soluble blue
nounDefinition of soluble blue
1
often capitalized S&B
: any of several water-soluble acid dyes made by sulfonating the Spirit Blues and used in the form of salts chiefly in writing inks, in laundry blues, and as biological stains
2
: a pigment dispersible in water that is made by treating an iron blue with sodium ferrocyanide or oxalic acid and is used chiefly in permanent writing inks and laundry blues