732 7. MERCURIALS 



Equilibria between Hg++ and Halide Ions 



In aqueous solution HgClg does not dissociate simply into Hg++ and CI" 

 ions, but forms a series of complexes, the relative concentrations of which 

 depend on the CI" concentration and the pH. The following species are the 

 most important: Hg++, HgCl+, HgClg, HgClg", and HgCl4=. This applies to 

 acid solutions where hydrolysis and hydroxyl complexes can be ignored 

 (see next section). Higher CI complexes with Hg++ can be neglected in 

 biological work, as can univalent Hgg^ + and its complexes (since no equilib- 

 rium with metallic mercury occurs). Sillen and his collaborators in Stock- 

 holm have summarized their extensive investigation of the halide complexes 

 of mercury (Sillen, 1949) and we shall follow their values for the equilib- 

 rium constants (it should be noted that their work was done at 25° so that 

 small corrections should be applied for solutions at other temperatures). 

 However, we shall differ in two ways from Sillen in the expression of the 

 constants. In the first place, we shall use dissociation rather than association 

 constants, in conformity to the usage throughout this book. In the second 

 place, we shall indicate the individual dissociations by ^'s and the cumu- 

 lative dissociations by /5's, in conformity with the usual terminology in 

 metal-ligand complexes and chelates (Bjerrum et al., 1957). The fundamental 

 dissociations and their constants can be formulated as in Table 7-2. The tight 

 binding of the first two Cl~ ions is evident, but the next two are bound only 

 weakly, due perhaps to the change in bond configuration and the increasing 

 negativity; that the latter is not a major factor is indicated by the similar 

 behavior of the ammonia complexes. The constants for the Br" and I" 

 complexes are much less than for CI", i.e., the former ions are more tightly 

 bound to Hg++, but such equilibria are seldom of importance in biological 

 systems. 



The relative concentrations of the various complexes depend in simple 

 solutions mainly on the Cl~ concentration. The fractions of the total mer- 

 cury in particular complexes may be calculated from the following equa- 

 tions: 



(7-1) 



where 



A = 1 + ^-— + — - — H — + 



(Cb)^ 



i5i /3, /33 ^4 



The CI" concentration varies over a wide range in the media used. In isolated 

 enzyme work it may be very low (unless KCl or NaCl is added); it is 102 mM 



