In chemistry, pyrophosphate is a phosphorus oxyanion. Compounds such as salts and esters are also called pyrophosphates. The group is also called diphosphate or dipolyphosphate, although this should not be confused with two phosphates. As a food additive, diphosphates are known as E450. A number of hydrogen pyrophosphates also exist, such as Na₂H₂P₂O₇, as well as the normal pyrophosphates.