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. 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. The blue dye has been extracted from lichens since the 16th century. The word "litmus" comes from the old Norse word for "to dye or color".
Litmus Test
To perform the test, simple place a drop of liquid sample on a small strip of paper or dip a piece of litmus paper in a small specimen of the sample. Ideally, you don't dip litmus paper in an entire container of a chemical.
The litmus test is a quick method of determining whether a liquid or gaseous solution is acidic or basic.
Limitations of the Litmus Test
The litmus test is quick and simple, but it suffers 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.
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