pH and pOH Calculator
Convert hydrogen ion concentration, hydroxide ion concentration, temperature, and ionic strength into pH and pOH values. Ideal for chemistry students, analysts, and anyone calibrating buffers or checking solution acidity.
About this calculator
pH is defined as the negative base-10 logarithm of hydrogen ion activity: pH = −log₁₀[H⁺]. At standard conditions (25 °C, pure water), pH + pOH = 14. This calculator extends that relationship with two corrections. An ionic strength correction factor adjusts for the fact that in concentrated electrolyte solutions, ion–ion interactions reduce effective activity. A temperature correction accounts for the shift in water's dissociation constant (Kw) with temperature — roughly 0.003 pH units per °C deviation from 25 °C. The combined formula used here is: pOH = 14 − log₁₀([H⁺] × γ) + (T − 25) × 0.003, where γ is the ionic strength correction and T is temperature in °C. Understanding these corrections is critical for accurate buffer preparation and physiological measurements.
How to use
Suppose [H⁺] = 1×10⁻³ M, ionic strength correction = 1.0 (ideal), and temperature = 37 °C. Step 1 — compute the ionic-strength-adjusted term: log₁₀(1×10⁻³ × 1.0) = −3. Step 2 — apply temperature correction: (37 − 25) × 0.003 = 0.036. Step 3 — pOH = 14 − (−3) + 0.036 = 14 + 3 + 0.036 = 17.036. Note: pH = −log₁₀[H⁺] = 3.0; pH + pOH = 14 at 25 °C shifts slightly at 37 °C. Enter the values above to verify the result.
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 (often expressed as an activity coefficient γ from the Debye–Hückel equation) accounts for electrostatic shielding in solutions containing high concentrations of dissolved salts. In dilute solutions (below ~0.01 M total ionic strength), γ ≈ 1 and can be ignored. In physiological saline, seawater, or industrial brines, ignoring it can introduce errors of 0.1–0.3 pH units. Include a correction factor greater than 1 when your solution contains significant background electrolytes.
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), so the sum drops below 14; at lower temperatures it rises above 14. Additionally, in non-aqueous or mixed solvents, the self-ionization constant changes entirely, making the 14 rule inapplicable. This calculator's temperature term adjusts the sum accordingly for aqueous solutions over a practical temperature range.