all species in the wastewater that it can react with including heavy metals and sulfide scavengers this eliminates the need to test the water and to re treat the water if a sufficient quantity of sulfide was not added in addition trace metal sulfides may coprecipitate or be enmeshed in the ferrous or ferric sulfide that is subsequently precipitated the sulfide reagent can be any metal sulfide which is soluble in water sodium sulfide and potassium sulfide may be preferred for economic reasons and because the counterions are innocuous 34 soluble in water 34 means that the reagent must be capable of providing a sufficient concentration of sulfide ions to the water a sufficient quantity of sulfide ions is about 0 001 or higher moles of sulfide ions per liter of water the sulfide reagent can be added as a solid though the preferred method is to add an aqueous solution hydrogen sulfide can be used though its toxicity may limit the utility of this sulfide reagent in many applications and environments and its solubility at moderate to low ph levels may not be adequate for metals precipitation the temperature for the method is not critical and can range from about 0 u00b0 c to about 100 u00b0 c the water cannot.