Understanding the Litmus Paper Test for Acids and Bases

Litmus was first used around 1300 CE, by the Spanish alchemist Arnaldus de Villa Nova.

The litmus test is a test used to identify and distinguish acidic and basic substances. It is one of the first tests taught to a chemistry student, and is the easiest to perform.

Here's some helpful tips on how to test acids and bases using the litmus paper test.

The litmus test is virtually always performed with a litmus paper (a litmus solution can also be used, as has been explained further on in this article, but a paper is much more practical). The dyes in a litmus paper react with acids and bases to produce specific colors; viz., red for acids and blue for bases. An ideal neutral litmus paper is violet.

► If a substance changes the color of a blue litmus paper to red, the substance is acidic. If a substance doesn't change the color of a red litmus paper, it may be acidic or neutral (salt), since neutral compounds have no effect on the litmus paper.

► Similarly, if a red litmus paper changes to blue, the substance is basic. Like with acids, a blue litmus paper remaining blue can mean that the substance is either basic or neutral.

► Litmus paper reacts to the temporary presence of acidity- or alkalinity-determining ions in an aqueous solution of a compound. This means that the color change is temporary, and is reversed as soon as sufficient amounts of the opposite is presented.

However, some substances cause color changes that are beyond the usual blue or red, and irreversible. Chlorine, for example, turns litmus paper very pale, by bleaching it.

► The test doesn't work with dry compounds. Common acids and bases are usually stored in the form of a solution, which facilitates the outcome of the test. If a litmus paper is wet, it can also test gases. For example, ammonia gas, whose aqueous solution is the basic ammonium hydroxide, turns a wet red litmus paper blue.

Similarly, fumes from concentrated hydrochloric acid, consisting of the hydrogen chloride gas, turn a wet blue litmus paper red. If water isn't present in either litmus paper or the compound, the paper would remain unchanged regardless of the pH of the compound.

Litmus paper is formed by coating filter paper with litmus, a mixture of dyes obtained from various lichens, primarily of the Roccella genus. Commonly used lichens are R. montagnei, Dendrographa leucophoea, R. tinctoria, R. fuciformis, Lecanora tartarea, Ochrolechia parella, etc.

These dyes are water-soluble, and can also be used to test a compound as a solution (without the intermediate step of having a filter paper absorb it), except that it won't be reusable in such a scenario. The color changes are the same as with paper; a blue litmus solution would turn red in the presence of acids, and vice versa.

Litmus paper doesn't show the pH of a compound. It may even not change color if the pH of the compound in question is between 4.5 and 8.3. It only indicates whether the compound is acidic or basic, in a very rudimentary fashion. Various more sensitive indicators, commonly seen in the form of a pH paper, are used to obtain the pH value of an acid or a base.