pH is a scale indicating the acidity or alkalinity of an aqueous solution, calculated as the negative common logarithm of the hydrogen ion concentration ([H⁺]). A pH below 7 is acidic; above 7 is basic.
Enter the hydrogen ion concentration (mol/L) to calculate both pH and pOH at once.
For example, if the hydrogen ion concentration is 0.0001 (10⁻⁴) mol/L, the pH is 4 and the pOH is 10.
pH measures how acidic or basic a solution is, on a logarithmic scale from 0 to 14 at room temperature. It is the negative base-10 logarithm of the hydrogen ion concentration, so each whole step represents a tenfold change.
Enter the hydrogen ion concentration [H⁺] to get the pH and pOH and see whether the solution is acidic or basic.
pH is the negative base-10 logarithm of the hydrogen ion concentration, so each unit is a tenfold change in acidity. Pure water at 25 °C is pH 7 with 10⁻⁷ mol/L of H⁺, lemon juice sits near 2, and household ammonia around 11. A change from pH 5 to pH 3 means one hundred times more H⁺, not twice as much.
pH = −log₁₀[H⁺] · pH + pOH = 14.00pH 7 is neutral at 25 °C, where [H⁺] = [OH⁻] = 10⁻⁷ mol/L. Below 7 is acidic, above 7 is basic, and pure water sits at exactly 7 under standard conditions.
Stomach acid is about 1.5, lemon juice around 2, coffee near 5, milk 6.5, blood tightly regulated at 7.35–7.45, soap 9–10 and household bleach about 13. Small pH changes in blood are medically significant.
A strong acid such as hydrochloric acid dissociates completely, so [H⁺] = 0.01 = 10⁻² and pH = 2. For a weak acid use Ka: 0.1 mol/L acetic acid with Ka 1.8 × 10⁻⁵ gives [H⁺] ≈ 1.3 × 10⁻³ and pH 2.87.
Yes. The formula has no built-in limit: 10 mol/L hydrochloric acid gives pH −1 and saturated sodium hydroxide exceeds 15. The 0–14 range is a convention for dilute aqueous solutions, not a physical boundary.
2026-10-19