Chlorine, lighter than air, greenish yellow color, pungent and poisonous gas. Chloride is a compound that chlorine forms with elements other than oxygen and fluorine. Chlorine is not found freely in nature. Chloride ion is mostly found in inland seas and ocean waters such as Caspian Sea and Dead Sea. It is also present in the karatuzu minerals combined with sodium.

Chlorine was first used as a chemical weapon during the first world war. When chlorine gas comes into contact with water, hydrochloric acid and oxygen are released. Causes serious respiratory distress. Air containing 2,5 milligrams of chlorine per liter can cause death very quickly.

Chloride is the most extracellular ion found in the human body. Chloride level being too high is due to the change in serum carbon dioxide level. If this value falls, the chloride level increases. Or if this value rises, the chloride level decreases. Serum chloride levels are low in kidney diseases which cause salt loss and excessive vomiting. This is very important for health.

Chloride measurements performed by laboratories are mostly used in the calculation of negative electrically charged ion deficit. As a result of chloride measurements, the presence of such ions which are not normally found in the body is determined.

Chloride is one of the most important inorganic anions (negative electrically charged ions) found in water and wastewater. If the chloride in water is due to the dissolution of the sodium chloride salt, the concentration of 250 mg chloride gives the salinity taste to the water. However, even with chloride, calcium and magnesium cations, the concentration of 1000 mg chloride does not cause salinity in the water.

Chloride content in wastewater is higher than drinking water. In seashore settlements, seawater is mixed with sewage and wastewaters contain high amounts of chloride. Some industrial wastewaters also contain chloride.

Different methods are used for the determination of chloride in water and wastewater in laboratories.