1395-18-2

Indicator instead of litmus. pH: 4.5 red, 8.3 blue. Usable with most mineral, some organic acids (not hydroxy acids) and some alkaloids; also used for preparing litmus media for bacteriologic purposes.

It crystallises from water as dark violet scales, or as a red powder on precipitation from H2O by addtition of EtOH. It is an indicator which is red at pH 4.5 and blue at pH 8.3. [cf Kolthopff & Rosenblum Acid-Base Indicators, Macmillan, NY pp160-162, 365-366 1937.] p,p'-Azoxyanisole (4,4'-dimethoxyazoxybenzene) [1562-94 -3] M 258.3, transition temperatures: 118.1-118.8o, 135.6-136.0o, pK25 -5.23 (20% aqueous EtOH + 80% aqueous H2S O4). Crystallise the dye from absolute or 95% EtOH, or acetone, and dry it by heating under vacuum or sublime it in a vacuum onto a cold finger. [Beilstein 16 II 326.]