By A.P. Mentzer – Updated Mar 24, 2022
Litmus paper is a quick, reliable indicator that reveals whether a solution is acidic or alkaline. When a substance dissolves in water, the resulting solution causes the paper to change color. While the test provides a rapid yes‑or‑no answer, it does not give a precise pH value.
Chemists define acids and bases in several ways. Svante Arrhenius, a 19th‑century pioneer, described acids as compounds that release hydrogen ions (H⁺) in water, and bases as those that release hydroxide ions (OH⁻). For instance, hydrochloric acid (HCl) dissociates to form hydronium (H₃O⁺) and chloride (Cl⁻). Ammonia (NH₃), a base, yields ammonium (NH₄⁺) and hydroxide (OH⁻). In everyday terms, acids taste sour—think lemon juice—while bases feel slippery, like soap.
The pH scale quantifies hydrogen ion concentration using a negative logarithm. It ranges from 0 to 14, with 7 denoting neutral water. Values below 7 indicate acidity; above 7 indicate alkalinity. Typical examples include lemon juice or stomach acid at pH ≈ 2, coffee at pH ≈ 5, blood at pH ≈ 7.4, household bleach at pH ≈ 9, and ammonia at pH ≈ 12.
Litmus paper is a classic acid‑base indicator. It comes in red, blue, and neutral (purple) forms, each derived from lichen dyes that shift color in response to pH. Red paper turns blue in basic solutions; blue paper turns red in acidic solutions. The neutral strip changes to either red or blue, depending on whether the tested solution is acidic or alkaline.
To use litmus paper, dip a strip into the liquid or apply a droplet with a pipette. Blue paper will turn red when exposed to a solution with pH 4–5 or lower. Red paper will turn blue when contacting a base with pH greater than 8. Solutions with pH between 5 and 8 will produce minimal color change, making it difficult to distinguish them using litmus alone.
Litmus paper excels at indicating the general direction of acidity or alkalinity. It is inexpensive, portable, and requires only a small sample volume. However, it cannot provide an exact pH value and is ineffective for solutions near neutral (pH 5–8). For precise measurements, a calibrated pH meter or pH‑specific indicators are recommended.
