[H⁺] = 10^(−pH)
The pH scale measures the acidity or basicity of a solution. pH = −log₁₀[H⁺], so [H⁺] = 10^(−pH). At 25°C, the ion product of water Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, giving pH + pOH = 14. Neutral water has pH 7 ([H⁺] = 10⁻⁷ M). Each pH unit represents a 10-fold change in [H⁺]: pH 3 is 10× more acidic than pH 4. Common pH values: stomach acid 1-2, lemon juice 2, vinegar 3, coffee 5, blood 7.4, seawater 8.1, bleach 12.5, drain cleaner 14. Accurate pH measurement uses glass electrodes (pH meters) or indicator dyes. In biological systems, even small pH changes can denature proteins and disrupt enzyme function, which is why buffer systems are essential.