Optical measurement of aqueous kinetics at micromolar concentrations

Abstract

The authors describe a device for the continuous measurement of micromolar concentrations of dissolved species in water. The device produces a collimated beam of ultraviolet light that alternates in wavelength over a wide range. When absorbing materials are placed in the beam, the alternation is converted to an amplitude modulation that may be detected by a phototube and lock-in amplifier. This arrangement has a noise level corresponding to an absorbance of 1*10^-4, or about 3*10^-8 mole l^-1 of sulphur(IV) in a 100 mm cell, if the integration time is one second. Some sample studies of the manganese catalysed oxidation of sulphur(IV) to sulphur(VI) are presented. Both the high and low sulphur concentration rate laws were observed for the manganese system with this apparatus. A dependence of the rate constant on the initial sulphur concentration in the 10^-4 to 10^-5 molar regime was also observed.