litmus

Litmus is a water-soluble dye derived from certain species of lichens, especially Roccella tinctoria, that has long been used as a simple pH indicator. In solution, litmus changes color in response to acidity or basicity, providing a quick, qualitative measure of how acidic or basic a liquid is.

The color change follows a roughly pH-dependent pattern: acidic solutions turn litmus red, while basic (alkaline) solutions turn it blue. The most commonly cited transition range is about pH 4.5 to 8.3, with red indicating strongly acidic conditions and blue indicating strongly basic conditions. Litmus is commonly encountered in two forms on paper: blue litmus paper and red litmus paper. Blue litmus turns red in acidic environments; red litmus turns blue in basic environments, allowing a simple, visual assessment of pH direction.

Applications and limitations: Litmus is widely used in classrooms and basic laboratories for quick, qualitative pH checks and to distinguish acids from bases. It does not provide precise pH values, and its results can be influenced by dye concentration, sample composition, and exposure to air or light. For accurate measurements, pH meters or other indicators with narrower ranges are preferred.

History and usage: Litmus has a long historical association with natural dyeing and laboratory testing, arising from its natural lichen-derived source. Today, it remains a common, if imprecise, tool for rapid screening of acidity versus basicity in educational and some practical contexts.