Indicator Equilibrium & Action
An acid-base indicator is a weak organic acid (\(\text{HIn}\)) whose un-ionised molecule and conjugate base ion exhibit distinctly different colours in solution:
\[\underbrace{\text{HIn}(\text{aq})}_{\text{Colour A}} \rightleftharpoons \text{H}^+(\text{aq}) + \underbrace{\text{In}^-(\text{aq})}_{\text{Colour B}}\]The colour change occurs over the range \(\text{pH} = \text{p}K_{\text{In}} \pm 1\), with end-point at \([\text{HIn}] = [\text{In}^-]\) (\(\text{pH} = \text{p}K_{\text{In}}\)).
How Indicators Work
Common IB Indicators
| Indicator | Acid Colour | Base Colour | pH Range | Best For |
|---|---|---|---|---|
| Methyl orange | Red | Yellow | 3.1 - 4.4 | SA + WB |
| Phenolphthalein | Colourless | Pink | 8.3 - 10.0 | WA + SB |
| Bromothymol blue | Yellow | Blue | 6.0 - 7.6 | SA + SB |
Matching pKIn to the Equivalence Point
An indicator is suitable if its colour change interval (\(\text{p}K_{\text{In}} \pm 1\)) coincides with the steep vertical section of the titration pH curve.
Weak Acid - Weak Base Titrations
Weak Acid - Weak Base Titrations: Because there is no sharp vertical pH jump, indicators cannot be used. A pH meter / data-logger must be used to locate the equivalence point.
Complete Indicator Selection Guide
| Titration Type | Equivalence pH | Suitable Indicator |
|---|---|---|
| Strong Acid + Strong Base | \(\approx 7\) | Phenolphthalein or Methyl orange |
| Weak Acid + Strong Base | > 7 | Phenolphthalein (\(\text{p}K_{\text{In}} \approx 9.3\)) |
| Strong Acid + Weak Base | < 7 | Methyl orange (\(\text{p}K_{\text{In}} \approx 3.7\)) |
| Weak Acid + Weak Base | Variable | None (use pH probe) |
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