Preservation of sulfidic waters containing dissolved As(iii)

Abstract

Solutions containing variable concentrations of sulfide (0 to 31 ppm) and arsenite (0.35 to 10 ppm) were subjected to several different preservation treatments. As predicted by equilibrium thermodynamics, at near-neutral pH the experimental solutions were undersaturated with respect to orpiment (As₂S₃). However, upon acidification to pH 2 with HCl or HNO₃, instantaneous precipitation of poorly crystalline As₂S₃ occurred in sulfidic solutions which resulted in the loss of dissolved arsenite. These results have implications concerning the practice of acidifying water samples containing hydrogen sulfide with HCl or HNO₃ for the purpose of preserving total arsenic values. If a near-neutral water sample contains 1 ppm arsenite and as little as 0.4 ppm sulfide, loss of dissolved arsenic will occur upon acidification. An alternative three-step preservation method involving base addition, oxidation, and acidification is proposed and investigated as an appropriate technique for preserving sulfide-bearing aqueous samples for total arsenic.