Equilibrium constant of the γ-sultone and extraction constant of sodium and potassium ions from water into benzene in presence of 18-crown-6 and bromocresol green

An extraction procedure for the spectrophotometric determination of sodium or potassium involving a 1:1:1 metal ion: crown ether: sulfonephthalein dye monoanion ion associate has been described in the literature. In a recent paper from this laboratory the total dye content of the organic phase resulting from partitioning bromocresol green (HBCG) alone between water and benzene was determined from back extraction into an aqueous phosphate buffer. In benzene, bromocresol green is present to a large extent as a colorless lactone form, or γ-sultone. The sultone equilibrium constant, Kdist (sultone)=[sultone]o/[BCG−]wγ(BCG−)wa(H+) was found to be 373±42 mol−1 dm3. In the present study, sodium was extracted from water into benzene with 18-crown-6 and BCG-, yielding Kdist (sultone)=232 mol−1 dm3 and the conventional extraction constant, Kex, of (5.49±0.49)×103 mol−2 dm6. Corresponding values from potassium extraction data are 256 mol−1 dm3 and (4.80±0.51)×106 mol−2 dm6, respectively. In the absence of ethanol, the calibration plot of sodium closely followed Beerʹs law, but that of potassium curved downward, approaching linearity in 15% ethanol. High 18-crown-6, low bromocresol green concentrations are analytically favorable for the extraction of potassium from the 15% aqueous ethanol mixture. All plots from the back-extractions (a measure of the total dye content in the benzene phase) were linear. Ethanol markedly reduced the reagent blank, when C(MCl)=0, as a result of the sultone being partitioned less favorably into benzene.