An apparatus and method of determining chemical dosage required to reduce sulfides in wastewater includes providing a headspace apart from the wastewater. The temperature of the wastewater and the concentration of sulfide gas in the headspace are measured. Dissolved hydrogen sulfide gas concentrations in the wastewater are calculated from the temperature of the wastewater and the concentration of sulfide gas in said headspace. When pH is monitored, dissolved sulfide concentrations can be calculated.