General Considerations on Neutralization Indicators

Neutralization indicators are organic substances that are acids or bases themselves. The color of their acidic form is different from that of the basic form. The ionization constant of the indicator couple is given by the equation 

These relations show that the acid and base concentrations ratio depends on the pH through the indicator pKa value. The solution color depends on the pH value. Some indicators exhibit only one colored form they are called unicolor indicators. For example, this is the case of phenolphthalein. The theory concerning them is the same as that concerning bicolor indicators except for the influence of their concentration on the color-change interval (see Sect. 8.4). 

Recall that assuming activities equal to concentrations, as was the case above, is not always legitimate. 

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