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

CHAPTER 15 ELECTRODE MEASUREMENTS

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

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

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

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

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

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

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

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 + ]

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

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

- 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

- [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

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 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 V = 0.177

pH GLASS ELECTRODE Glass Electrode Response at 25 o C E = constant + β( )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

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)

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)

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

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

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

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

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

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

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

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