Sulfide ion (S2-) is formed in wastewater by the action of anaerobic bacteria on organic matter. Sulfide can be readily protonated to form volatile, toxic H2S. In addition to the toxicity and noxious odor, sulfide and H2S cause corrosion problems because they can be easily converted to sulfuric acid when conditions change to aerobic. One common method to determine sulfide is by coulometric titration with generated silver ion. At the generator electrode, the reaction is Ag →Ag+ + e-. The titration reaction is S2- + 2Ag- → Ag,S(s).

(a) A digital chloridometer was used to determine the mass of sulfide in a wastewater sample. The chloridometer reads out directly III ng CI-. In chloride de: terminations, the same generator reaction is used, but the titration reaction is CI- + Ag+ →AgCl(s). Derive an equation that relates the desired quantity, mass S2 - (ng), to the chloridometer readout m mass CI- (ng).

(b) A particular wastewater standard gave a reading of 1689.6 ng CI-. What total charge in coulombs was required to generate the Ag+ needed to precipitate the sulfide in this standard?
The following results were obtained on 20.00-mL samples containing known amounts of sulfide.17 Each standard was analyzed in triplicate and t~= mass of chloride recorded. Convert each of the chloride results to mass S2- (ng).

(d) Determine the average mass of S2- (ng), the standard deviation, and the % RSD of each standard.

(e) Prepare a plot of the average mass of S2- determined (ng) versus the actual, mass (ng). Determine the slope, the intercept, the standard error, and the R2 value. Comment on the fit of the data to a linear model.

(f) Determine the detection limit (ng) and in parts per million using a k factor of2 (see Equation 1-12).

(g) An unknown wastewater sample gave an average reading of 893.2 ng CI-. What is the mass of sulfide (ng)? If 20.00 mL of the wastewater sample was introduced into the titration vessel, what is the concentration of S2- m parts per million?