ELECTROCHEMISTRY CHEM 4700 CHAPTER 5 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

CHAPTER 5 POTENTIOMETRY

- Based on static (zero-current) measurements - Used to obtain information on the composition of an analyte - Potential between two electrodes is measured Applications - Environmental monitoring - Clinical diagnostics (blood testing, electrolytes in blood) - Control of reaction processes POTENTIOMETRY

- Also known as indicator electrodes - Respond directly to the analyte - Used for direct potentiometric measurements - Selectively binds and measures the activity of one ion (no redox chemistry) Examples pH electrode Calcium (Ca 2+ ) electrode Chloride (Cl - ) electrode ION-SELECTIVE ELECTRODES (ISE)

Advanteages - Exhibit wide response - Exhibit wide linear range - Low cost - Color or turbidity of analyte does not affect results - Come in different shapes and sizes ION-SELECTIVE ELECTRODES (ISE)

- Made from a permselective ion-conducting membrane (ion-exchange material that allows ions of one electrical sign to pass through) - Reference electrode is inbuilt - Internal solution (solution inside electrode) contains ion of interest with constant activity - Ion of interest is also mixed with membrane - Membrane is nonporous and water insoluble ION-SELECTIVE ELECTRODES (ISE)

- Responds preferentially to one species in solution Internal reference electrode Ion-selective membrane Internal (filling) solution ION-SELECTIVE ELECTRODES (ISE)

- Selective (preferential) ion is C + - Membrane is made of poly(vinyl chloride) (PVC) - 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 (ISE)

- [C + ] inside the electrode ≠ [C + ] outside the electrode - Results in a potential difference across the membrane Generally (at 25 o C) - 10-fold change in activity implies 59/z i mV change in E - z i is the charge on the selective ion (negative for anions) - z i = +1 for K +, z i = +2 for Ca 2+, z i = -2 for CO 3 2- ION-SELECTIVE ELECTRODES (ISE)

- Let c i = molarity of C + - Activity (a i ) rather than molarity is measured by ISEs - Activity is the effective (active) concentration of analyte (effective concentration decreases due to ionic interactions) - z i = ionic charge (±) a i = γ i c i where γ i = activity coefficient (between 0 and 1) ION-SELECTIVE ELECTRODES (ISE)

Debye-Hückel Equation - Relates activity coefficients to ionic strength (at 25 o C) α = size of ion in picometers (1 pm = m) µ = ionic strength ION-SELECTIVE ELECTRODES (ISE)

Ionic strength - A measure of the concentration of all ions in solution with their charges taken into account c i = the concentration of the ith species Ionic strength of electrolytes 1:1 electrolytes (NaCl) µ = molarity 2:1 electrolytes (CaCl 2 ) µ = 3 x molarity 3:1 electrolytes (AlCl 3 ) µ = 6 x molarity 2:2 electrolytes (MgSO 4 ) µ = 4 x molarity ION-SELECTIVE ELECTRODES (ISE)

- For very dilute solutions a i ≈ c i - Activity coefficient decreases as ionic strength increases For z i = mV change in potential implies 4% change in activity For z i = mV change in potential implies 8% change in activity - This is known as Nernstian behavior ION-SELECTIVE ELECTRODES (ISE)

Selectivity Coefficient (k) - A measure of the ability of ISE to discriminate against an interfering ion - It is assumed that ISEs respond only to ion of interest - In practice, no electrode responds to only one specific ion - The lower the value of k the more selective is the electrode - k = 0 for an ideal electrode (implies no interference) ION-SELECTIVE ELECTRODES (ISE)

Selectivity Coefficient (k) For k > 1 - ISE responds better to the interfering ion than to the target ion For k = 1 - ISE responds similarly to both ions For k < 1 - ISE responds more selectively to ion of interest ION-SELECTIVE ELECTRODES (ISE)

Empirical Calibration Plot Potential (mV) p[C + ] Slope = 59/z i mV z i = charge of ion Called Nernstian slope - Used to determine the unknown concentration of analytes - Departure from linearity is observed at low concentrations ION-SELECTIVE ELECTRODES (ISE)

Three groups of ISEs - Glass electrodes - Liquid electrodes - Solid electrodes ION-SELECTIVE ELECTRODES (ISE)

GLASS ELECTRODES - Responsive to univalent cations - Employs thin ion-selective glass membrane

pH GLASS ELECTRODE - The most widely used - For pH measurements (selective ion is H + ) - Response is fast, stable, and has broad range - 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 - Thin glass membrane (bulb) consists of SiO 4 - Most common composition is SiO 2, Na 2 O, and CaO Glass membrane contains - dilute HCl solution - inbuilt reference electrode (Ag wire coated with AgCl)

pH GLASS ELECTRODE Glass Electrode Response at 25 o C (potential across membrane with respect to H + ) ΔpH = pH difference between inside and outside of glass bulb β ≈ 1 (typically ~ 0.98) (measured by calibrating electrode in solutions of known pH) K = assymetry potential (system constant, varies with electrodes)

pH GLASS ELECTRODE - Equilibrium establishes across the glass membrane with respect to H + in inner and outer solutions - This produces the potential, E - Linearity between pH and potential - Calibration plot yields slope = 59 mV/pH units - Electrode is prevented from drying out by storing in aqueous solution when not in use

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)

OTHEER GLASS ELECTRODES Glass Electrodes For Other Cations K + -, NH 4 + -, Na + -selective electrodes - Mechanism is complex - Employs aluminosilicate glasses (Na 2 O, Al 2 O 3, SiO 2 ) - Minimizes interference from H + when solution pH > 5 pH Nonglass Electrodes - Quinhydrone electrode (quinone – hydroquinone couple) - Antimony electrode

LIQUID MEMBRANE ELECTRODES - Employs water-immiscible substances impregnated in a polymeric membrane (PVC) - For direct measurement of polyvalent cations and some anions - The inner solution is a saturated solution of the target ion - Hydrophilic complexing agents (e.g. EDTA) are added to inner solutions to improve detection limits - Inner wire is Ag/AgCl

Ion-Exchange Electrodes - The basis is the ability of phosphate ions to form stable complexes with calcium ions - Selective towards calcium - Employs cation-exchanger that has high affinity for calcium ions (diester of phosphoric acid) - Inner solution is a saturated solution of calcium chloride - Cell potential is given by LIQUID MEMBRANE ELECTRODES

Other Ion-Exchange Electrodes - Have poor selectivity and are limited to pharmaceutical formulations Examples - IEE for polycationic species (polyarginine, protamine) - IEE for polyanionic species (DNA) - IEE for detection of commonly abused drugs (large organic species) LIQUID MEMBRANE ELECTRODES

Neutral Carrier Electrodes - Employs neutral carriers such as crown ethers and cyclic polyesters - Carriers envelope target ions in their pockets Used for clinical analysis - detection of blood electrolytes - detection alkali and alkaline earth metal cations LIQUID MEMBRANE ELECTRODES

Neutral Carrier Electrodes Examples of Carriers - Monensin for sodium - Macrocyclic thioethers for Hg and Ag - Valinomycin for potassium ions - Calixarene derivatives for lead - 14-crown-4-ether for lithium LIQUID MEMBRANE ELECTRODES

Anion-Selective Electrodes - For sensing organic and inorganic anions Examples of Anions - Phosphate - Salicylate - Thiocyanate - Carbonate LIQUID MEMBRANE ELECTRODES

SOLID-STATE ELECTRODES - Solid membranes that are selective primarily to anions Solid-state membrane may be - single crystals - polycrystalline pellets or - mixed crystals

SOLID-STATE ELECTRODES Examples - Most common is fluoride-ion-selective electrode (limited pH range of 0-8.5) (OH - is the only interfering ion due to similar size and charge) - Iodide electrode (high selectivity over Br - and Cl - ) Chloride electrode (suffers interference from Br - and I - ) Thiocynate (SCN - ) and cyanide (CN - ) electrodes

OTHER ELECTRODES - Coated-wire electrodes (CWE) - Solid-state electrodes without inner solutions - Made up of metallic wire or disk conductor (Cu, Ag, Pt) - Mechanism is not well understood due to lack of internal reference - Usually not reproducible For detection of amino acids, cocaine, methadone, sodium

APPLICATIONS OF ISEs - Used as detectors for automated flow analyzers (flow injection systems) - High-speed determination of blood electrolytes in hospitals (H +, K +, Cl -, Ca 2+, Na + ) - For measuring soil samples (NO 3 -, Cl -, Li +, Ca 2+, Mg 2+ ) - Coupling ion chromatography with potentiometric detection - Micro ISEs as probe tips for SECM - Column detectors for capillary-zone electrophoresis