pH / pOH Calculator
Convert pH, pOH, hydrogen ion and hydroxide ion concentration with a visual pH scale.
What you can convert
Choose the known value and enter either pH, pOH, hydrogen ion concentration, or hydroxide ion concentration. The result card converts the other three values and places the answer on a labeled acid-base scale.
| Known value | Example input | First calculation |
|---|---|---|
| pH | 3 | pOH = 14 - pH |
| pOH | 2 | pH = 14 - pOH |
| [H+] | 1e-3 | pH = -log10[H+] |
| [OH-] | 1e-5 | pOH = -log10[OH-] |
Formulas used
Worked examples
[H+] = 1 x 10^-3 M: pH = 3 and pOH = 11, so the solution is acidic.
pOH = 2: pH = 14 - 2 = 12, so the solution is basic.
pH = 7: pOH = 7, [H+] = 1 x 10^-7 M, and [OH-] = 1 x 10^-7 M at 25 C.
Why logarithms matter
The pH scale is logarithmic. A one-unit pH change means a tenfold change in hydrogen ion concentration. A solution at pH 3 has ten times more [H+] than pH 4, and one hundred times more [H+] than pH 5. The labeled scale helps make that direction clear.
Where this calculator is useful
- Converting strong acid or strong base concentration into pH or pOH.
- Checking acid-base homework that gives [H+] or [OH-] in scientific notation.
- Comparing acidic, neutral, and basic solutions on a visual scale.
- Preparing for buffer and Henderson-Hasselbalch calculations.
- Reviewing Kw and water autoionization at 25 C.
Common mistakes
- Forgetting the negative sign in pH = -log10[H+].
- Using pH + pOH = 14 outside the 25 C classroom assumption without checking temperature.
- Entering 10^-3 as 10-3 instead of 1e-3.
- Calling pH 6 neutral; at 25 C, pH 7 is neutral.
- Assuming each pH step is linear instead of tenfold.
Result checking
Acids have pH below 7 and bases have pH above 7 at 25 C. If [H+] is larger than 1 x 10^-7 M, pH should be below 7. If [OH-] is larger than 1 x 10^-7 M, pH should be above 7. Very concentrated solutions can fall outside the common 0 to 14 classroom scale.
Related Chemistry Tools
pOH
pOH measures hydroxide ion concentration: pOH = -log10[OH-]. At 25 C, pH + pOH = 14. A low pOH means a basic solution, while a high pOH means an acidic solution.
FAQs
What does pH stand for?
pH originally stood for the German 'Potenz Hydrogen' — power, or potential, of hydrogen. It was introduced by Søren Sørensen in 1909 as a way to express hydrogen-ion concentration on a manageable scale. Definition: pH = -log10[H+], where [H+] is the molar concentration of hydrogen ions in mol/L. At 25 °C the practical scale runs from 0 to 14: pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Each pH unit corresponds to a 10× change in [H+], so pH 3 has ten times the [H+] of pH 4. Reference points: stomach acid pH 1-2, lemon juice ≈ 2, black coffee ≈ 5, pure water 7, blood 7.35-7.45, household ammonia ≈ 11, 0.1 M NaOH ≈ 13.
Is more H acidic or basic?
More H+ means more acidic. Acidity is defined by hydrogen-ion concentration: a higher [H+] gives a lower pH (since pH = -log[H+]) and a more acidic solution. At 25 °C the ion product of water is constant: Kw = [H+][OH-] = 1.0 × 10^-14. So when [H+] increases, [OH-] must decrease to keep that product fixed. Examples at 25 °C: [H+] = 10^-1 M → pH 1 (very acidic); [H+] = 10^-7 M → pH 7 (neutral); [H+] = 10^-14 M → pH 14 (very basic). Strong acids release H+ on dissolution (HCl → H+ + Cl-); strong bases release OH- (NaOH → Na+ + OH-).
How to calculate pOH from pH?
At 25 °C, pOH = 14 - pH. This comes from the auto-ionization of water: Kw = [H+][OH-] = 1.0 × 10^-14. Taking -log of both sides gives pH + pOH = pKw = 14.00. Examples: pH 3 → pOH 11; pH 7 → pOH 7; pH 12 → pOH 2; pH 1.5 → pOH 12.5. From either value, [H+] = 10^-pH and [OH-] = 10^-pOH. The 14.00 sum is temperature-dependent because Kw is temperature-dependent. At 50 °C the sum is closer to 13.26; at 0 °C it is closer to 14.94. For room-temperature problems, 14.00 is the standard.
What is a pOH?
pOH is the negative base-10 logarithm of the hydroxide-ion concentration: pOH = -log10[OH-]. At 25 °C: pOH < 7 is basic, pOH = 7 is neutral, pOH > 7 is acidic — the opposite of pH. It is linked to pH through the auto-ionization of water: pH + pOH = 14.00 at 25 °C. When to use it: pOH is convenient for problems where you start from [OH-] (such as base solutions). For 0.01 M NaOH, [OH-] = 0.01 M → pOH = 2 → pH = 12. For weak bases use [OH-] ≈ √(Kb · C) and convert: pOH = -log[OH-]; pH = 14 - pOH.
What is the difference between pH and pOH?
Both are negative base-10 logarithms of an ion concentration; they just describe different ions. pH = -log[H+] tracks hydrogen-ion concentration. Low pH = acidic; high pH = basic. pOH = -log[OH-] tracks hydroxide concentration. Low pOH = basic; high pOH = acidic. At 25 °C they sum to 14: pH + pOH = pKw = 14.00. So an acidic solution might be characterized as pH 2 / pOH 12, a neutral one as pH = pOH = 7, and a basic one as pH 12 / pOH 2. Use pH when starting from [H+] or an acid concentration, and pOH when starting from [OH-] or a base concentration — they describe the same solution from opposite directions.
Does pH + pOH always equal 14?
Only at 25 °C. The sum equals pKw, and Kw is temperature-dependent because water self-ionization is endothermic. Approximate values: 0 °C → Kw ≈ 1.14 × 10^-15, pKw ≈ 14.94; 25 °C → Kw = 1.0 × 10^-14, pKw = 14.00; 50 °C → Kw ≈ 5.48 × 10^-14, pKw ≈ 13.26; 100 °C → Kw ≈ 5.13 × 10^-13, pKw ≈ 12.29. At 100 °C, pure water has pH = pOH = 6.14 — still neutral, even though pH ≠ 7. Neutrality is defined by [H+] = [OH-], not by any fixed pH value. Most introductory problems assume 25 °C, where pH + pOH = 14 holds.
What is the pOH of a 0.0235 M HCl solution?
pOH ≈ 12.37 at 25 °C. HCl is a strong acid, so [H+] = 0.0235 M. pH = -log(0.0235) = 1.629. pOH = 14.00 - pH = 14.00 - 1.629 = 12.371. Cross-check: [OH-] = Kw / [H+] = (1.0 × 10^-14) / 0.0235 = 4.26 × 10^-13 M, and -log(4.26 × 10^-13) = 12.37, consistent.
A solution has a pOH of 13.12 — this tells us?
It is a strongly acidic solution at 25 °C. pH = 14 - 13.12 = 0.88, so [H+] = 10^-0.88 ≈ 0.132 M. [OH-] = 10^-13.12 ≈ 7.6 × 10^-14 M. This corresponds to a roughly 0.13 M strong-acid solution — comparable to dilute battery acid or somewhat more concentrated than gastric acid. Litmus turns red, methyl orange turns red, and phenolphthalein stays colorless. Quick rule of thumb: pOH > 12 implies strongly acidic; pOH < 2 implies strongly basic.