Overview
This lecture explains how to estimate the pH of an unknown solution using different acid-base indicators and their color changes.
Steps to Predict pH Using Indicators
- Prepare a list of indicators used and the colors observed in the unknown solution.
- Draw a pH scale ranging from 0 (acidic) to 14 (basic).
- Mark the color-change ranges of each indicator on the pH scale.
- Note the pH range where each indicator displays the observed color in the unknown solution.
Example Analysis
- Phenol red turned yellow, indicating a pH range of 0 to 6.6.
- Methyl orange turned yellow, indicating a pH range of 4.4 to 14.0.
- Phenolphthalein was colorless, indicating a pH range of 0.0 to 8.2.
- Methyl red appeared red, indicating a pH range of 0.0 to 4.8.
- The unknown solution’s pH is determined by the overlap of these color ranges.
- The narrowest overlapping pH range from all indicators is 4.4 to 4.8.
Key Terms & Definitions
- Indicator — A substance that changes color depending on the pH of the solution.
- pH scale — A scale from 0 to 14 measuring acidity (low values) or basicity (high values).
- Phenol red — Indicator, yellow at pH 0–6.6.
- Methyl orange — Indicator, yellow at pH 4.4–14.0.
- Phenolphthalein — Indicator, colorless at pH 0.0–8.2.
- Methyl red — Indicator, red at pH 0.0–4.8.
Action Items / Next Steps
- Practice predicting pH using different indicator color results.
- Review indicator color ranges from the Chemistry 20 data booklet.