A stock solution of sulfide ions (0.1 mM) was prepared in waterand stored at 4◦C in the refrigerator. Standard test solutions wereprepared by appropriate dilution of the stock solution. For thedetection of sulfide ions, 100 L of 1.2 mM AgNO3, and 10 L ofNa2S solutions with different concentrations, 200 L of 1 mM TMB,was added sequentially in 690 L of 0.1 M NaAc buffer solutions(pH 4.0). After thorough mixing, the solution was incubating forat 30◦C for 5 min. Then, the solution was transferred for UV–visscanning.