Talanra, Vol. 33, No. IO, pp. 843-846, 1986 0039-9140/86 $3.00 + 0.00 Printedin Great Britain.All rightsreserved Copyright 0 1986 Pergamon JournalsLtd FORMATION CONSTANTS OF SOME MERCURY(I1) COMPLEXES DETERMINED FROM THEIR ANODIC POLAROGRAPHIC SIGNALS M. ESTEBAN, E. CASASSAS* and L. FERNANDEZ Departament de Quimica Analitica, Facultat de Quimica, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain (Received 13 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA November 1985. Revised 8 April 1986. Accepted 2 June 1986) Summary-The formation constants of some Hg(I1) complexes have been determined from the anodic polarographic signals for the oxidation of mercury in the presence of the following ligands: thiourea and its phenyl- and diphenyl-derivatives, thiocyanate, ethylenediamine, EDTA, methylthioacetic acid, 2,2’-thiobisacetic acid, 3,3’-thiobispropanoic acid, 2,2’-[l,l-methandiylbis(thio)]bisacetic acid and 2,2’-[1,2-ethandiylbis(thio)]bisacetic acid. The use of differential pulse polarography and a.c. polarography instead of d.c. polarography increases the accuracy and precision of the potential measurements and, as a consequence, of the stability constants determined. The results obtained by the different methods are compared. It is well-known that in the presence of most com- plexing agents, mercury from the dropping mercury electrode (DME) can be anodically oxidized to the bivalent state,’ leading to the overall electrode pro- cess: Hg+pL+HgL,+2e- (1) (ionic charges on the ligand and the complex are omitted). The half-wave potential of the process is related to the ligand concentration, [L], and to the formation constant of the complex, BP, by the well-known equation2,3 RT 52 = E&,+ + - In zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 2(P- 1) D l/2 2F p @‘)I’* /~,[L]‘P-I) (2) where D and D’ are the diffusion coefficients of the ligand and the complex, respectively: the ratio of these is usually taken as unity. The free ligand concentration in equilibrium with the Hg(II)-ions, [L], depends on the total concentration C, of ligand and the pH, according to the equation v-1 = W%,,, (3) where +a) is the proton side-reaction coefficient. Substituting [L] from equation (3) into equation (2) and rearranging, gives: (4) *Author for correspondence. When p = 1, i.e., only the HgL complex is formed, E,,* is independent of CL and is a function of only pH, as has been observed by several authors in the study of the Hg(I1) complexes of EDTA, DCTA, EGTA, DTPA4-’ and TTHA.8 Equation (4) can also be applied in the less common case of formation of a soluble Hg(1) complex. From equation (4) /?, values can be determined in two different ways. (a) From the intercept of the linear plot of E,,, vs. log CL at constant pH over a C,_ range where one particular complex predominates. Low pH values are to be preferred, to minimize complications due to formation of mercury hydroxo- complexes.’ If high pH values have to be employed, the theoretical treatment should include the ~(~~(~n) coefficients.’ From E,,, vs. log C, relationships a great number of jr values for Hg(I1) complexes have been determined, including those for the thiourea’&‘* and thiocyanate’.‘3 complexes. (b) From the intercept of the linear plot of El,* vs. log uLu,) at constant ligand concentration in a pH range where only one complex predominates. The main drawback of this method is that only a restricted pH range can be used if the hydroxo-complexes of mercury are to be neglected. The /I2 value of the Hg(en), complex’4~‘5 has been determined by means of this relationship. In the work reported here, the application of differential pulse polarography (dpp) and alternating current polarography (acp) to the determination of the formation constants of mercury complexes from the data for anodic oxidation of mercury is discussed. The potentials can be measured much more precisely by these techniques than by d.c. polarography (dcp). The theoretical treatment above starts from the 843