A Conceptual Chemistry Question...that I can't figure out...:(?

A common ysis scheme for determind the calcium content of a compound involves the precipitation of Ca2+ ions as CaC2O4 (calcium oxalate). This precipitate is appreciable soluble in water. CaC2O4(s)<------->Ca2+(aq) + C2O42-(aq) Yet it can be safely washed with an aqueous solution of Na2C2O4 (sodium oxalate) without risk of loss. Why is this?