Calcium Citrate is a popular calcium supplement. Its bio availability is superior to that of other calcium salts such as Calcium Carbonate and hence is a good source of Calcium for the human body. Calcium citrate is the calcium salt of citric acid. It is commonly used as a food additive, usually as a preservative, but sometimes for flavor. In this sense, it is similar to sodium citrate. Calcium citrate is also used as a water softener because the citrate ions can chelate unwanted metal ions.