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. In the process, a salt is also formed when the anion from acid and the cation from base react together.
Neutralization reactions are generally classified as exothermic since heat is released into the surroundings. However, an example of endothermic neutralization is the reaction between baking soda (sodium bicarbonate) and vinegar (acetic acid) (or any other weak acid).
The most common neutralizing chemicals are:

Caustic (NaOH) - also known as Caustic Soda
Calcium Hydroxide (CaOH2)
Calcium Carbonate (CaCO3) - also known as Lime or Limestone
Ammonium Hydroxide (NH4OH)
The basic principle of neutralization of a base or acid requires either hydroxide ions (OH-) in a base for neutralizing an acid or hydrogen ions (H+) in an acid for neutralizing a base.

Since most chemicals listed above will work to neutralize waste streams, cost considerations will often determine the selection. Sulfuric Acid (H2SO4) is by far the most common acid available and is generally less expensive. Concentration is also an issue. Sulfuric Acid is available in 98% concentrations and may be the most economical in this form but storage issues such as the types of tanks and secondary containment available, familiarity of operators in handling hazardous liquids, the dangers of refilling storage containers or procedures for transferring from bulk containers, may suggest 30% to 50% concentrations regardless of the increased costs.

Liquid Caustic (NaOH) is most common in 50% concentrations. Because of safety issues, some customers, to avoid a hazardous liquid, may opt for passive neutralization via Lime or Limestone in its solid, mineral form, despite its bulk and weight. Sodium Hydroxide is often preferred because of its solubility. Unfortunately, the neutralization process also forms salts that are very soluble in water. This high solids content can affect pump selection and maintenance. Temperature can also be an issue since 50% NaOH will begin to freeze at temperatures below 60F. This will obviously interfere with the process. Often 25% NaOH is recommended since this lowers the freezing point to below that of water.

In cement pouring operations large amounts of alkaline wastewater are generated. Discharge authorities demand that such wastewater be treated on site. Carbon Dioxide (CO2), which converts to Carbonic Acid (H2CO3) in water, is an excellent choice for such applications since the site is temporary, the gas is non-hazardous, can be used in-line assuming retention and mixing is considered and is self-buffering so regardless of dosage it will not lower the pH below 7.5-7.0.

• Chemical titration methods are used for analyzing acids or bases to determine the unknown concentration. Either a pH meter or a pH indicator which shows the point of neutralization by a distinct color change can be employed. Simple Stochiometric calculations with the known volume of the unknown and the known volume and molarity of the added chemical gives the molarity of the unknown.
• Excess gastric acid in the stomach (acid indigestion) is typically neutralized by the ingestion of sodium bicarbonate (NaHCO3) or another neutralizing agent such as an antacids.
• Neutralization can also be used to reduce the pain of insect and plant stings. Bee stings can be neutralized with alkalis and wasps with acids. Nettle stings can be neutralized with alkalis like the one found in dock leaves.
• In neutron wastewater treatment, chemical neutralization methods are often applied to reduce the damage that an effluent may cause upon release to the environment. For pH control, popular chemicals include Calcium Carbonate, Calcium Oxide, Magnesium Hydroxide, and Sodium Bicarbonate. The selection of an appropriate neutralization chemical depends on the particular application.