pH & pOH Calculator

For acid-base chemistry calculations.

Enter any one value to calculate the others. Use 'e' for scientific notation (e.g., 1.5e-5).

0°C 25°C (Standard) 100°C

pH Calculator for H+ and OH- Concentration

Use this pH calculator to convert between pH, pOH, hydrogen ion concentration and hydroxide ion concentration. At 25 C, pH + pOH = 14, [H+] = 10^-pH and [OH-] = 10^-pOH.

When you know pH

Find pOH, [H+] and [OH-] for acid-base homework, titration checks and lab calculations.

When you know concentration

Enter [H+] or [OH-] to calculate pH or pOH directly with logarithmic formulas.

AI Answer Summary

What is the relationship between pH and pOH?

pH measures hydrogen ion concentration, pOH measures hydroxide ion concentration, and at 25 C their sum is 14. A lower pH means a more acidic solution, while a lower pOH means a more basic solution.

Key formulas

pH = -log[H+], pOH = -log[OH-], and pH + pOH = 14 at 25 C.

Best for

Converting between pH, pOH, [H+] and [OH-] for acid-base calculations.

Temperature note

The pH + pOH = 14 shortcut assumes water at 25 C; other temperatures change Kw.

Worked Examples: pH and pOH

Example 1: pH from [H+] = 2.5×10−4 M

  1. Formula: pH = −log10[H+].
  2. Compute: pH = −log10(2.5×10−4) = −(log 2.5 − 4) ≈ −(0.398 − 4) = 3.60.
  3. At 25°C, pOH = 14 − pH = 10.40; solution is acidic.

Example 2: pOH from [OH−] = 3.2×10−6 M

  1. Formula: pOH = −log10[OH−].
  2. Compute: pOH = −log10(3.2×10−6) = −(log 3.2 − 6) ≈ −(0.505 − 6) = 5.50.
  3. Then pH = 14 − pOH = 8.50; solution is basic.

Example 3: Find [H+] from pH = 7.25

  1. [H+] = 10−pH = 10−7.25.
  2. Compute: 10−7.25 ≈ 5.62×10−8 M.
  3. Then [OH−] = Kw/[H+] = 1.0×10−14 / 5.62×10−8 ≈ 1.78×10−7 M.