_Procedure._—Add 500 cc. of the sample to 10 cc. of the standard iodine
solution and 1 gram of potassium iodide in a large glass-stoppered
bottle or flask. If the sample is to be collected from a tap lead the
water into the bottle through a rubber tube extending to the bottom of
the bottle so as to eliminate errors due to aeration. Shake the bottle,
allow it to stand for a few minutes, and then titrate the excess of
iodine with sodium thiosulfate in the presence of starch indicator.
Hydrogen sulfide (H_{2}S) in parts per million is equal to 0.34 times
the difference in cubic centimeters between the amount of iodine
solution added and the amount of N/100 thiosulfate used in the
titration.