Temperature dependence of (SCN)(2)(center dot-) in water at 25-400 degreesC: Absorption spectrum, equilibrium constant, and decay

The temperature dependence of the absorption spectrum of the formation and decay of (SCN)(2)(.-), a well-characterized dimer anion, was investigated a t temperatures from 25 to 400 degreesC. The absorption peak was found to sh ift to longer wavelength with temperature (red shift), from 470 nm at 25 de greesC to 510 nm at 400 degreesC. The equilibrium constants K-1 and K-2 for the reactions SCNOH.- reversible arrow SCN. + OH- and SCN. + SCN- reversib le arrow (SCN)(2)(.-), respectively, were found to decrease with temperatur e. Due to the considerable decrease of K2 with temperature, a rise in tempe rature shifts the reaction in favor of SCN., so the observed yield of (SCN) (2)(.-) high temperatures is strongly dependent on the SCN- concentration. As the SCN. concentration could be as high as or even higher than the (SCN) (2)(.-) concentration at high temperatures, a pseudo-first-order decay of S CN. has to be taken into consideration to account for the overall decay of (SCN)(2)(.-). Using the kinetic parameters obtained in this work and availa ble in the literature, the decay profiles of (SCN)(2)(.-) can be well repro duced for any temperature and KSCN concentration considered. A combination of the simulation and the experimental results reveals a decrease of epsilo n (max) of (SCN)(2)(.-) with temperature; the degree is similar to 30% for a rise from 25 to 400 degreesC.