10,11,12) Redox Potential
SHE half equation
2H+ + 2e- --> H2
What is E?
A measure of whether a solution is oxidising or reducing
What do we use instead of the SHE?
A mercury and mercurous chloride electrode with a E of +0.244v
Joule =
(C x V) The energy required to move electrical charge of 1C through and electrical PD of 1v
What is the slope?
(RT/nF)
What is the redox potential of SHE at pH7?
-0.43v relative to SHE at pH0
Nernst Equation
E = Em + (RT/(nF))ln([ox]/[red])
What is a redox titration?
E measured at different values of oxidised/reduced When 50/50 = Em
Oxidation is...
Loss of electron by a reductant
Reductants have what redox potentials?
NEGATIVE (compared to oxidants)
-n is?
Negative charge and no. of electrons
How do electrons move?
Net negative to net positive
Actual deltaG =
deltaGo' + RT ([products]/[reactants])
Prime
pH7 (H+ at 1 x10^-7 mol/L)
Which direction do electrons flow in the SHE pH7 and pH0 cell?
From pH7 to ph0 (also liberates H+ into pH7 solution as lower [H+] - Equilibrium to the left (forming the e- and H+)
Reduction is..
Gain of electrons by oxidant
What is the issue with Eo'?
At standard conditions, whereas inside a cell concentrations of reactants and products are likely to be different
More negative reactions go..
Backwards
What type of enzymes catalyse redox reactions?
Dehydrogenases (remove and add hydrogens), oxidases, reductases, peroxidases
How can redox control genes?
Electron arrangement means SH groups are allowed to form S=S bonds Also reduction potential of the cell
Actual E =
Eo' - (RT/(nF))ln([red]/[ox])
Redox Potential is..
Excess potential of a redox couple relative to the SHE at the START (after connecting to the half cell, current flows and properties change)
What is F?
Faraday constant is the amount of electrical charge, 96487 C/mol, used to convert E to deltaG
How do you work out PD? (DeltaE)
Flip the sign of the most negative and add them, then you can use the value to work out the free energy (DeltaG)
What are the issues with the SHE?
Hydrogen explosive, hard to store, pH 0 is not physiological
Exam question:
Plot E vs ln[ox]/[red] Find Em (y intercept) Find slope (triangle) and then calculate n (if given temperature) or T (if given n)
How can you calculate Em?
Plot E vs ln[ox]/[red] and Em is the Y INTERCEPT
Example of an oxidant in redox titration?
Potassium ferricyanide
What are reactive oxygen species?
Redox states of oxygen which are powerful oxidants that cause damage - Oxidative stress
Reactions always written as?
Reduction
Eh
Relative to the SHE
What must you do to plot the nernst equation?
Remove the negative and invert the natural log (xo/red) Plot E as a function of % redox couple reduced Sigmoid
When does Em = Eo'
When [ox]=[red]
When do you use E = Em + (2.303RT/(nF))log10([ox]/[red])?
When there are protons involved
Example of a reductant used in redox titrations?
Sodium dithionite and sodium ascorbate
Why is cytochrome c a good example?
The absorbance spectrum of cytochrome c changes strongly due to its haem cofactor iron being oxidised (ferrous 2+ is oxidised to ferric 3+)
The reaction is favourable if..
The deltaG is negative
What is Em?
The midpoint potential, E when [ox]=[red]
What is reduction potential?
The strength of reductant/oxidant (affinity for electrons)
What is the process of a redox titration?
Titrate a solution of a redox couple using small amounts of oxidant or reductant, measuring the potential at each point and estimating the amount oxidised/reduced
