Thallium ion-selective electrodes based on PVC and five kinds of thiaethers are described. Although a PVC membrane electrode containing 5.1 w/w% 1,4,8,11-tetrathiacyclotetradecane (TTCT) plasticized by DOP (61.5 w/w%) showed good performance for Tl+ sensitivity, potassium ion did not interfer. However, electrode containing other macrocyclic thiaethers do not show a Nernstian response to thallium activity. The size of the macrocyclic ring and a hydrophilic group introduced into the macrocyclic thiaether influenced the Nernstian slope as well as the linear activity range of the emf response. The additive effect of a lipophilic salt, potassium tetrakis(p-chlorophenyl)borate (KTpC1PB), in the membrane with respect to Tl+ selectivity was also observed.
The effects of a-substituents on sodium-ion selectivities of bis(12-crown-4-methyl) malonates, which are highly selective for sodium ions, were investigated by incorporating various substituents with high lipophilicity, bulkiness, polarity and back strain. The incorporation of geminal substituents such as long alkyl chains and benzyl groups into the a-position of the malonate was found to generally enhance the Na+ selectivity, especially against K+, of the bis(12-crown-4) derivatives. The a,a'-dibenzylmalonate derivative gave the best Na+ selectivity of all the bis(crown ether)s designed here.Keywords Bis(12-crown-4) derivatives, a-substituent effect, sodium ion selectivity, ion-selective electrode Monocyclic polyethers, so-called crown ethers, often possess only limited ion selectivities as neutral carriers for ion-selective electrodes. Bis(crown ether) derivatives, a class of macrobicyclic polyethers, are able to bind certain metal ions that are a little larger than their crown cavity powerfully by cooperative action of their two adjacent crown rings on cation complexation.l It was found by our group in 1979 that bis(15-crown-5) derivatives, when applied as neutral carriers for K+-selective electrodes, show prominent ion selectivities.2 Since then, extensive research has been done concerning neutral carrier-type ion-selective electrodes based on bis(crown ether) derivatives.3Malonate ester-type Bis(12-crown-4) derivatives, bis-(12-crown-4-methyl) malonates like 1-3 are much more selective for Na+ than the corresponding monocyclic crown ethers due to the above-mentioned bis(crown ether) effect.4 Specifically, Bis(12-crown-4-methyl) 2-dodecyl-2-methylmalonate 1, which is now commercially available, is an excellent neutral carrier for plasticized poly(vinyl chloride) (PVC) membrane Nat-selective electrodes of practical use. The study has suggested that the existence of the a-substituents on malonate derivatives is quite important for high Na+ selectivity of the malonate ester-type Bis(12-crown-4)s. This prompted us to modify the a-substituents of the Bis (12-crown-4) and to compare Na+ selectivities of the resulting bis(12-crown-4) derivatives as the neutral carriers. This paper is concerned with the design of Bis(12-crown-4-methyl) malonates carrying various a-substituents, 4 -15, and comparisons of their Na+ selectivities as neutral carriers for ion-selective electrodes.
ExperimentalSyntheses of Bis(12-crown-4) derivatives Malonic diethyl esters with two identical a-substituents were prepared by the reaction of diethyl malonate with two equimolar amounts of an alkyl bromide in the presence of sodium ethoxide. Similarly, the diethyl esters with two different a-substituents were obtained by the reaction of diethyl malonate successively with one equimolar amount of two different alkyl bromides. Diethyl benzalmalonate was prepared by the treatment of diethyl malonate with benzaldehyde in benzene in the presence of piperidine and was then hydrolyzed to its corresponding diacid with sodium hydroxide in ethanol. Th...
New optically active bis(12-crown-4) based electrodes for Na+ were evaluated. The best selectivity (log k = −2.3) was obtained when the sensors were used in association with NPOE as a plasticizing solvent mediator and NaHFPB as an anion excluder in PVC matrices. Na+ selectivity against K+ was augmented by a factor of nearly two using R,R- or S,S- bis(12-crown-4) compared with that of optically inactive bis(12-crown-4)s reported previously.
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