Measuring concentration using electrodes Indicator electrodes used with reference electrode to measure potential of unknown solution E cell = E indicator – E reference + E j (potential arising from salt bridge) E indicator - responds to ion activity - specific (one ion) or selective (several ions) Two general types of indication electrodes - metallic - membrane

Fig (p.660) A cell for potentiometric determination

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

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 - E 0 = V mercury electrode works for EDTA (ethylene-diamine-tetra-acetic acid) HgY e -  Hg (l) + Y 4- E 0 = V Y 4- : EDTA anion

2.3 Electrodes of the third kind - respond to different ion than metal electrode mercury electrode works for EDTA HgY e -  Hg (l) + Y 4- E 0 = V CaY 2-  Ca 2+ + Y 4- K f =  Ca2+  Y4- /  caY2-

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 {LaF 3 for F - } - Polycrystalline or mixed crystal: {Ag 2 S for S 2- and Ag} Noncrystalline - Glass:– {silicate glasses for H +, Na + } -Liquid: {liquid ion exchange for Ca 2+ }

3.1 Glass pH electrode Contains two reference electrodes E ind = E b +E ref2 E cell = E ind - E ref1 Fig (p.666) Glass-calomel cell for pH measurement

Fig (p.666) Glass pH electrode Combination pH electrode (ref + ind)

Membrane structure SiO 4 - 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

Fig (p.666) Silicate glass structure for a glass pH electrode

Boundary Potential E b Difference compared with metallic electrode: the boundary potential depends only on the proton activity

Asymmetry potential Fig (p.669) Potential profile across a glass membrane

Boundary Potential E b

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 a H+ - Maximize k H/Na  Na+ for other ions - modifying the glass surface (incorporation of Al 2 O 3 or B 2 O 3 )

Fig (p.670) Acid and alkaline error of selected glass electrode

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.

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 (p.677) Schematic of a gas-sensing probe for CO 2