Ultraviolet-visible spectroscopic method in determing reaction rate constant for the reduction of cr (vi) to cr (iii) using ferrous sulphate (feso4)

The kinetics of the reduction of hexavalent to trivalent chromium using ferrous sulphate (FeSO4) under acidic condition was studied using two methods of UV-Visible spectroscopy – the HACH and the Lambda Series. Potassium dichromate (K2Cr2O7) was used as source of Cr6+. Due to complexity and time consuming steps of the Lambda Series, the HACH Method was employed as screen test. As soon as the trial gave good results from the HACH method, the Lambda series was performed. Comparison of the results was carried out and several differences were noted. The study was composed of four parts – kinetics of the reduction of potassium dichromate by ferrous sulphate, determination of the rate and order of reaction, analysis of chromium hexavalent using the Diphenylcarbazide Method of HACH and Lambda Series UV-Vis spectrometer and determination of rate constant. There are several trials performed to arrive at the procedure for the reduction of hexavalent chromium via ferrous sulphate presented herein. Same is true with the rate and order of reaction. At a ratio of 1:10 of potassium dichromate to ferrous sulphate, a good result for the rate and order of reaction was obtained. Results from the procedures showed that the overall rate of reaction is first order – that is zero order with respect to K2Cr2O7 and first order with respect to FeSO4. In the kinetic study, a ratio of 3:5 K2Cr2O7 to FeSO4 also showed good results in which the rate constant was computed to be 1.7 x 10-4 per second. However this result was only obtained from the HACH Method. Lambda Series showed different results in the computation of rate constant.

Antonio G. Celajes and Jose S. Hilario
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