CHAPTER III. ELECTROGRAVIMETRY AND COULOMETRY I ...

CHAPTER III. ELECTROGRAVIMETRY AND COULOMETRY
I. Introduction
i/. Characteristics
- Reaction goes to completion in an electrochemical cell.
- Large A/V (electrode area/solution volume) conditions and as effective mass
transfer conditions as possible.
- Highly accurate and precise (often 0.1% or better).
- Requires no calibration against standards.
ii/. Classification of Techniques
- Electrogravimetry
- Constant-potential electrolysis.
- Constant-current electrolysis.
iii/. Units for Quantity of Electricity
a) Coulomb (C) - the quantity of charge that is transported in one second by a
constant current of one ampere.
Q = It
or Q = ∫ I⋅dt 0→t
b) Faraday (F) - the charge in coulombs associated with one mole of electrons.
1 F = 96,485 C
iv/. Faraday's Law of Electrolysis:
w = (QM)/(nF)
where w = weight of substance oxidized or reduced
M = molecular weight of the substance
Q = number of coulombs passing through the cell
II. Electrogravimetry
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Analyte is quantitatively deposited on a previously weighted electrode.
FIGURE 3-1. “Harris” Fig. 17-5 (p. 379).
Sensitivity is limited by the difficulty in determining the small difference in weight.
III. Coulometry
A form of chemical analysis based on counting the electrons used in a reaction.
i/. Potentiostatic Coulometry (Controlled-Potential Coulometry)
e.g., EXAMPLE 3-2. “Harris” Ex. (p. 380).
Current decreases with time owing to:
a) Depletion of sample in the bulk solution.
b) Concentration polarization.
FIGURE 3-3. “Bard” Fig. 10.3.1 (p. 378).
FIGURE 3-4. “Skoog” Fig. 21-1a (p. 519).
The total quantity of electricity Q(t) consumed in the electrolysis is given by the
area under the I-t curve: Q(t) = ∫ I(t)⋅dt.
With a two-electrode cell, the voltage between two electrodes is
E = E(cathode) − E(anode) − IR − overpotentials
As the analyte, say, Cu 2+ , is used up, the current decreases and both the ohmic and
overpotentials decreases in magnitude. If E and E(anode) are constant, then
E(cathode) shifts to more negative values and the solutes more easily reduced than
H + will be electrolyzed.
FIGURE 3-5. “Harris” Fig. 17-7 (p. 380).
To prevent the cathode potential from becoming so negative that unintended ions
are reduced, a three-electrode cell with a potentiostat to control the cathode
potential can be used.
FIGURE 3-6. “Harris” Fig. 17-3 (p. 377).
Working (indicator) electrode - reaction of interest takes place.
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Reference electrode - maintain a constant potential irrespective of changes in
current.
Counter (auxiliary) electrode - to complete the electrochemical cell but its potential
is rarely of interest.
ii/. Amperostatic Coulometry (Controlled-Current Coulometry)
FIGURE 3-7. “Bard” Fig. 10.4.1 (p. 385).
When applied current < limiting current, current efficiency = 100%
When applied current > limiting current, current efficiency < 100%
The selectivity is poorer than the controlled potential method.
iii/. Coulometric Titrations
Employs a titrant that is electrolytically generated by a constant current, i.e.,
unstable reagents such as Ag 2+ , Cu + , Mn 3+ , and Ti 3+ can be generated and used in a
single vessel.
FIGURE 3-8. “Harris” Fig. 17-9 (p. 382).
Some external means must be used to determine the end-point of the reaction, e.g.,
visual indicators, potentiometry, amperometry, and photometry.
e.g., 2Br − → Br 2 + 2e
H 2 C=CH 2 + Br 2 → H 2 BrCCBrH 2
iv/. Applications
a) Neutralization, Precipitation, and Complex-Formation Titrations
TABLE 3-9. “Skoog” Table 21-1 (p. 529).
b) Oxidation/Reduction Titrations
TABLE 3-10. “Skoog” Table 21-2 (p. 530).
v/. Comparison of Coulometric and Volumetric Titrations
Advantages:
a) Absolute technique needing no standard solution - minimizes errors in
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preparation and storage of standard solutions.
b) Very small amount of substances can be determined.
c) Substances that are unstable or inconvenient to use can be employed as titrants.
d) Easily automated.
e) Can be performed remotely.
f) More accurate.
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