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The reaction of chromate with diphenylcarbazide. II
Published in -
Volume: 10
Pages: 209 - 221
Job's method was employed for ascertaining the detailed mechanism of the chromato-carbazide reaction. Three different systems (1) chromatc-carbaxidc, (2) chromate-carbastone and (3) chromous-earbazone, leading to the formation of the same coloured complex were investigated. From studies in cquimolecular solutions, the maximum composition values for the reactions were found to be 2:3, 1:3 and 1:1, respectively. Proceeding from the assumption that the reaction (3) is a case of simple displacement, the stoichiometry of all the three systems has been formulated. The role of chromate in the chromatc-carbay.ide reaction, has been found to be primarily that of an oxidising agent followed by complex formation. The tentative formulation for the three systems is given below. (1) 2Cr+6 + 3 carbazide ⇌ 2 [Cr+2 carbazone-2 complex ] + carbacliazone + 4 H+ (2) Cr+6 + 3 carbazone ⇌ [Cr+2 carbazone-2 complex] + 2 carbadiazone + 2 H+ (3) Cr+2 + carbazone ⇌ [Cr+2 carbazone-2 complex] + 2 H+ The proposed mechanism seems to explain the relevant facts observed in the course of investigation of the three Systems, particularly the rapid discharge of the colour produced when excess of chromium was added and the greater sensitivity of carbaxide as compared to carbaxonc were accounted for, The determination of K values from studies in non-equimolccular solutions of the three reactions also corroborates the aforesaid mechanism. The apparent dissociation constant of the Cr+2-carbazonc complex was found to be 2.7.10-6 and the equilibrium constants of (1) and (2) were calculated as 5.7.10-12 and 2.3.10-7, respectively. The existence of bivalent Cr+2 ion in the complex molecule has also been independently confirmed from magnetochemical measurements. The molar susceptibility value of 4.6 Bohr magnetons calculated on the basis of 1:1 chromous-carbastonc ratio, indicates ionic bonding in the complex. © 1954.
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JournalAnalytica Chimica Acta