Abstract: Protonation constants and metal-complex stability constants of tannic acid, valonea tannin and black wattle tannin were determined by pH potentiometric titration under different temperatures in ionic strength I=0.1 mol/L NaCl solution. The complexation selectivities of different structures of tannins with Fe³⁺,Cu²⁺,Zn²⁺,Ni²⁺ ions were investigated and factors affecting protonation constant and stability constant were discussed. The results showed that the order of stability constants of the tannins complexed with Fe³⁺,Cu²⁺ ions was:black wattle tannin>valonea tannin>tannic acid, and that of tannins with Zn²⁺,Ni²⁺ ions was:valonea tannin>black wattle tannin> tannic acid. The rise of temperature was favorable for disassociation of phenol hydroxyl but unfavorable for the formation of complex. The purification of black wattle tannin by gel column chromatography showed little effect on the complex stability constants.

Key words: tannin, pH potentiometric titration, protonation constant, stability constant, metal ions