A red coloring medium made from phenol or carbolic acid. It
is soluble in alcohol and ether but insoluble in water. When
fabrics were steeped in this solution, the color was nonactinic
and therefore safe for darkrooms, windows, and doorways.
Aurin was also mixed with water-soluble gums and brushed
on the backs of collodion and silver bromide gelatin plates as
an antihalation layer.
It forms green crystals with a metallic luster, but the colour depends on the solvent used. When recrystallised from brine (saturated aqueous NaCl) acidified with HCl, it forms red needles, but when recrystallised from EtO/AcOH, the crystals have a beetle iridescent green colour. It has been recrystallised from Me2CO (although it dissolves slowly), methyl ethyl ketone, 80-95% AcOH and from AcOH/*C6H6. An aqueous KOH solution is golden yellow, and a 70% H2SO4 solution is deep red in colour. An alternative purification is to dissolve this triphenylmethane dye in 1.5% of aqueous NH3, filter, and heat to 70-80o, then acidify with dilute AcOH by adding it slowly with vigorous stirring, whereby the aurin separates as a brick-red powder or as purplish crystals depending on the temperature and period of heating. Filter off the solid, wash it with H2O and a little dilute AcOH, then H2O again. Stir this solid with Et2O to remove any ketones and allow it to stand overnight in the Et2O, then filter and dry it in air then in a vacuum. [Gomberg & Snow J Am Chem Soc 47 202 1925, Baines & Driver J Chem Soc 123 1216 1923, UV: Burawoy Chem Ber 64 462 1941, Beilstein 8 IV 2646.]
