05-Redox-FUNDAMENTALS OF AQUATIC CHEMISTRY.ppt

Groundwater pollution and remediationRedox Yilian Li OXIDATION-REDUCTION REACTIONS EQUILIBRIA DESCRIBED BY THE NERNST EQUATION Where: E0 = Standard potential for the reaction R = Universal gas constant = 8.314*10-3 kJ/(mol·K) T = Temperature in Kelvin n = Number of electrons transferred (n = 2 equiv/mole) F = Faraday constant = 96.42 kJ/(V·equiv) MEASURMENT OF OXIDATION-REDUCTION POTENTIAL NEED A BENCH MARK · Reference with respect to the H2 : H+ couple · Where PH2 = 1atm [H+] = 1 e.g. Measurement of Fe3+ : Fe2+ couple OXIDATION REDUCTION POTENTIAL - Eh · Eh: A measure of the quantity of oxidized species versus reduces species · Applicable only to rapidly reacting oxidation-reduction couples · Examples: Fe2+ → Fe3+ Mn2+ → Mn4+ · Not applicable to slow reacting oxidation-reduction couples · Examples: S2- → SO42- CH4 → CO2 REACTION RATES ARE VERY IMPORTANT VALUE OF MEASUREMENTS MADE WITH Eh ELECTRODES To use Eh measurements to describe metal speciation · Redox couple must be at equilibrium · Redox species must be electroactive at Pt electrode · Both sides of the redox couple must be present in significant concentrations AGREEMENT BETWEEN EhMEASUREMENTS AND CALCULATED Eh Electrode measurements of limited values for many systems e.g. O2 : H2O; CO2 : CH4; HS- : SO42- Eh AT EQUILIBRIUM · Eh decreases as the amount of oxidized species decreases EXAMPLE: What is Fe speciation? FeT = 10-1 molal Eh = 150 Mv E0(Fe2+/Fe3+) = +0.77V DEFINITION OF pe Also define pe where: Where: F = Faraday constant R = Universal gas co