Analysis of the mixture of aluminum and chromium(III) ions by kinetic method

By the aid of different rates of aluminum and chrome(III) ions for formation of 1.1 complex with xylenol orange reagent the kinetic analysis of the mixture having these two metal ions was investigated. Optimum pH, wavelength and temperature for the kinetic analysis were determined. For the determination of optimum pH in the pH range of 2-4, the mixture containing 1 ml, 3 x 10-4 M metal ion, 1 ml buffer and 1 ml 3x10-4 M xylenol orange solutions was heated to 100 ° C in a water bath. After cooling ıt was diluted to 10 ml. The absorption spectra of obtained solutions at different pH's were recorded using reference water. Maximum molar absorptivities for 1:1 complexes of investigated two metals were obtained at pH 2,5 acetate buffer. The isobestic point at which the 1:1 complexes formed by equal concentrations of chrome (III) and aluminum with xylenol orange showed the same molar absorptivities were found to be 561 nm at pH 2,5. The absorbance measurements for kinetic analysis were made at this wavelength. In order that the complex formation reactions proceed according to pseudo 1. order kinetics the molar concentration of the metal ion were taken as 50 times of the xylenol orange. The rates of complex formation reactions of aluminum and chrome ions with xylenol orange were determined by observing the absorbances of the complexes at the isobestic point against time. For this purposes a spectrophotometer equipped with cell holder for constant temperature and a thermostated water bath was used. To the mixture of 1 ml of 1,8 x 10-4 M xylenol orange and 1 ml of buffer solutions heated to required temperature 1 ml of the solution of 9x10-3 M metal ion which was heated to the same temperature was added. This mixture was poured into the cell which was kept at the same temperature. The absorbance values changing by time were recorded. Reaction rate constants at the working temperatures were determined by the rate curves, plotted ln ( A∞ - At) versus time. By the aid of Arrhenius graph the most suitable temperature for kinetic analysis was determined as 42 ° C. At this temperature the kinetic analysis of Al-Cr(III) binary mixtures having different mole fractions were performed using the Single Point method of Roberts and Regan. In the calibration curve 1/te[Mo] values were plotted against [ Ao] / [Mo] mole fractions.