The liberation of hydrogen sulfide on adding dilute hydrochloric acid to
a sludge indicates the presence of ferrous sulfide. As ferrous sulfide
quickly oxidizes on exposure to air a quantitative determination of this
constituent must be made immediately after collection of the sample.
_Procedure._—Heat a definite portion of the sludge with hydrochloric
acid in a flask. Pass the liberated gas through bromine water or
hydrogen peroxide. Determine gravimetrically the sulfate in the
oxidizing solution, and calculate the equivalent of ferrous sulfide by
multiplying the weight of barium sulfate by 0.376.