How this calculator works
Thermodynamic pH is defined from hydrogen ion activity. This concentration conversion assumes activity coefficients of one, which is suitable as an ideal dilute-solution approximation. The default pKw = 14 approximates water near 25 C; pKw changes with temperature. Negative pH is allowed because concentrated solutions can have activity above one.
Formula
pH = -log10([H+] / (1 mol/L)); pOH = pKw - pH.
Worked example
pH 3 corresponds to 0.001 mol/L H+. At pKw = 14, pOH 5 gives pH 9 and 10^-9 mol/L H+.
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Frequently asked questions
Can pH be negative?
Yes. Negative pH is possible, though activity effects become important.
Does neutral always mean pH 7?
Neutral water has pH = pKw/2; it is near 7 only when pKw is near 14.
Why show scientific notation?
It keeps very small concentrations legible without rounding them to zero.