Convert between pH, pOH and the hydrogen [H+] and hydroxide [OH-] ion concentrations at 25 °C, with the nature of the solution.
For concentrations type something like 1e-3 or 0.001.
Enter the values to see the result here.
Formula
At 25 °C:
pH=−log10[H+]pOH=−log10[OH−]pH+pOH=14
and the concentrations follow as [H+]=10−pH and [OH−]=10−pOH.
Worked example
A 0.010 M solution of sodium hydroxide, a strong base.
[OH−]=0.010 mol/L, so pOH=−log10(0.010)=2.
pH=14−2=12.
[H+]=10−12 mol/L.
The solution is basic. Diluting it tenfold lowers the pH by exactly one unit, to 11.
Common mistakes
Treating a pH difference of 1 as a small change. It is a tenfold change in concentration.
Using pH + pOH = 14 at temperatures far from 25 °C.
Taking the concentration of a weak acid as [H+]. Only a small fraction of it ionises.
Entering a concentration of 0 or a negative number. The logarithm is not defined.
Frequently asked questions
What does pH measure?
pH is the negative base-10 logarithm of the hydrogen ion concentration. Each unit is a tenfold change, so pH 3 is ten times more acidic than pH 4. Pure water at 25 °C is pH 7.
Why do pH and pOH add up to 14?
Because the ion product of water, Kw = [H+][OH-], is 1 × 10⁻¹⁴ at 25 °C. Taking the negative logarithm gives pH + pOH = 14. The sum changes with temperature, for example about 13.6 at 37 °C.
How do I find the pH of a strong acid solution?
A strong monoprotic acid such as HCl dissociates completely, so [H+] equals its concentration: 0.001 M HCl has a pH of 3. For H2SO4 and other polyprotic acids, count how many H+ each molecule gives, and weak acids need their Ka.