Chloride is a major element in seawater. Along with sodium (10,560 ppm), chloride (18,980 ppm) makes up about 86% of the salt dissolved in seawater. As seawater is diluted with freshwater the level of dissolved salts naturally decreases. Chlorinity is the measure of chlorine in a water sample and refers to the amount of chlorine in one kilogram of water. The relationship of salinity to chlorinity is described by the following relationship:
Salinity = 1.807 x Chlorinity
Chloride competes with nitrite for transport sites on the gill chloride cells and, due to this competition protects fish from nitrite toxicosis at neutral to alkaline pH conditions. In freshwater, addition of an appropriate amount of chloride can reduce fish fatalities from nitrite induced methemoglobinemia. The addition of 2 gram per liter NaCl to freshwater will generally protect fish temporarily from nitrite toxicosis. In situations where large volumes of water are concerned, measurement of nitrite and chloride levels in the water will allow for more precise dosage determination for chloride needed as a preventative for nitrite poisoning. Dosage calculation for chloride addition to control nitrite toxicosis are given in the Treatment Module.