Determination of trace amounts of aqueous sulfide by indirect CVAAS of mercury

Abstract

A method was developed by which sulfide ions could be quantitatively determined by the stoichiometric removal of mercuric ions from a standard solution, and the unreacted mercuric ions be reduced to elemental mercury to be measured by cold vapor atomic absorption. Supporting this method is the extreme insolubility of HgS in aqueous solution. It was found that the experimental conditions used (a pH of 3, ascorbic acid reducing agent, a 2 L/min. carrier gas flowrate, 12mm x 75 mm culture tube and serum sleeve cap) led to a technique capable of determining sulfide at the 6 ppb level (in sample) with a typical precision of ± 5%. Cationic (Cd2+, Ca2+, Cu2+, Fe3+, Mn2+, Ni2+, PB2+, and Zn2+) and anionic (F-, CL-, Br-, I-, CN-, CrO4 2-, CO3 2-, No3-, Po4 3-, So4 2-, and So3 2-) interfaces were evaluated to determine their "threshold" level of interference. The cupric (Cu2+) ion showed the greatest interference effect on the system of the cations tested, even when a 10-5 M stock Cu2+ solution was used. Zinc (Zn2+) exhibited the least interfering effect on the system by the cation tested, ad not even when a 10-2 M stock Zn2+ solution was used did the absorbance reading change. The most severe anionic interferences were produced by cyanide (CN-), iodide (I-), sulfide (SO3 2-), and (to a lesser extent) bromide (Br-). The determination of the 45.2 ppb sulfide standard was unaffected by chromate (CrO4 2-), even when a stock concentration of 10-2 M was used. The threshold interference levels of the other ions were also determined.