CHEMICAL PROPERTIES 745 



Table 7-5 

 Dissociation Constants for Complexes with Methylmekcuric Ion " 



Ligand pK 



CI- 5.45 



Br- 6.7 



I- 8.7 



OH- 9.5 



CN- 14.2 



SCN- 6.1 



Acetate 3 . 6 



Phenolate 6.5 



HEDTA^ 6.2 



Ammonia 8.4 



Pyridine .4.8 



Imidazole 7 . 3 



Histidine (NHj group) 8.8 



Histidine (imidazole group) 6.4 



" From Simpson (1961). 



inorganic Hg may be split off more readily than is usually supposed. The 

 exchange reaction: 



Hg2«3Cl2 + -OOC— 97— Hg+ ±? HgCl^ + -OOC— (p— Hg2«^ + 



is fairly fast, the rate constant being 5.4 liters mole^^sec"^ at 25° with an 

 activation energy of 12 kcal/mole (Cerfontain and van Aken, 1956), and 

 this indicates the instability of the C — Hg bond. It is possible to produce 

 Hg203-labeled p-MB by this reaction. This problem has assumed a good 

 deal of importance in diuretic action and will be discussed more fully in 

 this connection. 



The synthesis of p-MB has been described by Whitmore and Woodward 

 (1941). It may be purified by repeated solutions in dilute NaOH and preci- 

 pitations with excess HCl (Boyer, 1954). For the accurate titration of SH 

 groups it is suggested that the purity of the 2>-MB be checked by iodometric 

 titration or spectrophotometrically by absorption measurement at 232 m// 

 at pH 7 (£.v = 1-69 X 10*) or 234 m// at pH 4.6 (f^ = 1-74 x 10*). The 

 stability of 39-MB solutions has not been determined quantitatively, but 

 Cunningham et al. (1957) found that heating to 80o-82o for 90 min with 

 various buffers and at different pH's does not destroy more than 2-4%, 

 and MacDonnell et al. (1951) stated that solutions are stable for a month 

 at room temperature. Nevertheless, I would advise making solutions daily 

 for accurate work. 



