The association constant of the first chloro-copper(II) complex in water to 100°C

The equilibrium constant, K1, for the reaction Cu2+ Cl− ⇌ [CuCl]+ has been determined in the range 15–100°C from spectrophotometric data. The method of analysis, which is widely used, requires both slope and intercept from a reciprocal plot of absorbance against total metal ion concentration. The calculated values of K1, particularly at low temperature, are very sensitive to the chosen method of data treatment such that the application of weighting factors to the absorbance data for a given photometric accuracy can halve the values. The large variations in K1 found in the literature are likely to be caused by underestimation of the instability inherent in the method as much as by the neglect of chemical effects such as the formation of higher complexes. By appropriate weighting of the absorbance data an increase in K1 is apparent above 30°C and the method may have limited usefulness at high temperatures.