Abstract
The solvent extraction method has widely been applied to the analysis of trace elements, particularly in their colorimetric determination. However, in order to apply the extraction method in electroanalytical chemistry, the organic layer containing the extracted sample species has to be evaporated and the residue redissolved in water, or else the sample species must be back-extracted from the organic layer into a suitable aqueous solution.
In order to avoid these time-consuming procedures, the present author, with Fujinaga et al., has proposed the use of a ternary solvent mixture, consisting of chloroform, methylcellosolve and water, for the polarographic analysis and has investigated the polarographic behavior of diethyldithiocarbamate in this solvent mixture.
The solution for polarographic analysis can be prepared simply by mixing chloroform, into which the sample species has been extracted, with appropriate amounts of methylcellosolve, water and supporting electrolyte. This homogeneous ternary solvent mixture has a dielectric constant high enough to ensure the dissociation of supporting electrolytes.
A new cathodic stripping method, depending on the adsorption and subsequent reduction of adsorbed mercuric diethyldithiocarbamate, has been developed in this solvent for the determination of trace amount of mercury.
In the preceding report, the polarography of diphenylthiocarbazide in an aqueous solution of 50% methylcellosolve was studied in order to use diphenylthiocarbazide or diphenylthiocarbazone as an organic reagent for the inorganic polarographic analysis. When the concentration of diphenylthiocarbazide was larger than approximately 0.1 mM, two anodic waves could be observed in d.c. polarogram and two corresponding peaks in a.c. polarogram. Since the oxidation of thiol on a platinum electrode occurs at a potential of about + 1.0 V, the electrochemical process of diphenylthiocarbazide at the dropping mercury electrode was confirmed as follows, in acidic media: 2H4Dz-Hg_??_Hg(HDz)2+6H++6e (1)
The wave at the more negative potential was an adsorption pre-wave corresponding to the formation of the mercuric complex, Hg(HDz)2, adsorbed on the electrode surface. And in alkaline solution:
H4Dz+2OH-_??_H2O+2e (2)
H4Dz+Hg+4OH-_??_HgDz+4H2O+4e (3) in which H4Dz, H2Dz and Dz represent diphenylthiocarbazide, diphenylthiocarbazone and diphenylthiocarbodiazone, respectively.
As a result of the adsorption of HgDz on the surface of dropping mercury electrode, a minimum was observed at the plateau of the oxidation wave according to equation
(2). The total wave was due to the oxidation reaction of equation (3). A similar adsorption wave has been reported for gluthathione, thioglycolic acid, diethyldithiocarbamate, and 2, 3-dimercaptopropanol.
In the present paper, the polarographic and voltammetric behaviors of diphenylthiocarbazone and its complexes were investigated in the ternary solvent mixture containing 0.2 M sodium acetate, 0.1 M acetic acid and 0.1 M potassium chloride as the supporting electrolytes.
