Potentiometry (Ch 15-1 to 15-3)

• To describe the construction and half-cell reactions of a Ag|AgCl and a calomel reference (Do Exercise 15-1 & 15-2) • To determine ionic activities using a metal indicator electrode in combination with a reference electrode (Do Exercise 15-9) • To explain the cause of junction potential (Do Exercise 15-13)

Chem215/P.Li/Potentiometry/P 1

Classification of electroanalytical techniques

Electroanalytical chemistry

Non-faradaic methods Faradaic methods potentiometry potentiometric conductometry titration or electrodeposition

at non-stationary at stationary electrode electrode (e.g. ) (e.g. )

Chem215/P.Li/Potentiometry/P 2

1 Potentiometry

Potentiometry uses to measure voltages which are related to chemical information. Indicator electrode or : responds to analyte activity. Reference electrode: maintains a fixed (reference) potential.

Ag(s) AgCl(s)Cl - (aq) Fe3+ (aq), Fe2+ (aq) Pt(s) 144424443 144424443 reference electrode indicator electrode

Chem215/P.Li/Potentiometry/P 3

Reference electrodes Ag½AgCl reference electrode

See Fig. 15-3 for a commercial single-junction Ag½AgCl reference electrode

See Fig. 15-4 for a commercial double-junction Ag½AgCl reference electrode

There is an additional compartment to contain KCl solution or any other solution

(e.g. KNO3) that is more compatible with the analyte solution.

At 25 °C, E° = + 0.222V AgCl(s) + e- Ag(s) + Cl- (aq) E(saturated KCl) = + 0.197V 0.05916 E = E o - log[Cl - ] 1 0.197 = 0.222 - 0.05916log[Cl - ] \[Cl - ] = 2.65M

The advantage of using saturated KCl solution is the [Cl-] does not change even if some water evaporates. Chem215/P.Li/Potentiometry/P 4

2 Reference electrodes

Calomel electrode (calomel is Hg2Cl2)

- - ½Hg2Cl2(s) + e Hg(l) + Cl (aq) Fig. 15-5 for a commercial double-junction saturated At 25 °C, E° = + 0.268V calomel electrode (S.C.E.) E(saturated KCl) = + 0.241V

How to convert voltages between different reference scales? Fig. In Sec 15-1, voltage scale with reference to standard hydrogen electrode. S.C.E. A -0.461 B -0.417 Ag|AgCl +0.033 -0.011 S.H.E.

-0.220 0 +0.197 +0.230 +0.241

Chem215/P.Li/Potentiometry/P 5

Indicator electrodes

A) metal electrode: inert metals (e.g. Pt, Au and carbon); electroactive metals (e.g. Ag, Hg). B) Ion-selective electrode (I.S.E.): This is not based on processes. Instead, selective migration of one type of ion across an electrode membrane generates an electric potential (e.g. pH electrode, F- electrode).

Chem215/P.Li/Potentiometry/P 6

3 Indicator electrodes

+ - o Ag + e Ag E+ = 0.799V

- - ½Hg2Cl2(s) + e Hg(l) + Cl (aq) E-° = 0.241V

1 - + Hg(l) 2 Hg2Cl2 (s) Cl (aq) Ag (aq) Ag(s)

é 0.05916 1 ù + E = E+ - E- = ê0.799- log + ú - 0.241 = 0.558+ 0.05916log[ Ag ] ë 1 [Ag ]û

So the voltage of the cell provides a direct measure of [Ag+].

Chem215/P.Li/Potentiometry/P 7

Junction potential

Junction potential is developed when 2 dissimilar electrolyte solutions are in contact.

Fig. 15-7: development of junction potential caused by unequal mobilities of Na+ and Cl-.

Since Cl- diffuses faster than Na+, excess negative charge is developed on the side containing excess Cl-.

KCl and KNO3 are usually used in salt bridge because of the similar mobilities of + - - K , Cl and NO3 .

Since the junction potential is usually not known, its presence puts a fundamental limitation on the accuracy of direct potentiometric meaurements.

Chem215/P.Li/Potentiometry/P 8

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