ABC - Titration Indicators (Lesson)
Titration Indicators
In the previous lessons, the quantitative aspects of titrations were discussed by calculating the pH of the solutions at various points along a titration curve. In this lesson, acid/base indicators are discussed, including what their role is and how to choose the appropriate indicator for the titration being performed.
Acid-Base Indicators
In previous lessons, the equivalence point of a titration was discussed; the equivalence point is the stoichiometric point at which the number of moles of acid and base are equal to one another. In practical experience, however, most acids and bases are completely colorless, and this equivalence point is impossible to determine when titrating an acid or base that has an unknown concentration. In order to visually estimate when this point occurs during the course of a titration, an indicator is typically used.
An indicator is a substance that is added in a titration to signal the end of the titration by changing color. It must be noted that the equivalence point of a titration is not the same thing as an endpoint. The endpoint of a titration is the point where the indicator changes color, and this could possibly be right before or right after the actual equivalence point is reached. The goal of choosing the proper indicator for a titration is to choose one that will provide an endpoint that matches as closely as possible to the actual equivalence point. So how is this done?
A general rule of thumb is to choose an indicator whose pKa matches as close as possible to the pH of the solution at the equivalence point. A table showing different indicators and their pKa values is shown below:
| Indicator | pKa | Color Change |
|---|---|---|
| Alizarin yellow R | 11.0 |
yellow to red |
| Thymolphthalein | 9.9 | colorless to blue |
| Phenolphthalein | 9.5 | colorless to pink |
| Thymol blue | 9.2 | yellow to blue |
| Phenol red | 7.4 | yellow to red |
| Bromothymol blue | 7.3 | yellow to blue |
| Ethyl red | 5.4 | colorless to red |
| Resorcin blue | 5.3 | red to blue |
| Resazurin | 5.1 | orange to violet |
| Methyl red | 5.0 | red to yellow |
| Bromophenol blue | 4.1 | yellow to blue |
| Malachite green | 1.3 | yellow to turquoise |
| Methyl violet | 0.8 | yellow to blue |
Using this table the proper indicators for the following titrations can be chosen.
- A titration between HCl and NaOH. Because this is a titration between a strong acid and a strong base, the pH at the equivalence point will be 7.0 with no calculations required. After consulting the chart above, it can be seen that the two best choices of indicator for this titration would be either Phenol Red or Bromothymol Blue because their pKa values most closely match the pH at the equivalence point.
- A titration between formic acid and NaOH whose pH at the equivalence point is equal to 8.57. Calculations for this example (not shown here) reveal that the pH at the equivalence point is significantly basic. The best indicator to choose in this situation would be Thymol Blue, as its pKa is closest to the 8.57 pH at the equivalence point.
A chart showing additional common indicators is shown below, and it also contains a visual representation of their color changes at the endpoint.
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