PLANT BIOPHYSICS - Y. I. Posudin - 2004

II. TRANSPORT PROCESSES IN THE SOIL-PLANT-ATMOSPHERE SYSTEM

11. ELECTRICAL TRANSPORT

11.3. MEASUREMENT OF ACTION POTENTIALS

Electrochemical Methods rely on the ability of redox reactions to proceed not only upon direct contact but also when spatially separated. Consider a reductant Red₁ and an oxidant Ox₂ placed in separate vessels. If metal electrodes are immersed into the solutions and connected by an external conductor, while the solutions are joined by a salt bridge (Fig. 11.1), the reductant will donate electrons to the first electrode E₁ and undergo oxidation (Red->Ox₁). Electrons will flow through the external circuit to the second electrode E₂, which transfers them to the oxidant, causing the latter to be reduced (Red₇->Ox₂). Thus, the following reaction takes place:

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Fig. 11.1. Principle of operation of electrochemical methods (explanation in the text).

This process is accompanied by electron transport through the external circuit and ion transport through the internal circuit. If the external circuit prevents electron flow, such an electrochemical Cell can be used for potential measurements.

One of the electrodes must respond to Changes in the concentration of the substance being determined. This electrode is called the indicator or working electrode. To prevent it from reacting with sample components, it is made of noble metals (Hg, Pt, Au) or inert Materials (such as graphite).

The second electrode is called the reference electrode; it serves as a baseline reference for the parameter measured by the indicator electrode. Silver-silver chloride (AgCl) or calomel (Hg₇Cl₂) electrodes are commonly used. The appearance of a reference electrode is shown in Fig. 11.2.

Fig. 11.2. Appearance of the reference electrode: 1 - sleeve, 2 - Glass tube, 3 - electrode, 4 - stopper, 5 - cotton plug, 6 - electrolyte solution, 7 - silver chloride Column.

A typical setup for measuring resting and action potentials is shown in Fig. 11.3. The indicator electrode (Ag/AgCl or Hg₇Cl₂), which is in contact with the plant, transmits a signal to an electrometer with a high input impedance (200 TΩ). The electrometer is interfaced with a computer for signal Processing and analysis. The reference electrode makes contact with the Base of the plant.

Fig. 11.3. Typical setup for measuring resting and action potentials.