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- 1. Introduction to Electro analytical Chemistry Chapter five 1
- 2. Presentation out lines Electrochemical cells and cell potential Current in electrochemical cells Types of electro-analytical methods 2
- 3. Introduction Electro-analytical chemistry deals on measurement of electrical properties like current, resistance and voltage of the solution containing the analyte. Electro-analytical techniques are capable of producing low detection limits and a wealth of characterization information including :- stoichiometry and rate of interfacial charge transfer, rate of mass transfer, extent of adsorption or chemisorption, and the rates and equilibrium constants . 3
- 4. Conti.. 4 Electro-analytical Chemistry: .) group of analytical methods based upon electrical properties of analytes when part of an electrochemical cell General Advantages of Electrochemistry: a) selective for particular redox state of a species. e.g. CeIII vs. CeIV b) cost - $4,000 - $25,000 for a good instrument compared to $10,000 $50,000 - $250,000 for a good spectrophotometer c) measures activity (not concentration)‚ activity usually of more physiological importance d) fast e) in situ f) information about: ‚ oxidation states‚ stoichiometry‚ rates‚ charge transfer‚ equilibrium constants
- 5. Conti…. Construction of electrochemical cell using two electrodes allows the study of cell reaction, changes in concentration, and electrolysis. Important electrochemical reactions examples: ● acid-base reactions, where acid donates proton to base; ● precipitation reactions, where the reactants form an insoluble product; ● complexation reactions, where a ligand coordinates to an acceptor; ● oxidation-reduction reactions, where the oxidizing agent gains electrons from the reducing species. All of these reactions involve charged species and all may be studied by electrochemical methods. 5
- 6. Conti… Electro analytical methods are a class of techniques in analytical chemistry, which study an analyte by measuring the potential (volts) and/or current (amperes) in an electrochemical cell containing the analyte. The three main categories are potentiometry (the difference in electrode potentials is measured), coulometry (the cell's current is measured over time), and voltammetry (the cell's current is measured while actively altering the cell's potential). 6
- 7. 5.1 Electrochemical cells and cell potential Electrochemical cells An electrochemical cell consists of two electrical conductors called electrodes, each immersed in a suitable electrolyte solution. Cathode is an electrode where reduction take place. Anode is an electrode where oxidation take place. By convention, the cell is always written with the anode at left and the cathode at right. Anode │ solution │ cathode 7
- 8. Conti.. For a current to develop in a cell, it is necessary that the electrodes be connected externally with a metal conductor, the two electrolyte solutions be in contact to permit movement of ions from one to the other, and Electron transfer reaction can occur at each of the two electrodes. Electrochemical cells are either galvanic or electrolytic depending on cell reaction take place. Galvanic cell (voltaic cell):- self-powered device that produces electricity by means of chemical energy; it is limited by the supply of the chemicals contained inside the device. 8
- 9. The electrode reaction occurs spontaneously (∆G < 0) when the electrodes are externally connected by a conductor. The cathode is positive as compared to the anode. Electrolytic cell:- The cell reaction requires an external source of electrical energy for operation. The cell reaction is non-spontaneous (∆G>0). Cathode is negative as compared to the anode. In a galvanic cell, since the oxidation is spontaneous, there is an excess of electrons at the anode electrode. On the other hand, in an electrolytic cell where oxidation is forced to occur, there is a shortage of electrons and a positive charge. 9
- 10. Differentiate electrolytic and galvanic cells? 10
- 11. Daniel cell(galvanic cell) The two equilibria which are set up in the half cells are: 11 (a) Galvanic and (b) electrolytic cells
- 12. Two types of processes can conduct currents across an electrode/so/n interface. One kind involves a direct transfer of es via an oxidation rxn at one electrode and a reduction rxn at the other. Processes of this type are called faradaic processes because they are governed by Faraday’s law, which states that the amount of chemical rxn at an electrode is proportional to the current; the resulting currents are called faradaic currents. To understand the basic difference b/n a faradaic and a non faradaic current, imagine an e traveling down the external circuit to an electrode surface. When the e reaches the so/n interface, it can do 1 of only 2 things. roc Faradaic and nonfaradaic processes 12
- 13. Conti… 13 It can remain at the electrode surface and increase the charge on the double layer, which constitutes a non faradaic current. Alternatively, it can leave the electrode surface and transfer to a species in the solution, thus becoming a part of a faradaic current. Under some conditions, processes such as adsorption and desorption can occur, and the structure of the electrode-solution interface can change with changing potential or solution composition, these processes are called non faradaic processes.
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- 15. 15 Consider the voltaic electrochemical cell below
- 16. 16 Consider this galvanic cell.
- 17. a. Identify the anode and write the oxidation half-reaction. b. Identify the cathode and write the reduction half-reaction. 17
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- 19. 19 The following electrochemical cell was set up.
- 20. Exercises A solution is 10−3 M in Cr2O7 2− and 10−2 M in Cr3+. If the pH is 2.0, what is the potential of the half-reaction at 298K? 20 Mark on the diagram the following parts 1) The direction of the electron flow 2) The anode 3) The cathode 4) The half cell in which oxidation is taking place 5) The half cell in which reduction is taking place. 6) Calculate the reading on the Voltmeter. 7) Explain why the Voltmeter must have a very high resistance (what would happen if it did not?) 8) Explain why the salt bridge is needed. What care must be taken when choosing a suitable electrolyte to include in the salt bridge.
- 21. Answers 21
- 22. 7) A high resistance means a very small current flow. If the resistance is low then a higher current would flow from anode to cathode and the reading on the voltmeter would drop. 8) The salt bridge allows the movement of ions into the 2 half cells to maintain the electrical neutrality of the solution. In the cell where oxidation occurs the concentration of the positive aqueous ion increases making the solution more positive. In the cell where reduction occurs the concentration of the positive aqueous ion decreases making the solution more negative. 9) at the cathode 2H+ +2e➜H2 at the anode Mg ➡ Mg2+ + 2e 10) Over all equation 2H + Mg➜Mg2+H2 22
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- 25. Mass transport modes A faradaic current requires continuous mass transfer of reactive species from the bulk of the solution to the electrode surface. Three mechanisms bring about this mass transfer: 1. Diffusion: when there is a concentration difference b/n two regions of a solution, ions or molecules move from the more concentrated region to the dilute. This process is called diffusion, ultimately leads to a disappearance of concentration difference. 2. Migration: involves the movement of ions through a solution which results from electrostatic attraction or repulsion b/n a species and an electrode. 3. Convection: reactants can also be transported to or from electrode as a result of stirring, vibration or temperature gradients. 25
- 27. Currents in electrochemical cells Electro analytical methods involve electrical currents and current measurements. We need to consider the behavior of cells when significant currents are present. Electricity is carried within a cell by the movement of ions. With small currents, Ohm’s law is usually obeyed, and we may write E = IR where E is the potential difference in volts responsible for movement of the ions, I is the current in amperes, and R is the resistance in ohms of the electrolyte to the current. The measured cell potential normally departs from that derived from thermodynamic calculation. 27
- 28. Conti… This departure can be traced to a # of phenomena, including ohmic resistance and several polarization effects, such as charge-transfer overvoltage, reaction overvoltage, diffusion overvoltage, and crystallization overvoltage. Generally, these phenomena have the effect of reducing the potential of a galvanic cell or increasing the potential needed to develop a current in an electrolytic cell. 28
- 29. Ohmic Potential; IR Drop To develop a current in either a galvanic or an electrolytic cell, a driving force in the form of a potential is required to overcome the resistance of the ions to movement toward the anode and the cathode. This force follows Ohm’s law and is equal to the product of the current in amperes and the resistance of the cell in ohms. The force is generally referred to as the ohmic potential, or the IR drop. The net effect of IR drop is to increase the potential required to operate and electrolytic cell and to decrease the measured potential of a galvanic cell. Therefore, the IR drop is always subtracted from the theoretical cell potential. Ecell = Ecathode – Eanode - IR 29
- 30. Charge-Transfer Polarization Charge-transfer polarization arises when the rate of the oxidation or reduction reaction at one or both electrodes is not sufficiently rapid to yield currents of the size demanded. The overvoltage arising from charge-transfer polarization has the following characteristics: 1. Over voltages increase with current density (current density is defined as the amperes per square centimeter of electrode surface) 2. Over voltages usually decrease with increases in temperature. 30
- 31. 3. Over voltages vary with the chemical composition of the electrode. 4. Over voltages are most marked for electrode processes that yield gaseous products such as hydrogen or oxygen; they are frequently negligible where a metal is being deposited or where an ion is under going a change of oxidation state. 5. The magnitude of overvoltage in any given situation cannot be predicted exactly because it is determined by a number of uncontrollable variables. 31
- 32. 5.3.Types of electro analytical methods The various electro-analytical are classified into Interfacial and Bulk methods. 1. Bulk methods :- they are based on phenomena that measure properties of the whole solution. Example: Direct conductometry and conductometric titration 2.Interfacial methods:- they are more widely used than bulk methods, in which the signal is a function of phenomena occurring at the interface between an electrode and the solution in contact with the electrode. 32
- 33. Main Branches of Electro analytical Chemistry Key to measured quantity: I = current, E = potential, R = resistance, G = conductance, Q = quantity of charge, t = time, vol = volume of a standard solution, m = mass of an electrodispensed species Interfacial methods Bulk methods Static methods (I = 0) Dynamic methods (I > 0) Potentiometry (E) Conductometry (G = 1/R) Controlled potential Constant current Voltammetry (I = f(E)) Amperometric titrations (I = f(E)) Based on Figure 22-9 in Skoog, Holler and Crouch, 6th ed. Electro- gravimetry (m) Coulometric titrations (Q = It) 33
- 34. Interfacial methods can be divided into two major categories a) Static methods (direct potentiometry and potentiometric titration) :- no current passes b/n the electrodes and the concentrations of species in the electrochemical cell remain unchanged or static. b) Dynamic methods consist of Controlled potential (Constant Electrode potential coulometry, voltammetry, Amperometric titrations and electrogravimetry) and Constant current (coulometric titrations and electrogravimetry) In dynamic interfacial electrochemical methods, in which current flows and concentrations change as the result of a redox reaction. 34
- 35. Table 5.1 Electro analytical techniques 35
- 36. Limitation of electro analytical methods Response to activity rather than to the concentration of the analyte so difficult to characterize activity; Rely in almost cases on reactions on electrode surface; Signal doesn’t necessarily represent the bulk of the solution; Electrode reaction itself may alter the composition in the vicinity; Often reference electrode required, which are the potential source of measurement error. 36
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