# Convert Eh (mV) to pe – Geochemistry Tips!

Convert Eh (mV) to pe – Geochemistry Tips!

Well, we are going to do nothing fancy. We will be converting our field Eh (mV) measurements to pe. I just used simple formula and make the spread sheet in excel that I use for my own quick reference. If you have lost your calculator or could not remember the value of “F” or “R” then my spreadsheet might be handy for you. Feel free to use it and distribute with your friends.

Eh to pe
Simply change the Eh value in the first column. That will do. Rest has been set up for you!

Quick Background on Eh (reference)

Eh, or redox potential, is the electrochemical potential of a solution relative to the standard hydrogen electrode. The standard hydrogen electrode is a fictive solution with a hydrogen ion (H+)activity of 1 molal at equilibrium with 1 Atmosphere of H2 at 25 degrees C. In practice, the potential is usually measured by a platinum electrode relative to a reference electrode is measured and converted to the appropriate value.

The Eh and pH of a solution are related. For a half-cell equation (conventionally written as reduction, or with electrons on the right side):

aA + bB + n e- + h H+ = cC + dD

The half-cell standard potential Eo is given by:

Eo (volts) = -Delta G/nF

where Delta G is the Gibbs free energy change, n is the number of electrons involved, and F is Faradays Constant. The Nernst Equation relates pH and Eh:

Eh = Eo + (0.059/n) x log {([A]^a [B]^b) / ([C]^c [D]^d)} – (0.059 h/n) pH

where square brackets indicate activities and exponents are shown in the conventional manner (using ^). This equation is the equation of a straight line for Eh as a function of pH with a slope of -0.059h/n volt (pH has no units.) This equation predicts lower Eh at higher pH values – This is observed for reduction of O2 to OH- and for reduction of H+ to H2. If H+ were on the opposite side of the equation from H+, the slope of the line would be reversed (higher Eh at higher pH). An example of that would be the formation of magnetite (Fe3O4) from HFeO2-(aq) (Garrels and Christ):

3 HFeO2- + H+ = Fe3O4 + 2 H2O + 2 e-

where Eh = -1.1819 – 0.0885 log[HFeO2-] + 0.0295 pH. Note that the slope of the line is -1/2 the -0.059 value above, since h/n = -1/2.

Half-cell equations can be combined if one is reversed to an oxidation in a manner that cancels out the electrons to obtain an equation without electrons in it.

Eh-pH (Pourbaix) diagrams are commonly used in mining and geology for assessment of the stability fields of minerals and dissolved species (See Eh (geology) for a very limited discussion.) Under conditions where a mineral (solid) phase is the most stable form of an element, these diagrams show that mineral. As with results from all thermodynamic (equilibrium) evaluations, these diagrams should be used with caution. Although teh formation of a miineral or its dissolution) may be predicted to occur under a set of conditions, the process may be negligible becaus its rate is so slow. Under those circumstances, kinetic evaluations are necessary. However, the equilibrium conbdiutions can be used to ebaluate the direction of spontaneous changes and the magnitude of the driving force behind them.

Temperature and pressure can affect Eh values. Temperature changes of a few degrees typically do not affect Eh – pH diagrams significantly. Changes from increases in pressures of the order of tens of atmospheres are generally small when gaseous reactants are involved. Chapter 9 of Garrels and Christ discusses the effects of temperature changes on geochemical equilibria.

Editor

Ankan Basu is a Certified Professional Geologist (CPG) with 10+ years of experience in the field of geology, hydrogeology and geochemistry.