Introduction Perez-Benito et al. [1, 2] reported the oxidation of formic and oxalic acids by water soluble colloidal MnO 2 and these studies were extended by Tuncay et al. [3] and Kabir-ud-Din et al. [4] to micellar media. In case of lactic acid oxidation by colloidal MnO 2 , it was observed that the process took place in two steps [5] (nonauto- catalytic and autocatalytic) and that the second step disappeared completely in the presence of added man- ganese(II). We see that the above cited papers [3, 4] are the only few concerned with the effect of surfactant on the redox chemistry of colloidal MnO 2 [3, 4] while anionic-, cationic- and nonionic micellar effects on the kinetics and mechanism of organic and inorganic reac- tions have so far been extensively studied [6–12]. The oxidation of a-hydroxy acids has been particu- larly interesting because both C–H and C–C cleavage have been reported [13–15]. Recently [16], we have re- ported the oxidation of citric acid by colloidal MnO 2 that proceeded through the C–H and C–C bonds cleavage. We searched for another closely related sys- tem, where there is a possibility of C–C and C–H bond breaking, and such a situation is present in DL-malic acid. Therefore, in this paper we wish to report the results of investigations of the MnO 2 oxidation of Kabir-ud-Din S. M. Shakeel Iqubal Zaheer Khan Reduction of soluble colloidal MnO 2 by DL-malic acid in the absence and presence of nonionic TritonX-100 Received: 9 June 2004 Accepted: 4 September 2004 Published online: 1 December 2004 Ó Springer-Verlag 2004 Abstract Kinetics of oxidation of DL-malic acid by water soluble col- loidal MnO 2 (prepared from potas- sium permanganate and sodium thiosulfate solutions) have been studied spectrophotometrically in the absence and presence of nonionic Triton X-100 surfactant. The reac- tion is autocatalytic and manga- nese(II) (reduction product of the colloidal MnO 2 ) may be the auto- catalyst. The order of the reaction is first in colloidal [MnO 2 ] as well as in [malic acid] both in the absence and presence of the surfactant. The reaction has acid-dependent and acid-independent paths and, in the former case, the order is fractional in [H + ]. The effect of externally added manganese(II) is complex. The results show that the rate constant increases as the manganese(II) concentration is increased. It is not possible to predict the exact depen- dence of the rate constants on man- ganese(II) concentration, which has a series of reactions with other reactants. In the presence of TX-100, the observed effect on k w is catalytic up to a certain [TX-100]; thereafter, an inhibitory effect follows. The catalytic effect is explained in terms of the mathematical model proposed by Tuncay et al. (in Colloids Surf A Physicochem Eng Aspects 149:279 3). Activation parameters associated with the observed rate constants (k obs /k w ) have also been evaluated and discussed. Keywords Colloidal MnO 2 Æ DL-Malic acid Æ Kinetics Æ Surfactant Æ Triton X-100 Colloid Polym Sci (2005) 283: 504–511 DOI 10.1007/s00396-004-1215-z ORIGINALCONTRIBUTION Kabir-ud-Din (&) Æ S. M. S. Iqubal Department of Chemistry, Aligarh Muslim University, Aligarh, 202 002, India E-mail: kabir7@rediffmail.com Tel.: +91-571-2703515 Z. Khan Department of Chemistry, Jamia Millia Islamia, Jamia Nagar, New Delhi, 110 025, India