Abstract
Chloroplatinic acid, for example hydrogen hexachloroplatinate (IV) (H2[PtCl6]), undergoes a ligand substitution reaction in aqueous solution. Rate of the ligand substitution was estimated from measurement of ultraviolet-visible (UV-Vis) absorption spectrophotometry and ion-exchange chromatography under different chloride concentrations, light conditions, and time frames. [PtCl4]2- in the PtCl2 solution of 20w% hydrochloric acid indicated absorption peaks on its spectrum, that was distinguished from Cl- and Pt (IV) complexes. Spectrum of [Pt(OH)2Cl4]2- in the PtCl4 solution diluted with ultra pure water changed with passage of time under normal fluorescent lighting. Absorbance at the wavelength below 380nm decreased with time, while that increased at the wavelength above. Simultaneously, the concentration of Cl- measured by ion chromatography decreased with time. These aging characteristics were referred from reaction pathway of the ligand substitution of OH- for Cl-. Drop of downward slop on the spectra at lower wavelength meant precipitation of hydrogen hydroxoplatinate (H2[Pt(OH)6]) induced by the substitution reaction. Hydrochloric acid was added to the diluted chloroplatinic acid solutions after that had reached equilibrium, and then prepared solutions also underwent measurement of UV-Vis spectra periodically. Characteristic of the spectra of above prepared solutions aged for several days, conformed with that of H2[PtCl6] solution. Spectra of the chloroplatinic acid solution with [PtCl6]2- had a specific peak at 260nm, and the peak area absorbance was in inverse proportion to Cl- concentration measured by ion chromatography. Thus, the change of this absorbance indicates the substitution of OH- for Cl-. Chloroplatinic acid diluted with hydrochloric acid or sodium hydroxide solutions showed above-mentioned change patterns not only under lighting but also in the dark. To the contrary, that of diluted with ultra pure water showed no changes in the dark. The reaction rate of the substitution was accelerated by lighting and increasing of amount of additions.
