736 7. MERCURIALS 



Equilibria between Hg++ and Hydroxyl Ions 



It has generally been assumed that the Hg++ ion is hydrated and that 

 this ionizes according to the following equations: 



This is essentially saying that the hydrated ion is a dibasic acid (Hietanen 

 and Sillen, 1952). It is evident that at pH's near neutrality, Hg(0H)2 will 

 be the predominant form. For our purpose and comparison of these equilib- 

 ria with those for Cl~, it might be better to express the reactions as simple 

 complexing with OH" ions. Thus p^oH ~ 10.3, and p^qh — 11-4: 



(Hg++) (OH ) 



Hg+- + OH- ±. HgOH+ ZoH, = ^jrnl.^ = ^-Z^". = ^ ^ 1^"" 



* {HgOH + ) ' 



HgOH- + OH- ^ Hg(OH), ^OH, = ^^^h^qh^^^ = ^'^'^"^ = * "" ^^"" 



which may be compared to p^C^] = 6.74, and p^^ = 6.48 (Table 7-2). 

 Now in a Cb-free medium, even at pH 5, the ratio [Hg(0H)2]/(Hg++) will 

 be 5000, and at pH 7 will be 50,000,000, so that Hg++ will be negligible. 

 Although the affinity of Hg++ for OH" is greater than for CI", when CI" is 

 present in appreciable concentration (e.g. 10-150 mM) it will compete ef- 

 fectively for the Hg++ ion since at neutrality its concentration will be 10^ 

 to 10® times that of OH". Therefore one would predict that, in the usual 

 media for inhibition studies, the Cl~ complexes will predominate over the 

 OH" complexes although, particularly as the pH is increased above 7, it 

 is clear that complexes of the type HgCl(OH), HgCl(0H)2", HgCl2(0H)-, 

 and HgCl3(0H)= may contribute significantly to the total population. The 

 data given by Sneed and Brasted (1955) allow one to calculate the constants 

 for the following equilibria: 



HgCl- + OH- ^ HgCl(OH) ^"$^pinwfi"^ = 3.2xl0-« 



[HgCl(OH)] 



HgCl3- + OH- ^ HgCl3(0H)= ^S?.r)nMfi^ = ^"^ ^ 1^"* 



[HgCl3(0H)=] 



The affinities of the CI" complexes for OH" are thus of the same order of 

 magnitude, and less than for the Hg++ ion. At pH 7, (HgCl+)/HgCl(OH) = 

 0.32 and (HgCl3")/HgCl3(OH)= - 0.5, so it is seen that these OH" com- 

 plexes are indeed significant. The importance of these OH" and mixed com- 

 plexes for inhibition studies is, of course, the same as that of the CI" com- 

 plexes, but the concentration of Hg++ wiD be even less than calculated in 

 the previous section. 



