Hydrogen sulfide should be determined preferably in the field; the
procedure as far as the final titration with sodium thiosulfate must be
carried out in the field.
_Reagents._—1. N/100 sodium thiosulfate.
2. Standard iodine. A N/100 solution containing potassium iodide
standardized against the N/100 sodium thiosulfate. To standardize, add
10 cc. of the iodine solution to 500 cc. of boiled distilled water. Add
about 1 gram of potassium iodide, and titrate with N/100 sodium
thiosulfate in the presence of starch indicator. One cc. of N/100 iodine
is equivalent to 0.17 mg. H_{2}S.