Litmus paper: a simple pH indicator for acids and bases

Composition and properties

The reagent is a blended dye system rather than a single compound. Historically the colouring substances were extracted from lichen species and processed into a concentrated form. Today some preparations use partly synthetic dyes for consistency. The dye is water soluble and adheres to filter paper so that handheld strips can be dipped into solutions for a quick reading.

Key characteristics

• Simple, low-cost indicator suitable for classroom and field use.
• Blue litmus turns red under acidic conditions; red litmus turns blue in alkaline (basic) solutions.
• Effective as a qualitative test over a limited range (commonly cited around pH 4.5–8.3), not a substitute for a pH meter.

History and development

Colored extracts from lichens have been used for centuries as dyes and indicators. Over time these natural extracts were refined into standardized litmus preparations and applied to paper for practical testing. Modern manufacturing often blends traditional extracts with synthetic dyes to improve stability and reproducibility.

Uses and limitations

Litmus paper is widely used in education, basic laboratory work and quick field checks where a yes/no answer about acidity or alkalinity is sufficient. It is unsuitable for precise pH measurement, for strongly oxidizing or non‑aqueous media that alter the dye, or when detailed titration data are required. A liquid form, litmus solution, serves the same role when paper strips are not used.

Practical notes and distinctions

When selecting or using litmus, bear in mind how the strip is intended to be read and that the apparent colour can be affected by sample colour, concentration and temperature. For more precise pH values consider universal indicator papers or electronic pH meters. For background on composition see information about litmus mixtures and on dye sources see lichen-derived dyes. For dye chemistry and alternatives consult dye chemistry references and solubility data. For guidance on interpreting results and pH ranges see pH scale resources and general indicator use at acid–base testing guides.

Manufacturers and suppliers provide technical sheets and safety information; search product documentation or laboratory manuals and supplier pages such as manufacturing notes for procedural details.