pH is a logarithmic measure of the hydrogen ion concentration of an aqueous solution:
pH = -log[H+]
where log is the base 10 logarithm and [H+] is the hydrogen ion concentration in moles per liter
pH describes how acidic or basic an aqueous solution is, where a pH below 7 is acidic and a pH greater than 7 is basic. pH of 7 is considered neutral (e.g., pure water). Typically, values of pH range from 0 to 14, although very strong acids may have a negative pH, while very strong bases may have a pH exceeding 14.
The term "pH" was first described by Danish biochemist Søren Peter Lauritz Sørensen in 1909. pH is an abbreviation for "power of hydrogen" where "p" is short for the German word for power, potenz and H is the element symbol for hydrogen.
Why pH Measurements Are Important
Chemicals reactions in water are affected by the acidity or alkalinity of the solution. This is important not only in the chemistry lab, but in industry, cooking, and medicine.
pH is carefully regulated in human cells and blood. The normal pH range for blood is between 7.35 and 7.45. Variation by even a tenth of a pH unit may be fatal. Soil pH is important for crop germination and growth. Acid rain caused by natural and man-made pollutants changes the acidity of soil and water, greatly affecting living organisms and other processes. In cooking, pH changes are used in baking and brewing. Since many reactions in everyday life are affected by pH, it's useful to know how to calculate and measure it.
There are multiple methods of measuring pH.
Extremely acidic and basic solutions may be encountered in laboratory situations. Mining is another example of a situation that may produce unusually acidic aqueous solutions. Special techniques must be used to measure extreme pH values below 2.5 and above around 10.5 because the Nernst law isn't accurate under these conditions when glass electrodes are used. Ionic strength variation affects electrode potentials. Special electrodes may be used, otherwise it's important to remember pH measurements won't be as accurate as those taken in ordinary solutions.
Read more at http://chemistry.about.com/od/ph/a/Ph-Measurements.htm