Formula
pH uses the concentration in moles per liter:
pH = −log10(mol/L)
The result is rounded to two decimal places. At 25°C, with the ion product of water near 1.0 × 10^−14, pOH is 14 minus pH. A separate pOH page is not published.
Assumptions
- The number you type is the hydrogen ion concentration in mol/L, not a pH that you want to convert the other way.
- pOH uses 25°C and a water ion product of 1.0 × 10^−14.
- The concentration stands in for activity. Real solutions can differ.
Worked example
0.0000001 mol/L of hydrogen ions is pH 7.00.
Input definitions
- Hydrogen ion concentration
- Moles of H+ per liter. Pure water at 25°C is about 0.0000001.
Limitations
This does not measure a sample, identify an acid, or say whether a solution is safe to handle. A meter reading and this logarithm can differ when activity and concentration are not the same.
Questions
Why is 0.0000001 equal to pH 7?
The base-10 log of 0.0000001 is −7, and pH is the negative of that log.
Can pH be negative?
Yes. A concentration above 1 mol/L has a negative logarithm. Strong laboratory acids can be in that range.