pH Calculator

Use this pH Calculator to calculate pH, pOH, hydrogen ion concentration, or hydroxide ion concentration from the information you know. It supports strong acids, strong bases, weak acids, weak bases, direct ion concentrations, and pOH.

pH Calculation Formulas

pH = −log₁₀[H⁺]

pOH = −log₁₀[OH⁻]

At 25°C:

pH + pOH = 14

The inverse relationships are:

[H⁺] = 10^−pH

[OH⁻] = 10^−pOH

Acid and Base Methods

For a strong acid, the calculator assumes complete dissociation:

[H⁺] = acid concentration × H⁺ equivalents

For a strong base:

[OH⁻] = base concentration × OH⁻ equivalents

Weak acids and weak bases do not fully dissociate. Their results are calculated using the entered acid dissociation constant, K_a, or base dissociation constant, K_b.

Worked Example

Suppose a hydrochloric acid solution has a concentration of 0.10 M. HCl is treated as a strong acid with one H⁺ equivalent.

[H⁺] = 0.10 × 1 = 0.10 M

pH = −log₁₀(0.10)

pH = 1.00

At 25°C:

pOH = 14 − 1 = 13.00

Important Notes

• Concentration must be converted to molarity before applying the formulas.
• The relationship pH + pOH = 14 assumes an aqueous solution at 25°C.
• Strong-acid and strong-base modes assume complete dissociation.
• Weak-acid and weak-base results depend on the accuracy of the entered K_a or K_b.
• Buffers, mixed solutions, concentrated solutions, polyprotic acids, temperature changes, and ionic interactions may require more detailed calculations.
• Calculated pH may differ from measured pH because real pH is based on ion activity, not concentration alone.

Use a properly calibrated pH meter when an accurate laboratory, environmental, agricultural, pool, aquarium, or industrial measurement is required.

Sources

• IUPAC Gold Book — Definition of pH
• OpenStax Chemistry 2e — pH and pOH
• OpenStax Chemistry 2e — Relative Strengths of Acids and Bases