Indicators

Indicators change color depending on pH and can be used to monitor the pH of a solution.

A pH indicator contains a weak acid, represented by HIn, which changes color when converted to its conjugate base, the In ion. The color changes over the indicator’s pH interval, which depends on its pKa.

If a solution has a pH below the pKa of the indicator, the hydronium concentration is high. The equilibrium of the reaction shifts towards the left, increasing the concentration of HIn.

When a base is added, the concentration of hydronium decreases. This pH change converts HIn into the In ion, and the solution’s color shifts.

The HIn color dominates when the pH is at least one lower than the pKa; the In ion color dominates when the pH is at least one higher, and the color is a mix at pHs in between.

There are a variety of pH indicators which can be used to monitor the pH that change color over distinct pH ranges.

The point at which an indicator changes color is called the endpoint. The ideal indicator for a titration has an endpoint near the equivalence point.

The titration curve of a strong acid and base is steep due to the rapid pH change near the equivalence point. These titrations can use a relatively broad range of indicators.

During the titration of 0.1 molar hydrochloric acid with 0.1 molar sodium hydroxide, the pH quickly increases from 3 to 11. Either phenolphthalein, an indicator with the pH interval of 8.3 to 10, or methyl red, an indicator with an interval of 4.2 to 6, could be used here as their endpoints overlap with the steep portion of the curve.

In contrast, a titration involving a weak acid or base has a less steep curve than one with only a strong acid and a strong base; therefore, a relatively narrow range of indicators can be used.

During the titration of 0.1 molar acetic acid with 0.1 molar sodium hydroxide, the pH increases quickly from 7 to 11. Phenolphthalein can be used here as its endpoint overlaps with the equivalence point, but methyl red would be an ineffective indicator.