Take 70 mL of water from the source (under a stream) in a beaker, pour into an Erlenmeyer flask, add 2 mL of starch solution and immediately titrate with iodine solution in KJ until a blue color appears. The H2S content (expressed in mg/L) is calculated by multiplying the number of mL of iodine solution consumed in the titration by a factor of 0.5.