Recent polarographic works using double complexing agents are reported. When double complexing agents are used as supporting electrolyte, the following three cases occur. (1) A negatively reducible complex ion of smaller dissociation constant is predominantly formed in the solution. (2) Two complex ions are formed in a mixed base solution, and the negatively reducible complex ion is transfered to the positively reducible complex ion at the electrode surface as the reduction proceeds. (3) A new complex ion of mixed ligand is formed in a mixed base solution. Examples of (1) are as follows : (i) Simultaneous determination of Fe and Ni using the supporting electrolyte composed of 1M NH₄OH-1M NH₄Cl and excess 5-sulfosalicylic acid, or 1 M NH₄Cl-0.5M pyridine and excess 5-sulfosalicylic acid are reported. (ii) Polarographic determination of trace of Bi in the presence of a large amount of Cu was scucessfully carried out by using double complexing agent consisting of 0.1M EDTA, 0.1M KCN and 10% ethanol at pH 9.7 (Fig. 1). Example of (2) is as follows: Polarographic behaviour of UO₂²⁺ in each supporting electrolyte is shown in Fig. 2. Polarogram of UO₂²⁺ using 0.05M EDTA, 0.05M Na₄P₂O₇, 0.3M KCl and 0.01 % gelatin are separated into two waves(Fig. 2b). Some part of the pyrophosphate complex ion is transfered to the EDTA complex ion at the electrode surface as the reduction of EDTA complex ion proceeds. Examples of (3) are as follows : (i) The half-wave potentials of Cu, Bi and Pb are separated by the use of 0.5M Na citrate and 0.01MEDTA (pH 4.5) to -0.09, -0.64 and -1.1V. (vs. SCE), respectively (Fig. 3). (ii) A supporting electrolyte composed of 0.1M EDTA and 0.02M sodium citrate (pH 6.0) is used for the simultaneous determination of Ti and V (Fig. 4). A new Ti EDTA-cit-rate complex ion having mixed ligand is formed in this medium. Both ions give well defined reduction waves, and the half-wave potentials are -0.59 for Ti and -1.3 V. (vs. SCE) for V at pH 6.0 (Table 1). (iii) The base solution composed of 0.1 N HNO₃, 0.005M nitrilotriacetic acid arid 0.04% sodium carboxy methylcellulose is used for the simultaneous determination of Cu and Bi in Pb. As Bi forms a stable complex with nitrilotriacetic acid in this medium, the reduction step of Bi complex ion can be shifted from the copper step cvcn in the prescnce of large amounts of lead. The half-wave potentials of Cu and Bi are -0.004 V. and -0.14 V. (vs. Hg pool), respectively (Table 2). (iv) Simultaneous determination of Sn and Pb using the supporting electrolyte cornposed of 3MNaBr, 0.05M EDTA and 0.008% gelatin (pH 1.3-1.7) are reported (Fig. 5). A new Sn mixed complex using EDTA and Br as a ligand is formed in this medium, On the other hand, Pb is present as a bromide complex ion in this condition (pH 1.3-1.7). The half-wave potentials of Pb and Sn are -0.48 and -0.71V. (vs. SCE), respectively. (v) Sn (IV) in 1 MNaBr, 0.2M oxalic acid and 0.004% gelatin as a supporting electrolyte forms a well defined four electron reduction wave. Polarograms of Sn (IV) using oxalic acid and alkali halide (KCI, NaBr, KI) as double complexing agents are presented in Fig. 6.

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A review is given of adsorption of surface-active substances at mercury-solution interfaces, especially with regard to the structure of adsorbed layers. The difficulty in studying adsorption equilibrium at a mercury electrode surface exists in the fact that the amount adsorbed depends on the electrode potential as well as the bulk concentration of organic substances. It is generally accepted that both the Langmuir isotherm and the Gibbs isotherm are valid for adsorption of organic substances. The dependence of the amount adsorbed on the electrode potential has been explained quantitatively by Frumkin and Butler independently. The physical picture of the latter theory, regarding adsorbed molecules as dielectrics with a permanent dipole and a given polarizabillty, may be easier for chemists to follow, but no less important is the fundamental expression for charge density of the former theory. A linear relationship which is usually assumed to hold between the surface coverage and the double layer capacity should be tested by further experiments. Some conclusions on adsorption rate and mechanism of adsorption can be derived from the frequency dependence behavior of the heights of adsorption-desorption peaks, which appear on a differential capacity-potential curve. The dependence of peak potential on concen-tration has been successfully explained. The rate constant of the adsorption process has also been estimated by Lorenz for aliphatic alcohols and acids of low molecular weight, while the formation of a condensed adsorbed layer has been pointed out for higher homologues of those. The frequency effect observed for those substances has been interpreted semiquantitatively by assuming that a relatively slow association process may take place between adsorbed molecules after a rapid adsorption process. 4.3μF/cm², recently obtained by Laitinen and his co-worker for a monolayer of palmitic acid on a mercury-coated platinum electrode, would be reasonable for the differential capacity value of a monolayer film of higher fatty acids. In comparison to this value, 1.8 μF/cm² observed with nearly saturated solutions of higher aliphatic alcohols and acids, might indicate the formation of a bimolecular adsorbed layer. There seems to remain some ambiguity for the orientation of neutral molecules at an electrode surface, but this problem may be solved in the case of long chain surfactant anions, which have been lately studied in detail by Eda. With solutions of sodium dodecyl sulfate in sodium sulfate, four peaks are observed on the differential capacity-potential curve. The two peaks which appear at the most positive or negative potential may correspond to the adsorption-desorption reaction in an ordinary sense. The relatively small peak which appears on the positive branch near the electrocapillary maximum may be attributed to the reorientation of the adsorbed anions ; and therefore a bimolecular adsorbed layer may be formed on the more positively charged mercury surface than this peak potential. The other small peak on the negative branch may correspond to the phase separation, which means the transformation of a gaseous phaseinto a condensed phase.

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In the course of college lecture it is important to summarize what polarography is and to point out its characteristics. The characteristics of polarography are pointed out as follows : polarography is a) a part of chemistry in solution, b) chemistry in electrode reaction, that is, reduction-oxidation system and c) chemistry in heterogeneous system, and d) polarisation is most important in polarography. As for the concepts of currents in polarography, it is easy for students to understand diffusion current but not migration current in diffusion layer. The latter is clearly explained in figures 4 and 5. By way of this explanation it must be emphasized that migration current is due to migration froce in solution, but the current itself is electrolytic current. That is, migration current is due to migration in solution and electron transfer at the surface of electrode, so that it is also faradaic current.

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In non-aqueous polarography, the standardization of electrode potential involves some difficulties such as preparation of the reference electrode with non-aqueous solvent or evaluation of the junction potential between aqueous and non-aqueous solutions'. One of the methods is to compare the half wave potential with that of the reference substance which coexists in the electrolytic solution. For instance thallous ion is employed as pilot ion in aqueous media. Vlcek2) suggested lately the half wave potential of potassium ion can be used as reference in any solvent.
The redox-potential, and also the half wave potential, of ion should be affected by its solvation state even in the case where no chemical reaction with supporting electrolytes or with solvent molecules is involved. These circumstances are studied in water-acetonitrile mixtures in the present paper. Thermodynamic analysis of the half-wave potential is only possible in the reversible process, and so the reductions of thallous and cadmium ions are discussed here.

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