Inner salts of sulfonic acids Alkali Blue

Inner Salts of Sulfonic Acids (Alkali Blue Types) 541... 

Two types of insoluble triarylcarbonium compounds are used industrially as pigments. Both are salts of these basic dyes. So-called Alkali Blue type triarylcarbonium pigments are inner salts of sulfonic acids, while the second group comprises salts of complex inorganic anions of heteropolyacids. 

The thus prepared product, however, being the inner salt of a sulfonic acid, turned out to be entirely insoluble in water. The compound only became a suitable textile dye after it had been converted into its sodium salt, i.e., through application in a slightly alkaline medium (= Alkali Blue). 

---