Silver chloride

Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver (and chlorine), which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as a mineral chlorargyrite. It is produced by a metathesis reaction for use in photography and in pH meters as electrodes. Silver chloride is unusual in that, unlike most chloride salts, it has very low solubility. It is easily synthesized by metathesis: combining an aqueous solution of silver nitrate (which is soluble) with a soluble chloride salt, such as sodium chloride (which is used industrially as a method of producing AgCl), or cobalt(II) chloride. The silver chloride that forms will precipitate immediately. AgNO3 + NaCl -> AgCl(v) + NaNO3 2 AgNO3 + CoCl2 -> 2 AgCl(v) + Co(NO3)2 It can also be produced by the reaction of silver metal and aqua regia; however, the insolubility of silver chloride decelerates the reaction. Silver chloride is also a by-product of the Miller process, where silver metal is reacted with chlorine gas at elevated temperatures. Silver chloride has been known since ancient times. Ancient Egyptians produced it as a method of refining silver, which was done by roasting silver ores with salt to produce silver chloride, which was subsequently decomposed to silver and chlorine. However, it was later identified as a compound of silver and chlorine in 1566 by Georg Fabricius. Silver chloride has also been an intermediate in other historical silver refining processes. One such example is the Augustin process developed in 1843, where copper ore containing small amounts of silver is roasted in chloridizing conditions and the silver chloride produced is leached by brine, where it is more soluble. Silver-based photographic films were first made in 1727 by Johann Heinrich Schulze with silver nitrate. However, he was not successful in making permanent images, as they faded away.