Ampholytes isoionic point

Experimentally, the isoionic point is defined as the pH of an ampholyte solution which will not change when the concentration is increased by dissolving more of the pure ampholyte. 

The mathematical treatment of the isoelectric conditions (the pH of zero mobility) for an ampholyte is a much more intricate problem than that of the isoionic state. Since the difference is without practical importance, the isoionic state is chosen for the discussions below. Therefore no distinction is made in the following pages between isoelectric and isoionic point. Isoelectric point will be used also for isoionic points and the symbol pi will be used for both. 

Since pi is always larger than pK+, the second term in the denominator will always be larger than unity. This means that the upper limit for a is equal to Obviously a becomes larger as the difference between pK+ and the isoionic point diminishes. The conductivity of an ampholyte is directly dependent of the extent of its ionization. This means that the condition for high conductivity at the isoionic and isoelectric point is that the pair of pK values lying on either side of the pi and nearest to it are close together. 

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