By April Sanders
Updated Mar 24, 2022
pH paper is a paper or fabric strip saturated with one or more pH indicators. These color‑changing dyes reveal whether a tested solution is acidic or alkaline, and to what extent. The strip is dipped into the sample, held for the time specified on the instructions, then compared against a color chart to read the pH value.
Most commercial strips contain a universal indicator—a blend of weak acids that shifts through a spectrum of colors from red (pH 2) to green (pH 10). Natural dyes such as litmus, which turns red in acids and blue in bases, are often part of this blend. The result is a single strip capable of reporting a wide pH range.
Before saturation, the universal indicator solution is prepared in a non‑metallic container to avoid chemical reaction. The mixture typically includes distilled water, propan‑2‑ol (a flammable solvent), and a cocktail of weak acids. Some commercial formulations list LD50 values such as IVN rat (1088 mg/kg), ORL mouse (3600 mg/kg), ORL rat (5045 mg/kg), and SCU mouse (6 g/kg) to indicate toxicity levels—information that should only be handled by trained professionals.
After the solution is ready, the paper or fabric is fully immersed, thoroughly saturated, and immediately removed for drying in a chemical‑free environment. Once dry, strips are cut to uniform size and stored in a sealed container to preserve their sensitivity.
For those who prefer a natural approach, pH paper can be made at home using readily available materials:
Boil the cabbage leaves in distilled water, strain, and heat the liquid until one‑third evaporates. Submerge the paper in this liquid, let it soak until saturated, then remove and dry—ideally draped over a clean string or bar. The dried strips will appear gray; they turn red in acids and green in bases, mirroring the behavior of commercial universal indicators.
While homemade strips are convenient, keep in mind that their accuracy may vary. For critical measurements, laboratory‑grade strips are recommended.
