Answer reviewed 22 February 2023
Litmus paper affords a cheap, simple and reliable method of determining whether a solution is acidic, basic or neutral. Litmus papers are based on acidobasic indicators, which are very stable compounds. They can stay chemically stable for a long time provided they are stored properly, which is why some litmus papers do not indicate an expiration date.
The samples at your school, which have been kept dry and stored in the dark, are likely to still be viable. An inspection of the paper would indicate signs of deterioration, such as variations in colour or texture. A simple and easy way to determine whether the litmus paper is still usable is to immerse one end of it in some acid or basic solution and note if it changes colour as expected. As you are preparing for a significant practical activity, it would also be worthwhile testing the paper under the conditions of the practical activity to ensure that it performs well.
Science ASSIST recommends keeping litmus papers dry and in a tightly closed container in a cool and well-ventilated area, and also to keep the container away from light, oxidising agents, heat and sources of ignition.1
Litmus paper is made of wood cellulose paper infused with litmus—a mixture of weakly acidic blue organic dyes obtained from certain lichens.2
There are three different types of litmus paper:
- Blue litmus paper which turns red in acid
- Red litmus paper which turns blue in bases
- Neutral litmus paper (purple) which turns red in acids and blue in bases
To determine if a solution is neutral, you would test with blue paper (does not become red) and then with red paper (does not become blue). Neutral paper would remain purple.
1 ChemSupply, (2023), Safety Data Sheet: Litmus. Search https://www.chemsupply.com.au/ to source the latest Safety Data Sheet via the product information page.
2 How Products are Made. (nd). ‘Litmus paper’, Retrieved (22 February 2023) from the How Products are Made website, http://www.madehow.com/Volume-6/Litmus-Paper.html