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~~~ PERIODIC TABLE OF THE ELEMENTS ~~~
Note that elements towards the left possessing a few
electrons in higher energy orbitals tend to be
electron donors, whereas those towards the right
possessing nearly complete orbitals tend to be
electron acceptors or formers of strong covalent bonds.
~~~ CHARACTERISTICS OF THE PROGRESSIVE ADDITION ~~~
OF OXYGEN ATOMS TO THE SAME CORE ELEMENT
increased valence/oxidation state of core element
increased oxidant strength as electron acceptor
increased tendency to donate oxygen atoms
increased acidity of hydrated oxides
some disproportionate in acid solution
many are medicinal at low dosages
many are toxic especially at high dosages
~~~ EXAMPLES OF SERIES OF COMPOUNDS HAVING THE SAME ~~~
CORE ELEMENT IN VARIOUS OXIDATION STATES
Chlorine: HCl Cl2 Cl2O ClO2 Cl2O6 Cl2O7
NaClO NaClO2 NaClO3 NaClO4
Sulfur: H2S S8 SO2 H2SO3 SO3 H2SO4 H2SO5 S2O
H2S2O3 Na2S2O4 NaS2O5 H2S2O6 H2S2O7 H2S2O8
Selenium: H2Se Se H2Se2O3 SeO2 H2SeO3 SeO3 H2SeO4
Nitrogen: H3N H4N2 N2 N2O NO N2O3 NO2 N2O4 N2O5
H2N2O2 NaNO2 HNO3 HOONO
Carbon: H4C C CO CO2
Alkanes: RCH3 RCH2OH RCHO RCO(OH) RCO(OOH)
Manganese: Mn Mn(OH)2 Mn2O3 MnO2 K2MnO4 KMnO4
Chromium: Cr Cr++ Cr2O3 CrO3 K2CrO4 K2Cr2O7
Molybdenum: Mo MoO2 Mo2O5 MoO3 Na2MoO4
~~~ DEFINITIONS PERTAINING TO OXIDATION ~~~
oxy- or oxi- comes from the Greek word meaning to bite
= to take away or remove electron(s)
= to take away or remove hydrogen atom(s)
= to abstract electron(s) or hydrogen atom(s)
= to dehydrogenate
= to increase the valence
= to add atom(s) of oxygen
= any agent which takes away electron(s)
= any agent which abstracts hydrogen atom(s)
= the process of removing electron(s)
= the process of removing hydrogen atoms(s)
~~~ DEFINITIONS PERTAINING TO REDUCTION ~~~
= to decrease in size or volume
= to concentrate by boiling or evaporation
= to decrease the gap of a fracture
= to produce metal from ore
It was subsequently discovered that the donation of electrons
to ores containing cations of certain elements produced metals.
The definition of "REDUCE" was therefore expanded to mean:
= to donate or add electron(s) or hydrogen atom(s)
= to decrease the valence
= any agent which donates or adds electron(s)
= any agent which donates or adds hydrogen atom(s)
= the process of donating of adding electron(s)
= the process of adding hydrogen or hydrogenation
~~~ DEFINITIONS PERTAINING TO REDOX ~~~
"REDOX" is a contraction of the words reduction and oxidation.
The REDUCTANT donates electron(s) to the OXIDANT
which accepts them.
Such reactions are referred to as oxidation/reductions
or "REDOX REACTIONS" for short.
The reacting donor and acceptor combination considered together
are referred to as a "REDOX COUPLE".
Because the reductant/donor looses electrons in the process,
it is said to be "OXIDIZED".
Because the oxidant/acceptor gains electrons in the process,
it is said to be "REDUCED".
Thus the reductant reduces the oxidant and is oxidized,
while the oxidant oxidizes the reductant and is reduced.
~~~ HALF REACTIONS ~~~
"HALF REACTIONS" are a means to understand
and describe seperately the donating
and accepting aspects of redox reactions.
The following denotes a redox reaction involving
the transfer of one electron:
D + A ---> D+ + A-
This can be seperated into two component reactions.
1) D ---> D+ + e-
2) A + e- ---> A-
The tendency of D to denote can be measured in volts.
The tendency of A to accept can be measured in volts.
Such measurements for various elements and molecules
are published in tables.
~~~ REDOX TABLES ~~~
Redox tables list and describe numerous half-reactions.
These are set in order according to their standard potentials.
The hydrogen oxidation reaction is set at zero
for comparison purposes.
Therefore it always appears in the middle of the table.
Elements towards the left of the periodic table tend
to appear as stronger reductants in the redox tables.
Elements towards the right of the periodic table tend
to appear as stronger oxidants in the redox tables.
In most cases elements in the same column of the periodic
table behave as stronger oxidants as the atomic
They behave as stronger reductants as the atomic number
For most redox active agents, as the acidity level of the
solution increases, their strength as oxidants
Elements capable of binding oxygen atoms form complexes
of higher oxidant strength as the valence increases.
~~~ EXAMPLE OF A REDOX TABLE ~~~
|Reducing Reactants|| || Products ||Voltages|
|acetate- + 2e- + 2H+ || ---> || acetaldehyde || -0.58 |
|alpha-ketoglutarate2- + CO2 + 2e- + H+ || ---> || isocitrate3- || -0.38 |
|NADP+ + 2e- + H+ || ---> || NADPH || -0.324 |
|NAD+ + 2e- + H+ || ---> || NADH || -0.321 |
|1,3-bis-P-glycerate4- + 2e- + 2H+ || ---> || glyceraldehyde-3-p2- + Pi2- || -0.29 |
|lipoic acid + 2e- + 2H+ || ---> || dihydrolipoic acid || -0.29 |
|3-ketoacyl-SCoA + 2e- + 2H+ || ---> || 3-hydroxyacyl-SCoA || -0.238 |
|pyruvate- + 2e- + 2H+ || ---> || lactate- || -0.185 |
|oxalacetate2- + 2e- + 2H+ || ---> || malate2- || -0.166 |
|flavin + 2e- + 2H+ || ---> || leukoflavin || -0.12 |
|enoyl-SCoA + 2e- + 2H+ || ---> || acyl-SCoA || -0.015 |
|ubiquinone + 2e- + 2H+ || ---> || ubiquinol || +0.010 |
|fumarate2- + 2e- + 2H+ || ---> || succinate2- || +0.031 |
|dehydroascorbic acid + 2e- + 2H+ || ---> || ascorbic acid || +0.08 |
|cytochrome-c3+ + e- || ---> || cytochrome-c3+ || +0.24 |
|O2 + 4e- + 4H+ || ---> || 2 H2O || +0.816 |
~~~ NERNST EQUATION ~~~
This formula predicts which direction a redox reaction
will proceed and its electromotive force or voltage.
2.303 R T [oxidant]
E = E'o + ---------- x log-----------
n F [reductant]
At 25 F and for 2 electron transfers this simplifies to:
E = E'o + 0.03 x log [ox] / [red]
E'o can be found in the redox tables and represents the
actual measured voltage under standardized conditions:
pH = 7.0, T = 25 C, all concentrations = 1.0 M
Note that the relative concentrations of the OXIDANTS
and REDUCTANTS in the reaction mixture effect both the
VOLTAGE and the direction of the reaction.
~~~ EXAMPLES OF SOLUTIONS TO THE NERNST EQUATION ~~~
What are the voltages of the half cells containing the
following redox active agents at the concentrations given?
1) Nicotinamide-Adenine-Dinucleotide in a two to one ratio?
E'o = -.32 [NAD+]/[NADH] = 2 log 2 = .301
E = -.32 + .03 x .301 = -.32 + .009 = -.311
2) Flavin-Mono-Nucleotide in a three to one ratio?
E'o = -.12 [FMN]/[FMNH2] = 3 log 3 = .477
E = -.12 + .03 x .477 = -.12 + .014 = -.106
3) Ubiquinone in a four to one ratio?
E'o = +.10 [CoQ]/[CoQH2] = 4 log 4 = .602
E = +.10 + .03 x .602 = +.10 + .018 = +.118
4) Cytochrome c in a ten to one ratio?
E'o = +.24 [CytcFe+++]/[CytcFe++]=10 log 10 = 1.0
E = +.24 + .06 x 1.0 = +.24 + .06 = +.30
5) Dehydroascorbic Acid and Ascorbic Acid in a one to five ratio?
E'o = +.08 [DHAA]/[AA] = 0.2 log 0.2 = -.699
E = +.08 + .03x(-.699) = +.08 - .021 = +.059
6) Lipoic Acid and Dihydrolipoic Acid in a one to eight ratio?
E'o = -.29 [LA]/[LAH2] = 0.125 log 0.125 = -.903
E = -.29 + .03x(-.903) = -.29 - .027 = -.317
Where more OXIDANT is present the voltage is more POSITIVE.
Where more REDUCTANT is present the voltage is more NEGATIVE.
~~~ OXIDOSIS VERSUS REDOSIS ~~~
Numerous types and variable concentrations of oxidants
and reductants are present in biological fluids.
Many of these are interactive and as a result effect
the overall or combined redox potential of the fluid.
When greater than normal quantities of oxidants are present,
the fluid is said to be in a condition of "OXIDOSIS".
When greater then normal quantities of reductants are present,
the fluid is said to be in a condition of "REDOSIS".