Review of Polarography

When an aqueous solution freezes, solutes are expelled from ice crystals and accumulate in the grain boundary space. The solutes dissolve to form a freeze concentrated solution (FCS) at temperature higher than the eutectic point of the system. It is known that the FCS is involved in various reactions of environmental and atmospheric importance. We have found interesting phenomena occurring in the FCS using a variety of analytical methods, such as chromatography, electrophoresis, fluorescence spectroscopy and lifetime measurements, X-ray spectroscopy, voltammetry etc. Here, we present some of them and discuss the ice-confinement effects on the chemical processes in the FCS.

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The greatly enhanced oxidation ability of dilute aqueous nitric acid (0.10–2.0 mol dm^-3) containing bromide and iodide salts as well as chloride salts has been examined based on the dissolution kinetics of pure gold at 30–60 ℃. It has been found that bromide salts are more effective than chloride salts in gaining the ability of dissolving gold in dilute aqueous nitric acid solution. At 60 ℃, a piece of gold-wire (ca. 20 mg) is dissolved in 20 mL of as low as 0.10 mol dm^-3 HNO₃ solution containing 1.0–5.0 mol dm^-3 NaBr and the dissolution rate constant, log(k/s^-1), increases linearly (from -5.78 to -4.52) with the increasing NaBr concentration. The gold dissolution ability has been examined also with nitrous acid containing chloride and bromide ions at 35 ℃. The NaNO₂ solution containing twice or more amounts of HX (X = Cl, Br) gives the maximum efficiency for gold dissolution, according to the log(k/s^-1) values of the mixed solutions of NaNO₂ (0.10–2.0 mol dm^-3) and HX of various concentrations. The influence of oxidation by dilute nitric and nitrous acids on the gold dissolution is discussed from the standpoint of the redox potentials in ‘‘modified’’ aqueous solutions and not of the changes in the activity coefficients of ions. The corrosion of stainless-steel (SUS316L) in dilute nitric acid containing NaCl or NaBr has been also examined to find that the passive film on the surface is broken down by the evolved Cl₂ or Br₂ gas.

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