The Original Color of Litmus Paper and Its Applications
The original color of litmus paper is purple, derived from lichens, and it responds to pH levels by changing color: it turns red in acidic solutions and blue in alkaline solutions. This unique property makes litmus paper one of the oldest and most reliable pH indicators in scientific and practical applications.
Chemical Structure and Composition of Litmus Paper
Litmus paper is a water-soluble mixture of dyes extracted from various species of lichens. Each dye contributes to the overall purple color of the paper. The core chromophore in litmus components is 7-hydroxyphenoxazone, which is responsible for the color changes induced by varying pH levels.
Chemically, litmus can be prepared as an aqueous solution that functions similarly to the paper form. Under acidic conditions, the solution turns red, while under alkaline conditions, it turns blue. This solution provides a versatile method for pH testing in different settings.
Historical and Natural Sources of Litmus
Litmus was first used around 1300 AD by Spanish physician Arnaldus de Villa Nova. Over the centuries, the blue dye was extracted from various lichens, with special mention to species like Roccella tinctoria (South America), Roccella fuciformis (Angola and Madagascar), Roccella pygmaea (Algeria), and Roccella phycopsis. Lecanora tartarea (Norway and Sweden), Variolaria dealbata, Ochrolechia parella, and Parmotrema tinctorum are also important sources. Currently, Roccella montagnei (Mozambique) and Dendrographa leucophoea (California) are the main sources for litmus production.
Uses of Litmus Paper
The primary use of litmus paper is to test the acidity or basicity of a solution. Blue litmus paper turns red when it is exposed to acidic conditions, while red litmus paper turns blue when it is exposed to basic or alkaline conditions. This color change occurs within the pH range of 4.5 to 8.3 at 25 °C (77 °F), and neutral litmus paper remains purple. Wet litmus paper can also be used to test for water-soluble gases that affect acidity or alkalinity, as the gas dissolves in water to create the appropriate solution.
In addition to these applications, litmus paper can be prepared as an aqueous solution. Under acidic conditions, this solution turns red, and under alkaline conditions, it turns blue. This solution provides an alternative method for testing pH levels without the need for paper.
Chemistry and Reactions of Litmus
The litmus mixture contains approximately 10 to 15 different dyes and has a CAS number of 1393-92-6. The principal constituent of litmus has an average molecular mass of 3300, similar to but in different proportions compared to orcein, another related dye mixture.
The color-changing properties of litmus are attributed to the 7-hydroxyphenoxazone chromophore. Some fractions of litmus have been specifically named, such as erythrolitmin, azolitmin, spaniolitmin, and leucoorcein. Azolitmin shows nearly the same effect as litmus in color change experiments.
Mechanism of Color Change in Litmus Paper
Red litmus paper contains a weak diprotic acid. When it is exposed to a basic compound, the hydrogen ions react with the added base to form a conjugate base. The conjugate base has a blue color, thus turning the wet red litmus paper to blue in alkaline solutions. Conversely, when red litmus paper is in acidic condition, it will turn red due to the acidity acting on the chromophore.
Moreover, certain chemical reactions can cause irreversible color changes to litmus paper. For example, chlorine gas bleaches blue litmus paper white by reacting with the litmus dye, forming hypochlorite ions. This reaction is irreversible, so litmus does not act as a pH indicator in this situation.
Overall, litmus paper remains a fundamental tool in scientific research, education, and practical testing, due to its simplicity, reliability, and cost-effectiveness. Its applications are diverse, ranging from basic pH testing to complex environmental and industrial analyses.