pH / pOH Calculator

pH and pOH: Convert Acid-Base Values Quickly

pH measures hydrogen ion activity on a logarithmic scale. pOH does the same for hydroxide. For many classroom calculations, the relationship pH + pOH = 14 is used at 25 C.

This calculator lets you start with pH, pOH, [H+], or [OH-]. It also allows pKw adjustment for temperature-aware problems.

The logarithmic scale is the part that makes pH feel different from ordinary concentration arithmetic. A solution at pH 3 has ten times the hydrogen ion concentration of pH 4, and one hundred times the hydrogen ion concentration of pH 5. This is why small-looking pH changes can be chemically important in buffers, enzyme assays, water testing, soil chemistry, aquariums, and titration problems.

How to use this calculator

1. Select the value you already know.
2. Enter the numeric value.
3. Use pKw = 14 for common 25 C water problems, or choose a preset/custom value.
4. Review pH, pOH, [H+], [OH-], and the acidic/basic interpretation.

Interpretation

A pH below the neutral point is acidic. A pH above the neutral point is basic. The neutral point equals pKw / 2.

When pKw matters

The familiar neutral pH of 7 assumes water near 25 C, where pKw is approximately 14. At other temperatures, pKw changes, so the neutral point changes too. That does not mean warm pure water has become acidic or basic in the everyday sense; it means the balance point of hydrogen and hydroxide concentrations has shifted. Use the pKw field when a chemistry problem gives a different value or when a temperature-specific answer is expected.

Concentration notes

For classroom calculations, [H+] and [OH-] are normally entered in moles per litre. Very concentrated acids and bases, mixed solvents, high ionic strength solutions, and real analytical measurements can require activity corrections rather than simple concentration formulas. For typical homework, introductory lab work, buffer estimates, and quick checks, the pH, pOH, and pKw relationships used here are the standard starting point.

Worked example

If [H+] is 0.001 M, then pH = -log10(0.001) = 3. At pKw 14, pOH is 11 and [OH-] is 1 x 10^-11 M. The result is acidic because the pH is below the neutral point of 7.