What Is Litmus Paper?

You can make paper test strips to determine the pH of an aqueous solution by treating filter paper with any of the common pH indicators. 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 10-15 natural dyes obtained from lichens (mainly Roccella tinctoria) that turns red in response to acidic conditions (pH < 7) and blue under alkaline conditions (pH > 7). When the pH is neutral (pH = 7) then the dye is purple. The first known use of litmus was around 1300 AD by Spanish alchemist Arnaldus de Villa Nova.

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 more narrow 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 at neutral pH to greenish-yellow at pH = 12, plus you are more likely to find cabbage at the local grocery store than lichen.