The differential pulse (dp) polarograms of thiamine in neutral aqueous solutions exhibited six peaks at low depolarizer concentration (⋦10⁻⁴ mol dm⁻³) and only three peaks at concentrations ≥10⁻³ mol dm⁻³. Only one of these was found to correspond to the diffusion-controlled reduction of this compound at the dme and this was shown to be an irreversible two-electron process. The kinetic parameters derived from the dp polarograms were found to be in good agreement with those calculated from classical polarograms and were:E_1/2=−1·261 Vvs SCE,an_a=0·54 andD≈3·5×10⁻⁶ cm² sec⁻¹ for 10⁻³ mol dm⁻³ thiamine in 0·1 mol dm⁻³ acetate buffer (pH 6·5). The reduction product has been identified as dihydrothiamine. The effect of pH on the dpp of thiamine was studied in the pH range 0–7. In the pH region 5·5 to 7·0 only one peak attributable to the B₁⁺ form of thiamine is present. In the pH region 3·5–5·5 another dpp peak attributable to the protonated form (B₁H²⁺) of thiamine was also observed. At pHs less than 3 only one peak was observed which could be attributed to the doubly protonated form (B₁ H₂³⁺) of thiamine. Surfactants like triton-X-100 and CTABr were found to inhibit the electroreduction of thiamine due to the strong adsorption of these compounds on the dme. Thiamine itself was found to have an inhibitory effect on its own electroreduction, although to a smaller extent.

The effect of surfactants on the absorption and emission properties of thionine (TH⁺) have been studied in detail. Among the various surfactants investigated sodium lauryl sulphate (SLS) has marked effect on these properties. Changes in the absorption spectrum and the decrease in fluorescence intensity at [SLS] below the critical micelle concentration (CMC) are attributed to the formation of a dye-surfactant complex. At [SLS] above CMC, the restoration of dye spectrum with increased extinction coefficient at the λ_max and a small but definite red shift of the λ_max are interpreted as due to the incorporation of the dye into the SLS micelle. The absorbance and spectral shift data suggest the thionine cation to be localized near the micelle Stern layer in the case of SLS micelles but completely outside the micelle in the aqueous environment in the case of CTABr. From the absorbance and fluorescence data, the association constant for the formation of the TH⁺-SLS complex in the premicellar region, and the binding constant for the incorporation of the dye into the micelle in the micellar region have been computed. The values of both these constants were found to increase markedly in the presence of electrolytes.

One-electron reduction of thionine has been studied by using the technique of nanosecond pulse radiolysis and kinetic spectrophotometry. H,e_aq⁻ as well as radicals derived from methanol, ethanol, isopropanol, THF, dioxane andt-butanol by H atom abstraction were used as reductants. The rate constants for the transfer of electrons from these radicalts to thionine were directly determined from the pseudo first-order formation rates of the product, semithionine and the one-electron reduction potential of thionine estimated. The absorption spectrum of semithionine in its different conjugate acid-base forms was found to be in agreement with previously reported spectra and the decay of the species was second order. By monitoring transient absorbance changes as a function of pH, twopK_a values were observed and, based on the effect of ionic strength on the second-order decay constants of the species were assigned to the equilibria described.