Membrane experiment, basic

Without a membrane, an AC voltage Av is measured. This voltage is proportional to the current through the system, it is in-phase with the current, and the amplitude is independent on frequency. The solution behaves ohmic , obeying Ohm s law (Eq. (2.1)) like a perfect resistor. The voltage is due to the ohmic voltage drop caused by the current density in the bulk volume between the recording electrodes. 

Then the measured voltage increases, the phase of the recorded voltage lags behind the current, and the amplitude is frequency-dependent. 

The conductivity of the metal is more than 1000 times that of the electrolyte, and the metal is thin. The voltage drop in the metal itself must be very small, and the increased measured voltage must be caused by the two double layers formed on each side of the metal plate. These double layers clearly have capacitive properties, and they are in series with the metal. 

Let us suppose that there is a thin pore in the membrane. The walls of the pore must also be covered by a double layer. Will counter-ions in that double layer, and ordinary ions in 

The measured voltage difference is very large at low frequencies. At higher frequencies (e.g., 1 kHz), measured voltage is smaller and almost 90° after the applied voltage. 

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Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP < Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as ... 

For a lecithin with chain containing 18 carbon atoms, a value of d = 70 A is found, which is nearly equal to two times the length of a fully extended lecithin molecule. This and other findings support the conclusion that the membrane consists essentially of a bimolecular layer (bilayer) of oriented lipid molecules. The polar head groups of the lipid molecules point toward the aqueous medium, whereas the fatty acid chain forms the interior of the membrane. Some hydrocarbon solvent remains dissolved in the film, but otherwise the structure of the artificial bilayer closely resembles the arrangement of the lipid molecules in biological membranes. Experiments with artificial lipid membranes have indicated some of the basic mechanisms by which ions may cross biological membranes [323]. 

FIG. 17 Schematic illustration of the setup for a tip-dip experiment. First glycerol dialkyl nonitol tetraether lipid (GDNT) monolayers are compressed to the desired surface pressure (measured by a Wilhehny plate system). Subsequently a small patch of the monolayer is clamped by a glass micropipette and the S-layer protein is recrystallized. The lower picture shows the S-layer/GDNT membrane on the tip of the glass micropipette in more detail. The basic circuit for measurement of the electric features of the membrane and the current mediated by a hypothetical ion carrier is shown in the upper part of the schematic drawing. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

The above LLE experiments actually served as preliminary smdies for a supported liquid membrane (SLM) process that have later been described by Maximini et al. [39]. The basic principle of the SLM process is based on LLE yet it has... 

The calcium-independent ATPase of the lipid modified preparations is not only different from the calcium-dependent ATPase but also from the calcium-independent ATPase of native preparations — the basic ATPase — which has a lower nucleotide specificity126. The experiments in which the lipid matrix of the sarcoplasmic membranes has been replaced by lipid compounds not present in native membranes reveal a high degree of functional flexibility of the enzyme. On the other hand, a few residual lipids in the protein are sufficient to prevent these changes in the structure of the enzyme and to preserve its calcium sensitivity. 

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. 

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