On-site catalytic determination of trace manganese in fresh water using oxidation of malachite green with periodate.

An on-site determination method for trace manganese has been developed using the manganese-catalyzed oxidation of Malachite Green (MG) with potassium periodate. Absorbance measurement of MG was carried out using a laboratory-made palm-top size colorimeter with a red-green-blue light-emitting diode. The reciprocal value (1/τ) of the reaction time at a fixed absorbance value for red light was chosen as a kinetic parameter for simple on-site analyses. The value of 1/τ was proportional to the concentration of manganese in the range of 2 - 20 µg L(-1). At a reaction temperature (T) of 20°C or more, manganese was determined within a reaction time of 25 min. The calibration equation was approximated by 1/τ = (aT + b)[Mn] + (cT + d), where a to d were constants in a range of 10 - 40°C. The two equations for 30°C (for the laboratory-preparation of the calibration equation) and T give the value of 1/τ at 30°C as 1/τ(f) = (30a + b)(1/τ - cT - d)/(aT + b) + 30c + d. Without any temperature control, 1/τ(f) can be calculated by this equation and measurements of 1/τ and T. The calculation introduced analytical errors of within 1 µg L(-1). The proposed method was successfully applied to tap-, river- and lake-water samples.