The mixture was vacuum filtered,the residue was washed with 100 mL of ethanol and then dried at 50 ◦C for 24 h. About 0.1 g of the above dry residue was mixed with 15 mL of 2 N HCl and 1 mL of thioglycolic acid which was then boiled with occasional shaking for 4 h and centrifuged (7500 × g, 15 min)