when a person gives blood at a blood donation camp,a small amount of sodium citrate is added.Why is this done?

Blood is usually drawn from large vein of arm and is collected in a plastic bag. The plastic bag contain sodium citrate, phosphate, dextrose and adenine. The combination of these chemicals help to preserve blood and prevent it's clotting.

The sodium citrate acts as anticoagulant and prevents clotting of blood.