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pH-value (hydrogen ion exponent)
The pH value of the concentration of hydrogen ions in an aqueous solution is defined as follows:
[H +] is the molar concentration of hydrogen ions (mol/dm3). At 25 °C, [H+] = [OH-] = 10-7 mol/dm3 in a neutral solution and therefore neutral solutions have a pH value of 7. If the pH value of a substance is higher than that, we call it a base. Nowadays relative activity (aH) is usually used instead of molar concentration.