INDICATOR TRANSITION RANGE

The acid base indicator changes color over a pH range. This transition range depends on the ability of the observer to detect the small color changes. Our eyes can generally discern only one color if it 10 times as intense as the other. In this transition pH ranges indicator would have both forms are colored, only one color with more concentrated form is seen. With this basic we can calculate the pH transition of the indicator.

from the equation above we can predict that indicator changes its color from one color to another from pKa-1 to pKa + 1. During this transition the observed color is a mixture of the two colors, and in the midtransition the concentration of two form of indicator are equal then the pKa of indicator should be close to the pH of the equivalence point. If you on preparing acid base titration you better choose an indicator wit a pKa near the equivalence pH.

Here are ph transition ranges some indicators.