# pH Calculator

Free pH calculator. Convert between pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-].

## What this calculates

Calculate pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-] from any one of these values. Based on the water autoionization constant Kw = 10^-14 at 25°C.

## Inputs

- **Input Type** — options: pH, pOH, [H⁺] (mol/L), [OH⁻] (mol/L) — Select which value you know.
- **Value** — Enter pH (0-14), pOH (0-14), [H+] in mol/L, or [OH-] in mol/L.

## Outputs

- **pH** — The pH value (-log10[H+]).
- **pOH** — The pOH value (-log10[OH-]).
- **[H⁺]** — formatted as text — Hydrogen ion concentration in mol/L.
- **[OH⁻]** — formatted as text — Hydroxide ion concentration in mol/L.
- **Nature** — formatted as text — Whether the solution is acidic, neutral, or basic.

## Details

The pH scale measures how acidic or basic a solution is. It is a logarithmic scale based on the concentration of hydrogen ions.

Key Relationships (at 25°C)

- pH = -log₁₀[H⁺]

- pOH = -log₁₀[OH⁻]

- pH + pOH = 14

- [H⁺] × [OH⁻] = 10⁻¹⁴ (Kw, the water autoionization constant)

The pH Scale

- pH 0-6.9: Acidic (more H⁺ than OH⁻)

- pH 7.0: Neutral (equal H⁺ and OH⁻)

- pH 7.1-14: Basic/Alkaline (more OH⁻ than H⁺)

Common pH Values

- Battery acid: pH ~0

- Lemon juice: pH ~2

- Vinegar: pH ~2.5

- Coffee: pH ~5

- Pure water: pH 7

- Baking soda: pH ~9

- Ammonia: pH ~11

- Bleach: pH ~13

Important: These calculations assume 25°C (298.15 K). At other temperatures, Kw changes, and pH + pOH will not equal exactly 14.

## Frequently Asked Questions

**Q: What is pH?**

A: pH stands for 'power of hydrogen' (or 'potential of hydrogen'). It is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log10[H+]. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic. Each whole number change represents a 10-fold change in H+ concentration.

**Q: Can pH be negative or greater than 14?**

A: Yes, though it is uncommon. A pH below 0 means [H+] > 1 M (e.g., concentrated strong acids). A pH above 14 means [OH-] > 1 M (e.g., concentrated strong bases). The 0-14 range covers concentrations from 1 M to 10^-14 M at 25°C.

**Q: Why does pH + pOH = 14?**

A: At 25°C, water autoionizes: H2O ⇌ H+ + OH-. The equilibrium constant is Kw = [H+][OH-] = 10^-14. Taking -log10 of both sides: pH + pOH = 14. This only holds at 25°C; at other temperatures, Kw changes (e.g., at 37°C, Kw ≈ 2.4×10^-14, so pH + pOH ≈ 13.6).

**Q: How do I measure pH in practice?**

A: pH can be measured with a pH meter (electronic probe, most accurate), pH indicator paper/strips (color-changing, less precise), or liquid indicators (like phenolphthalein or litmus). For precise laboratory work, a calibrated pH meter is standard.

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Source: https://vastcalc.com/calculators/chemistry/ph
Category: Chemistry
Last updated: 2026-04-21