposition of hypophosphoric acid see A. Joly, Compt. 

 rend., cii, 110, 1886. and Jahresber. chern., i, 347, 1886. Also Amat, 

 Cornpt. rend., cxi, 676. 



f The fact that the mercurous chloride in equation (a) is necessarily repre- 

 sented by HgCl is not incompatible with the theory that the double molecule 

 Hg 2 Cl 2 is the normal one, since the HgCl may polymerize as soon as it is 

 formed. A similar explanation may be offered for representing hypophos- 

 phoric acid by H 2 P0 3 instead of H 4 P 2 6 . 



