Calcium-selective Membrane Research Articles

One-Component Room Temperature Vulcanizing-Type Silicone Rubber-Based Calcium-Selective Electrodes

New calcium-selective membranes for all-solid-state ion sensors are developed with a highly adhesive one-component room temperature vulcanizing-type silicone rubber (RTV-1-type SR) matrix. The membranes are formulated with 21.6 wt % bis(2-ethylhexyl) adipate, 0.3 wt % tetradodecylammonium tetrakis(p-chlorophenyl)borate (ETH 500), 0.1 wt % potassium tetrakis(p-chlorophenyl)borate, and 0.8 wt % calcium-selective neutral carrier ETH 129 or ETH 1001. Plasticizer added to the RTV-1-type SR matrix not only decreases the bulk membrane resistance but also increases the solubility of electroactive components incorporated in the membrane without significantly deteriorating its adhesive strength. It is found that the lipophilic salt ETH 500 remarkably enhances the calcium selectivity of ETH 129 or ETH 1001 ligands in the SR matrix; the selectivity coefficients ([Formula: see text] by separate solution method, where j = Li(+), Na(+), K(+), or Mg(2+)) for the optimized membranes were below 10(-5). Potentiometric characteristics of planar-type Ag electrodes coated with optimized RTV-1-type SR membranes, e.g., response slope 29.0 ± 0.5 mV/decade, detection limit below 5.0 × 10(-7) M a(Ca)((2+)), and 2-3 mV of potential drift per day, were virtually the same as those of the corresponding poly(vinyl chloride) membrane-based conventional electrodes, but with greatly enhanced sensor-to-sensor reproducibility and lifetime (3-9 weeks).

Electrochemical and thermal behaviour of calcium-selective membranes

An ion-selective membrane based on a polyimide polymeric matrix plasticized with diethylene glycol dibenzoate and containing calcium synthetic ionophores was used to obtain a potentiometric Ca2+ sensor. It gives a linear response over the pCa range 1–5 with a slope of 27.4 mV (pCa)–1. The sensor can be used for the pH range 3–8.5. The calcium-selective membranes were characterized in terms of their surface morphology and the structural parameter, the glass transition temperature. The ac impedance and FTIR analysis of the polyimide-based polymeric membrane were also studied.

Carrier mechanism of acidic ionophores in solvent polymeric membrane ion-selective electrodes

The behavior of cation-selective solvent polymeric membrane electrodes based on acidic ionophores is investigated by studying the selectivity coefficients as a function of cationic or anionic additive concentration. This technique allows discrimination between neutral and charged carrier-related mechanisms, resulting in the following findings. Ionophores with a carboxylic acid group act as neutral carriers and sulfonic acid derivatives behave as charged carriers irrespective of the plasticizer used. The Ca 2+ -selective organophosphate ionophore examined (bis-[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate) acts in low dielectric constant plasticizers in a mixed mode where the analyte ion forms two different complexes in the membrane phase-one with the charged form of the carrier and the other with the protonated form (neutral ionophore). From these results it can be seen that when developing novel ion-selective electrodes based on acidic ionophores, both cationic and anionic additives as well as plasticizers of high and low dielectric constant should be tested since completely different selectivity behavior could be obtained due to the charged and neutral forms of the carrier in the organic phase. The addition of potassium tetrakis[3,5-bis(bifluoromethyl)phenyl]borate to the calcium-selective membrane phase based on the organophosphate ligand mentioned above, the plasticizer dioctyl phenylphosphonate, and poly(vinyl chloride) improves the potentiometric properties of the electrodes with respect to membrane resistance and anion interference

Flow-through calcium-selective electrode: application in flow-injection analysis and ion chromatography

The use of solid contact flow-through calcium-selective electrodes as potentiometric detectors in flow-injection analysis and non-suppressed ion chromatography is discussed. Owing to the high selectivity of the membrane electrode based on tetratolyl- m-xylylenediphosphine dioxide, it can be used to monitor trace amounts of calcium ions in the presence of a 100-fold excess of alkali metal, ammonium and magnesium ions. The detection limit is about 1 × 10 −6 M. The composition and thickncss of the calcium selective membrane influence both the detection limit and selectivity of the electrode. The sensitivity of this potentiometric detector in ion chromatography relative to alkaline earth and heavy metals is significantly higher than that of a commercial conductivity detector.

The effect of changes in hydrodynamic conditions on the response of calcium-selective membrane electrodes with a macrocyclic carrier

Shifts of the potentials of a calcium ion-selective electrode based on the complex of 7,19-dibenzyl-2,2,3,3-tetramethyl-7,19-diaza-1,4,10,13,16-pentaoxacycloheneicosane-6,20-dione with calcium tetra(4-chlorophenyl)borate in various membrane solvents caused by changes in hydrodynamic conditions in the measured solution were examined. The use of 2-fluoro-2'-nitrodiphenyl ether as a membrane solvent combined with substitution of benzyl groups with octyl ones in the neutral carrier eliminates this unfavourable phenomenon while preserving the selectivity parameters of the electrode.

Potentiometrische Messungen an asymmetrischen calciumselektiven Membranelektroden

Abstract Potentiometrie measurements with asymmetric calcium-selective membranes using a three compartment flow-through cell were carried out. These membranes exhibited a behav iour which depended on their orientation with respect to the solutions. Whilst with one orientation of the membranes Nernstian behaviour was observed, in the opposite case con­ siderable asymmetry potentials appeared.

Potentiometric response of the calcium selective membrane electrode in the presence of surfactants

Potentiometric response of the calcium selective membrane electrode in the presence of surfactants

Factors affecting the response time of a calcium selective liquid membrane electrode

Factors affecting the response time of a calcium selective liquid membrane electrode