321 J. Indian Chem. Soc., Vol. 93, March 2016, pp. 321-326 Reduction of gold(III) by malonate in acetate buffer : Mechanism of the rate processes Pratik K. Sen* a , Anirban Sarkar a , Krishnendu Maiti a and Biswajit Pal* b a Department of Chemistry, Jadavpur University, Kolkata-700 032, India E-mail : senpk@yahoo.com b Department of Chemistry, St. Paul’s Cathedral Mission College, 33/1, Raja Rammohan Roy Sarani, Kolkata-700 009, India E-mail : palbiswajit@yahoo.com Manuscript received 08 June 2015, accepted 26 November 2015 Abstract : The kinetics of reduction of tetrachloroaurate( III) by sodium malonate has been spectrophotometrically studied in acetic acid-sodium acetate buffer in the pH range 4.20–4.83. The reaction is of first order with respect to Au III but of complex order with respect to substrate and H + . The rate of reaction increases with increasing ionic strength and increasing [H + ]. The rate is also enhanced with an increase in the dielectric constant of the medium. AuCl 3 (OH) – and malonate anion are presumed to be the predominant reactant species under experimental conditions. AuCl 3 (OH) – reacts with malonate ion to produce an intermediate species which in turn decomposes in the rate determining step to yield glycolic acid, CO 2 and Au I . Thermodynamic parameters associated with different reaction steps have been evaluated. Keywords : Reduction, sodium malonate, tetrachloroaurate( III), acetate buffer, kinetics and mechanism. Introduction Malonic acid is an important chemical which is widely used in medicine and in various chemical industries. While it is used in its ester form in the pharmaceutical industry, it’s pure form is highly reactive and unsafe. Malonic acid and its esters are characterized by the large number of condensation products. They are important intermediates in syntheses of vitamins, barbiturates and non-steroidal anti-inflammatory drugs 1 . The kinetics and mechanism of oxidation of malonic acid has been studied by different oxidants such as Cr VI 2 , Mn VII 3 , Mn IV 3 , Mn II -catalysed bromate 4 and Bi V 5 . The biological importance of gold(III) compounds have been reported 6–9 . The kinetics of oxida- tion of a few alcohols 10 , glycolaldehyde 11 , -hydroxy- acids 12 and oxalic acid 13 by Au III have already been re- ported. An attempt has therefore been made to investigate the mechanism of oxidation of malonic acid, another di- carboxylic acid, by tetrachloroaurate(III). It will also be interesting to find out whether Au III is reduced by a single two-electron transfer step, or two, one-electron transfer steps 14 . Experimental Reagents : All the reagents were of analytical grades. Sodium malonate was prepared by neutralising 0.8 mol dm –3 ma- lonic acid (extra pure AR, SRL, India) by 0.8 mol dm –3 sodium hydroxide. Au III solution was prepared by dis- solving approximately 1 g of chloroauric acid (AR, SRL, India) in 50 mL of 0.01 mol dm –3 HCl solution. The strength of this solution was determined by measurement of the absorbance of the solutions (after proper dilution in 0.01 mol dm –3 HCl and 1 mol dm –3 NaCl) at the absor- bance maximum (313 nm) 13 of AuCl 4 – . All the solutions were made in doubly distilled water. Kinetic measurements : The kinetics of the reaction was studied under pseudo- first order condition where [malonate] >> [Au III ], i.e. the substrate concentration was taken in large excess com- pared to the oxidant concentration. The course of the re- action was studied spectrophotometrically by measuring the absorbance of the mixture at 400 nm at known inter- vals of time. All the kinetic investigations were carried JICS-8