6.5 REDUCTION- OXIDATION POTENTIAL (ELECTRODE METHOD)

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18 — REDOX6.5.3.BINTERPRETATIONA rigorous quantitative interpretation of a measurement of Ehrequires interactive access to an aqueous speciation code. Exercisecaution when interpreting a measured Eh using the Nernstequation. The Nernst equation for the simple half-cell reaction(M I (aq) = MII (aq) + e− ) isEh = E o + 2.303RT/nF log (a II M(aq) / aI M(aq) )where:R = gas constant;T = temperature, in degrees kelvin;n = number of electrons in the half-cell reaction;F = Faraday constant; anda I and M(aq) aII = thermodynamic activities of the free ions M(aq) MI (aq)and M II and not simply the analytical concentrationsof total M in oxidation states I and II,(aq)respectively.Measurements of Eh are used to test and evaluate geochemicalspeciation models, particularly for suboxic and anoxic groundwatersystems. Eh data can be useful for gaining insights on theevolution of water chemistry and for estimating the equilibriumbehavior of multivalent elements relative to pH for an aqueoussystem. Eh can delineate qualitatively strong redox gradients; forexample, those found in stratified lakes and rivers with an anaerobiczone, in an oxidized surface flow that becomes anaerobic afterpassing through stagnant organic-rich systems, and in minedrainagedischarges.Reduction-Oxidation Potential (Electrode Method) U.S. Geological Survey TWRI Book 9 4/98
REDOX —19Contact the instrument manufacturer if the suggestions in table6.5–4 fail to resolve the problem. Check the voltage of the batteries.TROUBLESHOOTING 6.5.4 Always start with good batteries in the instruments and carryspares.Table 6.5–4. Troubleshooting guide for Eh measurement[±, plus or minus; mV, millivolts; emf, electromotive force]SymptomPossible corrective actionEh of ZoBell's solutionCheck meter operation:exceeds theoretical by±5 mV • Use shorting lead to establish meter reading at zero mV.Excessive drift• Check/replace batteries.Erratic performance • Check against backup meter.Poor response whenusing paired electodesCheck electrode operation:• Check that electrode reference solution level is to thefill hole.• Plug questionable reference electrode into referenceelectrode jack and another reference electrode in goodworking order of the same type into the indicatorelectrode jack of the meter; immerse electrodes in apotassium chloride solution, record mV, rinse off andimmerse electrodes in ZoBell's solution. The two mVreadings should be 0 ± 5 mV. If using different electrodes(Ag:AgCl and Hg:HgCl2), reading should be 44 ±5 mV for a good reference electrode.• Polish platinum tip with mild abrasive (crocus cloth,hard eraser, or a 400–600-grit wet/dry Carborundumpaper), rinse thoroughly with deionized water. Use aKimwipe if these abrasives are not available.• Drain and refill reference electrolyte chamber.• Disconnect reference electrode. Drain and refill electrolytechamber with correct filling solution. Wipe offconnectors on electrode and meter. Use backupelectrode to check the emf.• Read emf with fresh aliquot of ZoBell’s solution;prepare fresh ZoBell's solution if possible.• Recondition electrode by cleaning with aqua regia andrenewing filling solution—this is a last resort.U.S. Geological Survey TWRI Book 9 4/98Reduction-Oxidation Potential (Electrode Method)
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6.5 REDUCTION- OXIDATION POTENTIAL (ELECTRODE METHOD)

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