pH and indicators

Theory

The pH scale expresses acidity logarithmically. For a dilute aqueous solution, pH is approximately −log₁₀[H₃O⁺], using hydronium-ion concentration in mol/L. A neutral aqueous solution has pH 7 at 25 °C; neutral pH changes with temperature.

Each indicator changes colour across a particular pH range. Universal indicator mixes several indicators to cover a wider range. Match the sample colour to the chart for the product you are using, since products can differ. Treat the reading as an estimate: a properly calibrated pH meter gives greater precision.

pH ≈ −log₁₀[H₃O⁺]

Use [H₃O⁺] in mol/L for this dilute-aqueous approximation. The neutral pH value varies with temperature.