The complexation of mercury(II) contained in Hg(NO3)2⋅2H2O with 1-phenyl-2,3-dimethylpyrazoline-5-thione (PDPT) in 1.0 mol/L HNO3 at 273–338 K is studied by potentiometric titration using a redox electrode based on PDPT and the oxidized form of it. It is found that four complex species are formed in the studied system; they have the following compositions: HgL(H2O)32+, HgL2(H2O)22+, HgL3(H2O)2+, and HgL42+. For each complex species, stability constants are found and refined using the Bjerrum method. It is shown that the stepwise stability constants of mercury(II)–PDPT complexes decrease with an increase in temperature and the number of coordinated ligand molecules in the inner sphere of the complex. Using the stability constants, the thermodynamic functions of the complexation process are calculated. Distribution curves for all complex species formed in the Hg(II)–PDPT–1.0 mol/L HNO3 system at temperatures of 273–338 K are calculated. The distribution diagrams of mercury(II)–PDPT complexes as a function of organic ligand concentration are analyzed to identify the region of dominance of all complex species formed in the studied system and develop optimum procedures for synthesizing novel mercury(II) coordination compounds.
Beknazarova et al. (Sun,) studied this question.