Methyl Orange
Summary: Methyl orange is an indicator used in acid-base titrations. It is red in acidic solutions and yellow in alkaline solutions. Its colour change occurs at pH 3.1–4.4, making it ideal for strong acid-weak base titrations. Tags: igcse chemistry definition acids-bases indicators Created: 2026-07-14 Last Updated: 2026-07-23
Methyl orange is a synthetic indicator used in acid-base titrations. It is specifically mentioned in the IGCSE Chemistry 0620 syllabus as one of the indicators affected by acids and alkalis.
Colour Changes
| Solution Type | Colour of Methyl Orange |
|---|---|
| Acidic (pH < 3.1) | Red |
| Neutral (pH = 7) | Orange |
| Alkaline (pH > 4.4) | Yellow |
The colour change occurs over the range pH 3.1–4.4.
Why “Orange” at Neutral pH?
At pH 7 (neutral), methyl orange is in transition between its red (acid) and yellow (alkali) forms, giving an orange colour. This is where the indicator gets its name.
Comparison with Other Indicators
| Indicator | Acid Colour | Neutral Colour | Alkali Colour | pH Range |
|---|---|---|---|---|
| Methyl orange | Red | Orange | Yellow | 3.1–4.4 |
| Litmus | Red | Purple | Blue | 5–8 |
| Phenolphthalein | Colourless | Colourless | Pink | 8.3–10.0 |
| Thymolphthalein | Colourless | Colourless | Blue | 9.3–10.5 |
When to Use Methyl Orange
Methyl orange is the preferred indicator for titrations involving:
- Strong acid + weak base — the end point is in the acidic range (pH ~5)
- Strong acid + strong base — works well (end point pH ~7 falls within its colour change range)
It should NOT be used for:
- Weak acid + strong base — end point is alkaline (pH ~9) — use phenolphthalein instead
Key Facts
- Methyl orange is red in acid and yellow in alkali
- Colour change range: pH 3.1–4.4 (acidic range)
- Ideal for strong acid-weak base and strong acid-strong base titrations
- Named in the IGCSE 0620 syllabus as an indicator affected by acids and alkalis
- At neutral pH 7, it appears orange
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.