Composition and Stability of Lanthanum(III) -p-nitrobenzeneazochromotropic Acid (Chromotrope 2B) Chelate in Solution

A detailed study of the composition and stability of lanthanum(III)-p-nitrobenzeneazochromotropic acid (chromotrope 2 B) chelete has been made spectrophotometrically. The chelate is reddish violet in colour (\(\lambda\)_max at 540m\(\mu\)). The composition, as determined by different methods, is 1:1 (metal: chelating agent). The chelate is stable between pH 5.0 and 12.5. The values of log K, as determined by two different methods, are 4.5±0.1 and 4.9 ± 0.18 respectively at 25° and at pH 6.0.