Geology 56, Prof. Thomas Johnson, Fall 27 Class Notes: 36: Redox #2 Eh diagrams, and practical applications Reading: White, Section 3...3; Walther Ch. 4 Goals: Recognize that, because many redox reactions contain H + as a reactant or product, these reactions are sensitive to as well as pe/eh and must be included to fully describe the system. Use Eh diagrams as a way of charting the speciation of redoxsensitive elements Review field measurement methods for Eh Recognize that many redox reactions are kinetically sluggish and far from equilibrium at room temperature See how the free energy drop accompanying disequilibrium redox reactions provides energy for life. Why are so many redox reactions sensitive?. Many redox reactions also have H+ in them: a. SO 4 2 + 4H + +2e = H 2 SO 3 + H 2 O b. + 7H + +3e = + 4H 2 O c. H 3 AsO 4 + 2H + +2 e = HAsO 2 + 2H 2 O d. Fe 2 O 3 + 4H + +2e = 2FeOH + + H 2 O e. There is no way to write these reactions without H + or OH. Try it! The reason for this is that changes in the number of oxygens coordinating an element usually changes with changes in valence and, in water, the only way to get rid of O 2 is to make H 2 O. 2. H + is present in most redox reactions, so the equilibria are dependent. 3. So we must consider BOTH Eh and when we study the speciation of elements (protonation/deprotonation AND redox changes). 4. We use Eh diagrams to display the speciation of an element as its valence and the of the solution change:.2 = CrO 2 2 4 6 8 2 4 e. 5. What do the lines on this diagram mean? a. Uppermost line: Water is unstable above this line, so natural solutions can never go above it. This line represents the equilibrium: H 2 O = 2H + + /2 O 2 + 2e. A system in equilibrium with a pure O 2 atmosphere plots on this line. A system in equilibrium with the real atmosphere plots a little below it. b. Lowermost line: Water is unstable below this line, so natural waters never go below it. This line represents the equilibrium: H + + e <==> /2 H 2 c. Vertical lines: Give the values for the equivalence points for deprotonation reactions d. Diagonal lines: Give the and Eh values for the equivalence points for redox reactions f. Consider the line that separates 2 from. At = 4, if we start at Eh = volts and increase Eh, what happens? Do we see an abrupt change from to 2 exactly on the line? NO. The line gives the point at which the activities of and 2 are equal. Below the line, the / 2 activity ratio is > % abundance of species 2% % 8% 6% 4% 2% %.75 5.9 Eh
Above the line, the / 2 activity ratio is < g. Lines between solid phases and dissolved species. These lines represent the conditions at which the solid is in equilibrium with a solution containing the dissolved species with a concentration specified by the person who made the diagram (e.g., 6 molal). The solid phase exists only within the field bounded by this line. Thus, this line is a sharp division between fields; not fuzzy like the one between two aqueous species. However, the line s position depends on the aqueous concentration chosen by the person who made the diagram. Furthermore, the aqueous concentration in equilibrium with the solid decreases with increasing distance away from the line and into the field where the solid is present. If the dissolved concentration in your system is less than that specified by the diagram s maker, you would want to modify the diagram to make the solid field smaller. To summarize all this: When an activity diagram has a boundary between a solid phase and one or more dissolved species, the boundaries of the field where the solid is stable depend on the activity of the dissolved species. 6. How can we measure Eh in real solutions: a. First, recognize that Eh measurement has some serious problems see below. b. To measure Eh, we need an inert conductor to immerse into water, mud, etc. Do not use iron or copper they are reducing agents! Will change the Eh of your system. This is true for many other metals Platinum is the most common electrode material nearly inert. Possible reactions of Pt with O 2 and S(II) cause problems Pt redox occurs 2. Careful cleaning helps Glassy carbon electrodes better (?) Sometimes gold is used c. Connect it to what? Can t bring a standard hydrogen half cell into the field easily. d. Reference cell is often a calomel electrode Hg in contact with Hg + solution (Hg + concentration buffered) +268 mv E relative to the standard H + /H 2 couple e. Another option is the Ag + / Ag couple, E = +222mV 7. Problem # with field Eh measurements: Field Eh probes only respond to rapidly reacting redox species O 2, Fe(II), some others Slow redox reactions (see below) may not affect the probe at all e.g., SO 4 2, Fe(OH) 3(S), NO 3 Bacteria greatly influence redox reaction rates. See more below. 8. Problem #2: Kinetics of redox reactions are often slow and all the theory above applies only to reactions that are in equilibrium. a. Redox reactions involve electron transfers and, often, rearrangement of bonding environments (e.g., coordination changes) of the redoxactive element. b. Sometimes, one of the species reacting is a solid and the reaction is inhibited by the fact that reaction can occur only at the solid s surface and not everywhere within the solution. c. Therefore, it is usually not useful to measure Eh with a Pt electrode and then predict the speciation of all elements in the system based on that Eh. Eh is not as easily used as a master variable in most systems (whereas is a good master variable rapid equilibration). d. However, Eh measurements do have some meaning and can give useful information, especially when values change over time or space. e. Examples: Organic matter is usually a strong reducing agent but it is a slow one: It persists in contact with oxygen in air for years (strong disequilibrium) Organic matter tends to react slowly covalent bonds not easily broken This means the O 2 /O 2 redox couple is not in equilibrium with organic matter/co 2 couples (e.g., acetate/co 2 ).
The presence of high f O2 corresponds to a relatively high Eh, whereas acetate/co2 would give a low Eh. Neither value is very meaningful as a master variable in the system. Similarly, SO 4 2 reduction by microbes + organic matter is slow This means the SO 4 2 /H 2 S couple is not in equilibrium with the organic matter/co 2 couples Nitrate reduction by sulfide is slow The NO 3 /N 2 redox couple is not in equilibrium with the SO 4 2 /H 2 S couple. f. Does this mean Eh diagrams are useless? No. You can use them: To predict speciation for those redox couples that equilibrate rapidly. To determine the speciation that is stable, and toward which slow redox couples will tend to evolve, for various conditions (e.g., water in contact with air). To determine if a redox couple is out of equilibrium. If it is, then: a. The reaction has a drive (ΔG) to run forward or backward b. Microbes might live off it 9. How bacteria and other living things use redox disequilibrium to live: a. If a redox reaction is not in equilibrium, that means it has a nonzero ΔG between the reactants and products. If, somehow, one can harness this energy by speeding up (catalyzing) the reaction and somehow transferring some of the driving energy (ΔG) to some molecule for storage, then one has a way of using chemical energy to live. This is exactly what all living organisms do: They mediate redox reactions (for us, it is glucose + O 2 = CO 2 + H 2 O + energy) and through a complex chain of chemical reactions, they make ATP from the ΔG of the reaction. Here is more detail for those who will use this in their work: Transfer of electrons via enzymes to electron carrier molecules NAD + /NADH is a redox couple (NAD + is the oxidized form; E = +.). Organic matter or some other reduced substance is the electron donor that changes NAD + to NADH These molecules then set up H + concentration gradients that are used to make ATP from ADP ATP is then a stored power source for the cell Ultimately, electrons must be dumped somewhere (permanently) to complete the electron chain O 2, Fe(III), Mn(IV), etc (oxidized species) are the terminal (final) electron acceptors b. Overall: A redox reaction that is not in equilibrium is like a battery. The machinery of the cell provides a way of harnessing the battery s energy, storing it, and then using it to build itself larger, maintain its structures, and reproduce. c. In many cases where a redox reaction has a slow abiotic reaction rate, bacteria provide the main reaction mechanism. If you want to understand the redox speciation, you must understand the metabolism, nutrition, and growth of the microorganisms involved.. We can use individual redox pairs as a way of describing redox conditions in a system: a. Eh electrodes respond to abundant, rapidly equilibrating redox couples ONLY, and often the electrode responds slowly or incompletely. b. A better way to describe the redox state of a system is to measure the concentrations of redox couples for common elements that may be present in large enough abundance to dominate the redox state of the system. If oxygen is present, we simply measure D.O. (dissolved oxygen). o 2H + + /2 O 2 + 2e = H 2 O can be used to calculate a theoretical Eh o More generally, the presence of oxygen indicates oxidizing conditions o Water in equilibrium with the atmosphere starts out with roughly 5 mm (8 mg/l) dissolved O 2 o When measureable D.O. is present, Fe, Mn, Cr, N, S, and other redoxsensitive elements tend to be in their most oxidized form o Low D.O. tells you the system contains reductants such as organic matter, and that Fe, Mn, Cr, N, S, and other redoxsensitive elements might be undergoing reduction If Goethite + dissolved Fe(II) are present: FeO(OH) Fe(II) (aq) couple o Measure Fe(II) concentration o You can calculate Eh to get a quantitative redox indicator, but keep in mind it is not a good master variable
o More generally, the presence of substantial amounts of both Fe(II) and some Fe(III) phase means that the system is moderately reducing (not oxic, but also it has not bee strongly reducing for long enough to exhaust the Fe(III) phase. If MnO 2 and Mn(II) (aq) are present o Same approach: Measure Mn(II) concentration o o Mn(II) presence indicates at least moderately reducing conditions Generally, if both Mn(II) and MnO 2 are present, one can think of the system as mildly reducing If S(II) is present, conditions are strongly reducing Under strongly reducing conditions, H 2 fugacity is sometimes used as a measure of redox state. Redox zones in aquifers, soils and sediments: Even when redox equilibrium is not attained (often), there is still a strong tendency for a sequence of redox zones to be set up. **In almost every system, there is enough organic matter in sediments and aquifers to act as the dominant reducing agent. Bacteria are involved in most or all of these redox reactions. a. You should know where, on the Eh diagram, various environments tend to plot..2 = CrO 2 Oxic waters; young groundwater; many lakes and streams; dry soils Moist soils; groundwater systems with somewhat older water or much organic matter or other reductants (e.g., pyrite) Old and/or very organicrich groundwater Waterlogged, organic rich soils and sediments when left undisturbed for a long time 2 4 6 8 2 4 b. c. Dissolved O 2 zone: Water in equilibrium with the atmosphere has of order 5mM DO (dissolved O 2 ). a. Strong Oxidizing agent: O 2 H 2 O couple is about +2V (at 7., I think) b. So oxygen is then a very good Electron acceptor c. As long as D.O. is present in any appreciable quantity, there is a tendency for reduced species like Fe(II) to be oxidized if they are produced d. So even if Fe(III) reduction and other redox processes occur, the reduced products will not build up greatly d. Mn(IV) reduction zone: As soon as DO becomes extremely low, the occurrence of Mn(IV) reduction leads to a buildup of Mn(II) concentration e. NO 3 reduction also begins to occur shortly after oxygen is essentially gone: Eh = +2 for nitrate reduction to nitrite (which is metastable), Eh = +.74 for reduction to N 2 (stable) f. Fe(III) reduction zone follows, dissolved Fe(II) becomes larger g. SO 2 4 reduction zone follows, dissolved sulfide appears (though often limited by precip. Of sulfide minerals) h. Fermentation (methanogenesis) follows: Bacteria break down organic molecules into smaller ones to gain (a very small amount of) energy. Methans is abundant in this zone ***So instead of giving Eh values as indicators of the redox state of a system, it is perhaps more accurate to say which zone the system is in.**. Practical example: Cr(VI) reduction in groundwater. Mobility of Cr(VI) is very great: Highly soluble, weakly adsorbing. Also highly toxic. Cr(III) is insoluble, strongly adsorbing, and not very toxic In oxic waters, Cr(VI) is stable dangerous In anoxic conditions, if Mn(IV) is present, it can oxidize any Cr(III) present supports Cr(VI) still dangerous In anoxic conditions, if Mn(IV) is gone, Cr(VI) reduction can occur immobilizes Cr
If Fe(III) reduction occurs in an aquifer, Fe(II) released from this reaction reduces Cr(VI) to Cr(III) readily.2 2 CrO 2 2 4 6 8 2 4. Practical Example: Fe(III) vs. Fe(II) in magmas At high T, all redox couples should approach equilibrium with each other Neutral oxide components are convenient for igneous reactions Oxygen fugacity, f O2, used to describe redox state: Fe 2 O 3 = 2FeO + /2O 2 Ferrous iron (Fe(II) or FeO) stabilizes certain minerals olivine, pyroxene Ferric Iron (Fe(III) or Fe 2 O 3 ) stabilizes other minerals magnetite, amphiboles, micas This redox couple dominates and can act as a master redox variable in the mantle and all mafic rocks Variations in f O2 determine the ferric:ferrous ratio and thus control: o the stability of the various minerals that might precipitate out of the magma o the evolution of the magma s composition as mineral precipitate o Magma viscosity and other physical parameters