Chapter 15

Electrode Measurements

“Chem 7” test in hospital lab.Na+, K+, Cl-, CO2, glucose, urea, & creatinine

ion-selective electrodes

15-1 The Silver Indicator Electrode

Reference electrode: E is constantIndicator 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) E0+ = 0.799 V

Hg2Cl2(s) + 2e- 2Hg(l) + 2Cl- E- = 0.241 V

][Ag log 0592.0558.0241.0][Ag

1log 0592.00.799EEE

Ag+ + I- AgI (s)

(1) Before Ve Ksp = [Ag+][I-] [Ag+] = Ksp / [I-]

(2) At Ve

Ksp = [Ag+][I-] [Ag+] = [I-] = √Ksp

(3) After Ve

[Ag+] = mole of excess Ag+ / Vsoln

17sp 108.3

1

K

1K

][Ag log 0592.0558.0E

Titration of a halide ion with Ag+

Example at p.337

Use for titration of a mixture at p.140

Fig. 6-4 Fig. 6-5Ag+ + I- AgI(s) Ksp = 8.3x10-17

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.

Eobserved = Ecell + E junction

E junction is usually unknown inherent inaccuracy

ionic mobilities in Table 15-2junction potentials in Table 15-3

similar mobilities: K+ & Cl- junction E

Direct versus relative potentiometric measurements

Direct potentiometric measurement:inherent inaccuracyliq-liq junction: ex. S = 4% among 14measurements by direct potentiometry

(1) different indicator electrodes (2) varying liquid junction potentials

Relative potentiometric measurement: by titrationare relatively precisepermit an end point to be identified with little uncertainty.

1.22 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

pH(0.0592) constantE

• Structure of glass: network of SiO4

• Glass surface contains –O-