Electroanalytical techniques

Electrochemical methods are used to quantify a broad range of different molecules, including ions, gases, metabolites and drugs.

*Note: The basis of all electrochemical analysis is the transfer of electrons from one atom or molecule to another atom or molecule in an obligately coupled oxidation-reduction reaction la redox reaction).

It is convenient to separate such redox reactions into two half-reactions and, by convention, each is written as:

oxidized form + electron(s) (ne−)	reduction	reduced form
	oxidation

You should note that the half-reaction is reversible: by applying suitable conditions, reduction or oxidation can take place. As an example, a simple redox reaction occurs when metallic zinc (Zn) is placed in a solution containing copper ions (Cu), as follows:

⇒ Equation [34.2]

Cu2+ + Zn → Cu + Zn2+

The half-reactions are (i) Cu²⁺ + 2e⁻ → Cu and (ii) Zn²⁺ + 2e⁻ → Zn. The oxidizing power of (i) is greater than that of (ii), so in a coupled system, the latter half-reaction proceeds in the opposite direction to that shown above, i.e. as Zn − 2e⁻ → Zn²⁺. When Zn and Cu electrodes are placed in separate solutions containing their ions, and connected electrically (Fig. 34.1), electrons will flow from the Zn electrode to Cu²⁺ via the Cu electrode owing to the difference in oxidizing power of the two half-reactions.

By convention, the electrode potential of any half-reaction is expressed relative to that of a standard hydrogen electrode (half-reaction 2H⁺ + 2e⁻ → H₂) and is called the standard electrode potential, E⁰. Table 34.1 shows the values of E⁰ for selected half-reactions. With any pair of half-reactions from this series, electrons will flow from that having the lowest electrode potential to that of the highest. E⁰ is determined at pH = 0. It is often more appropriate to express standard electrode potentials at pH 7 for biological systems, and the symbol E^0' is used: in all circumstances, it is important that the pH is dearly stated.

Fig. 34.1 A simple galvanic electrochemical cell. The KCI salt bridge allows migration of ions between the two compartments but prevents mixing of the two solutions.

The arrangement shown in Fig. 34.1 represents a simple galvanic cell where two electrodes serve as the interfaces between a chemical system and an electrical system. For analytical purposes, the magnitude of the potential (voltage) or the current produced by an electrochemical cell is related to the concentration of a particular chemical species. Electrochemical methods offer the following advantages:

• excellent detection limits, and wide operating range (10⁻¹ to 10⁻⁸ mol L⁻¹);
• measurements may be made on very small volumes (µL) allowing small amounts (pmol) of sample to be measured in some cases;
• miniature electrochemical sensors can be used for certain in vivo measurements, e.g. pH, glucose, oxygen content.

Table 34.1 Standard electrode potentials* (E0) for selected half-reactions