pH

In chemistry, pH is a measure of the acidity or basicity of 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 very close to 7.

The pH scale is traceable to a set of standard solutions whose pH is established by international agreement. 

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.

Universal_indicator_paper

Universal pH paper provides a colour indication of pH of a subtance. image: wikipedia

pH measurements are important in medicine, biology, chemistry, agriculture, forestry, food science, environmental science, oceanography, civil engineering, chemical engineering, nutrition, water treatment & water purification, and many other applications.

Mathematically, pH is the negative logarithm of the activity of the (solvated) hydronium ion, more often expressed as the measure of the hydronium ion concentration.

sourcehttp://en.wikipedia.org/wiki/PH