Calculate pH from H⁺ ion concentration, or find concentration from pH. Covers pH, pOH, Kw, acidic and basic solutions, and buffer systems. Essential chemistry tool.
pH is a measure of hydrogen ion concentration in a solution. It ranges from 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is basic (alkaline). pH = -log₁₀[H⁺].
Pure water has a pH of exactly 7.0 at 25°C, making it neutral. This is because water self-ionises: H₂O ⇌ H⁺ + OH⁻, producing equal concentrations of 10⁻⁷ mol/L each.
pH = -log₁₀[H⁺]. For [H⁺] = 0.001 mol/L: pH = -log₁₀(0.001) = -(-3) = 3. This is mildly acidic, similar to vinegar.
pH measures hydrogen ion concentration. pOH measures hydroxide ion concentration. At 25°C, pH + pOH = 14 (the ion product of water). If pH = 4, then pOH = 10.
The safe drinking water pH range is 6.5 to 8.5. Below 6 causes corrosion of pipes and tooth enamel. Above 8.5 is bitter and may indicate mineral contamination. Blood pH is tightly regulated at 7.35 to 7.45.
A buffer resists pH change when acid or base is added. It contains a weak acid and its conjugate base. Blood uses a bicarbonate buffer system to maintain pH between 7.35 and 7.45.
The neutral point of water (pH 7) is only valid at 25°C. At 37°C (body temperature), neutral pH is 6.8. At 0°C, neutral pH is 7.47. Higher temperatures increase ionisation of water.