PHENOLPHTHALEIN, an organic compound of the acomatic series which has two (I) as an indicator for acids and bases in volumetric analysis; (2) as a purgative drug. The phthaleins are prepared by condensing phenols with phthalic anhydride, phenol itself giving rise to phenolphthalein together with a small quantity of fluorane, whilst resorcinol under similar conditions yields fluorescein (q.v.). The phthaleins on reduction yield phthalines which are derivatives of triphenylmethane car boxylic acid ; thus phenolphthalein itself gives phenolphthaline.
Phenolphthalein is obtained when phenol and phthalic anhy dride are heated with concentrated sulphuric acid. It crystallizes in colourless crusts (m.p. 250-253° C) and is nearly insoluble in water, but dissolves in dilute solutions of the caustic alkalis with a fine red colour, being reprecipitated from these solutions by the addition of mineral acid. It is to this remarkable colour change that its use as an indicator is due. It dissolves in concen trated caustic alkalis to a colourless solution which probably contains salts of a non-quinonoid character. On fusion with
caustic alkali, phenolphthalein yields benzoic acid and para dihydroxybenzophenone, which shows that in the original con densation the phthalic acid residue has taken the para position to the hydroxyl groups of the phenol.
Fluorane is a by-product of the condensation of phthalic acid and phenol, substitution occurring in the ortho position to the hydroxyl groups of the phenol, so that anhydride formation takes place between these hydroxyl groups. It dissolves in concentrated sulphuric acid with a yellowish-green fluorescence.
Phenolphthalein has extended use as a purgative under such synonyms as purgen, laxoin, laxatol, etc. The official dose is 2 to 5 grains (0.12 to 0.3 grams), but larger doses are given. Phenol phthalein is dissolved in the bile and by the alkali of the intestine, and being irritant to the latter it causes peristalsis. It is partly absorbed and partly excreted by the liver and so acts repeatedly on the intestine, acting for a few days as a mild aperient.