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Chemistry & Materials Science

pH Calculator

Calculate the pH, pOH, and hydroxide ion concentration from the given hydrogen ion concentration [H+] for a complete solution profile.

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Instantly convert hydrogen ion concentration [H+][H^+] into pH. This calculator also provides the corresponding pOH and hydroxide ion [OH−][OH^-] concentrations for a complete profile of your acid-base solution.

The pH Scale

The pH scale is a logarithmic measure of the acidity or basicity of an aqueous solution. Because hydrogen ion concentrations can range from 11 M down to 1×10−141 \times 10^{-14} M, a logarithmic scale provides a manageable number between 0 and 14.

pH=−log⁡10[H+]\begin{aligned} \text{pH} = -\log_{10}[H^+] \end{aligned}

Where:
[H+][H^+]=
Hydrogen Ion Concentration (mol/L)

Temperature Dependence

The standard pH scale from 0 to 14 is specifically calibrated for water at 25∘C25^\circ\text{C} (298 K), where the autoionization constant of water (KwK_w) is exactly 1.0×10−141.0 \times 10^{-14}.

Frequently Asked Questions

pH stands for 'potential of hydrogen' or 'power of hydrogen'. It is the negative base-10 logarithm of the hydrogen (or hydronium) ion concentration in a solution.

Yes. In extremely concentrated strong acids (where [H+][H^+] is greater than 1 M), the pH drops below 0. For example, a 10 M solution of HCl has a pH of -1.

At 25∘C25^\circ\text{C}, pure water autoionizes to produce exactly 1.0×10−71.0 \times 10^{-7} M of [H+][H^+] and [OH−][OH^-]. The negative logarithm of 1.0×10−71.0 \times 10^{-7} is exactly 7.

Because the scale is logarithmic (base 10), a pH of 3 is exactly 10 times more acidic than a pH of 4, and 100 times more acidic than a pH of 5.

In reality, bare protons (H+H^+) do not exist in water; they attach to water molecules to form hydronium ions (H3O+H_3O^+). However, [H+][H^+] is commonly used as a shorthand convention in chemistry.

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