NONLINEAR DYNAMICS IN CHEMISTRY DERIVED FROM SULFUR CHEMISTRY .5. COMPLEX OLIGOOSCILLATORY BEHAVIOR IN THE REACTION OF CHLORITE WITH THIOCYANATE

The reaction of chlorite and thiocyanate has been studied in the pH range 1-4. The stoichiometry of the reaction is 2ClO2- + SCN- + H2O --> SO42- + 2Cl- + HOCN + H+. In excess ClO2-, ClO2(aq) is formed as a product at the end of the reaction. ClO2 is formed from the reaction of excess ClO2- with HOCl: 2ClO2- + HOCl + H+ --> 2ClO2 + Cl- + H2O. At pH less than 4 and in excess SCN-, ClO2 is formed as an intermediate but is totally consumed at the end of the reaction. The reaction is very complex with oligooscillatory behavior in which ClO2 concentration goes through two maxima during the course of the reaction. The reaction is catalyzed by acid in pH range 2-4 but is retarded by acid in pH less than 2. The rate-determining step involves the reaction Cl(III) + SCN- + H+ --> HOSCN + HOCl. The acid retardation is due to the fact that ClO2- is more reactive than HClO2 and SCN- is a better nucleophile than isothiocyanic acid, HNCS. The reaction is autocatalytic in HOCl. The autocatalysis can be explained by using the asymmetric intermediate, Cl2O2, which produces two HOCl molecules after a two-electron reduction. The reaction Of ClO2(aq) with SCN- was also studied, and it gives an autocatalytic rate of decay of ClO2. The mechanism involves initially forming ClO2- in an one-electron reduction followed by HOCl autocatalysis. Direct reaction between ClO2- and SCN- could be followed by using the FeSCN2+-SCN- reaction, which also showed acid retardation and HOCl autocatalysis. A 21-reaction mechanism was used to simulate the ClO2-SCN- reaction while a 24-reaction scheme was used to simulate the ClO2--SCN- reaction. There is reasonable agreement between experiments and simulations in both cases.