THE pH SCALE
The pH is a number that describes the acidity or alkalinity of an aqueous solution. The range of the pH scale is from O to 14. Aqueous solutions having a pH from O to 7 are acidic, while aqueous solutions having a pH from 7 to 14 are basic (or caustic or alkaline). The pH of pure water is 7, since water is neither acidic nor basic.
In a chemical laboratory, the pH is frequently determined by means of an electrometric measurement using an apparatus called a pH meter. A pH meter is a voltage-measuring device attached to a pair of electrodes. When the tips of the electrodes have been immersed in a solution, the pH of the solution can be readily established simply by reading the scale.
The definition of the pH is the following: the negative power to which 10 must be raised to equal the hydrogen ion concentration. In algebra, we learn that if l0~ = y, then x is the logarithm of y to the base 10. Hence, if [H+] = 10-PH, then the pH is expressed as follows:
pH = -log [H+]
It is important to note that the pH scale is an exponential one. Thus, vastly different concentrations in the hydrogen ion may exist in given solutions than their pH values seemingly reflect.