Calcium citrate is the calcium salt of citric acid. It is commonly used as a food additive (E333), 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. Calcium citrate is also found in some dietary calcium supplements. Calcium makes up 21% of calcium citrate by weight. Like citric acid, calcium citrate has a sour taste. Like other salts, however, it also has a salty taste. For this reason, citrates such as sodium and calcium citrate are commonly known as sour salt.