contains a mixed resin 20 and 22 designed to trap undesirable impurities the impurities are then pumped out of the mixed resin 20 and 22 and across ion exchange membranes 24 and 26 by dc voltage ion exchange membrane 24 is a cation permeable membrane which allows the passage of cations but not anions into the concentrating compartment 14 opposite ion exchange membrane 24 is anion permeable membrane 12 the anion permeable membrane 12 allows anions but not cations to pass into the concentrating compartment 14 likewise ion exchange membrane 26 is an anion permeable membrane which allows passage of anions but notifications into the concentrating compartment 28 from the diluting compartment 18 cation permeable membrane 30 opposite membrane 26 allows the passage of cations but not anions into the concentrating compartment 28 the anode 32 and the cathode 16 located at opposed sides of the system together create the dc voltage which powers the ion transport across the described selectively permeable membranes through this process of pulling anion and cation impurities from the crude ioversol trapped in the mixed resin bed 20 and 22 ioversol is purified purified ioversol emerges by gravity or forced flow from the diluting compartment 18 each of the ion exchange membranes described act as check valves to prevent acid and other impurities from reentering the purified ioversol the same method holds true for the purification of a crude magnetic resonance imaging mri agent the use of mixed resins in the diluting compartment is key to the present process.