Masao Sakashita

The effect of molybdate anion on the ion-selectivity of hydrous ferric oxide films in chloride solutions

The ion-selective property of hydrous ferric oxide precipitate films has been investigated by measuring membrane potentials which arise across precipitate membranes of hydrous ferric oxide with and without adsorbed MoO42− ions and of ferric molybdate in solutions of NaCl, KCl, MgCl2, CaCl2, BaCl2, AlCl3, and FeCl3. The hydrous ferric oxide membrane was only permeable to Cl− ions in chloride solutions, whereas the membrane with adsorbed MoO42− ions was permeable to cations in NaCl and KCl solutions, and to both Cl− and cations in the presence of multivalent cations. The ferric molybdate membrane was permeable to Cl− and cations in NaCl and KCl solutions, and only to Cl− ions in the presence of multivalent cations. It is suggested that in chloride solutions, the corrosion of iron covered with a precipitate film of hydrous ferric oxide is accelerated by enrichment of Cl− ions under the film, which may decrease the local pH and introduce a positive diffusion potential in the film. The adsorption of MoO42− ions on the oxide changes the ion-selectivity of the precipitate film from the anion-selective to the cation-selective in solutions of NaCl and KCl. This cation-selectivity of the film may inhibit the corrosion of iron, because of H+ ions diffusing out of the film. The inhibitive effect of MoO42− ions would be reduced in the presence of multivalent cations.

Ion Selectivity of Precipitate Films Affecting Passivation and Corrosion of Metals

Abstract Ion selectivity in precipitate films of hydrated iron (III) and iron (III) molybdate has been studied by measuring the membrane potential and polarization property as a function of solution pH and ion valency. The hydrated iron (III) oxide membrane is found to be anion selective in 1-1 electrolyte solutions in the pH region lower than a specific pH. Beyond this pH, the membrane turns to be cation selective. This specific pH, at which the membrane potential is identical to the liquid junction potential, is named the point of iso-selectivity pHpis, and is evaluated in KCI solutions to be 10.3. The hydrated iron (III) oxide membranes with adsorbed divalent anions such as MoO42− ions are cation selective in monovalent electrolyte solutions even in the pH region lower than pHpis. This cation selectivity can be attributed to the strong adsorption of the divalent anions forming the negative fixed charges on the membranes. Under an applied membrane potential, the ionic current is rectified through the bi...

An examination of the electrode reactions of Te, HgTe and Cd0.2 Hg0.8Te with rotating-split-ring-disc electrodes

Abstract Te, HgTe and Cd0.2Hg0.8Te (CMT) have been examined with the rotating-ring-disc technique. The oxidation and reduction of these materials as well as of the anodic oxides have been studied in solutions of pH 1 to 13. The soluble oxidation products of four-valent Te and two-valent Hg may be determined quantitatively at a Pt-ring. Hg metal deposits at the disc are seen by the additional formation of soluble monovalent Hg22+-ions during oxidation. Passivating oxides are formed on HgTe and CMT in weakly acidic or alkaline solutions (pH 4.9. 8.4) or in 0.1 M KOH in 90% ethylene glycol and 10% water. From the characteristics of the reduction of the anodic oxide it is deduced that a compound oxide rather than an oxide mixture is present. The HgO- and CdO-content reduce significantly the dissolution of TeO2 presumably by the formation of tellurites. At sufficiently positive potentials (E≈0 V) only the HgO-content may be reduced to Hg. At more negative values (−0.3 V) HgTe and Te are formed. The reduction of HgTe and CMT leads to Cd- and Hg-metal deposits and soluble telluride.

Water transference through nickel hydroxide precipitate membrane

The transference of water that results from ion migration through the nickel hydroxide precipitate membrane was studied in chloride, perchlorate, nitrate, and sulphate solutions to estimate the transference number of water and the co-ion transport. In the systems of univalent anions, the moles of water transported per mole of electrons in 0.1 N solutions is almost identical to the hydration number of each anion. This water flow decreases gradually as the concentration of external solution increases, because of increase in the co-ion (cation) transport with increasing concentration of the solution. In the system of sulphate solutions the co-ion transport is remarkable, the transport number of Na+ ions being 0.03 in 0.01 N, 0.27 in 0.10 N, and 0.50 in 0.5 N Na2SO4 solution. This large co-ion transport in Na2SO4 solution is attributed to the partical replacement of hydroxyl groups on the membrane by SO2−4 ions, which then acts as a negative fixed charge. The order of the selectivity for co-ion transport is K+ > Na+ > Li+ > Ni2+ ⩾ Mg2+ in sulphate solutions and also in chloride solutions, although the transport number of the cations is much smaller in chloride solution than in sulphate solution.