Chapter 15 Electrode Measurements “Chem 7” test in hospital lab. Na +, K +, Cl -, CO 2, glucose, urea, & creatinine ion-selective electrodes

15-1 The Silver Indicator Electrode Reference electrode: E is constant Indicator electrode: E varies  In general, use chemically inert elements Pt, Au, C as Indicator electrodes for electron transfer.  In contrast, Ag is in rxn. & conjunction with S.C.E. Ag + + e -  Ag(s) E 0 + = V Hg 2 Cl 2 (s) + 2e -  2Hg(l) + 2Cl - E - = V

Ag + + I -  AgI (s) (1) Before V e Ksp = [Ag + ][I - ]  [Ag + ] = Ksp / [I - ] (2) At V e Ksp = [Ag + ][I - ]  [Ag + ] = [I - ] = √Ksp (3) After V e [Ag + ] = mole of excess Ag + / V soln Titration of a halide ion with Ag + Example at p.337

Use for titration of a mixture at p.140 Fig. 6-4 Fig. 6-5 Ag + + I -  AgI(s) Ksp = 8.3x Ag + + Cl -  AgCl(s) Ksp = 1.8x10 -10

15-2 What is a Junction Potential? The junction potential: (small: ~ mV) caused by unequal mobilities of Na + and Cl -, exists at each end of a salt bridge. E observed = E cell + E junction E junction is usually unknown  inherent inaccuracy ionic mobilities in Table 15-2 junction potentials in Table 15-3 similar mobilities: K + & Cl -  junction E 

Direct versus relative potentiometric measurements Direct potentiometric measurement: inherent inaccuracy liq-liq junction: ex. S = 4% among 14 measurements by direct potentiometry (1)different indicator electrodes (2)varying liquid junction potentials Relative potentiometric measurement: by titration are relatively precise permit an end point to be identified with little uncertainty  0.05

15-3 How Ion-Selective Electrodes Work  The electric potential difference across in ion- selective membrane is measured by two reference electrodes, which might be AglAgCl. If the [C + ] in the unknown solution changes, the voltage changes.  Almost all the analyte ion inside the membrane in Figure 15-6b is bound in the complex LC +, which is in equilibrium with a small amount of free C +. P.342

Two classes of indicator electrodes (1)Metal electrodes: redox reaction take place on surface (Chapter 14) (2)Ion-selective electrodes: selectively binds one ion-no redox chemistry: pH meter etc., (Chapter 15).

15-4 pH Measurement with a Glass Electrode The most widely employed ion- selective electrode [C + ] = [H + ]   = 0.98~1 asymmetry potential Calibrating electrode for 

Structure of glass: network of SiO 4 Glass surface contains –O -