The anion of saccharin [1] is a polyfunctional ligand that binds to a plethora of metal ions. [2, 3] The solid hydrate of sodium saccharinate ("sodium saccharin" or "saccharinum soluble"; Scheme 1) is an edible chemical which constitutes more than half of the worlds artificial sweeteners, but which is suspected to be carcinogenic.[1] Although the compound has been extensively used for more than 150 years, its exact structure has not been unequivocally established. In most chemical catalogues it is listed as the dihydrate Na(sac)·2 H 2 O (sac = C 7 H 4 NO 3 S À ). An earlier effort to determine its crystal structure failed because of an unusually large monoclinic unit cell [4] and partial dehydration. Air-stable triclinic Na(sac)· 2 = 3 H 2 O is obtained upon recrystallization from ethanol. [5,6] Herein we report the first determination of the structure of the commercial artificial sweetener sodium saccharinate. The relative stability and the mechanisms of isobaric dehydration (heating at constant pressure) and isothermal dehydration (evacuation at constant temperature) of the two hydrates are also revealed.Recrystallization of sodium saccharinate from different solvents yielded two hydrates. Whereas the physicochemical analysis of the hydrate obtained from 95 % ethanol was consistent with the triclinic Na(sac)· 2 = 3 H 2 O described previously, [5,6] the monoclinic hydrate that was obtained from water was identical to the unpurified commercial product. Precise thermoanalytical measurements of the latter showed 1.87 ( 15 = 8 by diffraction, see below) water molecules per formula unit.The otherwise stable crystals of Na(sac)· 2 = 3 H 2 O are dehydrated [7] in two well-defined steps when heated under isobaric conditions. The intermediate Na ( The changes in the IR spectra [3, 8] and the final spectra obtained after evacuation and moderate heating of Na(sac)· 15 = 8 H 2 O are nearly identical (Figure 1). This observation indicates that the isobaric dehydration and the initial isothermal dehydration both proceed to give the same product, which was identified by thermal analysis to be Na(sac)· 9 = 8 H 2 O. Comparison of the infrared spectra obtained (Figure 2) confirms that the product is not the stable triclinic Na(sac)· 2 = 3 H 2 O. Figure 2 also shows that the isothermal dehydration commences more gradually than the isobaric one. The absence of additional spectral changes upon further evacuation indicates that prolonged isothermal evacuation does not affect Na(sac)· 9 = 8 H 2 O. The crystallographic data for the two hydrates and a detailed discussion of the monoclinic structure are given in Scheme 1. The atom-labeling scheme for the monoclinic hydrate of sodium saccharinate.

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