pH is a measure of the
acidity or
alkalinity of a solution. Aqueous solutions at 25 ℃ with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). The pH of 7.0 is defined as 'neutral' at 25 ℃ because at this pH the concentration of
H3O+ equals the concentration of
OH− in pure water. pH is formally dependent upon the
activity of
hydronium ions (H3O+), but for very dilute solutions, the
molarity of H3O+ may be used as a substitute with little loss of accuracy. (H+ is often used as a synonym for H3O+.) Because pH is dependent on ionic activity, a property which cannot be measured easily or fully predicted theoretically, it is difficult to determine an accurate value for the pH of a solution. The pH reading of a solution is usually obtained by comparing unknown solutions to those of known pH, and there are several ways to do so.
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