Current causing compositional changes

Ohm s law was discussed in some depth in Chapter 2, when we saw that a potential applied across a resistor will induce a current. Ohm s law is stated quantitatively as V = IR (equation (2.3)). If the potential V is an emf, and the solution between the two electrodes has a resistance of R, then a current I will flow (it s an ohmic current) - this is often termed the IR drop. As should be clear by now, this current passage through a voltammetric cell is to be avoided wherever possible, since such passage causes compositional changes. 

It seems unlikely that feedbacks due to species replacement have begun since the beginning of the Mauna Loa record because compositional changes due to climate change will take decades and have not yet been documented on a wide scale. However, this fact increases the importance of such feedbacks to future trends in atmospheric CO2 when these feedbacks become important it is highly unlikely that positive and negative feedbacks will exactly cancel each other and more likely that one or the other will prevail and cause deviations from current trends in atmospheric CO2. 

Current Passage through a Cell Causing Internal Compositional Changes... 

An electrical potential difference between the electrodes of an electrochemical cell (called the cell potential) causes a flow of electrons in the circuit that connects those electrodes and therefore produces electrical work. If the cell operates under reversible conditions and at constant composition, the work produced reaches a maximum value and, at constant temperature and pressure, can be identified with the Gibbs energy change of the net chemical process that occurs at the electrodes [180,316]. This is only achieved when the cell potential is balanced by the potential of an external source, so that the net current is zero. The value of this potential is known as the zero-current cell potential or the electromotive force (emf) of the cell, and it is represented by E. The relationship between E and the reaction Gibbs energy is given by... 

Figure 3.15 A flame ionization detector. Hydrogen and oxygen are introduced into the gas mixture as it emerges from the column to allow it to be burnt in the detector. Some molecules are ionized in the flame and cause a current to flow between the two polarized electrodes. The degree of ionization varies with the composition of the gas mixture and the resulting changes in current can be monitored.

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