Membranes plasticization

One example of a liquid-based ion-selective electrode is that for Ca +, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a nonconducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di- -octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca + and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride... 

It has been found that the influence of the membrane composition on the ISE selectivity is mainly governed by substitution degree of the nitrogen atom, but virtually does not depend upon the length of hydrocarbon substituents. The key factor in the selectivity of DBP-plasticized membranes with both kinds of ion-exchangers and of the o-NPOE-TNOBS-membrane is the ionic... 

As indicated earlier, heavy contamination can be buried, sealed or removed. Burying of the material should be well below the root growth zone, and this is normally taken as 3.0 m below the final ground-surface level. Sealing for heavy contamination to prevent vertical or lateral leaching through groundwater flow can be with compacted clay or proprietary plastic membranes. Removal from site of the contaminants is normally only contemplated in a landscaped scheme where the material, even at depth, could be a hazard to public health directly or phytotoxic to plant life. 

While it would be difficult to enumerate all of the efforts in the area of implants where plastics are involved, some of the significant ones are (1) the implanted pacemaker, (2) the surgical prosthesis devices to replace lost limbs, (3) the use of plastic tubing to support damaged blood vessels, and (4) the work with the portable artificial kidney. The kidney application illustrates an area where more than the mechanical characteristics of the plastics are used. The kidney machine consists of large areas of a semi-permeable membrane, a cellulosic material in some machines, where the kidney toxins are removed from the body fluids by dialysis based on the semi-permeable characteristics of the plastic membrane. A number of other plastics are continually under study for use in this area, but the basic unit is a device to circulate the body fluid through the dialysis device to separate toxic substances from the blood. The mechanical aspects of the problem are minor but do involve supports for the large amount of membrane required. 

Plastic membranes are being used in systems that could well pave the way to large-... 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

The drawback has been overcome by a technique that effects downward flow of transfer solution by capillary action under alkaline conditions, accomplishing efficient transfer of DNA to either nitrocellulose or plastic membranes in 2.5 hours (C2). 

An ISE with a plastic membrane directly attached to a graphite rod is described in [4]. 

As follows from the hydrodynamic properties of systems involving phase boundaries (see e.g. [86a], chapter 2), the hydrodynamic, Prandtl or stagnant layer is formed during liquid movement along a boundary with a solid phase, i.e. also at the surface of an ISE with a solid or plastic membrane. The liquid velocity rapidly decreases in this layer as a result of viscosity forces. Very close to the interface, the liquid velocity decreases to such an extent that the material is virtually transported by diffusion alone in the Nernst layer (see fig. 4.13). It follows from the theory of diffusion transport toward a plane with characteristic length /, along which a liquid flows at velocity Vo, that the Nernst layer thickness, 5, is given approximately by the expression,... 

An elecrochemical device for detecting dissolved oxygen content. The well known Clark electrode consists of a platinum wire tip surrounded by a thin film of electrolyte solution that is shrouded by a plastic membrane. The membrane is permeable to oxygen, but impermeable to... 

Shoukry et al. have prepared plastic membrane ion-selective electrodes for the determination of procaine and other anaesthetic compounds [69]. The electrode selective for procaine was prepared with the use of a membrane containing 15% of the procaine tetraphenylborate ion pair with 40% of dioctyl phthalate and 45% of poly vinyl chloride (PVC). The membrane was attached as a disc (12 mm diameter, 0.3 mm thick) to the polished PVC cap of the electrode tube, which contained an internal solution of 0.1 M sodium chloride made 1 mM in the same drug, and in contact with a Ag-AgCl wire. Linear response ranges were determined to be 20.0 pM to 16 mM for procaine over the pH range of 3.1 to 7.9. The electrodes could also be used in the potentiometric titration of the drug with 0.01 M sodium tetraphenylborate. 

If you define chemical breakthrough as the time it takes for the half-maximal concentrations to pass across the plastic membrane, will a 1 millimeter thick LLDPE sheet be effective at retaining benzene for 20 years ... 

A novel capillary electrophoresis method using solutions of non-crosslinked PDADMAC is reported to be effective in the separation of biomolecules [211]. Soil studies conducted with PDADMAC report the minimization of run-off and erosion of selected types of soils [212]. In similar studies, PDADMAC has found to be a good soil conditioner [213]. The use of PDADMAC for the simultaneous determination of inorganic ions and chelates in the kinetic differentiation-mode capillary electrophoresis is reported by Krokhin [214]. Protein multilayer assemblies have been reported with the alternate adsorption of oppositely charged polyions including PDADMAC. Temperature-sensitive flocculants have been prepared based on n-isopropylacrylamide and DADMAC copolymers [215]. A potentiometric titration method for the determination of anionic polyelectrolytes has been developed with the use of PDADMAC, a marker ion and a plastic membrane. The end-point is detected as a sharp potential change due to the rapid decrease in the concentration of the marker due to its association with PDADMAC [216]. 

Due to the inherent plasticity of plastic membranes and their solubility in organic solvents these membranes can be rather easily placed in different electrode body designs. Moreover, plastic membranes may be casted for all major electrolytes by the same... 

The design of an integrated, maintenance-free pH-ISS is far simpler with a plastic membrane than with a glass. 

Two types of electrodes are used in clinical analyzers, based either on a sodium glass or plastic membrane. As in the case of the pH sensor,... 

An electrode with a plastic membrane containing valinomycin as the active carrier is now predominantly used in clinical analyzers. Nearly four decades of experience with this sensor have proven that it fulfils all demands concerning sensitivity, selectivity and lifetime. An anionic interference that can be observed during measurements in undiluted urine may be eliminated by the use of silicone rubber instead of polyvinyl chloride in the membrane or by pre-dilution of urine. Despite some experimental trials, no other ionophore has replaced valinomycin as the active compound in potassium ISEs. This is basically due to the better stability and lipophilicity of this compound in comparison to the others proposed. 

At present the plastic membrane with ionophore ETH 2137 is used. The introduction of ionophore ETH 2137 generated a stable selectivity towards sodium ion allowing for numerical correction of sodium influence using the values of independently measured sodium. Thus a complete procedure covering therapeutic concentration range of lithium (which should be <1.2mmol/L) is available. 

The development of a sensor for ionized magnesium turned out to be one of the most difficult challenges of recent years. Several carriers have been designed for this purpose but none have been satisfactory. The first report of a successful measurement of ionized magnesium in an automated clinical analyzer (Thermo, prev. KONE) was published only in 1990 [30]. The ionophore ETH 5520 was used as the active compound. Two other carriers have been used since then ETH 7025 (Roche, former AVL), and a derivative of 1,10-phenenthroline (Nova). All of the magnesium sensors are based on a plastic membrane. Numerical compensations of the influence of calcium ion and the ionic strength are used due to insufficient selectivity of the magnesium sensors. 

The most widely used sensor for chloride ions in clinical analyzers is based on an ion-exchanger, a quaternary alkylammonium chloride, dispersed in a plastic membrane. It is not an ideal sensor due to the interference of lipophilic anions (e.g., salicylates, bromides) and lip-ophylic cations (e.g., bacteriostatic agents, anesthetics) and a relatively poor selectivity towards hydrogen carbonates (bicarbonates). However, compared to charged anion- and neutral carrier-based membranes that have been tested, it is still the best-suited for automated analyzers. 

Ephedrine is the pharmacologically active enantiomers of ephe-drine. The enantiopurity tests for (-)-ephedrine can be performed using a peroctylated y-cyclodextrin based plastic membrane electrode [26], Bis (l-butylpentyl)adipate (BBPAP) was used as plasticizer and 10 3 mol/L NH4C1 as inner solution. The slope of the electrode is 60 mV/decade of concentration, and the potentiometric enantioselectivity coefficient is less than 10 4 The limit of detection is of 10 7 mol/L magnitude order. 

A crown ether (19-[(10-undecen-l-yl)oxy]-4R,14R-(-)4,14-diphenyl-3,6,9, 12,5-pentaoxa-21-azabicyclo[15.3.1]heneicosa-l(21),17,19-triene) was used in a plastic membrane as chiral selector for the design of EPMEs [21]. These electrodes can differentiate between S- and R-l-phenylethylammo-nium ions. The values of the slopes obtained when different plasticizers were used for EPMEs design were between 51.3 and 60.6 mV/decade of... 

A convenient technology to get an array of potentiometric sensors is to use the classical PVC membrane, as a wide experience exists on their formulation and response characteristics, together with the commercial availability of membrane components. Figure 30.7 depicts the composition of a typical PVC membrane used to develop potentiometric sensors [76]. The components are normally dissolved with the aid of a volatile solvent (for example, tetrahydrofuran) and after its evaporation, a plasticized membrane results. 

Plastic membrane 60 Plasticized poly(vinyl chloride) 75 Platinum complex 634... 

Figure 8.16 Cross-sectional diagram of a single-use disposable powder injection system highlighting the major components. When the actuator button is depressed, the driver gas (He) is released into the surrounding rupture chamber. At a specific pressure, the plastic membranes of the drug cassette burst and the drug particles are entrained in the gas flow, which is accelerated through the convergent-divergent nozzle. [From Hickey (2001). Reproduced with permission from Euromed Communications.]...

In order to avoid the distortion caused by these two effects, the usual approach is to compensate the resistance Ru by a positive feedback loop (this is imperative in systems like plasticized membranes for which the uncompensated resistance can be of the order of megaohms [32-34]). Another possibility is to use microelectrodes, for which a decrease in the measured current is obtained which minimizes the ohmic drop and charging current distortion (see Sects. 2.7 and 5.4.1). 

Oxygen-sensitive membrane electrodes are commercially available. The electrode in such a system is covered with an oxygen-permeable plastic membrane, thus protecting it from impurities. The current is proportional to the activity of dissolved molecular oxygen, and at low concentration, to the amount of DO. Before the measurement of DO in the sample, calibrate the electrodes using standards of known DO concentrations (determined from iodometric titrations). 

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