Add 3 mL of a 250 g/L solution of barium chloride R to 4.5 mL of sulfate standard solution (10 ppm SO4) R1. Shake and allow to stand for 1 min. To 2.5 mL of this suspension, add 15 mL of the solution to be examined and 0.5 mL of acetic acid R. Prepare a standard in the same manner using 15 mL of sulfate standard solution (10 ppm SO4) R instead of the solution to be examined.