A CaCl2 solution was prepared by dissolving CaCl2.2H2O (233 kg) in water (723 kg). A 3-chloro-1,2-propanediol (CPD) solution was prepared by dissolving 3-chloro-1,2-propanediol (176 kg) in water (176 kg). The 4000 L glass-lined reactor was charged with water (907 kg) and 5,5′-[(1,3-dioxo-1,3-propanediyl)diimino]bis[N-(2,3-dihydroxypropyl)-2,4,6-triiodo-1,3-benzenedicarboxamide] (563 kg). The pH of the slurry was adjusted to 5.5 to 7.0 with 37% HCl or 30% NaOH as needed. 50% NaOH solution (128 kg) was charged to the reaction mixture while maintaining its temperature to ≦30° C. The CaCl2/water solution and then the CPD/water solution were each charged to the reaction mixture. The reaction mixture was then heated at 50° C. and stirred for ≈6 hours. 50% NaOH was added as needed to maintain the pH of the reaction at ≈10. When the reaction was complete, acetic acid (28 kg) was added to adjust the pH of the reaction mixture to 5. Using a jacket temperature of 75° C., water (1682 kg) was distilled off under vacuum. While cooling to 10° C., ethanol (2585 kg) was added to the residue in the reaction vessel. After stirring at 10° C. for 2 hours, the crude solid was collected by filtration. The solids were washed with ethanol (500 kg). The crude solid was dried at 55° C. under vacuum to give 783 kg of crude 5,5′-[(1,3-dioxo-1,3-propanediyl)bis[(2,3-dihydroxypropyl)imino]]bis[N-(2,3-dihydroxypropyl)-2,4,6-triiodo-1,3-benzenedicarboxamide]. The crude product contained 55% w/w 5,5′-[(1,3-dioxo-1,3-propanediyl)bis[(2,3-dihydroxypropyl)imino]]bis[N-(2,3-dihydroxypropyl)-2,4,6-triiodo-1,3-benzenedicarboxamide] which had a chromatographic purity of 77 area %.
