Chapter 4d 1

pe-pH and Eh-pH Diagrams

The oxidations and reduction of water (to oxygen and hydrogen, respectively)limits the range of redox potentials for a natural water system.

The upper and lower boundary of the water stability field is defined by pe or Ehand pH values for which water is in equilibrium with O2(g) and H(g).

2H2O(l) = O2(g) + 2H2(g)

The equilibrium constant for this reaction is:

K= [O2][H2]2=10-83.1

The partial pressures of each of these gases can not exceed 1. If we set the PO2 to 1atm, then PH2 must be 10-41.65. At PH2=1 atm, PO2=10-83.1.

The upper boundary, where PO2 to 1 atm, is defined by the equation for theoxidation of water:

1/2O2(g) + 2e- + 2H+ = H2O

The Nerst equation for this reaction is:

Eh = Eo + 0.0592/2*log[O2]1/2[H+]2

E0=ªGRo/nF=1.23V

Chapter 4d 2

pe = 20.77-pH

The upper limit of pe is 20.77 at a pH of zero, and 6.77 at a pH of 14.

While the lower boundary is defined by the pe or the Eh and pH values for whichliquid water is in equilibrium with H2(g) at 1 bar pressure.

H+ = e- = 1/2H2(g)

logK = 0

pe=-pH

The lower limit of pe is zero at a pH of zero and -14 at a pH of 14.

Between these two lines are the potential pe-pH possible for a water body.

The higher the pe, the more oxidizing. Lower pe values refer to reducing conditions.

Inside these two lines we can put stability fields of various redox species.

See the example handouts for the stability fields between the iron species hematite,magnetite, and Fe2+ and Fe3+.

Each of the lines represent a Nerst equation of a redox equation (reaction) in whichthe there are equal concentrations of the species on either side of the line.

Note that the boundary between Fe3+ and hematite is a vertical line, independent ofpH.

Inside of the lines are areas in which particular species predominate.

Some of the species are readily soluble (Fe3+, Fe2+), while others are typicallyinsoluble (hematite, magnetite).

From left to right the pH changes. The Soluble species Fe3+, Fe2+ are only occur at

Chapter 4d 3

very low, acidic conditions.

At higher pH, the solid phases are more stable.

pe changes from top to bottom from more oxidizing to reducing. The highest oxidation states are found at the top, and the lowest states at thebottom.

The different lines represented by 10-3 to 10-9 represent concentrations (activities)of Fe3+ or Fe2+ in solution. Note that the activities increase of these dissolvedspecies in equilibrium with the solids hematite and magnetite increases as the pHdecreases.

A similar diagram is shown in Figures 4-3 and 4-4 in your text.

We can use these fields to investigate iron transport in the subsurface.

In a soil with lots of organic matter near the surface, the pH of the infiltratingrainwater is about 5, with an oxidizing pe of about 10. In this environment, iron isleached from the soil as Fe2+. As the water continues to migrate through the soil, itcomes in contact with buffering agents, say calcium carbonate materials, and thepH rises, the water moves into the stability field of Fe(OH)3, and ferrihydrite isdeposited in the soil.

We often can’t measure the pe of a water, but we can attempt to measure the Eh.Therefore, Eh-PH diagrams are used.

Remember that Eh = 0.059 pe at 250C.

In Eh-pH diagrams, the stability field lies between Eh of +1 and Eh of -1 V. Note that the lines on an Eh-pH diagram are based on equilibrium reactions. However, most redox reactions are not at equilibrium due to slow kinetic.

Although we may try to measure the Eh of natural waters, the value we measuremost likely is a mixture of redox couples, say not just Fe species, but also nitrogen,C, Mn, and S. Therefore, it is often difficult to place your water sample with ameasured Eh on a theoretical stability field.

People are moving away from measuring Eh in the field. Other techniques

Chapter 4d 4

involve, measuring the amount of dissolved oxygen or hydrogen gas.

Understanding the redox conditions of natural waters, however, are important inunderstanding its water chemistry, biological activity, and mobility and toxicity ofions.

Figure 4-9 in text shows the Eh-Ph diagram of Nitrogen

This figure was drawn using a PN2=0.77 atm, which is at the Earth’s surface. Notethat in most groundwaters, this often lower.

Under oxidizing conditions (high Eh) NO3- is most stable.

Throughout most of the stability field N2 gas is stable. There is a small range of Eh-pH conditions that NO2

- is stable but this is in the N2stability field. This is because NO2

- is a metastable species (kinetic intermediary)between the reduction of NO3

- and N2 gas. Under reducing conditions and low to moderate pH, NH4+ is stable. Under high pH conditions, NH3 is stable.

Redox-Conditions of Natural Waters

In natural waters, the major oxidizing agent is oxygen, and the major reducingagent is organic matter.

When dead organic matter is introduced into a water system separated from theatmosphere (soil, or groundwater system),first it depletes the available oxygen and the Eh drops, at this point anaerobic conditions predominate, then nitrate (NO3

-), nitrite (NO2-), and then sulfate (SO4

2-) are reduced. If the redox conditions further, then bacterial reduction of carbonate species to CH4 (methanogenesis or methanefermantation) may occur. (See figure 4-12 in your text)

Each one of these steps involves a reduction of organic matter with a drop in pe(see figure 11.12 in Langmuir) and a corresponding change in pH.

The largest change in pe or Eh in a system is due to the loss of oxygen (see figure8-2) in your text.

Chapter 4d 5

Air contains about 286 mg/L of O2 at 250C, about 21% by volume.

At saturation with atm. Oxygen and 1 bar pressure, water holds 8.25 mg/L of O2(2.58 x 10-4 mol/L).

Assuming aerobic decay of organic matter, it takes only 3.1 mg/L of DOC(dissolved organic carbon) to consume all of the 8.25 mg/L of oxygen.

The limiting factor of oxygen supply in the subsurface is its transfer rate. Oxgyen can move through soils at about 1 ft/day. Barometric pressure changes canact to pump air deeper into soil gas.

The diffusion coefficient of oxygen is about 2.05 x 10-1 cm2/s in air and 10-5 cm2/s in water.

The turbulent nature of stream flow can act to aerate stream water.

In stagnant waters or in groundwater, oxygen is replenished mainly by diffusion,which is slow compared to its removal by the reduction of organic matter.

Redox conditions in Groundwater

The redox conditions in groundwater is dependent upon its recharge rate, itsresidence time, the amount of available oxygen, and the amount of decayingorganic matter.

Most shallow groundwaters have short residence times and have some measureableoxygen content and plot near the top of the pe diagram.

Many groundwaters are free of oxygen, may have slightly elevated concentrationsof Mn and Fe, but are potable.

Old groundwaters are often characterized by high concentrations of sulfides makesthese groundwaters not suitable for drinking.

Groundwaters or sediments affected by seawsater also tend to be well below thesulfate reduction level, and the pe is very low. Although the residence times of thesediments and muds are low, the amount of organic matter in them allows for the

Chapter 4d 6

low pe conditions.

In deep or older groundwaters, any remaining organic matter will most likely berefractory (not easily utilized by bacteria), such as coal and redox conditions oftendon’t change in these systems due to biological activity.

Contamination in groundwater can create redox zones from anoxic near the pointof contamination to oxic downgradient.

Typically the wastes are anoxic (such as from the bottom of a landfill), and as theplume moves away from its source it is gradually oxidized by a transfer of oxygenfrom mixing with recharge and unpolluted water.