Shake the sample vigorously and immediately pipet a volume of sample with ZnS in suspension containing less than 1.5 mg S-2 (100.0 mL max) into a 250-mL Erlenmeyer flask, and adjust the volume to approx. 100 mL. 6.2 Prepare a blank of approx. 100 mL demineralized water, and carry it through the procedure with the sample. 6.3 Add 10.0 mL 0.010N I2 and mix. 6.4 Without delay add 10 mL concentrated HCl. 6.5 Immediately titrate the excess 12 with 0.010N Na2S2O3, adding 2 to 3 mL starch indicator solution as the end point is approached (light-straw color).