Litmus paper is a straightforward chemical indicator used to determine if a substance is acidic or alkaline. The paper is treated with a dye derived from lichens, which reacts predictably to the presence of hydrogen ions (H+) or hydroxide ions (OH-) in a solution. This reaction is a qualitative measurement related to the potential of hydrogen, known as the pH scale. The pH scale measures how acidic or basic a substance is, with pure water considered neutral.
Applying the Litmus Paper
Using litmus paper requires a small amount of liquid sample to initiate the chemical reaction in the dye. For substances already in liquid form, dip one end of the paper strip directly into the solution for a moment.
When testing dry or solid samples, use distilled water to slightly dampen the material first. Alternatively, a single drop of the liquid being tested can be placed directly onto the paper using a clean dropper or glass rod. The resulting color change is almost immediate, so observation must occur instantly after contact. Minimize physical handling of the paper to avoid contamination from oils or residue on the fingers.
Decoding the Color Change
Interpreting the litmus test relies on observing the final hue of the paper against a known reference color. The paper is available in two forms: blue litmus and red litmus. An acidic substance, such as lemon juice, turns blue litmus paper red, signaling the presence of excess hydrogen ions.
This color shift occurs because the excess hydrogen ions (H+) interact directly with the blue dye molecules embedded in the paper. This interaction modifies the dye’s chemical structure, causing it to reflect red light instead of blue.
Conversely, alkaline substances, also known as bases, cause the opposite reaction when applied to red litmus paper. A base, like household ammonia, will turn red litmus paper a deep blue color.
The blue shift is a result of the high concentration of hydroxide ions (OH–) in the basic solution, which act as proton acceptors. These ions react with the red dye molecules, causing them to display a blue hue.
A substance is categorized as acidic if it turns blue litmus red and alkaline if it turns red litmus blue. The third possible result is no color change on either type of paper, which indicates a neutral substance. For example, when testing pure water, a red litmus strip will remain red, and a blue litmus strip will stay blue. This lack of reaction means the solution has a balanced concentration of H+ and OH– ions, typically corresponding to a pH value near 7.
Why Litmus Only Tells Half the Story
While effective for simple classification, litmus paper is limited because it only provides a binary result: acid or base. It cannot provide information regarding the strength of the acid or base being tested. For instance, a strong acid (pH 1) and a weak acid (pH 6) will both produce the exact same red result on blue litmus paper.
Litmus paper only indicates that the substance falls on one side of the neutral point, rather than providing a specific measurement of strength. For measurements requiring numerical precision, a different tool, such as universal pH paper, is used. Universal paper is impregnated with a mixture of indicator dyes, allowing it to display a wide spectrum of colors that correlate directly to specific numerical pH values from 1 to 14.