ELECTROCHEMISTRY CHEM 4700 CHAPTER 5

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

2. CHAPTER 5 POTENTIOMETRY

3. 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

4. ION-SELECTIVE ELECTRODES (ISE) - 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 (Ca2+) electrode Chloride (Cl-) electrode

5. 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

6. 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

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

8. 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)

9. ION-SELECTIVE ELECTRODES (ISE) - [C+] inside the electrode ≠ [C+] outside the electrode - Results in a potential difference across the membrane Generally (at 25 oC) - 10-fold change in activity implies 59/zi mV change in E - zi is the charge on the selective ion (negative for anions) - zi = +1 for K+, zi = +2 for Ca2+, zi = -2 for CO32-

10. ION-SELECTIVE ELECTRODES (ISE) - Let ci = molarity of C+ - Activity (ai) rather than molarity is measured by ISEs - Activity is the effective (active) concentration of analyte (effective concentration decreases due to ionic interactions) - zi = ionic charge (±) ai = γici where γi = activity coefficient (between 0 and 1)

11. ION-SELECTIVE ELECTRODES (ISE) Debye-Hückel Equation - Relates activity coefficients to ionic strength (at 25 oC) α = size of ion in picometers (1 pm = 10-12 m) µ = ionic strength

12. ION-SELECTIVE ELECTRODES (ISE) Ionic strength - A measure of the concentration of all ions in solution with their charges taken into account ci = the concentration of the ith species Ionic strength of electrolytes 1:1 electrolytes (NaCl) µ = molarity 2:1 electrolytes (CaCl2) µ = 3 x molarity 3:1 electrolytes (AlCl3) µ = 6 x molarity 2:2 electrolytes (MgSO4) µ = 4 x molarity

13. ION-SELECTIVE ELECTRODES (ISE) - For very dilute solutions ai ≈ ci - Activity coefficient decreases as ionic strength increases For zi = 1 - 1 mV change in potential implies 4% change in activity For zi = 2 - 1 mV change in potential implies 8% change in activity - This is known as Nernstian behavior

14. 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)

15. 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

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

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

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

19. 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 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

20. pH GLASS ELECTRODE - Thin glass membrane (bulb) consists of SiO4 - Most common composition is SiO2, Na2O, and CaO Glass membrane contains - dilute HCl solution - inbuilt reference electrode (Ag wire coated with AgCl)

21. pH GLASS ELECTRODE Glass Electrode Response at 25 oC (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)

22. 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

23. 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)

24. OTHEER GLASS ELECTRODES Glass Electrodes For Other Cations K+ -, NH4+-, Na+-selective electrodes - Mechanism is complex - Employs aluminosilicate glasses (Na2O, Al2O3, SiO2) - Minimizes interference from H+ when solution pH > 5 pH Nonglass Electrodes - Quinhydrone electrode (quinone – hydroquinone couple) - Antimony electrode

25. 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

26. LIQUID MEMBRANE ELECTRODES 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

27. 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)

28. 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

29. 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

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

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

32. 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

33. 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

34. APPLICATIONS OF ISEs - Used as detectors for automated flow analyzers (flow injection systems) - High-speed determination of blood electrolytes in hospitals (H+, K+, Cl-, Ca2+, Na+) - For measuring soil samples (NO3-, Cl-, Li+, Ca2+, Mg2+) - Coupling ion chromatography with potentiometric detection - Micro ISEs as probe tips for SECM - Column detectors for capillary-zone electrophoresis