Mercurous chloride (mercury [I] chloride), Hg2Cl2, is a white powder that is nearly insoluble in water. It is also called calomel. Mercurous chloride reacts with ammonia to produce a black solid, and this reaction has been widely used in the identification of dissolved mercury ions in water sources. Mercurous chloride finds uses as a purgative (laxative), and in the preparation of insecticides and medicines. It has also been used to treat infections of intestinal worms and as a fungicide (a substance used to kill fungi and prevent fungal growth) in agriculture.
Mercurous chloride has been most often used as a treatment for intestinal worms. In the past, large doses were often used to stimulate the intestines and remove blockages, although it is rarely used in medicine today due to the toxicity of mercury. When it is used as a laxative, if the treatment fails to work, large doses of other laxatives and water must be used to insure that no mercury is allowed to accumulate in the body. Mercury toxicity often results in severe neurological damage (vapors of mercury are far more dangerous than solid mercury compounds, although all mercury compounds are considered highly poisonous). Because of the extremely low solubility of mercurous chloride in water, very little is usually absorbed by the body, making it safer than most people would expect.
In laboratories, calomel electrodes are commonly found in pH meters and this is how mercurous chloride is widely used today.
See also Mercury (element).