5.5 HOW DO YOU MEASURE REDOX?
Redox potential (Eh) is commonly measured through the use of a reference electrode (calomel) plus a platinum electrode. Redox measurements are reported in volts or millivolts. Values greater than zero represent the oxidized state, whereas values less than zero represent the reduced state. The range of redox values (Eh) is determined by the points at which water is oxidized or reduced (it is pH dependent). Calibration of a potentiometer to measure the Eh of an unknown (e.g., soil sample) can be made by using reagents of known redox potential. First, attach a platinum (Pt) electrode to the plus terminal (in place of the glass electrode) of a pH meter with a millivolt scale and a saturated calomel electrode to the negative terminal. Second, lower the calomel and Pt electrodes into a pH 4 suspension of quinhydrone in 0.1 M K acid phthalate solution and adjust the potentiometer to read +213 mY. This reading is equivalent to an Eh of 463 mV or pe of7.85 at 25°C (Bartlett, 1981).
A more extensive calibration procedure involves a mUltipoint approach using quinhydrone (Q2H2)' which dissolves congruently, giving equimolar quantities of quinone (Q) and hydroquinone (Q2H)
The half-cell reaction, as shown at the beginning of this chapter, is Q + 2H+ + 2e-¢::} QH2
~,H2Q = 0.70 V The half-cell Nernst expression is
Since (H2Q) = (Q)
Eh = 0.70 - (0.059/2)log{(1)/(H+)2}
or
Eh = 0.70 - 0.059 pH
(5.66)
(5.67)
(5.68)
(5.69)
(5.70) A plot of pH versus Eh will produce a straight line with a slope of -0.059 and a y intercept of ~.~Q = 0.70 V. The relationship between pH and Eh is given for a number of reagent-grade couples in Figure 5.11. However, because the Eh instrument also
254
>
E
o W
REDOX CHEMISTRY
200~--~1-~~~----~--~r-~----+---~----4
-100r---+---~4_----~~--~~--~~~_+----~
-200r---+---~~--~--~~~~~----~r_--~
-300~--~r---~----_+~~~--~ct_----~--~
~OO~----r_----~----~----~--~--~~~--~
-500~--~----~----~---L--~~----~--~
0 2 4 6 8 10 12 14
pH
Figure 5.11. Relationship between pH and Eh for various redox couples (adapted from Kokholm,1981).
5.5 HOW DO YOU MEASURE REDOX? 255 contains a reference electrode (E;ef' calomel), it has to be included in the equation given above:
Ehmeas = E,Jef + 0.70 - 0.059 pH (5.71)
and, therefore, Eh actual (Ehaet) is given by
(5.72) where for the calomel electrode at 25°C, E;ef = 244.4 mY. To generate the calibration equation, one may take quinhydrone (Q2H2) and dissolve it in buffers of 9-2 pH. The Eh measurements ofthe various pH-buffered, redox-coupled (Q,Q2H) solutions should be in agreement with those of hydroquinone (Q2H2) in Figure 5.11. If they are not, potentiometer adjustments should be made to read appropriately.
5.5.1 Redox in Soils
Redox (Eh) measurements are based on sound scientific theory. Yet their accuracy is questionable because (1) the electrodes could react with gases such as 02 or H2S and form coatings of oxides or sulfides, (2) Eh measurements in soil most often represent mixed potentials owing to the heterogeneity of soil, (3) the degree and variability of soil moisture influences Eh, and (4) soil spatial variability is inherently very large.
Redox reactions in soil could also be affected by specific localized electron-transfer effects and one may find it difficult to assign absolute meaning to a given Eh measurement. Nevertheless, field Eh measurements provide an excellent tool for detecting relative redox changes in the environment as a function of varying conditions (e.g., flooding).
Commonly, soils vary in Eh from approximately 800 m V under well-oxidized conditions, to -500 mVunder strongly reducing conditions (Table 5.7; Figs. 5.12 and 5.13). The Eh values in a soil appear to be related to the redox reactions controlling it.
A number of such reactions are shown in Table 5.8. It appears that a particular redox reaction with a given Keq produces a given Eh plateau, which is referred to as poise (Fig. 5.14). Poise relates to Eh as buffer capacity relates to pH. It is defined as the potential of a soil to resist Eh changes during addition or removal of electrons. This occurs because Eh is related to ratios of oxidized species to reduced species.
In soils, two major sources/sinks for electrons are 02 and plant organic residues.
Generally speaking, 02 acts as an electron acceptor in soils, while plant organic residues act as electron donors. In the case of 02' the half-reaction is described by Equation 5.1, while the half-reaction of organic residue (e.g., sugar) is described by Equation 5.2.
Neither of these reactions is reversible. For this reason, under well-aerated conditions, 02 buffers the soil against reduction and plant organic residue buffers the soil against oxidation (Bartlett, 1981). Soil flooding is a mechanism by which 02 is excluded from this soil, which allows the soil to be reduced, owing to the presence of organic residues, an electron source. During the process of reduction, however, the various inorganic
256
>
- E
REDOX CHEMISTRY
1400r---__________________________ ~
1200
1000
800
600
400
ã200
o
pH = 6.5
HC03 - = 10-2.37 M
10-6 M. Mn
pH
Limits of Eh-pH for soils
14
Figure 5.12. Relationship between pH and Eh in soils (from Blanchar and Marshall, 1981, with permission).
5.5 HOW DO YOU MEASURE REDOX? 257
+1000
""Y. . .
+800 '. ' . •
+600 +400
.<=
+200
LLJ
0 -200 -400 -600
o 2 4 6 8 10 12
pH
Figure 5.13. Relationship between pH and Eh for soils (from Baas Becking et aI., 1960, with permission).
15
10
Cl> 5
a.
o
-5
-
f-
f-
-'.
"-
I I I I
02 - H2O
- Mn02'" Mn2•
Fe203 - Fe2•
SO~ - H2S
I I I
Fermentation
t
Amount of Organic Matter Consumed (Electron Source)
- - -
- -
0.885
0.59
-~ o
0.295<::"
..c W
o
-<:l.295
Figure 5.14. Change in pe for various redox couples as a function of organic matter (noted on a relative scale) decomposed (adapted from Drever, 1982, with permission).
258 REDOX CHEMISTRY TABLE 5.7. Range of Eh Measurements of Soil-Water Systems
Very well-oxidized soil Well-oxidized soil Poorly oxidized soil Much-reduced soil Extremely reduced soil
800mV 500mV 100mV -200 mV -500mV
mineral redox couples come into play, introducing poise (Fig. 5.14), which resists rapid Eh changes. The reverse is observed when Eh goes up during reoxygenation of soil.
Redox reactions in soils are affected by a number of parameters, including tempera- ture, pH (see Chapter 7), and microbes. Microbes catalyze many redox reactions in soils and use a variety of compounds as electron acceptors or electron donors. For example, aerobic heterotrophic soil bacteria may metabolize readily available organic carbon using NO;-, NO;: , N20, Mn-oxides, Fe-oxides and compounds such as arsenate (AsO~-) and selenate (SeO~-) as electron acceptors. Similarly, microbes may use reduced compounds or ions as electron donors, for example, NH4, Mn2+, Fe2+, arsenite (AsO;:), and selenite (SeO~-).
Table 5.8. Principal Electron Acceptors in Soils, Eh of These Half-Reactions at pH 7, and Measured Potentials of These Reactions
Reaction
O2 Disappearance
1/2 O2 + 2e- + 2H+ = H20 NO} Disappearance
NO} +2e-+2W=NOz +HzO Mn2+ Formation
Mn02 + 2e- + 4H+ = Mn2+ + 2H20 Fe2+ Formation
FeOOH + e- + 3H+ = Fe2+ + 2H20 HS- Formation
So,; + 9W + 6e-= HS- + 4HzO H2 Formation
H+ + e-= 1/2H2
CH4 Formation (example of fermentation) (CHzO)n = nl2 CO2 + nl2 CH4
Measured Redox Eh pH 7 (V) Potential in Soils (V)
0.82 0.6 to 0.4
0.54 0.5 to 0.2
0.4 0.4 to 0.1
0.17 0.3 to 0.1 -0.16 o to -0.15 -0.41 -0.15 to -0.22
-0.15 to -0.22
PROBLEMS AND QUESTIONS 259 PROBLEMS AND QUESTIONS
1. Explain the role of frontier electron orbital configuration on environmental chemistry.
2. Explain, using equations, how redox chemistry affects mineral solubility.
3. The equation Eh = O.77pe'+p 2+ + 0.059{log[(Fe3+)/(Fe2+)]} relates Eh (V) to the proportionality of Fe3+ to F~i+. Assuming that the total concentration of iron (Fe3+ plus Fe2+) is 10-3 M, calculate the concentrations of Fe3+ and Fe2+, when activities equal concentrations, for Eh of (a) 0.77 V, (b) 0.40 V, (c) 0.10 V, (d) 1.00 V, and (e) 1.20 V.
4. Consider all the parameters given in problem 3, with the only difference that activities are not equal concentrations (as expected). Calculate the concentrations of Fe3+ and Fe2+ assuming YPe'+ = 0.3 and YPe2+ = 0.6 at Eh of (a) 0.77 V, (b) 0.40 V, (c) 0.10 V, (d) 1.00 V, and (e) 1.20 V [hint: see Chapter 2 about the use ofYi (i = any ion) in equilibria-type equations]. What can you conclude about the role of ion activity in redox systems?
5. Based on the equation Eh = O.77FeJ+ Pe2+ + 0.059 {log[(Fe3+)/(Fe2+)]}, calculate Eh at the following Fe3+ to Fe2+ ratios:' (a) 0.05, (b) 0.1, (c) 0.4, (d) 0.7, and (e) 0.95.
6. What does it mean when the stability redox line of a couple is beyond the upper or lower stability limits of water?
7. The redox equation describing the transformation ofN a+ to N a metal is as follows:
Na+ + e- <=> Na(metal), J!l = -2.71 or log Keq = -47. Produce the equation that relates Eh to Na+/Na(metal) and explain if one could find Na(metal) in soil.
8. The redox equations describing the H2S-S0~-couple and the Mn02-Mn2+ cou- ple are as follows: SO~-+ lOH+ + 8e-= H2S + 4H20 and MnOis) + 4H+ + 2e =
Mn2+ + 2H20. Produce the necessary redox constants by assuming that Mn2+ =
10-3 M, SO~-= 10-5 M, and H 2S = 10-2 M and plot the corresponding stability lines in terms of Eh-pH. Explain whether Mn02 could oxidize H2S to SO~-in the absence of02.
9. A soil contains the mineral Mn02 in rather large quantities; someone decides to dispose FeCl2 in this soil. Based on redox reactions and some assumed concen- trations of Mn2+, Fe2+, and Fe3+ one may find in a normal soil, explain what type of reactions would take place and what type of products these reactions would produce.
10. Propose a scheme using redox chemistry for the effective removal of N03 from soil.
6 Pyrite Oxidation Chemistry