O-carboxyphenyl azo chromotropic acid (sodium salt) as an analytical reagent

Summary

0-carboxyphenyl azo chromotropic acid (sodium salt), named as chromotrope 2 C, is used as a new colorimetric reagent for the determination of micro amounts of thorium and aluminium. The blue-violet and red-violet complexes show maximum absorption at 590 nm and the colour systems obey Beer's law from 0.1 to 8 ppm for thorium and 0.1 to 1ppm for aluminium. However, their optimum concentration ranges are from 1.6 to 8 ppm for thorium and 0.2 to 0.8 ppm for aluminium, where the percent relative errors per 1% absolute photometric error are, respectively, 3.06 and 2.94. The composition of the complexes, as elucidated by the continuous variation method, suggests a metal to reagent ratio of 2∶3 for thorium and a ratio of 1∶1 for aluminium. The instability constants for the complexes are of the order of 4.044×10⁻¹⁰ and 1.006 × 10⁻⁶ at 30°C.