pH Calculator
pH, pOH and both ion concentrations, on a log scale.
pH, pOH and both ion concentrations, on a log scale. Handles the case where the obvious answer is wrong.
pH
3
acidic · pOH 11
pH = −log₁₀[H⁺], so the scale is logarithmic: each whole unit is a tenfold change in concentration. pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6, which is why small-sounding pH shifts matter so much in soil, water and blood. The 14 in pH + pOH is temperature dependent — at 50 °C neutral water sits nearer 6.6 while still being perfectly neutral.
How the pH Calculator works
pH = −log₁₀[H⁺], so the scale is logarithmic: each whole unit is a tenfold change in concentration. pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6, which is why small-sounding pH shifts matter so much in soil, water and blood.
Also known as: poh calculator · hydrogen ion concentration calculator · how to calculate ph · ph formula
A logarithmic scale
pH = −log₁₀[H⁺], so each whole unit is a tenfold change in concentration. pH 4 is ten times more acidic than pH 5 and a hundred times more than pH 6.
That compression is why pH shifts which sound small are not. A lake dropping from pH 6 to pH 5 has become ten times more acidic, which is the difference between healthy and hostile for most fish.
Why 7 is neutral, and when it is not
Pure water dissociates slightly, producing equal amounts of H⁺ and OH⁻. At 25 °C that equilibrium gives 10⁻⁷ M of each, which is where the 7 comes from.
Raise the temperature and water dissociates more, so both concentrations rise and neutral pH falls. At 50 °C neutral water sits nearer 6.6 — still perfectly neutral, because the two ions remain equal, but no longer at 7.
The pH + pOH = 14 relationship carries the same caveat. It is a 25 °C convention rather than a law, and the sum shifts with temperature for the same reason.
Frequently asked questions
How do I calculate pH?
Take the negative base-10 logarithm of the hydrogen ion concentration. A concentration of 10⁻³ M gives a pH of 3.
What is pOH?
The same measure for hydroxide ions. At 25 °C, pH + pOH = 14, so a pH of 3 means a pOH of 11.
Why is pH 7 neutral?
Because pure water at 25 °C dissociates just enough to give 10⁻⁷ M of each ion. That 7 is temperature dependent — at 50 °C neutral water sits nearer 6.6 while remaining perfectly neutral.
Can pH go below 0 or above 14?
Yes. Concentrated strong acids can reach negative pH and concentrated bases can exceed 14. The 0–14 range is where most aqueous chemistry happens, not a hard limit.
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