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===pH===
In chemistry, pH is the negative log of the activity of the free (dissociated) hydrogen ion (H+) in an aqueous solution. Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline. Pure water has a pH of 7.
The pH scale is traceable to a set of standard solutions whose pH is established by international agreement <ref name="bates">Bates, Roger G. Determination of pH: theory and practice. Wiley, 1973.</ref>. Primary pH standard values are determined using a concentration cell with transference, by measuring the potential difference between a hydrogen electrode and a standard electrode such as the silver chloride electrode. Measurement of pH for aqueous solutions can be done with a glass electrode and a pH meter, or using indicators.