How to use
- Type into whichever of the four boxes you have a value for, and leave the other three empty.
- Leave pKw at 14.00 unless you are working somewhere other than 25 °C. At 37 °C it is nearer 13.6.
- The other three appear underneath. Filling in a second box gives an error rather than an answer, because there would be two of them.
💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
Two relations do all of it: a concentration is ten to the minus p, and pH plus pOH is pKw. What deserves the attention on this page is not the arithmetic but the word concentration, because that is not what pH is defined as.
Four quantities, two relations.
- Concentration from p
- [H⁺] = 10^(−pH) [OH⁻] = 10^(−pOH)
- p from concentration
- pH = −log₁₀[H⁺] pOH = −log₁₀[OH⁻]
- The two halves
- pH + pOH = pKw (14.00 at 25 °C)
Reference points at 25 °C
| pH | [H⁺] (mol/L) | pOH | |
|---|---|---|---|
| 1 M strong acid | 0 | 1 | 14 |
| Gastric acid | 1.5 | 3.2 × 10⁻² | 12.5 |
| Neutral | 7 | 1 × 10⁻⁷ | 7 |
| Blood | 7.4 | 4.0 × 10⁻⁸ | 6.6 |
| 1 M strong base | 14 | 1 × 10⁻¹⁴ | 0 |
Each step of one pH unit is a factor of ten in concentration. That is worth keeping in front of you when a reading drifts by 0.3: it is a factor of two.
Practical notes
- ⚠️⚠️ IUPAC defines pH by activity, not by concentration — Gold Book P04524 gives pH = −lg[a(H⁺)] and adds that pH cannot be measured independently, because calculating the activity involves the activity coefficient of a single ion, so it can be regarded only as a notional definition. JIS Z 8802:2011 3.1 defines it operationally instead: the value obtained from the EMF measured with a glass electrode against the standard buffers.
- What this page computes, [H⁺] = 10^(−pH), is therefore the concentration you get by taking the activity coefficient as 1. That is close enough in dilute solution and increasingly wrong as ionic strength rises. In a 1 M salt solution it is not a good number.
- ⭐ pKw is an input because it moves with temperature. At 25 °C it is 14.00; at 37 °C it is about 13.6. A buffer worked out at body temperature against 14.00 is out by roughly 0.4 in pOH. The full temperature and density dependence is in Bandura and Lvov, J Phys Chem Ref Data, which is beyond what a converter carries.
- Neutral is not always pH 7. Neutral means [H⁺] equals [OH⁻], which is pKw ÷ 2 — so it is 7.00 at 25 °C and about 6.8 at 37 °C. Blood at pH 7.4 is more alkaline than neutral, but by less than the number suggests.
- pH outside 0 to 14 is real and the tool computes it. Fuming acids and concentrated alkali sit outside that range, which is a consequence of pKw at 25 °C rather than a law. What the tool refuses is a concentration of zero or below, where the logarithm is not defined.
FAQs
It is the concentration you get by assuming the activity coefficient is 1. pH is defined by activity, and activity and concentration part company as ionic strength rises — in a dilute buffer the difference is small, in a concentrated salt solution it is not. The number here is useful and it is an approximation, and the page says so rather than quietly presenting it as a measurement.
Because 14.00 is the value at 25 °C and almost nobody works only at 25 °C. At 37 °C it is about 13.6, so a pOH calculated against 14.00 is out by 0.4 — a factor of two and a half in hydroxide. Fixing it as a constant would hide a temperature dependence that changes the answer.
No. Neutral means [H⁺] equals [OH⁻], which puts it at pKw ÷ 2 — about 6.8 at body temperature. Blood at 7.4 is alkaline relative to neutral, but by about 0.6 rather than the 0.4 you would read off from a fixed 7.00.
Yes, and the tool will compute it. Concentrated strong acids reach below zero and concentrated alkali above 14. The 0-to-14 range is what falls out of pKw at 25 °C, not a rule about how acidic things can be. Measuring out there is another matter — a glass electrode is calibrated against buffers inside the range.
Because the logarithm of zero is not a number. A concentration of exactly zero would be a pH of infinity, and no solution of water has it — the autoprotolysis of water alone keeps [H⁺] near 10⁻⁷. Refusing is more honest than printing Infinity.
No. JIS Z 8802 defines pH by what a calibrated glass electrode reads against standard buffers, so the meter is where a pH comes from. This page converts a pH you already have into the other three numbers.
Often used together
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