DESCRIPTION

Lithol Rubine BK occurs as a red powder. It consists of the calcium salt of 3-hydroxy-4-[(4-methyl-2-sulfophenyl)azo]-2-naphthalenecarboxylic acid and subsidiary coloring matters together with water, calcium chloride, and/or calcium sulfate as the principal uncolored components. Lithol Rubine BK is produced by diazotizing 2-amino-5-methylbenzenesulfonic acid with hydrochloric acid and sodium nitrite. The resulting diazo compound is coupled in alkaline medium with 3-hydroxy-2-naphthalenecarboxylic acid and the resulting dye is converted to the calcium salt with calcium chloride. It is soluble in strong acids, slightly soluble in hot water (90°), and insoluble in cold water and ethanol.