| pH is a measure of the acidity of a solution, in terms of activity of hydrogen ions (H+). For dilute solutions, however, it is convenient to substitute the activity of the hydrogen ions with the molarity (mol/L) of the hydrogen ions (however, this is not necessarily accurate at higher concentrations).
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| In aqueous systems, the hydrogen ion activity is dictated by the dissociation constant of water (Kw = 1.011 x 10-14 M2 at 25°C) and interactions with other ions in solution. Due to this dissociation constant, a neutral solution (hydrogen ion activity equals hydroxide ion activity) has a pH of approximately 7. Aqueous solutions with pH values lower than 7 are considered acidic, while pH values
higher than 7 are considered basic.
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| The concept was introduced by S.P.L. Sorensen in 1909, and is purported to mean "pondus hydrogenii" in Latin. However, most
other sources attribute the name to the French term pouvoir hydrogene. In English, pH can stand for "hydrogen power," "power of hydrogen," or "potential of hydrogen." All of these terms are technically correct.
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| Though pH is generally expressed without units, it is not an arbitrary scale; the number arises from a definition based on
the activity of hydrogen ions in the solution. The pH scale is a reverse logarithmic representation of relative hydrogen proton (H+) concentration. Most scales are linear in nature and progress in a smooth incremental manner. On the pH scale,
a shift up in value by one number represents a ten-fold decrease in value. For example, a shift in pH from 2 to 3 represents
a decrease in total concentration of ten times less H+ concentration, and a shift from 2 to 4 represents a one-hundred fold
decrease (10 X 10) in H+ concentration.
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