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pH is a measure of the acidity or alkalinity of a solution. Solutions with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline). pH 7 is considered neutral because it is the accepted pH of pure water at 25 °C, although, due to the self-ionization of water, this is not completely accurate. pH is formally dependent upon the activity of hydrogen[1] ions (H<sup>+</sup>),[2] but for very pure dilute solutions, the molarity may be used as a substitute with some sacrifice of accuracy.[3] Because pH is dependent on activity, a property which cannot be measured easily or 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 of doing this.