pH vs pOH: What's the Difference?
You've got comfortable with pH, and then a question asks for pOH and it feels like a whole new topic. It isn't. pOH is the same idea pointed at the other ion, and the two are locked together by one equation.
The short answer: pH measures the concentration of hydrogen ions (H⁺) in a solution, while pOH measures the concentration of hydroxide ions (OH⁻). In water at 25 °C they are always linked by pH + pOH = 14, so knowing either one gives you the other by subtraction.
Quick comparison at a glance
| Feature | pH | pOH |
|---|---|---|
| Measures | H⁺ concentration | OH⁻ concentration |
| Formula | pH = −log[H⁺] | pOH = −log[OH⁻] |
| Low value means | Acidic | Basic (alkaline) |
| High value means | Basic (alkaline) | Acidic |
| Neutral water at 25 °C | 7 | 7 |
| Typical range | 0–14 | 14–0 |
| Which is quoted in daily life | Almost always this one | Rarely |
| Relationship | pH + pOH = 14 | pH + pOH = 14 |
What pH is
pH is the negative logarithm of the hydrogen ion concentration:
pH = −log[H⁺]
The log is what makes the scale manageable. Hydrogen ion concentrations in real solutions span from about 1 mol/L down to 0.00000000000001 mol/L, and nobody wants to write that. The log squashes fourteen orders of magnitude into the numbers 0 to 14.
It also means each pH unit is a factor of ten. A solution at pH 3 has ten times more H⁺ than one at pH 4, and a hundred times more than pH 5.
What pOH is
pOH does exactly the same job for hydroxide ions:
pOH = −log[OH⁻]
The direction is what trips people up. A low pOH means lots of OH⁻, which means a basic solution — the opposite of what a low pH means. Household bleach has a pH around 13 and a pOH around 1.
pOH is genuinely useful when a question hands you a base. If you dissolve 0.01 mol/L NaOH, you know [OH⁻] straight away, so pOH = 2 falls out in one step. Getting pH from there is a subtraction rather than a detour through the water equilibrium.
Why they add up to 14
Water quietly ionises:
H₂O ⇌ H⁺ + OH⁻
At 25 °C the product of the two concentrations is a fixed constant, the ion product of water:
[H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴
Take the negative log of both sides and the multiplication becomes addition:
pH + pOH = 14
That's the whole derivation. It also explains why pure water is pH 7: if the two concentrations are equal and multiply to 10⁻¹⁴, each must be 10⁻⁷.
One caveat worth knowing: 14 is a room-temperature number. The ion product rises as water gets hotter, so at 50 °C pH + pOH ≈ 13.3, and neutral water sits at pH 6.6. It's still neutral — [H⁺] still equals [OH⁻] — it just isn't 7 any more. Exam questions almost always specify 25 °C for this reason.
How to convert between them
Given pH, want pOH: subtract from 14.
Given pOH, want pH: subtract from 14.
Given [H⁺], want pH: take −log.
Given [OH⁻], want pOH: take −log.
Given [H⁺], want [OH⁻]: divide 1.0 × 10⁻¹⁴ by [H⁺].
Worked examples
Try each before reading the answer.
1. A solution has [H⁺] = 1 × 10⁻³ mol/L. Find pH and pOH.
pH = −log(10⁻³) = 3. pOH = 14 − 3 = 11. Acidic.
2. A 0.01 mol/L NaOH solution. Find pOH and pH.
NaOH is a strong base, so [OH⁻] = 0.01 = 10⁻². pOH = 2, and pH = 14 − 2 = 12. Strongly basic.
3. A solution has pOH = 4.5. Is it acidic or basic?
pH = 14 − 4.5 = 9.5 — basic. A low pOH always means basic.
4. Ammonia cleaner measures pH 11.6. Find [OH⁻].
pOH = 14 − 11.6 = 2.4, so [OH⁻] = 10⁻²·⁴ ≈ 4 × 10⁻³ mol/L.
5. Pure water at 25 °C.
[H⁺] = [OH⁻] = 10⁻⁷, so pH = pOH = 7. Neutral.
Common mistakes to avoid
- Reading pOH like pH. Low pOH means basic, not acidic. The scale runs the opposite way; sketch both scales stacked if it helps.
- Using pH + pOH = 14 at any temperature. It's a 25 °C relationship. At other temperatures the sum changes, and so does the neutral point.
- Forgetting the negative sign in the log. pH = −log[H⁺]. Drop the minus and every answer comes out negative.
FAQ
What is the difference between pH and pOH?
pH measures hydrogen ion concentration and pOH measures hydroxide ion concentration. At 25 °C they always add to 14, so a solution with a high pH has a low pOH.
Does pH + pOH always equal 14?
Only at 25 °C. The ion product of water changes with temperature, so at higher temperatures the sum is slightly less than 14.
Is a low pOH acidic or basic?
Basic. Low pOH means a high hydroxide concentration. Bleach, for example, has a pOH of about 1.
Why do we use pOH at all if pH is enough?
Because base calculations start from [OH⁻]. Working out pOH first and subtracting from 14 is quicker and less error-prone than converting concentrations twice.
The takeaway
pH and pOH are two windows onto the same solution — one counts H⁺, the other counts OH⁻ — and at room temperature they're tied together by pH + pOH = 14. Start from whichever ion the question gives you, then subtract.
Background → What Is pH? The pH Scale Explained Simply and Acids vs Bases: What's the Difference?. Related → [Strong vs Weak Acids] (sibling) explains why concentration and strength aren't the same, and [What Is a Buffer?] (sibling) shows how solutions hold pH steady. On the ions → What Is an Ion? Cations, Anions, and Charges.
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