Preparation of hexacyanocobaltic(III) acid pentahydrate
A saturated aqueous solution containing 10g of the potassium hexacyanocobaltate(III) salt are placed in a 50ml suction flask fitted with a stopper and delivery tube at least 10mm wide. The vessel is cooled in tap water while a steady stream of hydrogen chloride is introduced. [CAUTION: Use an empty trap in the apparatus to avoid suck-back.] When precipitation appears complete the flow of gas is stopped and the white solid, consisting of potassium chloride and the complex acid, is filtered off and thoroughly drained on the filter. The residue is treated in a stoppered flask with two 20ml portions of absolute alcohol and refiltered. The potassium chloride remains behind whereas the acid passes into solution and is recovered as colorless glistening needles by evaporating the alcohol at room temperature with a stream of dry air or nitrogen. The material should be stored in tightly stoppered all-glass bottles out of the light. Yield about 8g The free acid is completely ionized in aqueous solution and may be used to prepare cobalticyanides of other cations by the usual acid-base reactions.
Inorganic laboratory preparations, by G. G. Schlessinger, 86, 1962