Abstract
The kinetics of reduction of Cr(VI) by thiourea has been studied spectrophotometrically in an HClO4 medium. The reaction follows overall third order kinetics, first order each in Cr(VI), thiourea and HClO4. The reaction is HClO4 catalyzed. Addition of Mn(II) and Ce(IV) ions largely decrease the reaction rate. The reaction rate is increased by addition of EDTA. The spectrophotometric study indicates that the reduction of chromium(VI) follows the sequence
Cr-VI R/--> Cr-IV R/--> Cr-III
where, R=reductant (thiourea). The reaction is considered to proceed through the formation of a ternary EDTA-chromium(VI)-thiourea complex which decomposes giving the reduced Cr(IV)-EDTA complex, followed by fast reduction with thiourea. The reduction products of chromium(VI) have been found to be Cr(III)-thiourea (lambda(max) = 425 and 600 nm) and Cr(III)-EDTA (lambda(max) = 410 and 550 nm), respectively, in the absence and presence of EDTA.