2.3.7. Environmental Applications of Redox Measurements
2.3.7.3. Applications to Sterilization. Checking the potency of disinfecting regents represents another application where the monitoring of redox potential is beneficial
Strong oxidizing agents such as hypochlorite or ozone are commonly used to kill bacteria in food industry and healthcare facilities. For example, fruit and vegetable crops are frequently washed after harvesting in order to remove pesticides and kill pathogens, such as Salmonella [27]. The right level of hypochlorite is best determined by theEH and pH of the solution rather than the total concentration of chlorine. Too much hypochlorite is actually less effective. Agronomists recommend recycling the rinse water and adding fresh hypochlorite as needed to maintain the redox potential between+0.7 and+0.65 V at a pH level of 6.5. Under those conditions, HOCl kills Salmonella and many other microbes within 30 seconds [27].
k k
NADP+ NADPH
O– O
O
O
NH2 N+ O
O
P
–O O
OH OH
NH2 N N N N
O O
OH O
O O
OH OH
O
O O
O
O
OH O
O P
–O OH
N
N N
N H H O
N
NH2
NH2 O–
P P
–O
O P
–O OH
P
Eo′(pH = 7.0) = –0.320 V[9]
Ubiquinone Ubiquinol
O
O O
OH 10
H O OH
O
O H
10
Eo′(pH = 7.0) = 0.045 V[9]
Glutathione, GS Diglutathione, GSSG NH+ 3 O
O
O O–
O O O
O O
–O O
O O
–O
O– O O–
S S –O
H N
H H N
N N H
N H
N H NH3 NH3 +
+
SH
Eo′(pH = 7.0) = –0.24 V[24]
Ascorbic acid Dehydroascorbic acid (reduced)
HO HO
HO HO
HO OH
H O H
O O O
Eo′(pH = 7.0) = 0.47 V[26]
(oxidized)
R-group Reduced form (FADH2)
Oxidized form (FAD) Flavin adenine
Dinucleotide
NH HN R
N N
N NH O
R
NH N O
O O
OH
OH OH
HO
HO
O N N
N N NH2 OO
O
O O
OH OH P P
Eo′(pH = 7.0) = –0.219 V[9]
FIGURE 2.19 Some important redox agents in biochemical reactions. NADP+/NADPH, ubiquinone/
ubiquinol, and FAD are important enzyme cofactors that are recycled in many different metabolic pro- cesses. Ascorbic acid (vitamin C) is a cofactor for a few enzyme reactions, but is very important as a reducing agent that scavenges highly reactive oxidizing species that are often by-products of enzy- matic and nonenzymatic reactions [24, 25]. Glutathione is also an important scavenger of reactive oxidizing species. It is present in cells at a relatively high concentration of 1–15 mM [26].
k k
Anode Cathode Membrane
e– ORP
IC
ORP IC
O2 N2
(a) (b) (c)
FIGURE 2.20 Methods for controlling the redox potential of growth media in fermentation baths.
(a) Direct application of a voltage from an external power source using electrodes. (b) Controlled addi- tions of separate oxidizing or reducing reagents. (c) The direct addition of O2 or its removal using N2. Source: Chen-Guang Liu et al., 2017 [23]. https://www.intechopen.com/books/fermentation- processes/fermentation-and-redox-potential. Licenced under CC BY 3.0.
PROBLEMS
2.1 Calculate the reference electrode potential for a Ag/AgCl electrode in a solution with a 0.10 M KCl activity.
2.2 Is H2O2 a more powerful oxidizing agent in an acid solution or a basic solution?
Justify your answer with a quantitative argument.
2.3 If a null-point potential measurement device has a current meter capable of detect- ing a microamp of current, what is the maximum resistance that it can have in order to keep the error in the voltage (between point A and point B in Figure 2.1)
≤0.05 mV?
2.4 The limit to the precision of the potential measured by a null-point apparatus (as in Figure 2.1) is associated with the precision of the variable resistor used to adjust the voltage at point A. Precision variable resistors are commonly available with a rotary control that moves the sliding contact from the bottom position to the other extreme in 10 turns. Assuming that the dial on the rotary control can discriminate between settings that are 1∘ apart and assuming that the voltage on the battery is 1.5000 V, what is the uncertainty in volts in the voltage provided at point A?
2.5 What is the difference in mV in the voltage that one would observe for a Ag/AgCl reference electrode prepared using 0.50 M chloride concentration compared to 0.50 M chloride activity? (See Appendix A for help.)
2.6 (a) Write the Nernst equation for the half reaction for the reduction of permanganate ion to MnO2.
MnO4−+4H++3e−⇄MnO2+2H2O E∘ =1.70 (b) Calculate the formal potential for this half reaction at pH=7.0.
2.7 Ferric ion is not very soluble at neutral pH. (TheKsp for Fe(OH)3 is∼3×10−38.) Yet evidence of iron is often observed in water pumped directly from simple,
k k hand-operated pumps in rural locations. (It is noticeable in the taste of the water
and in the dark reddish-orange stain on the concrete support where water from the pump splashes.) Use the concept ofEHto describe the mechanism that solubilizes iron in the water deep in the ground water and leads to the rust color on the concrete base at the surface.
2.8 Consider the titration of samples solutions of∼0.1 M Sn2+in 0.5 M H2SO4solution using 0.1 M Cr2O72−titrant in 0.5 M H2SO4.
(a) What would the equivalence point potential be (neglecting the effect of activity coefficients and any junction potential)?
(b) What would the equivalence point potential be if the sulfuric acid concentra- tion were 0.05 M and all other conditions were the same (neglecting the effect of activity coefficients and any junction potential)?
2.9 Temperature affects a reference electrode in several ways. Consider a drop in tem- perature from 25 to 20∘C for a saturated calomel electrode (SCE). Explain the differ- ent mechanisms that lead to a change in the electrode’s potential. (Can you describe four different influences?)
2.10 Consider the reduction of hydroxyl amine in an acid solution:
NH3OH++2H++2e− ⇄NH4++H2O Eo=1.35 V (a) Write the Nernst equation for the half reaction.
(b) Calculate the formal potential for the same half reaction at a pH of 2 and assum- ing unit activity coefficients.
2.11 If a cell potential was measured to be+0.150 V using a saturated calomel reference electrode, what would the measured potential be if the reference electrode were replaced by a Ag/AgCl electrode in a 0.10 M KCl solution? (Assume that the junction potential is negligible in both cases.)
2.12 What potential would be expected for a cell with a platinum indicator electrode in an aqueous solution of 1.65 mM Br2, 0.1 M HCl, and 0.753 mM NaBrO3 versus a saturated calomel electrode (SCE) (neglecting activity coefficients and junction potentials)? See Appendix D for the appropriate standard electrode potential.
2.13 What would be expected for a cell with a platinum indicator electrode in an aqueous solution of 1.65 mM Br2 and 0.753 mM NaBrO3 versus a saturated calomel elec- trode (SCE) accounting for activity coefficients in a supporting electrolyte solution of 0.100 M HCl (neglecting the junction potentials)?
2.14 Consider the salt bridge for a Ag/AgCl reference electrode with a 1.0 M KCl inter- nal solution dipping into sample solution with a 1.0 M NaCl supporting electrolyte solution. Explain the effect of the Na+ion, K+ion, and Cl−ion on the junction poten- tial.
2.15 Using the Henderson equation and table of diffusion coefficients in Appendix C, calculate the junction potential for a salt bridge with 3 M KCl solution contacting a sample solution with 0.1 M NaOH.
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