Spectrophotometric determination of gallium (III) with Methylthymol Blue

The reaction betw een gallium (III) and Methylthymol Blue has been studied spectrophotometrically to establish the optimum conditionsfor determining small amounts of gallium. Depending on the pH value of a solution, gallium (III) reacts with Methylthymol Blue to form two sorts of complexes. The 1 to 1 complex which gives the maximum absorbance in the pH range from 1.5 to 1.8 has an absorption maximum at about 565mμ against a reagent blank. The 1 to 2 complex which gives the maximum absorbance in the pH range from 4.5 to 5.5 has an absorption maximum at about 515mμ. Beer's law is obeyed over the ranges from 7 to 84μg of gallium in 25ml for the 1 to 1 complex, and from 7 to 70 μg of gallium for the 1 to 2 complex. The apparent molar extinction coeficients of the two complexes at their absorption maxima were calculated to be 17, 700 and 11, 200, respectively. The effect of diverse ions on the absorbance of the 1 to 1 complex was examined. Such anions as oxalate, NTA and EDTA inhibited the formation of the complex.