for chemistry students Bsc and Msc

1. COULOMETRY
CLASS PRESENTATION ASSIGNMENT CHE 8.10
19/11/2019
BY
KIRUI KIBET DERRICK
PGC/CHE/003/019
LECTURER : DR. PHANICE WANGILA
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2. Coulometry is an electrochemical method
which we measure the current required to
exhaustively oxidize or reduce the analyte.
There are two forms of coulometry
.controlled potential (potentiostatic)
.controlled current (amperostatic)
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3. Controlled potential coulometry
Important points
 A constant potential is applied to the electrochemical cell.
 Maintaining constant potential at the working electrode ensures 100%
current efficiency, the analyte’s quantitative oxidation or reduction,
without simultaneously oxidizing or reducing an interfering species.
 The current flowing through an electrochemical cell under a constant
potential is proportional to the analyte’s concentration. Therefore a
decrease in analytes concentration results in current decrease.
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4. A graph showing current-
versus-time profile for
controlled-potential
coulometry
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5. From the graph
 te-electrolysis time, t = 0 is time before reaction and Integrating the area under the
curve from t = 0 until t = te, gives the total charge.
 If current varies with time, as it does in controlled potential coulometry, then the total
charge is given by where Q is total charge in coulombs, thus for varying current
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6.  From Faradays equation Q = nFN where n-number of electrons transfered
per mole of analyte, F-Faradays constant, N-mole of analyte
 A coulomb is also equivalent to an A×s; thus for a constant current, i, the
charge is given as Q = ite
Selecting a Constant Potential
The ideal constant potential selected is such that the reduction or oxidation
goes to completion without interference reactions from other components in
the sample matrix.
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7.  The potential needed for a quantitative reduction can be calculated by the
Nernst equation
 For a given metal analyte denoted as M, we define a quantitative reduction
as one in which 99.99% of the analyte M2+ is reduced to M then the
concentration of M2+ at the end of the electrolysis must be [M2+ ]≤ 10-4 [
M2+]0 where [M2+]0 is the initial concentration of the analyte in sample.
 Overpotential is the difference between the potential actually required to
initiate an oxidation or reduction reaction, and the potential predicted by
the Nernst equation
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8. Minimizing Electrolysis Time
 change in current as a function of time is approximated by an exponential
decay, thus current at time t is i=i0 e-kt where k constant directly
proportional to area of working electrode and inversely proportional to
the volume, i0 is initial current.
 For an exhaustive electrolysis process, i≤(10-4) i0
Substituting and solving the two immediatiate previous equations and
solving for te
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9. Instrumentation
 The working electrode has two types Platinum and Mercury electrodes.
For negative potential requirement preferred is Hg electrode and for
positive potential Pt electrode used.
 Auxiliary electrode usually Pt wire separated from analyte solution by salt
bridge.
 Reference electrode saturated calomel or Ag/AgCl electrodes
 Modern instruments use electronic integration to monitor charge as a
function of time. The total charge at the end of the electrolysis then can
be read directly from a digital readout or from a plot of charge versus
time
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10. 
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TW O CELL ASSEMBLIES FOR POTENTIOSTATIC COULOMETRY SHOW ING PLATINUM AND
MERCURY AS W ORKING ELECTRODES

11. Advantages and applications
 Inorganic application-most of metal ions can be determined by this step. As
many as 55 elements on the periodic table can be determined by reduction of
metal ion to the metal. The formation of amalgam with mercury is crucial.
 Analysis of radioactive materials -widely used for determination of uranium
and plutonium. Reduction of U2+ to U4+ can be carried out in H2SO4 medium
with a mercury pool cathode (− 0.6 V vs. SCE).
 Micro analysis- the technique is useful in cases where the analyte cannot be
easily weighed thus advantage over gravimetry
 Multistep controlled potential analysis-determination of several metal ions in
solution is made possible ; solution having Cu2+ and Bi3+ cathode, potential
controlled at 0.08v so that the Cu2+ is reduced to Cu. When current goes to
constant low, potential is controlled at new level to reduce the Bi3+ to Bi
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12.  Electrolytic determination of organic compounds:Trichloroacetic acid and
picric acid are quantitatively reduced at a mercury cathode. Coulometric
methods permit the analysis of these compounds with an accuracy of
0.1%.
Cl3CCOO− + H+ + 2e Cl2HCCOO− +Cl-
 Electrolytic synthesis of new organic compounds :Synthesis of new species
and novel chemical compounds are possible. No chemical reagents are
required since electron itself is the reagent for carrying out these reactions.
No contamination of the products takes place.
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13. Controlled current coulometry
important points
 A constant current is passed through the electrochemical cell.
 Has advantage of constant current result in more rapid analysis since
current does not decrease over time.
 Second advantage is with a constant current the total charge is simply
the product of current and time .
Graph of current vs
time
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14. Disadvantages and their solutions
 As electrolysis proceeds the analytes concentration decreases and
therefore current decreases since current function of concentration.
 Solution to maintain 100% current efficiency, introduction of reagent,a
mediator,whose products from competing oxidation reactions react
rapidly and quantitatively with the remaing analyte oxidizing the analyte
and itself reverting to its initial reduced state.
 The need for method to determine when analyte has been exhaustively
been analysed
 Solution is use of visual indicators like in titrimetry
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15. instrumentation
 Controlled current coulometry carried out using galvanostat and working
electrode usually Pt. here working electrode also called generator
electrode since mediator reacts here to generate species that react with
the analyte.
 The counter electrode is isolated from the analytical solution by a salt
bridge
 The products generated at the anode and cathode pass through
separate tubes. The appropriate oxidizing or reducing reagent can be
selectively delivered to the analytical solution.
 The other necessary instrumental component for controlled-current
coulometry is an accurate clock for measuring the electrolysis time, te,
and a switch for starting and stopping the electrolysis.
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16. 
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SCHEMATIC DIAGRAM OF
COULOMETRIC TITRATION

17. Method for the external generation of
oxidizing and reducing agents in
coulometric titrations.
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18. Coulometric titration
 Given equations Q = nFN and Q = ite combining and solving for N gives
 Comparing with equation for relationship of moles for strong acid strong
base titration, N= (Mbase)(Vbase) where (Mbase) molarity of base, (Vbase) volume
of base .
 Controlled-current coulometric methods commonly are called coulometric
titrations because of their similarity to conventional titrations. A titrant in a
conventional titration is replaced by a constant-current source whose current
is analogous to the titrant’s molarity. The time needed for an exhaustive
electrolysis takes the place of the volume of titrant, and the switch for starting
and stopping the electrolysis serves the same function as a buret’s stopcock.
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19. Advantages of coulometric titration
 Preparation, standardisation and storage of standard solutions are not
necessary in coulometric titrations. This is useful especially for preparation
of unstable reagents such as dipositive silver ions.
 It is easy to handle small quantities of reagents by coulometric titrations.
Byproper choice of current, micro quantities of substance can be analysed
with greater accuracy and ease.
 Coulometric titrations can readily be adapted to automatic titrations as the
current control is easily done.
 A number of automatic coulometric titrators are readily available in the
market to monitor environmental pollutant.
 In water titrators – Karl Fischer reagent is generated electrolytically to
determine trace level concentrations of water content /moisture
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20.  Precision Precision is determined by the uncertainties of measuring
current, time, and the end point in controlled-current coulometry and of
measuring charge in controlled-potential coulometry. Precisions of ±0.1–
0.3% are routinely obtained for coulometric titrations, and precisions of
±0.5% are typical for controlled-potential coulometry.
 Sensitivity For a coulometric method of analysis, the calibration sensitivity
is equivalent to nF in equation 11.25. In general, coulometric methods in
which the analyte’s oxidation or reduction involves a larger value of n
a greater sensitivity.
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21.  Selectivity Selectivity in controlled-potential and controlled-current
coulometry is improved by carefully adjusting solution conditions and by
properly selecting the electrolysis potential
 Accuracy the accuracy of a controlled current coulometry is determined
by current efficiency,accuracy with which time and current can be
measured and accuracy of endpoint. For current maximum error +/-
and for apparatus +/- 0.1%
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22. Thank you for your attention
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