pH and pOH Calculator
Convert hydrogen ion concentration into pH or pOH, with an activity correction for ionic strength and the temperature dependence of water's ion product Kw. Ideal for chemistry students, analysts, and anyone checking solution acidity.
Last updated: September 2026
Formula below · 2 sources (pubchem.ncbi.nlm.nih.gov, Wikipedia) · Updated Sep 2026
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About this calculator
pH is defined from the hydrogen ion activity: pH = −log₁₀(γ·[H⁺]), where γ is the activity coefficient. In dilute solutions γ ≈ 1 and pH = −log₁₀[H⁺]; at higher ionic strength ion–ion interactions lower γ (the Debye–Hückel effect), so the same concentration gives a slightly higher pH. pOH follows from water's ion product: pH + pOH = pKw. pKw is 14.00 only at 25 °C — it falls as water warms because autoionisation increases. This calculator uses the Harned–Owen fit pKw = 4470.99/T − 6.0875 + 0.01706·T (T in kelvin), which gives 14.94 at 0 °C, 14.00 at 25 °C and 13.62 at 37 °C, so pOH = pKw − pH. Temperature changes pKw (and therefore pOH and the neutral point, pKw/2), not the pH you compute from a given H⁺ activity.
How to use
Example 1 — pH. [H⁺] = 1×10⁻³ M in a dilute solution (γ = 1.00). Choose Show = pH. pH = −log₁₀(1×10⁻³) = 3.00. Example 2 — pOH at body temperature. Same solution, temperature = 37 °C, Show = pOH. pKw at 310.15 K = 4470.99/310.15 − 6.0875 + 0.01706 × 310.15 = 13.62, so pOH = 13.62 − 3.00 = 10.62 (at 25 °C it would be 11.00). Example 3 — Ionic strength. [H⁺] = 1×10⁻³ M with γ ≈ 0.85: pH = −log₁₀(0.85 × 10⁻³) = 3.07.
Frequently asked questions
How does temperature affect pH and why does it matter for biological samples?
Water's autoionization constant Kw increases with temperature, which lowers the neutral pH point below 7.0 at temperatures above 25 °C. For example, at 37 °C (body temperature) neutral pH is approximately 6.8. This means a blood pH of 7.4, which is alkaline at 37 °C, would read differently on a room-temperature calibrated meter. Always temperature-correct pH measurements when working with biological fluids or reactions sensitive to small pH shifts.
What is ionic strength correction and when should I include it in pH calculations?
Ionic strength correction (an activity coefficient γ from the Debye–Hückel or Davies equation) accounts for electrostatic shielding in solutions containing dissolved salts. In dilute solutions (below ~0.001 M total ionic strength), γ ≈ 1 and can be ignored. At I ≈ 0.003 M γ is about 0.95 and at I ≈ 0.03 M about 0.85; in physiological saline or seawater it is lower still (≈0.75), which shifts pH by 0.1 or more. Because pH is defined from activity, γ is always 1 or less, and a smaller γ raises the computed pH.
Why is pH + pOH not always equal to 14?
The relationship pH + pOH = 14 holds strictly at 25 °C because that is where pKw = 14.00 for pure water. At higher temperatures Kw increases (pKw decreases: 13.62 at 37 °C, about 13.26 at 50 °C), so the sum drops below 14; at lower temperatures it rises above 14 (14.94 at 0 °C). Additionally, in non-aqueous or mixed solvents, the self-ionization constant changes entirely, making the 14 rule inapplicable. In pOH mode this calculator uses the temperature-dependent pKw, valid for water between about 0 and 60 °C.