The abbreviation pH stands for “power of hydrogen”. It describes how concentrated the hydrogen ions in a substance are. A pH value below seven shows that the substance is acid. Above seven, it is an alkali.
The pH value describes the activity of hydrogen ions in aqueous solutions typically on a scale of 0 to 14. Based on this pH scale, liquids are characterized as being acidic, alkaline or neutral: a solution which is neither acidic nor alkaline is neutral. This corresponds to a value of 7 on the pH scale. Acidity indicates a higher activity of hydrogen ions and a pH measurement value lower than 7. Alkaline solutions are characterized by a lower hydrogen ion activity or higher hydroxide ion activity.
The pH scale is logarithmic. A difference of one pH measurement unit represents a tenfold, or ten times increase or reduction of hydrogen ion activity in the solution. This explains how a solution’s aggressiveness increases with the distance from the neutral point.
One of the keys to understanding pH measurements is the term “activity”, because the activity is temperature dependent it is not the same as the solution’s concentration. Activity, a, is defined as the product of the activity coefficient, y, which is always smaller than 1, and the actual concentration, c, of the concerned compound (a=y * c).
Activity is the effective concentration of a chemical compound, or more precisely its particles in the solution. In a real solution the activity is constantly smaller than the actual concentration. This is true because only in an ideal (infinitely thinned) solution the saluted particles do not affect each other. In this case they are spread apart because many molecules of the solvent are between them. The difference between activity and concentration becomes apparent in real solutions of ions, because ions interact with each other as a result of their electric charge. To describe or calculate the characteristics of a solution as exactly as possible the activity and not the concentration must be used in the mass action law.
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