5. Using Nernst Equation to calculate redox potentials; becoming familiar with environmental redox data. It is often noted in natural systems at ambient temperatures and pressures that redox couples are out of equilibrium. One way to assess whether or not equilibrium has been achieved is to calculate electrochemical potentials for several individual redox couples based on measured concentrations and see how they compare with each other. The potentials calculated from measured concentrations can also be compared with other measurements of water redox state, such as dissolved oxygen or Eh electrode measurements. The table below lists some measured concentrations of species in waters from an acid mine site (assume T = 25C and activitymolarity). Use this data and the standard electrode potentials (E) to answer the following (as always, show your work): Species Concentration (mg/L) Fe2+ 37.5 Fe3+ 2.5 HUS SO42- 3500 O2(aq) 9 pH 4.0 a What is the Eh of the system based on the measured concentrations of Fet and Fet? Use the Nernst equation to solve for Eh. b. A dissolved oxygen meter can be used to measure directly (using an electrode) the concentration of O2(aq) (reported in mg L-') in water. Using the measured dissolved O2(aq) concentration of the acid mine water and the measured pH, calculate the Eh. Assume that the standard potential for this half-reaction is: 2H,0=02(aq) + 4H+ + 4e E = 1.23 V Use the Nernst equation to solve for Eh. c. Sulfate (SO4) was measured for this site, but H2S was not. Based on your calculations of Eh above (you should have two different determinations of Eh), use the Nernst equation to estimate the equilibrium dissolved H2S(aq) concentration. To solve this, use the reaction: H2S(aq) + 4H20 = SO42- + 10H+ + 8e- E = +0.303 V Would you expect to be able to measure the equilibrium dissolved concentration of H2S(aq)? d Discuss why, in many natural waters, measurements of dissolved redox species generally give lower oxidation potentials than expected from equilibration of atmospheric O2(g) with water