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.