Sodium citrate may refer to any of the sodium salts of citric acid (though most commonly the third):
The three forms of the salt are collectively known by the E number E331. Sodium citrates are used as acidity regulators in food and drinks, and also as emulsifiers for oils. They enable cheeses to melt without becoming greasy.
Sodium citrate is used to prevent donated blood from clotting in storage. It is also used in a laboratory, before an operation, to determine whether a person's blood is too thick and might cause a blood clot, or if the blood is too thin to safely operate.