Comparison of dibenzo‐16‐crown‐5 compounds with pendent amide groups as sodium ionophores in ion‐selective electrodes and in solvent extraction

Potentiometric selectivities for alkali metal, alkaline earth metal and ammonium ions for a series of N,N-dialkyl sym-( R)dibenzo- 16-crown-5-oxyacetamides in solvent polymeric membrane electrodes are determined. For N, N-dipentyl sym-(R)dibenzo- 16-crown-5-oxyacetdamides, the Na+/K+ selectivity increases with variation of the R group in the order: hydrogen « propyl (linear alkyl) < isopropyl, neopentyl (branched alkyl). Solvent extractions of sodium and potassium picrates from aqueous solutions into 1,2-dichloroethane were also performed. Ratios of the extraction constants for Na+ and K+ (log K/K) are similar in magnitude and show the same trends for structural variation within the ionophore as do the potentiometric selectivities between Na+ and K+ (log K) in the polymeric membrane electrodes. From the picrate extraction results. it is deduced that the enhancement in potentiometric selectivity for Na+ over K+ when the R group is changed from hydrogen to propyl arises from increased binding of Na+ and diminished complexation of K+. On the other hand, the enhanced potentiometric selectivity for Na+ over K+ when a linear alkyl R group is replaced by a branched alkyl group appears to result from decreased complexation of K+ only.