H2so4 acid

All chemicals were of analytical reagent grade, and were dissolved in deionized (DI) water. The desired concentration of H2SO4 was prepared by diluting concentrated H2SO4 acid (98%, Merck, Germany) with DI water; 2.0 g L -1 of a DMPD solution was prepared by dissolving 0.50 g of the DMPD (Fluka, Switzerland) in 250 mL of 9 mol L -1 H2SO4 as a working solution A. A 16 mmol L -1 ammonium ferric sulfate solution, as an oxidizing reagent, was prepared by dissolving 1.95 g of NH4Fe(SO4)2•12H2O (Qrec, New Zealand) in 250 mL of 0.1 mol L -1 H2SO4 as a working solution B. Both solutions A and B were merged as an acceptor solution in the GD-FI, as depicted in Fig. 1. A standard stock solution of 25 mmol L -1 sulfide was prepared daily by dissolving 0.1952 g of sodium sulfide (Na2S•xH2O, Loba chemie, India) in 50 mL of 25 mmol L -1 NaOH to maintain the sulfide ion form. The exact concentration of sulfide was determined by using iodometric titration. Desired concentrations of the standard solutions were prepared by appropriately diluting the stock solution with 25 mmol L -1 NaOH. The canal water samples were collected in the Chiang Mai area and preserved by adding 2.5 mL of 5 mol L -1 NaOH for a 100 mL water sample to maintain the sulfide form and prevent H2S gas forming, which may be lost from the sample. The collected water samples were kept at 4°C in a refrigerator. 2