AbstractThymolphthalein (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 reagentin thecolorimetric 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 20–170ugferricyanide per 25ml.The molar absorbitivity is (4.28+0.02)×103cm2mg−1