Electrochemical method of analysis


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Electrochemical method of analysis

  2. 2.  Introduction  Electrochemistry is branch of chemistry concern with the interaction of electrical and chemical effects  A large part of this field deals with the study of chemical changes caused by the passage of an electrical current and the production of electrical energy by chemical reaction.
  3. 3. It is named electrochemistry because its originated from the study of the movement of electrons in an oxidation–reduction reaction.  Electrochemical methods: are analytical techniques that use a measurement of potential, charge, or current to determine an analyte’s concentration or to characterize an analyte’s chemical reactivity.
  4. 4.  It is a qualitative and quantitative methods of analysis based on electrochemical phenomena occurring within a medium or at the phase boundary and related to changes in the structure, chemical composition, or concentration of the compound being analyzed.  These methods are divided into five major groups: potentiometry, voltammetry, coulometry, conductometry, and dielectrometry.
  5. 5.  1. Applications Obtaining thermodynamic data about a reaction. 2. To generate an unstable intermediate such as radical ion and study its rate of decay or it is spectroscopic properties. 3. They use to analyze a solution for trace amount of metal ions or organic species.
  6. 6. 4. The electrochemical properties of the system themselves are of primary interest, for example, in the design of a new power source or for the electrochemical methods have been developed.
  7. 7.  Type of electrochemical techniques 1. Bulk techniques, in which we measure a property of the solution in the electrochemical cell. An example is the measurement of a solution’s conductivity, which is proportional to the total concentration of dissolved ions,
  8. 8. 2. Interfacial techniques, in which the potential, charge, or current depends on the species present at the interface between an electrode and the solution in which it sits. An example is the determination of pH using a pH electrode.
  9. 9.  Despite the difference in instrumentation, all electrochemical techniques share several common features. (1) The electrode’s potential determines the analyte’s form at the electrode’s surface (2) The concentration of analyte at the electrode’s surface may not be the same as its concentration in bulk solution;
  10. 10. (3) Current is a measure of the rate of the analyte’s oxidation or reduction; and (4) We cannot simultaneously control current and potential.
  11. 11. Interfacial Electrochemical Techniques  The interfacial electrochemical techniques is divided into Static techniques and dynamic techniques  Static technique the current is not pass through the analyte’s solution. Potentiometry, in which we measure the potential of an electrochemical cell under static conditions, is one of the most important quantitative electrochemical methods
  12. 12.  Dynamic techniques, in which we allow current to flow through the analyte’s solution, it comprise the largest group of interfacial electrochemical techniques e.g. Coulometry, in which we measure current as a function of time, Amperometry and voltammetry, in which we measure current as a function of a fixed or variable potential
  13. 13. Controlling and Measuring Current and Potential  we cannot simultaneously control both current and potential  if we choose to control the potential, then we must accept the resulting current, and we must accept the resulting potential if we choose to control the current.
  14. 14.  The second electrode, which we call the counter electrode, completes the electrical circuit and provides a reference potential against which we measure the working electrodes potential. Ideally the counter electrode’s potential remains constant so that we can assign to the working electrode any change in the overall cell potential.
  15. 15.  Electrochemical measurements are made in an electrochemical cell consisting of two or more electrodes and the electronic circuitry for controlling and measuring the current and the potential.  The simplest electrochemical cell uses two electrodes. The potential of one electrode is sensitive to the analyst’s concentration, and is called the working electrode or the indicator electrode.
  16. 16.  If the counter electrode’s potential is not constant, we replace it with two electrodes: a reference electrode whose potential remains constant and an auxiliary electrode that completes the electrical circuit.  Because we cannot simultaneously control the current and the potential, there are only three basic experimental designs
  17. 17. (1) Measure the potential when the current is zero, (2) Measure the potential while controlling the current, (3) Measure the current while controlling the potential  Each of these experimental designs relies on Ohm’s law, which states that a current, i, passing through an electrical circuit of resistance, R, generates a potential, E.  (E =iR) Each of these experimental designs uses a different type of instrument
  18. 18.  Type of Electrochemical Methods 1. Potentiometry methods: it measures the potential of a solution between two electrodes. The potential is then related to the concentration of one or more analytes. The cell structure used is often referred to as an electrode even though it actually contains two electrodes: an indicator electrode and a reference electrode.
  19. 19.  Potentiometry usually uses electrodes made selectively sensitive to the ion of interest, such as a fluoride-selective electrode. The most common potentiometric electrode is the glass-membrane electrode used in a pH meter.
  20. 20. 2. Voltammetry method: is based on the applies a constant and/or varying potential at an electrode's surface and measures the resulting current with a three electrode system. Voltammetry, with its variety of methods, constitutes the largest group of electrochemical methods of analysis and is commonly used for the determination of compounds in solutions (for example, polarography and amperometry).
  21. 21. 3. Coulometry methods: based on the measurement of the amount of material deposited on an electrode in the course of an electrochemical reaction in accordance with Faraday’s laws. A distinction is made between coulometry at constant potential and coulometry at constant current.
  22. 22.  Coulometry uses applied current or potential to completely convert an analyte from one oxidation state to another. In these experiments, the total current passed is measured directly or indirectly to determine the number of electrons passed. Knowing the number of electrons passed can indicate the concentration of the analyte or, when the concentration is known, the number of electrons transferred in the redox reaction.
  23. 23. 4. Conductometry methods: in which the electrical conductivity of electrolytes (aqueous and non-aqueous solutions, colloid systems and solids) is measured  It is based on the change in the concentration of a compound or the chemical composition of a medium in the interelectrode space;
  24. 24. Potentiometric titration  Is a technique similar to direct titration of a redox reaction. No indicator is used, instead the potential across the analyte, typically an electrolyte solution is measured. To do this, two electrodes are used, an indicator electrode and a reference electrode.
  25. 25.  In potentiometry we measure the potential of an electrochemical cell under static conditions. Because no current—or only a negligible current—flows through the electrochemical cell, its composition remains unchanged. For this reason, potentiometry is a useful quantitative method.
  26. 26. Potentiometric Measurements  A potentiometer is used to determine the difference between the potential of two electrodes. The potential of one electrode—the working or indicator electrode—responds to the analyte’s activity, and the other electrode— the counter or reference electrode—has a known, fixed potential.
  27. 27. Potentiometric Electrochemical Cells  The electrochemical cell consists of two halfcells, each containing an electrode immersed in a solution of ions whose activities determine the electrode’s potential. A salt bridge containing an inert electrolyte, such as KCl, connects the two half-cells.
  28. 28.  The ends of the salt bridge are fixed with porous frits, allowing the electrolyte ions to move freely between the half-cells and the salt bridge. This movement of ions in the salt bridge completes the electrical circuit as shown in the Figure below.
  29. 29.  By convention, we identify the electrode on the left as the anode and assign to it the oxidation reaction; thus Zn(s) ↔ Zn2 (aq) +2e −  The electrode on the right is the cathode, where the reduction reaction occurs Ag +(aq) + e− ↔ Ag (s)
  30. 30.  Advantages of potentiometric titrations over 'classical' visual indicator methods are: 1. Can be used for coloured, turbid or fluorescent analyte solution. 2. Can be used if there is no suitable indicator or the colour change is difficult to ascertain. 3. Can be used in the titration of polyprotic acids, mixtures of acids, mixtures of bases or mixtures of halides.
  31. 31.  Types  of Potentiometric Titration Depending on the type of the reactions involved to which potential measurement can be applied for end point detection, potentiometric titrations can be classified into followings: (a) (b) A cid-Base Titration Complexometric Titration (c) Oxidation-Reduction Titration (d) Precipitation Titration
  32. 32.  Location  of the End Point Titration Curve: It is obtained by plotting the successive values of the cell emf on y=axis and corresponding values of volume of titrant added on the x-axis. This gives an S-shaped curve. The central portion of this curve which shows the steeply rising portion corresponds to the volume for the end point of the titration.
  33. 33.  When there is a small potential change at the end point like in the titration of weak acid with strong base, titration of very dilute solution etc, it is difficult to locate end point by this method.
  34. 34. Titration method of locating end point
  35. 35. b) Analytical or Derivative Method: The end point can be more precisely located from the first or second derivative curves. The first derivative curve involves the plot of slope of the titration curve (ΔE/ΔV-ration of change in emf and change in volume added) against the volume of the titrant added.
  36. 36.  Most frequently ΔE/ΔV is plotted against the average volume of titrant added corresponding to the values of emf taken. Volume on the x- axis corresponding to the peak of the curve is the end point of the titration.
  37. 37. First derivative Curve
  38. 38.  In second derivative curve we plot the slope of first derivative curve (Δ2E/ΔV) against volume. The point on volume axis where the curve cuts through zero on the ordinate gives the end point. This point corresponds to the largest steepest point on titration curve and maximum slope of the ΔE/ΔV curve.
  39. 39.  This mentioned methods need values of potential corresponding to very small change in volume of titrant added near the end point for good result. In the immediate area of the end point the concentration of the original reactant becomes very small, and it usually becomes impossible for the ions to control the indicator electrode potential.
  40. 40. Second derivative curve