]> pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose pH is established by international agreement. [Wikipedia] Any process that decreases the pH value of a subtance. In chemistry, neutralization is a chemical reaction (also called a water forming reaction since a water molecule is formed during the process) in which an acid and a base or alkali (soluble base) react to produce salt and water (H2O). During the process, hydrogen ions H+ (a bare proton) from the acid (proton donor) or a hydronium ion H3O+ and hydroxide ions OH_ or oxide ions O2_ from the base (proton acceptor) react together to form a water molecule H2O. [Wikipedia] An acid as a compound which donates a hydrogen ion (H+) to another compound (called a base). [Wikipedia] A base is most commonly thought of as an aqueous substance that can accept hydrogen ions. [Wikipedia]