Sulfide is often present in groundwater, especially in hot springs, and it is common in wastewaters, coming in part from the decomposition of organic matter, sometimes from industrial wastes, but mostly from bacterial reduction. Hydrogen sulfide escaping into the air from sulfide-containing wastewater causes odor nuisances. This compound is also found in stagnant aquaculture situations where seawater or brackishwater is involved. Bacteria in muds and mangrove roots reduce sulfate from oceanic sources to sulfide. Sulfide is quite toxic to both aquaculture species and to humans.
Ammonium phosphate is added after color development to remove the ferric chloride color.
You are required, though, to increase the volume of reagents to accommodate a 25 mL sample. Fill a clean sample cell with 25 mL of sample.