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SummaryAddition of zinc ion in a concentration higher than 0.5 mM to the supporting electrolyte of 1 M potassium thiocyanate results in the appearance of an anodic wave due to the formation of the ternary complex of zinc tetrathiocyanatomercurate (II). This wave is very faint in DC step, but very prominent in AC peak. Evidences for the adsorption process involved in the anodic depolarization are given.Formation of the slightly soluble precipitate of tetrathiocyanatomercurate with bivalent metal ion is especially useful for the spot test of zinc1,2) and the solubility product data for zinc and cobalt are compiled.3) As well known, some anion forming slightly soluble precipitate or stable complex with mercury ion shows anodic wave, the polarographic properties of which have been clarified by Revends4) and Kolthoff with Miller.5) Especially the stepwise formation of mercury(II)-thiocyanate complex is investigated by Nyman and Alberts.6) It is now expected that in the presence of bivalent cation thiocyanate ion would depolarize the mercury anode resulting in the precipitation of tetrathiocyanatomercurate (II) and consequently an anodic wave of new type would appear. In the present paper an example of anodic wave due to the formation of a three-component complex is reported. ExperimentalApparatus The DC and AC polarograms are recorded by means of a Shimadzu polarograph, type RP-2, together with an AC attachment, type BF-l, the circuitry of which is proposed by Ishibashi et a1.The capillary characteristics were : m=1.484 mg sec-1 in pure water at 18°C, t=3.80 sec in the electrolysis solution at open circuit at h=58 cm.