EXTRACTION OF CESIUM NITRATE FROM CONCENTRATED SODIUM-NITRATE SOLUTIONS WITH 21-CROWN-7 ETHERS - SELECTIVITY AND EQUILIBRIUM MODELING

The extraction of cesium nitrate from a high concentration of sodium n itrate by a family of 21-crown-7 ethers in 1,2-dichloroethane has been investigated. Dibenzo-21-crown-7 (DB21C7) and bis[4(5),4'(5')-tert-bu tylbenzo]-21-crown-7 (BtBB21C7) ethers have higher selectivity for ces ium but lower extraction efficiency than dicyclohexano-21-crown-7 (DC2 1C7) ether. As measured by the distribution coefficient ratio D-Cs/D-N a, the cesium selectivity averaged 78 and 93 for DB21C7 and BtBB21C7, respectively. However, in the case of DC21C7, the cesium selectivity w as lower and decreased approximately three-fold from 10 to 3 as cesium loading increased. Alkyl substitution on the benzo group has only a s mall effect an the extraction behavior. It was Shown by use of the equ ilibrium modeling program SXLSQI that the extraction of sodium and ces ium could be adequately modeled by augmenting the previously determine d cesium nitrate equilibrium model with a 1:1 Na+:crown organic-phase complex. No evidence for a mixed-metal species was found. In accord wi th our previous study, cesium nitrate extraction entails formation of a 1:1 Cs+:crown complex, but a minor 1:2 complex also forms in the cas e of DC21C7. Although the sodium and cesium organic-phase complexes ar e largely ion-paired with nitrate anion under the experimental conditi ons employed, observed slope behavior suggests slight dissociation of the nitrate anion, and in fact, improved fits are obtained assuming io n-pair dissociation in the modeling. The modeling further suggests tha t the three-fold loss of selectivity toward cesium in the case of DC21 C7 is related in part to the diminishing contribution of the 1:2 Cs+:c rown complex as the crown ether is loaded with cesium.