pH Calculator
Calculate pH from H⁺ concentration or convert between pH, pOH, and ion concentrations. Essential for chemistry coursework and laboratory work.
pH Chemistry Guide
The pH Scale
pH = −log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. The scale runs from 0 (most acidic) to 14 (most basic), with 7 being neutral at 25°C. Each unit change represents a 10-fold difference in H⁺ concentration: pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6. Common pH values: battery acid 0, stomach acid 1.5–3.5, lemon juice 2, coffee 5, rain water 5.6, pure water 7, blood 7.4, baking soda 8.3, bleach 12.5, drain cleaner 14.
Calculating pH
From [H⁺]: pH = −log([H⁺]). If [H⁺] = 0.0001 mol/L = 10⁻⁴, pH = 4. From pH to [H⁺]: [H⁺] = 10⁻ᵖᴴ. The pOH relationship: pH + pOH = 14 (at 25°C). For a weak acid: using the simplified formula when Ka << acid concentration, [H⁺] ≈ √(Ka × C). For example, acetic acid (Ka = 1.8×10⁻⁵) at 0.1 mol/L: [H⁺] = √(1.8×10⁻⁵ × 0.1) = 1.34×10⁻³, pH = 2.87.
Buffers and pH Stability
A buffer solution resists pH change when acid or base is added. Buffers are mixtures of weak acids and their conjugate bases (or weak bases and their conjugate acids). Blood (pH 7.35–7.45) is maintained by the bicarbonate buffer system. Even small deviations in blood pH (±0.2–0.3) cause serious physiological problems — acidosis (pH under 7.35) or alkalosis (pH over 7.45) can be life-threatening. Buffers are used extensively in biology labs, medicine (IV solutions), food (preservation), and photo
pH in Everyday Life
Aquarium keeping: most freshwater fish need pH 6.5–7.5; some cichlids need 8.0+. Soil pH: most plants prefer 6.0–7.0; acidic plants (blueberries, rhododendrons) prefer 4.5–5.5; lawns prefer 6.0–7.0. Swimming pools: target pH 7.2–7.6 for sanitiser effectiveness and bather comfort. Skin: normal skin pH is 4.5–5.5 — alkaline soaps disrupt this barrier. Wine: typically pH 3–4. Bread dough: typically pH 5.5–6.0. Sourdough: pH 3.5–4.5.
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