With ordinary acid base indicators, the color change is not sharp and abrupt, but it extends over 2 units of pH. However, sometimes, it is desirable to have sharp color change over a narrow and selected range of pH. This can be achieved by the use of mixture of indicators whose pKind values are close together and the overlapping colors are complementary at an intermediate pH value.
For example:
A mixture of 3parts of phenolphthalein and 1 part of naphtholphthalein shows a change in color from pale rose to violet at pH = 8.9. The mixed indicator is useful for the titration of phosphoric acid to diproti stage. Similarly, a mixture of 3 parts of thymol blue and 1 part of cresol red shows change in color from yellow to violet at pH = 8.3. This mixed indicator is useful for the titration of CO32- to HCO3– stage.
