1. ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 15 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 15 ELECTRODE MEASUREMENTS

3. INDICATOR ELECTRODES Chemically Inert Electrodes - Do not participate in the reaction Examples Carbon Gold Platinum ITO

4. INDICATOR ELECTRODES Reactive Electrodes - Participate in the reaction Examples Silver Copper Iron Zinc

5. INDICATOR ELECTRODES - Respond directly to the analyte Two Classes of Indicator Electrodes - Metal Electrodes - Surfaces on which redox reactions take place Examples Platinum Silver

6. INDICATOR ELECTRODES - Respond directly to the analyte Two Classes of Indicator Electrodes - Ion-Selective Electrodes - Selectively binds one ion (no redox chemistry) Examples pH electrode Calcium (Ca 2+ ) electrode Chloride (Cl - ) electrode

7. DOUBLE-JUNCTION REFERENCE ELECTRODES - With the use of reference electrodes - KCl solution may slowly leak into solution through the porous plug (salt bridge) - Cl - may introduce errors (e.g. consumes Ag + when reagent is Ag + solution) - Double-junction reference electrode prevents direct leakage into reagent

8. JUNCTION POTENTIAL - When two dissimilar electrolyte solutions come in contact - Potential difference develops at the interface - Voltage is very small usually in millivolts - Very common at the ends of salt bridges - Observed voltage measurements may include junction potential

9. JUNCTION POTENTIAL E observed = E cell + E junction - A result of unequal ion mobilities - K + and Cl - have similar mobilities - Reason why KCl is used in salt bridges

10. POTENTIOMETRY - The use of voltage measurements for quantification Direct Potentiometric Method - Measures absolute potential (concentration) - A metal in contact with a solution of its cation - Associated with errors due to junction potentials Examples - Silver wire for measuring [Ag + ] - Potassium ion-selective electrode for measuring [K + ] - pH electrode for measuring [H + ]

11. POTENTIOMETRY - The use of voltage measurements for quantification Relative Potentiometric Method - Measures changes in potential (concentration) - Relatively precise and accurate Example - Measuring changes in potential during titration

12. ION-SELECTIVE ELECTRODES - Responds preferentially to one species in solution Internal reference electrode Ion-selective membrane Filling solution

13. - Selective (preferential) ion is C + - Membrane is made of poly(vinyl chloride) - Membrane is impregnated with nonpolar liquid - Membrane contains ligand L (ion-selective ionophore) - Membrane contains the complex LC + - Membrane contains hydrophobic anion R - (ion exchanger) ION-SELECTIVE ELECTRODES

14. - [C + ] inside the electrode ≠ [C + ] outside the electrode - Produces a potential difference across the membrane ION-SELECTIVE ELECTRODES - n is the charge on the selective ion (negative for anions) n = +1 for K + n = +2 for Ca 2+ n = -2 for CO 3 2- at 25 o C

15. pH GLASS ELECTRODE - The most widely used - Selective ion is H + - Glass membrane (bulb) consists of SiO 4 - pH changes by 1 when [H + ] changes by a factor of 10 - Potential difference is 0.05196 V when [H + ] changes by a factor of 10 For a change in pH from 3.00 to 6.00 (3.00 units) Potential difference = 3.00 x 0.05196 V = 0.177

16. pH GLASS ELECTRODE Glass Electrode Response at 25 o C E = constant + β(0.05916)ΔpH ΔpH = pH difference between inside and outside of glass bulb β ≈ 1 (typically ~ 0.98) (measured by calibrating electrode in solutions of known pH) constant = assymetry potential

17. pH GLASS ELECTRODE Sources of Error - Standards used for calibration - Junction potential - Equilibration time - Alkaline (sodium error) - Temperature - Strong acids - Response to H + (hydration effect)

18. COMPOUND ELECTRODE - Electrode surrounded by a membrane - Membrane isolates the analyte to which the electrode responds Examples - Gas sensing electrodes NH 3, CO 2, NO x, H 2 S, SO 2 - Enzyme electrodes (highly selective)

19. ELECTROCHEMICAL METHODS Applications - Biosensors (analyte sensors) (Glucose sensors) - Chromatography detectors - Solar energy storage systems - Microelectronics - Electrocatalysis of fuel cells and batteries

20. Electrogravimetric Analysis - Chemically inert cathode with large surface area is used (in the form of gauze) - Analyte is electroplated (deposited) on a preweighed cathode - Cathode is weighed again - Mass of analyte is determined by difference Cu 2+ (aq) + 2e - → Cu(s) (deposited on cathode) ELECTROCHEMICAL METHODS

21. Coulometric Analysis - Amount of analyte is determined from electron count - Electric current and time required to generate product are measured - Number of electrons is determined from current and time - Number of moles of analyte is determined from electron count Reaction of I 2 and H 2 S I 2 + H 2 S → S(s) + 2H + + 2I - ELECTROCHEMICAL METHODS

22. Three Electrode Cells - Reference electrode - Working (indicator) electrode - Auxiliary (counter) electrode - Current flows between working and auxiliary electrodes - Voltage is measured between working and reference electrodes ELECTROCHEMICAL METHODS

23. Amperometry - The electric current between the pair of electrodes is measured - Voltage is fixed - Current is proportional to the concentration of analyte Biosensors (glucose monitors) ELECTROCHEMICAL METHODS

24. Voltammetry - Voltage between two electrodes is varied as current is measured - Oxidation-reduction takes place at or near the surface of the working electrode - Graph of current versus potential is obtained (called voltammogram) - Peak current is proportinal to concentration of analyte ELECTROCHEMICAL METHODS

25. Voltammetry Polarography - Uses dropping-mercury electrode Square Wave Voltammetry - Uses waveform which consists of square wave superimposed on a staircase ELECTROCHEMICAL METHODS

26. Voltammetry Stripping Voltammetry - Analyte is concentrated into a drop of Hg by reduction - Analyte is reoxidized by making potential more positive - Current is measured during oxidation Cyclic Voltammetry (CV) - Electrode potential versus time is linear - Current versus applied voltage gives a cyclic voltammogram trace - Used to study electrochemical properties of analytes ELECTROCHEMICAL METHODS