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pH & pOH Calculator

Convert between pH, pOH, hydrogen ion concentration, and hydroxide ion concentration for an aqueous solution at 25°C.

pH
7
pOH
7
[H⁺]
1.0000e-7 mol/L
[OH⁻]
1.0000e-7 mol/L
Classification
neutral

How it works

pH and pOH are defined as the negative base-10 logarithm of a solution's hydrogen ion and hydroxide ion concentration: pH = −log₁₀[H⁺], pOH = −log₁₀[OH⁻]. At the standard reference temperature of 25°C, water's self-ionization constant Kw = [H⁺][OH⁻] = 1.0×10⁻¹⁴ ties the two together as pH + pOH = 14, so knowing any one of the four quantities — [H⁺], [OH⁻], pH, or pOH — is enough to derive the other three.

A pH below 7 is acidic (more free H⁺ than a neutral solution), a pH above 7 is basic/alkaline (more free OH⁻), and exactly 7 is neutral — the pH of pure water at 25°C, where [H⁺] = [OH⁻] = 1×10⁻⁷ mol/L.

Because pH + pOH = 14 specifically comes from Kw's value at 25°C, this calculator's results assume that standard reference temperature; Kw (and so the neutral point and the 14 constant) shifts at other temperatures.

FAQ

Does pH + pOH always equal 14?

Only at 25°C, the standard reference temperature this calculator assumes. Kw actually decreases at lower temperatures and increases at higher ones (it's about 1.1×10⁻¹⁵ at 0°C and nearly 1.0×10⁻¹³ at 60°C), which shifts both the 14 constant and where 'neutral' pH actually falls — pure water at 0°C is closer to pH 7.47, not 7.

Why does the calculator show ion concentrations in scientific notation?

Hydrogen and hydroxide ion concentrations routinely span many orders of magnitude (from around 1 mol/L in a strong acid down to 1×10⁻¹⁴ mol/L in a strong base), so scientific notation keeps very small values readable instead of a long string of leading zeros.

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