Spectrophotometric determination of copper with tiron

The reagent Tiron forms a soluble greenish-yellow complex with cupper. The sensitivity of the colour reaction is 0.24 mμ of copper/cm₂. The colour system at a pH of 6.1 to 6.9 is stable for over 24 h and shows a maximum absorption at 375 mμ. It obeys Beer's law at the latter wave length within a copper concentration of 10 to 160 p.p.m. The optimum range, however, is from 50 to 160 p.p.m. of copper, where the percent relative analysis error per 1% absolute photometric error is 2.94. JOB's method of continuous variation and the molar ratio method of YOE et al.² indicate that in solution the complex contains the metal and the reagent in a ratio of l : 2, with a dissociation constant on the order of 5.3 ? 10^-6.