Acid-Base Indicators — Chemistry Mnemonic
Target: mnemonic for acid base indicators pH range chemistry
Why is this hard to memorize?
Acid-base indicators change colour at specific pH ranges — and NEET/JEE test which indicator to use for which titration. The key indicators are: Litmus (red below 5, blue above 8), Methyl orange (red below 3.1, yellow above 4.4), Phenolphthalein (colourless below 8.2, pink above 10), Methyl red (red below 4.4, yellow above 6.2), and Universal indicator (full rainbow). Choosing the right indicator depends on the titration type: strong acid + strong base → any indicator, strong acid + weak base → methyl orange, weak acid + strong base → phenolphthalein.
Classic mnemonics you should know
"SA+SB = Any, SA+WB = Methyl Orange, WA+SB = Phenolphthalein"
The indicator must change colour near the equivalence point pH. SA+SB(pH~7 → any works), SA+WB(pH~5 → methyl orange changes at 3-4), WA+SB(pH~9 → phenolphthalein changes at 8-10).
"MO: Red(acid)→Yellow(base), PP: Clear(acid)→Pink(base), MR: Red(acid)→Yellow(base)"
Methyl Orange(MO): Red↔Yellow. Phenolphthalein(PP): Colourless↔Pink. Methyl Red(MR): Red↔Yellow. PP is the odd one out — it is colourless in acid (not red).
"MO at 3-4, MR at 4-6, PP at 8-10 — "MO(34) MR(46) PP(810)""
Memorize as number pairs: Methyl Orange(3.1-4.4), Methyl Red(4.4-6.2), Phenolphthalein(8.2-10). Notice they go in order of increasing pH range.
The complete list
- Litmus (pH 5-8, red↔blue)
- Methyl orange (pH 3.1-4.4, red↔yellow)
- Methyl red (pH 4.4-6.2, red↔yellow)
- Phenolphthalein (pH 8.2-10, colourless↔pink)
- Bromothymol blue (pH 6-7.6, yellow↔blue)
- Universal indicator (full pH range, rainbow)
Frequently asked questions
Which indicator should I use for a strong acid + weak base titration?
Methyl orange. The equivalence point pH is around 5 (acidic side) because the salt of a weak base hydrolyzes to give an acidic solution. Methyl orange changes colour at pH 3.1-4.4, which is near this equivalence point. Phenolphthalein would not work because it changes at pH 8-10, far from the equivalence point.
Why is phenolphthalein colourless in acid but pink in base?
Phenolphthalein exists in a closed lactone form (colourless) in acidic conditions. In basic conditions (pH > 8.2), the molecule opens up to a quinoid form with extended conjugation, which absorbs light in the visible range and appears pink/magenta.
Can I use phenolphthalein for a weak acid + weak base titration?
No. Weak acid + weak base titrations have no sharp pH change at the equivalence point, so no single indicator gives a clear colour change. These titrations are best monitored with a pH meter or potentiometric method, not visual indicators.
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