Chapter 23 Potentiometer 1 Principles

1. Chapter 23 Potentiometer1 Principles Measuring concentration using electrodes Indicator electrodes used with reference electrode to measure potential of unknown solution Ecell = Eindicator – Ereference+ Ej (potential arising from salt bridge) Eindicator - responds to ion activity - specific (one ion) or selective (several ions) Two general types of indication electrodes - metallic - membrane

2. Fig. 23-1 (p.660) A cell for potentiometric determination

3. 2 Metallic Indicator Electrodes 2.1 Electrodes of the first kind - respond directly to activity of electrode ion copper indicator electrode Cu2+ + 2e- Cu(s) Problems: simple but not very selective some metal electrode can not be use in acidic solutions some easily oxidized (deaerated solutions)

4. 2.2 Electrodes of the second kind - respond to anion activity through formation of complex silver electrode works as halide or halide-like anions AgCl(s) + e- Ag(s) + Cl- E0 = +0.222 V mercury electrode works for EDTA (ethylene-diamine-tetra-acetic acid) HgY2- + 2e- Hg (l) + Y4- E0 = +0.21 V Y4-: EDTA anion

5. 2.3 Electrodes of the third kind - respond to different ion than metal electrode mercury electrode works for EDTA HgY2- + 2e- Hg (l) + Y4- E0 = +0.21 V CaY2-Ca2+ + Y4- Kf = Ca2+Y4-/caY2-

6. 3 Membrane indicator Electrode (ion-Selective Electrode) Membrane • Minimal solubility – solids, semi-solids and polymer • Some electrical conductivity • Selective reactivity with the analyte Types (see Table 23-2 for examples) Crystalline • Single crystal {LaF3 for F-} • Polycrystalline or mixed crystal: {Ag2S for S2- and Ag} Noncrystalline • Glass:– {silicate glasses for H+, Na+} - Liquid: {liquid ion exchange for Ca2+ }

7. 3.1 Glass pH electrode Contains two reference electrodes Eind = Eb+Eref2 Ecell = Eind - Eref1 Fig. 23-4 (p.666) Glass-calomel cell for pH measurement

8. Combination pH electrode (ref + ind) Fig. 23-3 (p.666) Glass pH electrode

9. Membrane structure SiO4- frame work with charge balancing cations In aqueous, ion exchange reaction at surface H+ + Na+Glass- H+Glass- + Na+ H+ carries current near the surface Na+ carries charge in interior

10. Fig. 23-4 (p.666) Silicate glass structure for a glass pH electrode

11. Boundary Potential Eb Difference compared with metallic electrode: the boundary potential depends only on the proton activity

12. Asymmetry potential Fig. 23-6 (p.669) Potential profile across a glass membrane

13. Boundary Potential Eb

14. Sources of uncertainty in pH measurement with glass-electrode 1. Alkaline error 2. Others {Problems, #23-8) Glass electrodes for other ions (Na+, K+, Cs+,…): - Minimize aH+ • Maximize kH/NaNa+ for other ions • modifying the glass surface (incorporation of Al2O3 or B2O3)

15. Fig. 23-7 (p.670) Acid and alkaline error of selected glass electrode

16. 3.2 Crystalline membrane electrode (optional) • Usually ionic compound • Single crystal • Crushed powder, melted and formed • Sometimes doped with Li+ to increase conductivity • Operation similar to glass membrane Fluoride electrode At the two interfaces, ionization creates a charge on the membrane surface as shown by The magnitude of charge depend on fluoride ion concentration of the solution.

17. 4 Molecule Selective Electrodes 4.1 Gas sensing probes simple electrochemical cell with two reference electrodes and gas-permeable PTFE membrane - allows small gas molecules to pass and dissolve into internal solution - analyte not in direct contact with electrode – dissolved Fig. 23-12 (p.677) Schematic of a gas-sensing probe for CO2

19. 5 Instruments