W. Simon

Lipophilic and immobilized anionic additives in solvent polymeric membranes of cation-selective chemical sensors

Abstract Lipophilic borate salts are frequently used as anionic additives in potentiometric and optical cation-selective sensors based on solvent polymeric membranes. The lifetime of such membranes may be limited owing to chemical decomposition and leaching of the components. Borate salts, in particular, are decomposed in the presence of acids in the membrane. Adequately substituted borate salts and sulphonic acids, such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate and dinonylnaphthalenesulphonic acid (DNSS), are shown to be sufficiently stable as membrane additives. Furthermore, lipoholic mobile or immoblizied sulphonic acids [DNNS or poly(2-acrylamido-2-methyl-1-propanesulphonic acid-co-styrene), respectively were also tested as anionic additives. Their influence on the selectivity behaviour of the sensor is attributed to their strong association with positively charged species in the membrane phase. It may be kept small by choosing ionophores that from stable complexes with the analyte.

Lipophilic salts as membrane additives and their influence on the properties of macro- and micro-electrodes based on neutral carriers

Abstract The influence of salts containing lipophilic cations and anions on the electrical resistance of the membranes of calcium ion-selective macro- and micro-electrodes based on a neutral carrier is described. The resistance of macroelectrodes was decreased by a factor of about 50 or of about 3 compared to membranes without and with potassium tetrakis- (p-chlorophenyl)borate, respectively. No significant reduction of the membrane resistance was achieved for microelectrodes. The lower detection limit and the Ca2+/K+ selectivity factor were improved for both types of electrode.

Estimation of the chemical shifts of olefinic protons using additive increments—II : The compilation of additive increments for 43 functional groups

Abstract Additive shielding parameters for olefinic protons were derived for 36 functional groups by means of a least squares procedure applied to 4298 chemical shifts. Additional shielding parameters for a further 7 functional groups were obtained by assuming simple additivity. A number of conspicuous exceptions to the calculated values are listed.

Theoretical aspects of bulk optode membranes

Abstract The principles of optodes, which are based on a reversible mass transfer of analyte from the sample into the bulk of the sensing layer, have been elucidated in detail. It is demonstrated that the sensor response of many different types of bulk optode membranes described in the literature can be related to the same formalism. A universal description is given that is independent of the type of the optical transduction process and can be based on absorption (transmission or attenuated total reflection mode) or on fluorescence. The theoretical performance of bulk optodes based on plasticized PVC is described in terms of selectivity, response time, dynamic measuring range, sensitivity and lifetime, and whatever a comparison with experimental data is given. The geometrical arrangement and optimization of the membrane composition for a set of specific requirements are discussed in detail. By means of the spin-on technique, a simple manufacture of very thin optode membranes is described. With this technique, highly homogeneous membranes of thickness from 0.5 to to μm can be produced in only a few seconds.

Plasticizers for liquid polymeric membranes of ion-selective chemical sensors

Abstract A variety of 55 different plasticizers based on various structural elements were investigated in respect to their applicability in liquid polymeric membranes of chemical sensors. The plasticizers and the overall performance of the membrane were judged in respect to the properties of lipophilicity, solubility, exudation and selectivity, respectively. A strategy for the design of plasticizers is derived from theoretical considerations and verified by experiments. The selectivity data were obtained by measurements with a standard Mg 2+ selective electrode optimized for clinical applications. A plasticizer needs several large alkyl and/or aryl residues to exhibit sufficient lipophilicity. In order to avoid crystallization or exudation, the use of branched alkyl chains as well as the link or the substitution of the structural elements by an adequate number of polar or polarizable groups is encouraged. However, to ensure high selectivities, plasticizers should not contain functional groups which may act as coordination sites and therefore compete with the carrier. In contrast to earlier publications it was observed that other properties than the dielectric constant of the plasticizer are more important for the selectivity of the membrane. A correlation between selectivity coefficient and the dielectric constant of the plasticizer could not be found.

A hydrogen ion-selective liquid-membrane electrode based on tri-n-dodecylamine as neutral carrier

Abstract An electrically neutral hydrogen ion-selective carrier, tri-n-dodecylamine, is used as the active component for liquid-membrane electrodes. The observed ion selectivities are superior to those of previously described pH-sensitive liquid-membrane electrodes. The electrode characteristics permit the study of intracellular as well as extracellular hydrogen ion activities.

Ion specific electrochemical behavior of macrotetrolides in membranes

Abstract Electrochemical cells using macrotetrolides (nonactin homologs) on inert supports as membranes show a specificity for cations comparable to the ion specific behavior observed in the metabolism of rat liver mitochondria.

Highly Selective Potassium Ion Responsive Liquid-Membrane Electrode

Abstract The performance of a new liquid-membrane electrode using valinomycin as membrane component is described with respect to selectivity, working range, speed of response and accuracy. The electrode makes possible the measurement of potassium ion activities in the range of 10−1 M to at least 10−6 M in unbuffered systems with a selectivity of potassium over sodium ions of more than 4,000. The electrode was used for the direct titration of potassium ions.

Improved Calcium Ion-Selective Electrode Based on a Neutral Carrier

Abstract The performance of a liquid-membrane electrode using a synthetic, neutral carrier in o-nitro-phenyl-n-octyl-ether as membrane component in a PVC matrix, in the presence of sodium tetra-phenyl-borate, is described. Selectivities, working range, lifetime and anion interference are discussed. The electrode makes possible the measurement of calcium ion activities in the range 10−1M to 10−6M in unbuffered and in the range 10−1M to 10−8M in calcium buffered systems respectively. The selectivities of calcium over protons, magnesium, sodium and zinc are 25,000, 26,000, 2,800 and 7,100, respectively, and are far superior than those reported for other calcium sensors.

Reduction of the Anion Interference in Neutral Carrier Liquid-Membrane Electrodes Responsive to Cations

Abstract Based on A theoretical treatment of neutral carrier liquid-membrane electrodes, propositions are derived to reduce the interference of the cation response by lipid-soluble anions in the sample solution. The performance of such electrodes is shown to be improved by incorporating lipophilic anions, such as tetraphenylborate, into the membrane. There is A quantitative agreement between the theoretical predictions and the results thus obtained for carrier based calcium- and potassium-selective electrodes.