| number quantifying the acidic or the alkaline character of a solution, equal to the negative of the decimal logarithm of ion activity aH+ of the hydrogen cation H+ pH = −10 lg aH+
Note 1 to entry: At about 25 °C aqueous solutions with: - pH < 7 are acidic;
- pH = 7 are neutral;
- pH > 7 are alkaline.
At temperatures far from 25 °C the pH of a neutral solution differs significantly from 7. Note 2 to entry: For more details, see ISO 80000-9:2009, Annex C. Note 3 to entry: This entry was numbered 111-15-37 in IEC 60050-111:1996. It has been modified.
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