pH Calculator

pH describes how acidic or basic a solution is, on a scale where each step is ten times stronger than the last. Start from whichever value your problem gives you ([H⁺], pH, [OH⁻] or pOH) and this calculator finds the other three, says whether the solution is acidic, neutral or basic, and marks it on the pH scale.

How to Use It

1. Choose what to Start From: [H⁺], pH, [OH⁻] or pOH.
2. Enter its value. Type tiny concentrations in E notation, such as 1e-3 for 0.001 M.
3. Press Calculate to see all four values and where the solution sits on the scale.

How It Works

pH and pOH are negative logarithms of the ion concentrations, in mol/L:

In water at 25 °C, $[\mathrm{H^+}][\mathrm{OH^-}] = 1.0 \times 10^{-14}$, so:

Going the other way, $[\mathrm{H^+}] = 10^{-\text{pH}}$. Below 7 is acidic, 7 is neutral and above 7 is basic.

Worked Example

A solution has $[\mathrm{H^+}] = 1 \times 10^{-3}$ M:

So $[\mathrm{OH^-}] = 10^{-11}$ M, and at pH 3 the solution is acidic.

Tips and Common Mistakes

• Remember the minus sign. pH is the negative log; forgetting it gives −3 instead of 3.
• One pH step is a factor of 10. pH 2 has ten times the [H⁺] of pH 3.
• Use mol/L. Convert millimolar to molar first: 5 mM is 0.005 M.
• pH + pOH = 14 only at 25 °C. At other temperatures, the neutral point shifts.
• Round pH to match your data. The number of decimal places in pH matches the significant figures in [H⁺].