![ 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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Coulometry.pptx presentation assignment copy
- 1. COULOMETRY CLASS PRESENTATION ASSIGNMENT CHE 8.10 19/11/2019 BY KIRUI KIBET DERRICK PGC/CHE/003/019 LECTURER : DR. PHANICE WANGILA 1
- 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) 2
- 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. 3
- 4. A graph showing current- versus-time profile for controlled-potential coulometry 4
- 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 5
- 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. 6
- 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 7
- 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 8
- 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 9
- 10. 10 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 11
- 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. 12
- 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 13
- 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 14
- 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. 15
- 16. 16 SCHEMATIC DIAGRAM OF COULOMETRIC TITRATION
- 17. Method for the external generation of oxidizing and reducing agents in coulometric titrations. 17
- 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. 18
- 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 19
- 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. 20
- 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% 21
- 22. Thank you for your attention 22