This document summarizes a lecture on pharmaceutical analysis and reference electrodes. It discusses the standard hydrogen electrode, including its construction, working principles, and advantages and disadvantages. The standard hydrogen electrode is the reference electrode used to establish standard-state potentials for other half-reactions, though it is not commonly used for routine analytical work due to various disadvantages. It consists of a platinum electrode immersed in a solution with a hydrogen ion activity of 1.00 and hydrogen gas fugacity of 1.00, connected via a salt bridge. When connected to another electrode, the potential of that electrode can be measured relative to the standard hydrogen electrode.

1. Lecture- 6
Subject – Pharmaceutical Analysis-I
Course – B. Pharm 1st Year
Faculty Name – Umesh Kumar

2. Unit- V

3. Contents
 Reference Electrodes
◦ Standard Hydrogen
 Construction
 Working
 Advantages
 Disadvantages

4. 1. Reference Electrode
 In a potentiometric electrochemical cell one half-cell
provides a known reference potential and the potential of
the other half-cell indicates the analyte’s concentration.
 The ideal reference electrode provides a stable, known
potential so that any change in Ecell is attributed to
analyte’s effect on the potential of the indicator
electrode.
 In addition, the ideal reference electrode should be easy
to make and to use.

5. A. Standard Hydrogen Electrode
 Although we rarely use the standard hydrogen electrode
(SHE) for routine analytical work, it is the reference
electrode used to establish standard-state potentials for
other half-reactions.
 The SHE consists of a Pt electrode immersed in a
solution in which the activity of hydrogen ion is 1.00
and in which the fugacity of H (g) is 1.00.
 A conventional salt bridge connects the SHE to the
indicator half-cell.

6. Construction of Hydrogen Electrode;

7. Working of Hydrogen Electrode;
 When the standard hydrogen electrode is connected to
any electrode by connecting through salt bridge, the
potential of that electrode can be measured;
 When a zinc electrode is connected by potassium
chloride salt bridge, we can write,
Pt, H2 I H+ (a = 1) II Zn++ II Zn
NHE (a = Unknown)

8. Advantages of Hydrogen Electrode;
 Used as a standard in pH measurement.
 It can be used over a wide pH range.
 It exhibits no salt error.
 It establishes equilibrium rapidly, and gives accurate
results.

9. Disadvantages of Hydrogen Electrode;
 It can not be used in solutions containing strong
oxidizing or reducing agents.
 It can not be used in solutions containing metal ions that
are below hydrogen in potential series.
 It gets rapidly poisoned by a number of substances like
proteins, tannins, mercury salts, etc.
 It is cumbrous to prepare, and use in routine analysis

10. To be continued in Next video