# What is the pH?

Finds pH, pOH, the hydronium ion concentration [H₃O⁺] and the hydroxide ion concentration [OH⁻] of a water solution at 25 °C from any one of them, and says whether it is acidic, neutral or basic.

- Page: https://www.acalculator.org/chemistry/ph-calculator
- JSON spec: https://www.acalculator.org/chemistry/ph-calculator.json
- Version: 6ccffaa67b72

## Default answer

Example with the default inputs ([H₃O⁺] (mol/L) 0.0012): The pH is 2.92: the solution is acidic.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| ph | pH | The pH: −log of the hydronium ion concentration. |
| poh | pOH | The pOH: −log of the hydroxide ion concentration. |
| h | [H₃O⁺] (mol/L) | The hydronium (hydrogen) ion concentration in moles per liter, such as 1.2e-3. |
| oh | [OH⁻] (mol/L) | The hydroxide ion concentration in moles per liter, such as 0.0125. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| pH | pH | The pH: −log of the hydronium ion concentration. |
| pOH | pOH | 14.00 − pH at 25 °C. |
| h | [H₃O⁺] (mol/L) | 10^−pH moles per liter. |
| oh | [OH⁻] (mol/L) | 10^−pOH moles per liter. |
| nature | The solution is | Acidic below pH 7, neutral at pH 7, basic (alkaline) above pH 7, at 25 °C. |

## Method

pH = −log₁₀[H₃O⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = 14.00 at 25 °C, so [H₃O⁺] = 10^−pH and [OH⁻] = 10^−pOH.

## Assumptions

- A water solution at 25 °C, where Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ and pH + pOH = 14.00.
- Concentrations stand in for activities, as in introductory chemistry; very strong solutions differ.
- pH and pOH from −2 to 16; concentrations from 10⁻¹⁶ to 100 mol/L.

## Worked examples

1. h = 0.0012 gives ph = 2.920819, poh = 11.079181, nature = acidic. Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02).
2. ph = 7.3 gives h = 0, poh = 6.7, nature = basic. Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02) (5 × 10⁻⁸ M to one figure).
3. oh = 0.0125 gives poh = 1.90309, ph = 12.09691, nature = basic. Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02).
4. h = 0 gives ph = 7, poh = 7, oh = 0, nature = neutral. Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02).

## FAQ

### How do I calculate pH from the hydrogen ion concentration?

Take the negative base-10 logarithm: pH = −log[H₃O⁺]. For stomach acid with [H₃O⁺] = 1.2 × 10⁻³ M, pH = −log(0.0012) = 2.92. [H⁺] and [H₃O⁺] mean the same thing here.

### How do I get the concentration from the pH?

Raise 10 to the power −pH: [H₃O⁺] = 10^−pH. Blood at pH 7.3 has [H₃O⁺] = 10^−7.3 ≈ 5.0 × 10⁻⁸ M.

### How are pH and pOH related?

At 25 °C, water gives [H₃O⁺] × [OH⁻] = 1.0 × 10⁻¹⁴, so pH + pOH = 14.00. A 0.0125 M KOH solution has pOH = −log(0.0125) = 1.903 and pH = 14.00 − 1.903 = 12.10.

### What pH is acidic, neutral or basic?

At 25 °C a solution is acidic below pH 7, neutral at pH 7 (pure water, [H₃O⁺] = 1.0 × 10⁻⁷ M), and basic or alkaline above pH 7.

### How do I find the pH of a strong acid or base?

A strong acid such as HCl gives up all its H⁺, so [H₃O⁺] equals the acid’s concentration: type it as [H₃O⁺]. A strong base such as NaOH or KOH gives [OH⁻] equal to its concentration: type it as [OH⁻]. A weak acid needs its Ka, which this page does not use.

### Can pH be negative or above 14?

Yes, for very concentrated strong acids (above 1 M) or bases. This page accepts pH from −2 to 16, though at such strengths the simple concentration formula is only approximate.

### Does temperature change the answer?

Yes. Kw changes with temperature, so pH + pOH = 14.00 holds only at 25 °C. This page uses the 25 °C value.

## Sources

- OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10). https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02)