A.) Introduction:

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Coulometric Methods
A.) Introduction 1.) Coulometry
electrochemical method based on the quantitative
oxidation or reduction of analyte -
measure amount of analyte by measuring amount of
current and time required to complete
reaction lt charge current (i) x time in
coulombs - electrolytic method ? external
power added to system 2.) Example -
Coulometric Titration of Cl- - use Ag
electrode to produce Ag Ag (s) ? Ag
e- Ag Cl- ? AgCl (ppt.) - measure Ag
in solution by 2nd electrode - only get
complete circuit when Ag exists in solution -
only occurs after all Cl- is consumed - by
measuring amount of current and time required
to complete reaction can determine amount of Cl-
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Typical coulometric titration cell.
e.g. At the generator electrode (anode) Ag (s)
Ag e- (oxidation of silver to silver
ion)
At the cathode Possible reaction 2H 2e- H2
(g) (hydrogen evolution) Therefore, need
sintered glass to separate the species generated
in the other electrode (e.g. cathode, hydrogen
gas) to prevent reactions with the titration
species such as Ag.
cathode
anode
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3.) Based on Measurement of Amount of
Electricity (or charge, in coulombs) Required to
Convert Analyte to Different Oxidation
State - Q It for constant current with
time where Q charge required
(coulombs amp . sec) I current (amp.) t
time of current (sec) for variable current
with time Q IIdt Relate charge
(coulombs, C) to moles of e- passing electrode by
Faraday constant Faraday (F) 96,485
Coulombs (C)/mole e- F 6.022 x 1023 e-/ mole
e- x 1.60218 x 10-19 C/ e- 96,485
Coulombs/mole e- If know moles of e- produced
and stoichiometry of ½ cell reaction Ag (s) ?
Ag e- (11 Ag/e-) gives moles of analyte
generated, consumed, etc.
t
0
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Example Constant current of 0.800 A (amps.)
used to deposit Cu at the cathode and O2 at anode
of an electrolytic cell for 15.2 minutes. What
quantity in grams is formed for each
product? ½ cell reactions Cu2
2e- Cu (s) (cathode) 2H2O 4e- O2
4H (anode) To solve Q i.t Q (0.800
A)(15.2 min) (60 sec/min) Q 729.6 C
(amp.sec) amount Cu produced (729.6 C)(1
mole e-/96,485 C)(1 mole Cu/2 mole e-)(63.5g
Cu/mole Cu) 0.240 g Cu amount of O2
produced (729.6 C)(1 mole e-/96,485 C)(1 mole
O2/4 mole e-)(32.0g O2/mole O2) 0.0605 g O2
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4.) Two Types of Coulometric Methods a)
amperostatic (coulmetric titration) - most
common of two b) potentiostatic Fundamental
requirement for both methods is 100 current
efficiency - all e- go to participate in the
desired electrochemical process - If not, then
takes more current ? over-estimate amount of
analyte B) Amperostatic Methods (Coulometric
Titrations) 1.) Basics titration of analyte
in solution by using coulometry at constant
current to generate a known
quantity of titrant electrochemically -
potential set by contents of cell -
Example Ag (s) ? Ag e- for
precipitation titration of Cl- - To
detect endpoint, use 2nd electrode to detect
buildup of titrant after
endpoint.
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2.) Applications a) Can be used for Acid-Base
Titrations - Acid titration 2H2O 2e- ?
2OH- H2 titrant generation reaction -
Base titration H2O ? 2H ½ O2 2e- titrant
generation reaction b.) Can be used for
Complexation Titrations (EDTA) HgNH3Y2-
NH4 2e- ? Hg 2NH3 HY3- HY3- ? H
Y4- c.) Can be used for Redox Titrations
Ce3 ? Ce4 e- Ce4 Fe2 ? Ce3 Fe3
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3.) Comparison of Coulometric and Volumetric
Titration a) Both Have Observable Endpoint -
Current (e- generation) lt serves same function
as a standard titrant solution - Time lt
serves same function as volume delivered -
amount of analyte determined by combining
capacity - reactions must be rapid, essentially
complete and free of side reactions b.)
Advantages of Coulometry - Both time and
current easy to measure to a high accuracy -
Dont have to worry about titrant stability -
easier and more accurate for small quantities of
reagent lt small volumes of dilute solutions ?
problem with volumetric - used for
precipitation, complex formation
oxidation/reduction or neutralization
reactions - readily automated c) Sources of
Error - variation of current during
electrolysis - departure from 100 current
efficiency - error in measurement of
current - error in measurement of time -
titration error (difference in equivalence point
and end point)
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4.) Change in Potential During Amperostatic
Methods a) In constant current system, potential
of cell will vary with time as analyte is
consumed. - Cell seeks out electrochemical
reactions capable of carrying the supplied
current Cu2 2e- ? Cu (s) initial
reaction - Nernst Equation Ecathode
EoCu2/Cu 0.0592/2 log (1/aCu2)
Note Ecathode depends on aCu2. As aCu2
decreases ? (deposited by reaction) Ecathode
decreases.
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- When all Cu2 is consumed, current is
carried by another electrochemical
reaction lt generation of H2 (g) if
reduction 2H 2e- ? H2 (g) lt breakdown
of water if oxidation 2H2O ? H2O2 2H
2e- H2O2 ? O2 2H 2e- -
Not a problem as long as (1) other species
dont co-deposit (2) there isnt a large
excess of species being used in titrant
generation vs. titrated analyte
e.g., Ag (s) vs. Cl- in solution (in AgCl
precipitation experiment)
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C) Potentiostatic Coulometry 1.) Basics
-detection of analyte in solution by using
Coulometry at fixed potential to
quantitatively convert analyte to a given
form lt current controlled by contents of
cell. 2.) Instrumentation requirements -
electrochemical/electrolysis cell - a
potentiostat (apply the required
potential/voltage to the system) - an integrator
(analog or digital) for determination of the
charged consumed
Equivalent circuit
Practical Circuit of a Potentiostat and an
Electrochemical/Electrolysis
Electrochemical cell
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2) Advantages - more specific than
amperostatic coulometry lt avoids redox of
species that may interfere with constant current
coulometry - can be used for over 55 elements
without major interference 3) Disadvantages -
does take longer than amperostatic titration lt
current (i) decreases with time lt conversion
becomes slower as less analyte around to oxidize
or reduce
It Ioe-kt k 25.8 DA/Vd where It current
at time t (A) I0 initial current (A) t time
(sec) D diffusion coefficient (cm2/s) A
electrode surface area (cm2) V volume of
solution (cm3) d thickness of the surface
layer where concentration gradient exists (cm)
typical values of D are in the range of
10-5cm2/s typical values of d is 2 x 10-3 cm
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Example (using the two equations) Deposition of
Copper Cu2 2e- Cu (s) After 30 min,
current decreases from the initial 1.5 A to 0.08A
By this time, approx. 96 of the copper has been
deposited.

• 4) Other Applications of constant potential
  coulometry
• electroplating, apply the correct potential, the
  metal of interest will be deposited.
• e.g. gold plated onto silver (vermeil) as
  jewelry
• e.g. zinc plated onto steel for anti-corrosion
  (zinc as sacrificial cathodic coating)

The term "vermeil" refers to a silver item,
containing no less than 92.5 silver, that has
been plated with a gold or gold alloy that is no
less than 10 karat, to a thickness of not less
than 2.5 microns.