Chapter 24, Problem 24.13QAP

Sulfide ion (S^2- ) is formed in wastewater by the action of an aerobic 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 $\to$ Ag⁺ + e^-. The titration reaction is S^2- + 2Ag⁺ $\to$ Ag₂S(s).
(a) A digital chloridometer was used to determine the mass of sulfide in a wastewater sample. The chloridometer reads out directly in ng Cl^-.In chloride determinations, the same generator reaction is used,but the titration reaction is Cl^- + Ag⁺ $\to$ AgCI(s). Derive an equation that relates the desired quantity, mass S^2- (ng), to the chloridometer readout in mass Cl^- (ng).
(b) A particular wastewater standard gave a reading of 1689.6 ng Cl^-. What total charge in coulombs was required to generate the Ag⁺ needed to precipitate the sulfide in this standard?
(c) The following results were obtained on 20.00-mL samples containing known amounts of sulfide.17
Each standard was analyzed in triplicate and the mass of chloride recorded. Convert each of the chloride results to mass S^2- (ng).

(d) Determine the average mass of S^2- (ng), the standard deviation, and the %RSD) of each standard.
(e) Prepare a plot ofthe average mass of S^2- determined (ng) versus the actual mass (ng). Determine theslope, 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 of 2 (see Equation 1-12).
(g) An unknown wastewater sample gave an average reading of 893.2 ng Cl. 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 S^2- n parts per million?