Lecture 2 Transduction Elements PDF

Lecture 2 Transduction Elements Dr. Ashish A Prabhu Assistant Professor Department of Biotechnology 1 NIT Warangal  Electrochemical Transducers There are three basic electrochemical processes that a...

Lecture 2 Transduction Elements Dr. Ashish A Prabhu Assistant Professor Department of Biotechnology 1 NIT Warangal  Electrochemical Transducers There are three basic electrochemical processes that are useful in transducers for sensor application. Potentiometry:The measurement of a cell potential at zero current. Voltammetry(amperometry): In which an oxidizing (or reducing) potential is applied between the cell electrodes and the cell current is measured. Conductometry: Where the conductance (reciprocal of resistance) of the cell is measured by an alternating current bridge method. 2 Potentiometry and Ion selective electrodes Cells and Electrodes: Fig1: A metal electrode dipped into an Fig 2: Two half-cell electrodes combined, electrolyte solution - one half-cell. making a complete cell. 3 Electrochemical cell: Daniel cell Cells and Electrodes: Cell involves copper and zinc electrodes in solutions of copper(II) and zinc(II) sulfates, with a porous pot for the bridge. This cell has been used as a practical battery and has an emf of 1.10V. We can consider the half-cell reactions as follows: cu2++ 2e- =cu (1) Zn2+ + 2e- = Zn (2) Subtracting Equation (2)-(1) Cu2++Zn =cu+Zn2+ 4 Electrochemical cell: Daniel cell  The Gibbs free energy for this reaction is negative.  The Gibbs free energy is simply related to the emf of the cell by the following expression. ΔG = -nFE n =number of electrons transferred (in this case, n = 2), F = Faraday constant (= 96487 C mol-'), and E =emf of the cell.  Thus, if ΔG is negative, then E is positive. 5 Electrochemical cell: Daniel cell  We can determine ΔG for Eq (1) and (2) separately.  If we can deteremine ΔGCu+2 and ΔGZn, we can determine Ecu and Ezn.  But simple separation is not possible.  The hudrogen can be oxidized to H+ ions by removing an electron  Which is more usually written as  The ΔG for this reaction is zero and For any standard State, the Gibbs free energy is designated as ΔGO. The standard electrode potential for hydrogen is therefore: 6 Electrochemical cell: Daniel cell  We can show the half-cell reactions as before:  (3) (4)  Subtracting Eq (4) by Eq (3) We get:  Thus and Fig 3: A hydrogen electrode connected with another half-cell 7 Electrochemical cell: Daniel cell  We can show the half-cell reactions for Zn electrode : (5) (6)  Subtracting Eq (6) by Eq (5)We get:  Thus and Combining the half-cells for copper and zinc gives the cell emf cell as follows: 8 Reference Electrodes  The standard hydrogen electrode is a reference electrode (RE) to which other electrodes may be referred.  Qualities of RE (1) Non-polarizable. (2) Reproducible electrode potential. (3) Low co-efficient of variation with temperature. The Silver-Silver Chloride Electrode  Sparingly soluble in water  The half cell reaction can be given as This electrode consists of a silver wire coated with silver chloride 9 dipping into a solution of sodium chloride.  The saturated-calomel electrode  Colomel electrode refers to mercurous chloride (Hg2Cl2).  The half cell reaction is given as  This electrode consists of a mercury pool in contact with a paste made by mixing mercury(II) chloride powder and saturated potassium chloride solution. the whole being in contact with a saturated solution of potassium chloride.  The potential difference between an indicator electrode and the reference electrode to give the cell emf. Fig 4: Schematic of a saturated-calomel electrode  Quantitative Relationships: The Nernst Equation  The effect of different concentrations on the electrode potential is very essential for the analytical application of potentiometry.  The Nernst equation is used for this purpose.  The basic Nernst equation is a logarithmic relationship derived from fundamental thermodynamic equations such as the following:  The nernst equation is as follows: where aOx and aRare activities 11  Quantitative Relationships: The Nernst Equation  Alternatively the nerst equation is represented in terms of logarithm to base 10  It is same form as the Henderson-Hasselbach equation for the pH of mixtures of acids and bases  If R=8.314 J K-1 mol -1, F= 96480 C mol-1, T=298 K, The nernst equation can be simplified as 12 Quantitative Relationships: The Nernst Equation  The reduced species, R, is often a metal, in which case it has a constant concentration (activity) of 1, so the equation simplifies further to the following: For practical situation we can write the above equation as we can plot a graph of E against log[Ox], which would normally give a straight line of slope S, with an intercept of K. 13  Quantitative Relationships: The Nernst Equation  If we now incorporate the reference electrode potential (EREF)n d the liquid junction potential (Elj,), as shown in Figure 5 we have the following: Fig 5: A reference electrode combined Therefore: with another half-cell Hence: 14  Quantitative Relationships: The Nernst Equation  If instead of a reference electrode, we incorporate a similar half-cell with the same redox couple but with a different concentration of Ox. The half cell reaction can be given as : (7) (8) Subtracting Eq (8) by (7), we get Fig 6: Schematic of a concentration cell; RE1, and RE2 represent reference electrodes.  Now, if [Ox12 is kept constant (perhaps at a reference concentration), we then obtain: 15 where the constant is -Slog [OX]2  Quantitative Relationships: The Nernst Equation  We can write the voltage (emf) of the cell as follows: And so: Where: Where, a1 and a2 are the activities of the test and reference (standard) solutions, respectively. 16  Nernst Equation Applications The Nernst equation can be used to calculate:  Single electrode reduction or oxidation potential at any conditions  Standard electrode potentials  Comparing the relative ability as a reductive or oxidative agent.  Finding the feasibility of the combination of such single electrodes to produce electric potential.  Emf of an electrochemical cell  Unknown ionic concentrations  The pH of the solution and solubility of sparingly soluble salts can be measured with the help of the Nernst equation. 17  Numerical -1 1. The standard electrode potential of zinc ions is 0.76V. What will be the potential of a 2M solution at 300K? 2. What is the Cell Potential of the electrochemical cell in Which the cell reaction is: Pb2+ + Cd → Pb + Cd2+ ; Given that Eocell = 0.277 volts, temperature = 25oC, [Cd2+] = 0.02M, and [Pb2+] = 0.2M. 3. The Cu2+ ion concentration in a copper-silver electrochemical cell is 0.1M. If Eo(Ag+/Ag) = 0.8V, Eo(Cu2+/Cu) = 0.34V, and Cell potential (at 25oC) = 0.422V, find the silver ion concentration. 18  Ion selective electrodes  Designed to respond to one particular ion more than others.  This is a potentiometric device, i.e. the potential of the electrode, measured against an appropriate reference electrode, is proportional to the logarithm of the activity (or concentration) of the ion being tested.  Usually responds rapidly, with a linear range of about 10-6 to 1 M.  It on operates on the principle of a concentration cell, in that it contains a selective membrane which develops a potential if there is a concentration difference across the membrane of the ion being tested. 19  Practical Aspects of Ion-Selective Electrodes  In order to obtain consistent, reproducible results with the lowest detection limits, certain precautions have to be observed.  The following factors may need to be observed: (i) The ionic strength needs to be kept constant from one sample to the next. This can simply be done by adding a fairly high, constant concentration of an indifferent electrolyte, i.e. one that does not interfere in any way, to each sample and each standard. (ii) The pH may need to be controlled at a certain level. This is more important with some ionic samples than others, e.g. fluorides. (iii) It may be possible, and desirable, to add components that minimize or eliminate interfering ions.  Appropriate mixtures to provide these properties are usually called ionic-strength adjusters (ISAs) or more fully, total-ionic- 20 strength adjustment buffers (TISABs).  Measurement and Calibration Calibration Graphs and Direct Reading  A series of standard solutions are made up with an added ISA and the potentials are measured.  Then, a calibration graph is plotted of voltage against log (concentration).  The sample is treated in the same way and its log (concentration) value is then read from the graph.  New calibration graphs should be prepared regularly. 21  Measurement and Calibration Standard Addition  The sample is prepared as before and its voltage is read.  Then, a known amount of a standard of higher concentration, usually about 10 times the expected sample concentration, is added and a second voltage reading is taken.  The data are then fitted to an equation, which should include a correction for dilution by the added standard. If Cu, is the unknown concentration in Vu, ml of solution and Cs, is the added standard concentration in Vs, ml of solution, then we have: (9) 22  Measurement and Calibration Standard Addition (10) Subtracting Eq (10) by (9), we get This Equation can be rearranged to give Hence Cu can be obtained. 23  Measurement and Calibration Gran Plot  This is an extension of the standard addition method, using multiple standard additions.  The procedure is the same as in the standard addition method except that several additions are made (say, five or more). we have the following: And therefore: Taking anti-log, we obtain 24  Measurement and Calibration Gran Plot  The procedure is the same as in the standard addition method except that several additions are made (say, five or more). we have the following: Fig 7: Representation of gran plot And therefore: where K’ = 1OK/S. A plot 10 E/S against Cs. The plot is a straight line, with a negative intercept of - Taking anti-log, we obtain Cu, as when 10E/S = 0, Cu, = -Cs. 25  Numerical 2 The following data were obtained for the calibration of a calcium ISE and an unknown sample S What is the concentration of calcium in the sample S? 26  Thank You 27