Can I use litmus paper to test unknown liquid that contains a concentrated strong acid (example $\ce{H2SO4}$)? Can concentrated strong acid destroy litmus paper?

  • $\begingroup$ To test for concentrated sulphuric ac in such a way you can use also ham or bananas.... an unknown liquid cannot contain a concentrated acid, at most can contain a strong acid in high concentration. Regarding your last line: what destroys litmus paper and bananas is not an acid-base reaction but water withdrawal (of water already in the sample) and oxidation. Here sulphuric ac. acts as a strong oxidizer in acidic medium as well as dessicant. $\endgroup$
    – Alchimista
    Jun 6, 2019 at 10:10

1 Answer 1


Yes, concentrated strong acid can destroy litmus paper.

Paper is mostly cellulose, which is a carbohydrate polymer. Concentrated sulfuric acid will pull the hydrogen and oxygen out of it as water, leaving carbon behind. The indicator dye that's supposed to change color probably won't fare too well, either.

You can dilute the unknown by adding a drop or two of it to a small amount of distilled water, then use the litmus paper to check that.

A search for "concentrated sulfuric acid on paper" will yield many images and videos showing what the acid does to paper.

  • $\begingroup$ thanks jeffB . will the result be same if I replace litmus paper by ph paper? $\endgroup$
    – Light
    Jun 5, 2019 at 15:11
  • 1
    $\begingroup$ Paper. Yes the result will be the same. And even if the paper and the dye were not irrevocably damaged, you could get unexpected results with the dye in either strong acid of strong base. See what can happen to phenolphthalein at extreme pH. Again use a diluted solution to keep things where your textbook tells them they are. $\endgroup$ Jun 5, 2019 at 15:59
  • $\begingroup$ The "dilute" solutions to which Oscar refers are still strong enough to be a safety concern. A 0.1 molar solution of either is still nasty stuff requiring appropriate safety precautions. $\endgroup$
    – MaxW
    Jun 5, 2019 at 18:32
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    $\begingroup$ @maxw as do all chemicals, really. Even supposedly innocuous orange oil is not a fun experience if it gets in your eye. $\endgroup$ Jun 5, 2019 at 23:58

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