pKa Calculator

pKa Calculator — calculate pka using standard chemistry formulas. Worked example with units.

A pH calculator converts between pH, hydrogen ion concentration [H⁺], hydroxide ion concentration [OH⁻], and pOH. The pH scale runs from 0 (strongly acidic) to 14 (strongly basic/alkaline), with 7 as neutral at 25°C. Each pH unit represents a 10-fold change in [H⁺] — a solution at pH 4 has ten times more hydrogen ions than a solution at pH 5, and 100 times more than a solution at pH 6.

pH measurement is critical in chemistry, biology, food production, water treatment, agriculture, pharmaceuticals, and environmental science. Blood pH is maintained at 7.35–7.45 — deviations of just 0.2–0.3 units outside this range are life-threatening.

Enter the hydrogen ion concentration [H⁺] in mol/L to calculate pH and pOH.
Enter pH to calculate [H⁺], [OH⁻], and pOH.
For weak acids/bases, enter the concentration and Ka/Kb to estimate the equilibrium pH.
For buffer solutions, use the Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]).

pH formulas

pH = −log₁₀[H⁺] (where [H⁺] is in mol/L)

[H⁺] = 10^(−pH)

pOH = −log₁₀[OH⁻] | pH + pOH = 14 (at 25°C)

Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]) — for buffer calculations

Weak acid approximation: [H⁺] ≈ √(Ka × C), so pH ≈ ½(pKa − log C)

Reading pH results

Common pH reference points

pH 0–2: strongly acidic (battery acid, gastric acid). pH 2–4: weakly acidic (vinegar pH 2.4, lemon juice pH 2.0, coffee pH 5, rain pH 5.6). pH 7: neutral (pure water). pH 7–9: weakly basic (blood pH 7.4, sea water pH 8.1, baking soda pH 8.3). pH 10–12: strongly basic (bleach, ammonia). pH 12–14: highly caustic (drain cleaner, oven cleaner). Soil pH affects nutrient availability — most crops grow best at pH 6–7.

Chemistry tips and best practices

Temperature affects pH — pure water is pH 6.77 at 100°C and 7.47 at 0°C; standard pH = 7 is defined at 25°C.
For strong acids (HCl, H₂SO₄, HNO₃), [H⁺] = molarity directly at low concentrations. For weak acids, Ka equilibrium calculations are required.
Buffer systems resist pH change — important in biology (blood uses carbonate/bicarbonate buffer) and chemistry (phosphate buffers for lab assays).
Swimming pool ideal pH: 7.2–7.6 — too low (acidic) irritates skin and corrodes equipment; too high (basic) reduces chlorine effectiveness.

Stomach acid (gastric acid) has a pH of 1.5–3.5 — acidic enough to dissolve certain metals and denature proteins.
The US EPA drinking water standard for pH is 6.5–8.5 — outside this range, pipes corrode and taste is affected.
Ocean pH has fallen from approximately 8.25 pre-industrial to 8.10 today — a 0.15 unit drop representing a 40% increase in [H⁺] due to CO₂ absorption (ocean acidification).

Common mistakes to avoid

Confusing pH with concentration directly — pH 3 is NOT three times more acidic than pH 6; it is 1,000 times more acidic ([H⁺] difference = 10³).
Assuming pH + pOH = 14 at all temperatures — this identity only holds at 25°C; at other temperatures, Kw changes.
Treating weak acids as strong — weak acids do not fully dissociate; using molarity directly as [H⁺] overstates acidity.

pH calculations are theoretical values based on ideal solution behaviour. Real-world pH measurement requires calibrated pH meters or indicators, and accounts for activity coefficients at high ionic strength. For regulatory water quality, food safety, or pharmaceutical applications, use certified laboratory methods.