Chemistry
pH from hydrogen ion concentration — plus pOH, hydroxide concentration, and whether the solution is acidic, neutral, or alkaline.
Enter a concentration above zero, in moles per litre — 0.0001 means 10−4 mol/L. Results update as you type. All calculations run locally in your browser.
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pH = −log10[H⁺]. At 25 °C, pH + pOH = 14, and [H⁺] × [OH⁻] = 10−14, so pOH = 14 − pH and [OH⁻] = 10−14 ÷ [H⁺]. Below 7 is acidic, 7 is neutral, above 7 is alkaline.
[H⁺] = 0.0001 mol/L = 10−4 mol/L: pH = −log10(10−4) = 4.00. pOH = 14 − 4.00 = 10.00, and [OH⁻] = 10−14 ÷ 10−4 = 10−10 mol/L. The solution is acidic — about the acidity of tomato juice.
This page takes the hydrogen ion concentration as given; it does not work out how much a weak acid dissociates, which needs the acid's equilibrium constant. The pH + pOH = 14 relation holds at 25 °C; at other temperatures the neutral point shifts (at 100 °C, neutral pH is about 6.14). Activity effects in concentrated solutions are ignored, so results are exact for dilute solutions and approximate for strong ones. This page is for calculation, not for handling chemicals — follow lab safety rules for real solutions.
Because concentrations span many orders of magnitude: stomach acid at pH 2 has 10,000 times the hydrogen ions of a pH 6 solution, not three times. Each whole pH step means a tenfold change.
Only at 25 °C. Neutral means [H⁺] = [OH⁻], and that balance point moves with temperature, so neutral water at 100 °C has pH 6.14.
Yes, for very concentrated acids or bases — a 10 mol/L strong acid has a nominal pH of −1. The 0–14 range is a rule of thumb for dilute solutions, not a law.
Two views of the same solution: pH tracks hydrogen ions, pOH tracks hydroxide ions. At 25 °C they always add to 14, so either one determines the other.
Meters measure activity, not concentration; temperature, calibration, and dissolved salts all shift readings. For weak acids, incomplete dissociation is usually the bigger cause — the concentration you put in is not the free [H⁺].
Pure water pH 7, coffee about 5, vinegar about 2.4, baking soda solution about 8.3, household bleach about 12.5.
Results are estimates only and do not constitute professional advice.