A paper in the Journal of Chemical Education describes an experimental method for determining the concentrations of both insoluble metal sulfides, such as FeS2, and gaseous H2S in anaerobic sediments. Samples of sediment are placed in a flask along with 100 mL of water. After agitating the sediment using a magnetic stir bar, nitrogen gas is bubbled through the flask and the N2 and any gaseous H2S are swept into a second flask, which contains 50.00 mL of a basic buffer that converts the H2S to S2-. The concentration of S2- in this second flask is then determined using a 2- ion-selective electrode. Next, to release the sulfide from the insoluble metal sulfides, 50 mL of concentrated HNO3 is added to the first flask. The H2S that forms is, once again, swept into the second flask using N2 where its concentration is determined with the same ion-selective electrode.
(a) To calibrate the apparatus a standard solution of Na2S is placed in flask A and acidified with HNO3. The resulting H2S is swept into flask B with N2 and the potential of the ion-selective electrode is measured. The author’s report that the electrode’s response is
E = –0.6535 – 0.0323log[S2-]
When a 34.88-g sample of sediment is first placed in flask A, the potential of the ion-selective electrode is –0.7193 volts. How many μmol of H2S are present in this sample of sediment as a gas?
(b) After acidifying the sample with HNO3, the potential of the ion-selective electrode changes to –0.7302 volts. How many μg of insoluble sulfide are present in this sample of sediment, assuming that it is present as FeS2?
(c) What is the total concentration of sulfide (that present as H2S and that present as insoluble sulfides) in the sediment sample expressed as μg H2S/g sediment?
(d) The purpose of the N2 is to extract the H2S (although H2S is a gas it has some solubility in water) from the solution in flask A. It is unlikely that the extraction is 100% efficient. Would the failure to extract all H2S lead to a determinate error? Explain.
(e) The procedure indicates that 100 mL of H2O is placed in flask A, while 50.00 mL of buffer is placed in flask B. Note the difference in significant figures. Why does the volume in flask B need to be measured more precisely?
(f) Hydrogen sulfide is a diprotic weak acid with Ka1 = 9.5 x 10-8 and Ka2 = 1.3 x 10‑14. The solution in flask B is prepared by adding 20 g of NaOH to 1 liter of water. What is the pH of this solution and why was it chosen?
(g) The theoretical response of a sulfide ion-selective electrode is
E = K – 0.02958log[S2-]
while the author’s report the response as
E = –0.6533 – 0.0323log[S2-]
Does the disagreement between the theoretical and experimental slope lead to a determinate error? Explain.