Ferrous citrates are all highly unstable in air, converting to ferric citrates. Juices containing citrates are thought to reduce the risk of stone formation. Double nickel citrates exist, including tetraanion citrate when pH is over 9.5. The buffering properties of citrates are used to control pH in household cleaners and pharmaceuticals. Ferric citrates play important roles in the metabolism of iron by living organisms. Common sequestering agents are disodium EDTA, citric acid (and citrates), tartaric acid, and lecithin. Ferrous citrates are all paramagnetic, reflecting the weak crystal field of the carboxylate ligands.