Page 30 - Pharmaceutical analytical chemistry |

the color of the more concentrated form is seen. So, we can calculate the
pH transition range required to go from one color to the other.

▪ When the color of the un-ionized form is seen, [In-] / [HIn] = 1/10
pH = pKind + log 1/10 = pKind - 1

▪ When the color of the ionized form is observed, [In-] / [HIn] = 10/1
pH = pKind + log 10/1 = pKind + 1

Therefore, in going from one color to the other, the pH has changed from
pKind -1 to pKind + 1. This is a pH change of 2, and most indicators
require a transition range of about two pH units. Thus, the transition
range of indicator = pKind ± 1.

Types of acid – base indicators

1) Phthalein indicators
These are made by condensing phthalic anhydride with various phenols.
Most of them are one-color indicators. They are colorless in acid medium
and exhibit a variety of colors in alkaline medium.
Example: Phenolphthalein (pH range 8 - 9.8).

2) Sulfonphthalein indicators
Many of them are two-color indicators, one color in somewhat acidic
solutions and the other in neutral or moderately basic media.
Example: Phenol red (pH range 6.8 - 8.4).

3) Azo indicators
Most azo indicators exhibit a color change from red to yellow with
increasing basicity, their transition ranges are generally on the acidic side.
Example: Methyl orange (pH range 3.1 - 4.4) and Methyl red (pH range
4.2- 6.3).

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