pH Calculator
Calculate pH, pOH, and hydrogen/hydroxide ion concentration.
pH is typically between 0 and 14, but values outside this range are allowed.
pH
pH
pOH
[H⁺] (mol/L)
[OH⁻] (mol/L)
Calculation Steps
The pH Scale
The pH scale measures how acidic or basic an aqueous solution is. It runs from 0 to 14 at 25 °C, where 7 is neutral (pure water). Values below 7 are acidic, and values above 7 are basic (alkaline). The scale is logarithmic, so each whole-number change in pH represents a tenfold change in hydrogen ion concentration.
Acidic
pH < 7
Neutral
pH = 7
Basic
pH > 7
The pH Formulas
At 25 °C, the four quantities are related through the following equations. Knowing any one of them lets you calculate the rest:
pH = −log₁₀[H⁺]
[H⁺] = 10^(−pH)
pOH = −log₁₀[OH⁻]
[OH⁻] = 10^(−pOH)
pH + pOH = 14
[H⁺] · [OH⁻] = 1 × 10⁻¹⁴
- • pH = negative log of hydrogen ion concentration
- • pOH = negative log of hydroxide ion concentration
- • [H⁺] = hydrogen ion concentration (mol/L)
- • [OH⁻] = hydroxide ion concentration (mol/L)
Acids vs. Bases
Acids release hydrogen ions (H⁺) when dissolved in water, increasing [H⁺] and lowering the pH. Bases release or generate hydroxide ions (OH⁻), increasing [OH⁻] and raising the pH. Strong acids and bases dissociate completely, while weak acids and bases only partially dissociate.
| Substance | Approx. pH | Type |
|---|---|---|
| Battery acid | 0 | Strongly acidic |
| Lemon juice | 2 | Acidic |
| Pure water | 7 | Neutral |
| Baking soda | 9 | Basic |
| Bleach | 13 | Strongly basic |
Note: This calculator assumes dilute aqueous solutions at 25 °C, where the ion product of water Kw = 1 × 10⁻¹⁴ and pH + pOH = 14. At other temperatures Kw changes, so the neutral pH and these relationships shift. For precise analytical work, account for temperature, ionic strength, and activity coefficients.