Methyl orange colour change

The methyl orange colour change is from red in acid to yellow in alkali. In between, over a pH range of about 3.1 to 4.4, it looks orange, and that orange colour marks the end point of a titration. Because it changes colour in the acidic range, methyl orange is used for titrations involving a strong acid.

Methyl orange colours

Notice that methyl orange is already yellow at pH 7. It doesn't tell you a solution is neutral: it tells you whether the pH is below or above its narrow orange range. These values are from the Royal Society of Chemistry's indicator table.

Methyl orange colour change in a titration

Which way the colour changes depends on what is in the flask:

• Alkali in the flask, acid added from the burette: the solution starts yellow. At the end point it turns orange. Add too much acid and it goes red.
• Acid in the flask, alkali added from the burette: the solution starts red, turns orange at the end point, and goes yellow if you overshoot.

Aim for the first permanent orange colour. Near the end point, add the solution drop by drop and swirl the flask after each drop. A white tile under the flask makes the colour change easier to see.

Check your understanding.

Methyl orange vs phenolphthalein

Universal indicator isn't used for titrations, because it changes gradually through many colours instead of giving one sharp change at the end point.

Choosing an indicator: when to use methyl orange

During a titration, the pH changes slowly at first, then shoots up or down sharply near the equivalence point, the point where exactly enough acid has been added to react with the alkali. A good indicator must change colour within that steep section. At A-level you choose it from the pH curve:

Example 1: hydrochloric acid and ammonia

Ammonia solution is titrated with hydrochloric acid. The salt formed, ammonium chloride, makes the solution slightly acidic at the equivalence point, and the steep part of the pH curve lies in the acidic range. Methyl orange changes colour between 3.1 and 4.4, inside that steep section, so it is suitable. Phenolphthalein would change too early, while the pH is still high.

Example 2: ethanoic acid and sodium hydroxide

Here the equivalence point is above pH 7 and the steep section is in the alkaline range. Methyl orange would change colour too early, while the pH is still rising slowly, giving a titre that is too small. Phenolphthalein, which changes between 8.3 and 10.0, is the right choice.

Watch a pH curve build up as you add solution in the Titration Lab, and see how the result feeds into a calculation in the titration calculator guide.

Why methyl orange changes colour (A-level)

Methyl orange is a weak acid, which we can write as HIn. The acid form and its conjugate base have different colours:

Adding acid raises the concentration of H⁺, so the equilibrium shifts to the left and the red HIn form dominates. Adding alkali removes H⁺ (it reacts with OH⁻), so the equilibrium shifts right and the yellow In⁻ form dominates. When the two forms are present in similar amounts, the mixture looks orange.

Common mistakes

• Saying methyl orange is orange in alkali. It is yellow in alkali; orange is only the end point colour.
• Giving the colours the wrong way round. Red in acid, yellow in alkali.
• Using methyl orange for a weak acid and a strong base. Its range is too low; use phenolphthalein.
• Thinking yellow means neutral. Methyl orange is yellow anywhere above about pH 4.4.
• Writing the end point as "red to yellow". At the end point you look for the first permanent orange.

Practice questions

1. What colour is methyl orange in a solution of pH 2?
2. What colour is methyl orange in pure water?
3. Hydrochloric acid in a flask with methyl orange is titrated with sodium hydroxide. Describe the colour change at the end point.
4. Explain why methyl orange is not suitable for titrating ethanoic acid with sodium hydroxide.

Answers: 1) Red. 2) Yellow (pH 7 is above 4.4). 3) Red to orange. 4) The steep part of the pH curve is in the alkaline range, above methyl orange's range of 3.1 to 4.4, so methyl orange would change colour before the equivalence point.

Practise with a short timed paper

Titrations come up in GCSE Chemistry, and pH curves and indicators in A-level Chemistry. The daily A-level Chemistry: Year 2 and GCSE Chemistry Higher papers are short timed papers of 8 questions, with a new one every day.

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