PREPARATIONS 277 ethereal solution becomes colourless. Separate the ether, dry it over calcium chloride for about 20 minutes, and then filter it through a fluted filter-paper directly into a ioo ml. conical flask. Fit the latter with a delivery knee-tube and water-condenser, and then, observing all the usual precautions (p. 163), distil off the ether on a water-bath until about 10-15 m ^ remain. Pour this solution whilst still hot into a small evaporating basin and swirl it gently around: the hot ether rapidly evaporates and the ethane tetracarboxylic ester crystallises out. Allow it to stand exposed to the air for about 10-15 minutes, and then well press the crystals between several layers of thick drying-paper to remove any liquid residue. Recrystallise from a small quantity of rectified spirit or of light petroleum (b.p. 60-80°). The ester is obtained as colour-less crystals, m.p. 76°: yield, 6*o-6'5 g. This tetra-ethyl ester is difficult to hydrolyse: the corresponding tetra-
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