This is an undoubted acid, readily soluble in water, of decidedly
tanning properties, precipitating also the alkaloids and albumin.
Catechu-tannic acid being the first anhydride of catechin, there
are several more substances of that class; one of them is called
_Catechuretin_. This blackish brown almost insoluble substance is
obtained by heating catechin with concentrated hydrochloric acid at
180°:
2(C₁₉H₁₈O₈)-4 OH₂ = C₃₈H₂₈O₁₂.
Catechin, by melting it with caustic potash, affords Protocatechuic
acid, C₆H₃(OH)₂COOH, and Phloroglucin, C₆H₃(OH)₃: