Litmus is a long-standing tool in chemistry, functioning as an indicator to quickly determine the acidity or alkalinity of a substance. The measurement of acidity is based on the pH scale, which quantifies the concentration of hydrogen ions in a solution. Indicators are special dyes that exhibit different colors in response to these changes in ion concentration, providing a simple visual method for classification. Litmus allows for a rapid assessment of whether a solution is acidic (pH below 7) or basic (pH above 7).
The Litmus Test: The Color in Alkali
When litmus is introduced to an alkali, or basic, solution, the resulting color is definitively blue. An alkali increases the concentration of hydroxide ions (\(\text{OH}^-\)) in a solution, corresponding to a pH value greater than 7. Substances that are alkaline often feel slippery, such as soap or baking soda solutions.
This color change is a reliable, quick test for alkalinity. If red litmus paper is used, the paper will turn blue when dipped into the alkali. If blue litmus paper is used, it will remain blue, confirming the basic environment.
Understanding Litmus and Its Forms
Litmus is a natural, water-soluble dye extracted from various species of lichens, which are composite organisms combining a fungus and an alga. The dye is a complex mixture of 10 to 15 different colored compounds. Lichens have been a source for this dye since at least the 16th century.
The most common way litmus is used is in the form of litmus paper, which is filter paper saturated with the dye solution. Litmus can also be used in its liquid, aqueous form. The paper strips are the most convenient and portable method for general testing.
The Contrast: Litmus in Acid and Neutral Solutions
To understand the indicator fully, it is helpful to know what happens in non-alkaline environments. When litmus is exposed to an acid (pH less than 7), the color changes to red. Acids are characterized by a high concentration of hydrogen ions (\(\text{H}^+\)), which the litmus dye molecule interacts with. This shift from blue to red indicates an acidic substance.
The color of litmus in a neutral solution (pH of exactly 7) is purple or mauve. In distilled water or other neutral substances, the solution exhibits this color. Although litmus paper is often prepared in red or blue forms, a neutral litmus paper would be purple. The color change range occurs over a pH range of approximately 4.5 to 8.3, transitioning gradually between red and blue.
The Chemistry Behind the Change
The color change observed in litmus is rooted in a reversible chemical reaction that depends on the concentration of hydrogen ions (\(\text{H}^+\)). Litmus is a weak acid, meaning it can donate a proton when in a solution. The core of the dye molecule responsible for its color is called a chromophore.
When the solution is acidic and has a high concentration of \(\text{H}^+\) ions, the litmus molecule is in its protonated form, appearing red. When the solution is alkaline, excess hydroxide ions (\(\text{OH}^-\)) remove the \(\text{H}^+\) from the litmus molecule (deprotonation). This structural change alters the electronic configuration of the chromophore, causing it to appear blue.