Litmus paper is one of the oldest and simplest chemical indicators used to classify substances based on their relative acidity or basicity. This tool consists of porous cellulose paper treated with a natural, water-soluble dye mixture extracted primarily from lichens. The dye functions by undergoing a distinct color transformation when exposed to different concentrations of hydrogen ions (\(\text{H}^+\)) in a solution, or pH. By observing this color shift, a user can quickly determine a substance’s general position on the pH scale.
Identifying a Base
The specific observation of red litmus paper turning blue provides a definitive answer: the substance being tested is a base, also known as an alkali. Basicity is the chemical property associated with a pH value greater than 7.0 on the standard scale. The color change indicates that the solution contains a higher concentration of hydroxide ions (\(\text{OH}^-\)) than hydrogen ions (\(\text{H}^+\)).
The dye molecules within the red litmus paper react chemically with these excess hydroxide ions. This reaction causes the dye to lose a proton, which alters the molecular structure. The resulting structural form of the dye produces the characteristic blue color.
Numerous common household items will trigger this reaction due to their alkaline nature. These include cleaning agents like household ammonia and various bleaches. Common staples such as baking soda (sodium bicarbonate) and diluted soap solutions are also mildly basic. The intensity of the blue color often relates to the strength of the base.
How Litmus Paper Works
The dye mixture impregnated into the paper is derived from species of lichens, such as Roccella tinctoria, and is a complex blend of indicator compounds. This mixture acts as a weak diprotic acid. In its acidic form, which is the state on red litmus paper, the dye exists as a molecule capable of donating a hydrogen ion.
When this molecule encounters a basic solution, the excess hydroxide ions readily accept the hydrogen ion from the dye. This removal of a proton restructures the dye molecule into its conjugate base form. This new molecular configuration reflects the blue wavelength of light, causing the paper to change color.
The transition from red to blue does not occur instantaneously at a single pH point, but rather over a narrow range of \(\text{pH}\) values. The color transformation generally takes place between approximately \(\text{pH}\) 4.5 and \(\text{pH}\) 8.3. Below \(\text{pH}\) 4.5, the dye remains in its red form, and above \(\text{pH}\) 8.3, it is completely in its blue form. This specific color range makes litmus paper a reliable but broad indicator.
The Full Range of Litmus Results
The blue color change from red litmus paper only represents one possible outcome of a litmus test. The opposite result occurs when blue litmus paper is introduced to an acidic substance. The excess hydrogen ions in the acid donate a proton to the blue-colored dye molecule, reverting it back to its red, acidic form. This change from blue to red signifies that the substance is an acid.
A third possibility is a neutral substance, such as distilled water. When either red or blue litmus paper is exposed to a neutral solution (near \(\text{pH}\) 7), no significant color change is observed. The paper confirms the solution is neither sufficiently acidic nor basic to force the dye to change its molecular structure. Since the transition range spans from \(\text{pH}\) 4.5 to 8.3, a substance within this window may only produce a faint purple or no change at all.