Spectrophotometric determination of trace amounts of free cyanide in Prussian blue

Abstract

Trace amounts of free cyanide in Prussian blue are hydrolysed into hydrocyanic acid. The latter is captured by a lithium picrate solution contained in a test-tube, which is placed in the reaction vessel. The colour change due to the resulting lithium isopurpurate is measured spectrophotometrically at 500 nm. This method can detect cyanide down to a level of 2.5 µg in 100 mg of Prussian blue and is accurate and reproducible.