13548-43-1

Crystallise the double salt from a saturated aqueous solution at 55o by cooling slowly with rapid mechanical stirring. The resulting fine crystals are filtered on a Büchner funnel, partly dried on a porous plate, then equilibrated for several months in a vacuum desiccator over crude chromium ammonium sulfate (partially dehydrated by heating at 100o for several hours before use) [Johnson et al. J Am Chem Soc 75 3922 1953].