LITMUS – pH INDICATOR

One of the first indicators used for this purpose was litmus. Litmus paper is paper that has been treated with a specific indicator—a mixture of 10 to 15 natural dyes obtained from lichens (mainly Roccella tinctoria) that turns red in response to acidic conditions (pH 7). When the pH is neutral (pH = 7), then the dye is purple.

History

The first known use of litmus was around 1300 CE by Spanish alchemist Arnaldus de Villa Nova. The blue dye has been extracted from lichens since the 16th century. The word "litmus" comes from the old Norse word for "dye" or "color." While all litmus paper acts as pH paper, the opposite is not true. It's incorrect to refer to all pH paper as "litmus paper." Usually, litmus paper is either red or blue. Red paper turns blue when the pH is alkaline, while blue paper turns red when the pH turns acidic. If the paper turns purple, this indicates the pH is near neutral. Red paper that does not change color indicates the sample is an acid. Blue paper that does not change color indicates the sample is a base. Remember, acids and bases refer only to aqueous (water-based) solutions, so pH paper won't change color in non-aqueous liquids such as vegetable oil. Litmus paper may be dampened with distilled water to give a color change for a gaseous sample. Gases change the color of the entire litmus strip since the whole surface is exposed. Neutral gases, such as oxygen and nitrogen, do not change the color of the pH paper. Litmus paper that has changed from red to blue can be reused as blue litmus paper. Paper that has changed from blue to red can be reused as red litmus paper.

Limitations

The litmus test is quick and simple, but it suffers from a few limitations. First, it's not an accurate indicator of pH; it does not yield a numerical pH value. Instead, it roughly indicates whether a sample is an acid or a base. Second, the paper can change colors for other reasons besides an acid-base reaction. For example, blue litmus paper turns white in chlorine gas. This color change is due to bleaching of the dye from hypochlorite ions, not acidity/basicity.

Alternatives to Litmus Paper

Litmus paper is handy as a general acid-base indicator, but you can get much more specific results if you use an indicator that has a narrower test range or that offers a wider color range. Red cabbage juice, for example, changes color in response to pH all the way from red (pH = 2) through blue (neutral pH) to greenish-yellow (pH = 12), plus you are more likely to find cabbage at the local grocery store than lichen. The dyes orcein and azolitmin yield results comparable to those of litmus paper.