 The sample is placed in a flask connected to an apparatus for trapping hydrogen sulfide gas (H2S).Oxygen-free water and hydrochloric acid (HCl) are added, and the sediment is stirred for one hour while argon or nitrogen gas is bubbled through.When HCl is mixed with metal sulfides in the sediment, a reaction occurs that generates H2S and liberates the metal ion into aqueous form, as shown in Equation 4.