pH & pOH Calculator
Convert
The pH scale measures how acidic or basic a solution is: pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in mol/L. Its partner pOH = −log₁₀[OH⁻] tracks the hydroxide ions, and at 25 °C the two always satisfy pH + pOH = 14 because the ion product of water Kw = [H⁺][OH⁻] = 10⁻¹⁴. Enter any one of the four — pH, pOH, [H⁺] or [OH⁻] — and this calculator returns the other three and labels the solution acidic (pH < 7), neutral (pH = 7) or basic (pH > 7). Concentrations are shown in scientific notation so very small ion amounts stay readable.
pH
7
Neutral
pOH
7
[H⁺]
1.000e-7 mol/L
[OH⁻]
1.000e-7 mol/L
pH = −log₁₀[H⁺]; pH + pOH = 14 at 25 °C. Each pH unit is a 10× change in hydrogen-ion concentration.
How to use
- Choose which quantity you know: pH, pOH, [H⁺] or [OH⁻].
- Enter its value (concentrations in mol/L).
- Read pH, pOH, both ion concentrations and whether the solution is acidic, neutral or basic.
Frequently asked questions
- What is the pH formula?
- pH = −log₁₀[H⁺], where [H⁺] is the hydrogen-ion concentration in mol/L. A [H⁺] of 1×10⁻³ mol/L gives pH 3.
- How are pH and pOH related?
- At 25 °C, pH + pOH = 14, because the ion product of water Kw = [H⁺][OH⁻] = 1×10⁻¹⁴. So pOH = 14 − pH.
- What pH is acidic, neutral or basic?
- At 25 °C pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic (alkaline). The scale is logarithmic, so each unit is a 10× change in [H⁺].
- Does temperature change this?
- Yes — Kw and therefore neutral pH shift with temperature. This calculator uses the standard 25 °C value (Kw = 10⁻¹⁴, neutral pH 7).
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