pH, pOH and ion concentration in one place.
pH is just -log10 of the hydrogen ion concentration. Pick the value you already know, hydrogen ion concentration, pH, hydroxide concentration or pOH, and this works out the other three and tells you whether the solution is acidic, neutral or basic. Figures assume a temperature of 25 C.
The numbers
Enter a pH between 0 and 14. Concentrations accept scientific notation such as 1e-3.
What the result means
Where it lands on the pH scale
The 0 to 14 scale, acidic on the left, basic on the right
pH scorecard
pH of everyday substances
Typical values at room temperature. The row closest to your result is highlighted. Notice how each step down in pH is ten times more hydrogen ions.
| Substance | Typical pH | [H+] (mol/L) | Nature |
|---|
pH, explained
The logarithmic scale behind pH
pH measures how acidic or basic a water based solution is. It is defined as pH = -log10 of the hydrogen ion concentration in moles per litre, so a solution with [H+] of 1e-3 mol/L has a pH of 3. Because the scale is logarithmic, each whole number is a tenfold change: a pH of 4 has ten times more hydrogen ions than a pH of 5, and a hundred times more than a pH of 6. The everyday range runs from 0 to 14, though very strong acids and bases can sit slightly outside it.
pH, pOH and the water constant
Water constantly splits into hydrogen and hydroxide ions. At 25 C the product of the two concentrations is fixed: [H+] times [OH-] = 1e-14, a value called the ion product of water, Kw. Taking negative logarithms of both sides gives the simple partner rule pH + pOH = 14. That is why knowing any one of pH, pOH, [H+] or [OH-] is enough to pin down the other three. Raise the temperature and Kw shifts, so the neutral point drifts below 7, which is why these figures are quoted at 25 C.
Common questions
What is pH?
pH is a measure of how acidic or basic a solution is. It equals the negative base-10 logarithm of the hydrogen ion concentration, written pH = -log10[H+]. A low pH means a lot of hydrogen ions and a strong acid, while a high pH means very few hydrogen ions and a strong base.
What is the difference between pH and pOH?
pH tracks the hydrogen ion concentration and pOH tracks the hydroxide ion concentration. At 25 C they always add up to 14, so pOH = 14 - pH. If you know one you instantly know the other.
What does a pH of 7 mean?
A pH of 7 is neutral at 25 C, the point where hydrogen and hydroxide ions are present in equal amounts, each at 1e-7 mol/L. Pure water sits here. Below 7 is acidic and above 7 is basic, also called alkaline.
How do I convert pH to hydrogen ion concentration?
Reverse the logarithm: [H+] = 10^(-pH). For a pH of 3 that gives 10^(-3), or 1e-3 mol/L. Going the other way, take the negative logarithm of the concentration to get the pH.
Does temperature affect pH?
Yes. The ion product of water, Kw, rises with temperature, so the neutral point drops below 7 in hot water even though it is still neutral. All the relationships on this page assume 25 C, the standard reference temperature.
Can pH go below 0 or above 14?
It can. The 0 to 14 range covers most everyday solutions, but a very concentrated strong acid can show a negative pH and a very concentrated strong base can exceed 14. The formula still holds; the numbers simply fall outside the usual scale.
Results use the standard definitions of pH and pOH for dilute aqueous solutions at 25 C, with the ion product of water taken as 1e-14. Real measurements depend on temperature, ionic strength and activity effects, so for laboratory or clinical work verify against a calibrated meter and your own reference values.