Prepd by the action of fuming sulfuric acid on citric acid: Willsttter, Pfannenstiel, Ann. 422, 5 (1921); Ingold, Nickolls, J. Chem. Soc. 121, 1642 (1922); R. Adams et al., Org. Synth. coll. vol. I, 10 (2nd ed., 1941). By the action of Aspergillus niger on ammonium citrate mixed with some citric acid: Walker et al., J. Chem. Soc. 1927, 3050; also by B. pyocyaneus: Butterworth, Walker, Biochem. J. 23, 931 (1929). Decomposition studies: Wiig, J. Phys. Chem. 32, 961 (1928). Intermediate in synthesis of tropinone: R. Robinson, J. Chem. Soc. 111, 762 (1917); C. Schpf, G. Lehmann, Ann. 518, 1 (1921).