236 PRACTICAL ORGANIC CHEMISTRY Filter the product at the pump, using an alkali-resisting filter-paper, or a sintered glass filter-funnel. Wash the crystals on the filter with a small quantity of ethanol to remove the purple colour, and then drain thoroughly. To obtain the free acid, dissolve the potassium salt in 50 ml. of cold water, filter the solution if a small undissolved residue remains, and then boil the clear solution gently whilst dilute sulphuric acid is added until the separation of the acid is com-plete. Cool the solution and filter off the pale orange-coloured crystals of the benzilic acid: wash the crystals on the filter with some hot distilled water, drain well, and then dry in a desiccator. Yield of crude acid, 4 g. Recrystallise from benzene (about 50 ml.) to which a small quantity of animal charcoal has been added, filtering the boiling solution through a preheated funnel fitted with a fluted filter-paper, as the benzilic acid readily crystallises as the solution cools; alternatively, recrystallise from much hot
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