Lecture 20 summary

Reference electrodes. Because the standard hydrogen electrode is not a very convenient half cell for routine measurements, others have been employed as an alternative reference. With the potential difference between the reference and standard hydrogen electrode (S.H.E.) known, voltages measured in combination with the reference can easily be converted to the scale based on S.H.E.

The silver-silver chloride electrode is a common reference electrode. (See Harris, Figs.14-8 and 14-9, p.304)

The calomel (Hg₂Cl₂) reference is another common type of reference electrode. The calomel electrode with saturated KCl(aq) solution is called the saturated calomel electrode (abbreviated S.C.E.). (See Harris, Fig.14-10, p.305)

Voltage conversions (see Harris, Fig. 14-11, p.305). The silver-silver chloride (saturated KCl) electrode has a standard potential of +0.197 V; the saturated calomel electrode has a standard potential of +0.241 V. These values are of course with reference to the standard hydrogen electrode (S.H.E., 0.000 V by definition).

Example: Problem 14-19 (Harris, p.308) (a) Write the half-reactions for the silver-silver chloride and calomel reference electrodes. (b) Predict the voltage for the cell below.

What is a junction potential?

A junction potential is a small (in the range of several to tens of millivolts) potential difference that arises at the interface of two electrolyte solutions due to the differences in ion mobilities. For example, at the junction of a sodium chloride (NaCl) solution and water, a small potential difference arises because chloride diffuses faster into the water than the sodium ion does. Therefore, a small region at the leading edge of the diffusion front is enriched in chloride, while a small region just behind is enriched in sodium ion. This is the source of the junction potential. (See Harris, Fig.15-4, p.315.)

How ion-selective electrodes work

(See Harris, Fig.15-4, p.315.)

pH measurement

Glass combination electrode for pH measurements (see Harris, Fig.15-7, p.319)

Line diagram for glass combination pH electrode (see Harris, p.319)

Response of glass electrode; calibration

Errors in pH measurements

There are seven principal sources of error in pH measurements (see Harris, p.321-322)

• Standards
• Junction potential
• Drift in junction potential
• Sodium error
• Acid error
• Equilibration time
• Hydration