Litmus paper is a simple tool used to distinguish between acidic and basic solutions. As a pH indicator, it changes color in response to a substance’s acidity or alkalinity. This allows for a quick assessment of whether a solution is acidic, basic, or neutral, playing a role in various scientific and educational contexts. Its primary function is to provide a qualitative indication, not a precise numerical pH value.
Properties and Function of Litmus Paper
Litmus paper is made from absorbent paper treated with a natural, water-soluble dye extracted from lichens. This dye, known as litmus, is sensitive to hydrogen ion (H+) concentration, which determines a solution’s pH.
The chemical principle behind the color change involves the litmus dye acting as a weak diprotic acid. In the presence of a base, hydrogen ions from the litmus acid react with the base, causing the dye to lose a proton and form a blue-colored conjugate base. Conversely, in an acidic solution, the dye accepts protons, leading to its red form. The color transition typically occurs within a pH range of 4.5 to 8.3 at 25 °C.
Litmus paper commonly comes in two forms: red and blue. Blue litmus paper turns red in acidic conditions, specifically when the pH falls below 4.5. Red litmus paper turns blue when exposed to basic or alkaline solutions, generally at a pH above 8.3. There is also a neutral or purple litmus paper, which turns red in acid and blue in base.
Using Litmus Paper and Interpreting Results
To test a solution, a strip of litmus paper is dipped into the sample, or a drop is placed onto the paper. The color change occurs almost immediately upon contact. For gases, the litmus paper should first be dampened with distilled water before exposure.
Interpreting results depends on the type of litmus paper used and the observed color change. If blue litmus paper turns red, the solution is acidic. If red litmus paper turns blue, the solution is basic or alkaline. When neither red nor blue litmus paper changes color, the solution is neutral.
While litmus paper is effective for quickly determining if a substance is acidic or basic, it has limitations. It does not provide a specific numerical pH value, only an approximate indication of acidity or alkalinity. This means it cannot differentiate between varying strengths of acids or bases, only indicating if a solution falls into one of these categories. Furthermore, certain chemical reactions, such as exposure to chlorine gas, can cause a color change, bleaching the dye and making it unreliable.