Box 14-4 Concentrations in the Operating Cell

Why doesn t operation of a cell change the concentrations in the cell Cell voltage is measured under conditions of negligible current flow. The resistance of a high-quality pH meter is 1013 ft. If you use this meter to measure a potential of 1 V, the current is 

The rate at which Cd2+ is produced is 2.5 X 10 20mol/s, which has a negligible effect on the cadmium concentration in the cell. The meter measures the voltage of the cell without affecting concentrations in the cell. 

If the salt bridge were left in a real cell for very long, concentrations and ionic strength would change because of diffusion between each compartment and the salt bridge. We assume that cells are set up for such a short time that mixing does not happen. 

Equation 14-22 is true at any time. In the special case when the cell is at equilibrium, E = 0 and Q = K, the equilibrium constant. Therefore, Equation 14-22 is transformed into these most important forms at equilibrium  

Equation 14-24 allows us to deduce the equilibrium constant from E°. Alternatively, we can find E from K with Equation 14-23. 

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