Study on the precipitation reaction between baicalin and berberine by HPLC.

The solubility product equilibrium constant for the precipitation between baicalin and berberine was investigated because of the current interest in decocting process of complex prescription of Chinese herbal medicine. HPLC was used for determining two compounds' equilibrium concentrations at different precipitate conditions to calculate thermodynamic constants and study kinetic process. The analysis was performed on a Kromasil C(18) column with TEA-adjusted 0.02 mol/L H(3)PO(4) (pH 4.82)-acetonitrile (75:25) as mobile phase at a flow-rate of 1.0 ml/min, with detection at 254 nm. According to the experiment result, the molar ratio of baicalin and berberine in precipitate is about 1:1. The experimental K(sp) values are (1.01 +/- 0.12) x 10(-9)mol(2)/L(2) at 20 degrees C in 0.02 mol/L NaH(2)PO(4)-Na(2)HPO(4) (pH 4.82), and (3.20 +/- 0.46) x 10(-9)mol(2)/L(2) at 40 degrees C in the same buffer. The precipitate reaction is an exothermic process and occurs immediately, even though the precipitate cannot be observed in time because the precipitate is light, yellow, flocculous and suspending in the yellow solution.