Indian Journal of Chemistry Vo l. 40A, August 2001. pp . 874-877 Kinetics of copper ion catalysed peroxydisulphate oxidation of glyoxylate B P Panda & N K Mohanty* P.G. Department of Che mi stry, Sambalpur University Jyoti Vi har 768019, Orissa, India Received 9 June /999: revised 3 May 2001 Kinetics of copper ion ca tal ysed peroxydisulphate oxidation of glyoxyla te ha s been inve st igated in th e pH ran ge 3.6 to 5.4 and ionic str ength of 0.2 to 0.5 M. The reaction is first ord er each in th e concentrations of glyoxy lat e, peroxyd isu lphate and Cu 1 +. The rate is invari ant wi th pH indi ca tin g th at glyoxy late and glyoxylic acid do not differ in reactivi ty. Kin e ti c re sult. are consistent with a mec hani sm involving a co mpl ex formed between catalyst, subst ra te and ox idant spec ies on the rate-limiting process. The rate an d activat ion parameters of the reaction have been eva lu ated. High pos itive entropy of activation is in keeping wi th th e fo rmation of more number of product species and less effective so lvati on of the trans iti on sta te . Peroxyd isul phate ion, S 2 0 8 2 - is known to be a versatile ox idising agent for many reactions. Despite i ts relatively high oxida ti on potential ' of 2.0 I v. its reaction with many substrates is quite slow indicating its large kinetic stability and sometimes the reaction tak es place o nl y in th e p re sence of metal ion catalyst. Although uncatalysed 2 - 4 and si lve r ion catalysed 5 - 8 peroxydisulp hate oxida ti on reac ti ons have been d I . . I d 9 " 11 repo rte rece nt y, report on cupnc ton cat a yse pe roxydisulphate oxidation reJ.ctions are scanty. In the prese nt investigation, therefor e. we report the kine ti cs of copper ion ca tal ysed peroxydisulphate ox idation of glyoxy lat e. Ex perimental All the chemicals were of reagent grade. Sodium glyoxyla te sol ution was prepared by neutralisation of glyoxylic acid with sodium hydroxide solution a nd was estimated by ion exchange method : 2 us in g Dowex 50WX8 ca ti on exchange res in . Potassium peroxydtsulphale so lu tion was prepared by di ssolving the sol id in water and estimated by standard method. Sodium perchlorate sol uti on was prepared by di ss olving the solid sample in water and the solution was also standardised by io n exchange method. Sodium perchlorate sol ution thus prepared was used to maintain io nic strength. The pH of the reaction medium was maintained by using acetate buffer. A standard copper sulphate solution was used as catalyst. All the solutions were prepared in di stilled water. The kinetics of the reaction was followed by estimating unreacted peroxydisulphate in the reaction medium by iodometric method 13 . The reaction ha s been studied under pseudo-first order condition with ten fold excess of glyoxylate. The ki netic results are found to fit the pseudo-first order plot. The pse ud o- first order rate constant (kobs) was calcu lated from such pl ot by the me th od of least squares. The rate constants calculated f: ·nm duplicate runs agreed within 3%. Results and discussion Kinetics of copper ion catalysed peroxydisulphate oxidation of glyoxylate has been investigated in the pH range 3.6 to 5.2 and ionic strength 0.2 to 0 .5 ."vf. The reaction has been smdied under pseudo-first order cond iti on l[glyoxylatej >> [S 2 0 l- ]). The observed pseudo-first order rate consta nt (k 00 ,) was found to vary with [glyoxylate] and [Cu 2 +]. Plots of kubs versus at consta nt a nd th at between kn b' versus [S 2 0l-J at constant fCu 2 +] were linear passtng through the origin (Fig. I). Such findings indicated that the reaction was first order each in [glyoxylatej a nd [Cu 2 +]. Inva ri ance of kob s with that the reaction was also first order in [S 2 0 /] . A variation in p H in th e range 3.6 to 5.2 by employing acetate buffu indicated that there was no effect of pH on ra te. pKa (Ka =ac id dissociation constan t) of glyoxylic ac id bei ng 14 .) .3. it follows that re ative proporttons of CHOCOOH a nd CHOCoo· · vary in such p H range. S in ce the rate constant, kob' i. invariant with pH it fo llows that rea::tivity of CHOCOOH and CHOCOO- are not differe nt. For studying the effect of [Cu 2 +]. [S 2 0 t]. [S ub st rate] and ion ic streng th on rate, th e p H has been kept at 5.2. At this pH, no protonation of CHOCOO- occurs. To account for a ll the k in e ti c res ult s, the following me chanism has been proposed. K' CHOCOO- + H(OHhCOO- gem-diol structure K" i ( ) Cu 2 + + CH(OHhCOO- ,. \ .. \- )