Hydrogen Ion Concentration Calculator
Convert pH, pOH, hydrogen ion concentration, and hydroxide ion concentration at 25 °C.
Acid-base conversion
Enter one known property to calculate all four related values using pH + pOH = 14.
About hydrogen ion concentration
Hydrogen ion concentration is a quantitative measure of acidity in an aqueous solution. pH expresses this concentration on a logarithmic scale: pH = -log10([H+]). Rearranging gives [H+] = 10^(-pH). A change of one pH unit corresponds to a tenfold change in hydrogen ion concentration, which is why a pH 3 solution has ten times the hydrogen ion concentration of a pH 4 solution.
Hydroxide ions provide the complementary measure of basicity. At 25 °C, the ionic product of water is Kw = [H+] times [OH-] = 1.0 times 10^-14. The corresponding logarithmic quantities obey pH + pOH = 14. This calculator uses those relationships to convert any one of pH, pOH, hydrogen ion concentration, or hydroxide ion concentration into all four values.
A neutral aqueous solution at 25 °C has equal hydrogen and hydroxide ion concentrations of 1.0 times 10^-7 mol/L, producing pH 7 and pOH 7. A lower pH indicates greater hydrogen ion concentration and acidity, while a higher pH indicates lower hydrogen ion concentration and greater basicity. Values outside the familiar zero-to-fourteen range are possible in sufficiently concentrated solutions, although activity effects then make elementary concentration formulas less accurate.
The constant 14 is temperature dependent. It follows from pKw, the negative base-ten logarithm of water's ion product, and pKw changes as temperature changes. At temperatures other than 25 °C, neutral pH is not necessarily exactly 7. Precise physical chemistry calculations should use the appropriate Kw and activities rather than assuming ideal molar concentrations.
These conversions are useful in analytical chemistry, water-quality monitoring, titration work, environmental science, biology, and laboratory preparation. They do not by themselves predict the pH of a weak acid, weak base, buffer, or mixed solution from its formal concentration; those situations also require equilibrium constants, mass balance, and sometimes charge balance. Use this tool when one of the four directly related acid-base measures is already known, and interpret highly concentrated or nonideal solutions with suitable thermodynamic methods.
Hydrogen ion concentration examples
| Known value | Converted values | Interpretation |
|---|---|---|
| pH = 7 | [H+] = 1.0 × 10^-7 mol/L | Neutral water at 25 °C has equal hydrogen and hydroxide concentrations. |
| pOH = 4 | pH = 10; [OH-] = 1.0 × 10^-4 mol/L | The solution is basic under the 25 °C convention. |
| [H+] = 0.001 mol/L | pH = 3; pOH = 11 | A concentration of one thousandth molar corresponds to pH 3. |
How to convert acid-base values
- Choose whether the known value is pH, pOH, hydrogen concentration, or hydroxide concentration.
- Enter the known number in the value field.
- Select Calculate concentrations to convert the value at 25 °C.
- Read the pH, pOH, hydrogen ion, and hydroxide ion results together.
Hydrogen ion concentration FAQ
How do I convert pH to hydrogen ion concentration?
Raise ten to the negative pH power. For pH 3, the hydrogen ion concentration is 1.0 times 10^-3 mol/L.
Why do pH and pOH add to 14?
At 25 °C, water's ionic product has a pKw of 14. Taking negative logarithms of the equilibrium relationship gives pH + pOH = 14.
Is pH 7 always neutral?
It is neutral at approximately 25 °C under common ideal assumptions. Water's ion product changes with temperature, so neutral pH also changes.
Can pH be below zero or above fourteen?
Yes, concentrated acids and bases can produce values outside that range. Activity corrections become important, so simple concentration calculations may be less accurate.
Can this calculator find pH from an acid's starting molarity?
Only when the actual hydrogen ion concentration is known. Weak acids and bases require equilibrium calculations involving their dissociation constants.