Why donated blood is treated with sodium citrate ?

Donated blood is treated with sodium citrate because in the donated blood fibrinogen is removed to avoid the blood clotting. Here, sodium citrate works as an anticoagulant. 

A blood thinning medicine called citrate is added to the cell-separating machine. Citrate reduces the ionized calcium levels in the blood, which prevents the blood from clotting. When the blood is returned to the donor, the donor also receives the citrate. The citrate level in serum and urine typically returns to baseline within 4 hours after the infusion has stopped. 2 In the urine, the acute citrate load produces cation excretion including calcium, magnesium, sodium, and potassium.

Citrate exerts its anticoagulant effect through reversible chelation of circulating divalent cations, including Ca²⁺ and Mg²⁺, and sequestration of these ions from their normal physiological function. ... Approximately 18-20% of infused citrate remains unmetabolized and is excreted by the kidneys