Thymolphthalin as an Analytical Reagent: 1. Spectrophotometric Determination of Ferricyanide and Ferrocyanide

Abstract Thymolphthalein (I) as an acid‐base indicator is successfully reduced in sodium hydroxide medium and zinc dust into a colorless reagent (Thymolphthalin (III)). The latter is used as an analytical reagent in the colorimetric determination of some mildly strong oxidising agents such as potassium ferricyanide. Ferricyanide, ferrocyanide and a mixture of both are spectrophotometrically determined at 592 nm. Dichromate oxidises ferrocyanide to ferricyanide without interference on the reaction. The results obtained showed that the system does obey Beer's law over the concentration range 20–170 ug ferricyanide per 25 ml. The molar absorbitivity is (4.28+0.02)×10 3 cm 2 mg −1 mole ‐ 1 .