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   ex Na / K   ex 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   Na,K Pot ) 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.