Sodium-selective membrane systems

Without question, sodium and potassium have been the analytes receiving the most attention in conjunction with the development of new analyzers. Almost all instruments on the market utilize the potassium-selective membrane system based on the antibiotic valinomycin in a PVC membrane matrix. For blood measurements, such a membrane is quite adequate. However, in undiluted urine samples, a negative error in the measurement of potassium has been reported (KIO). Apparently, this interference comes from a negatively charged lipophilic component of the urine which can partition into the PVC membrane, reducing the membrane potential (i.e., the membrane is not permselective). Fortunately, this problem can be overcome by incorporating the valinomycin in a silicone rubber-based membrane matrix (A4) into which the unknown anionic component apparently has a less favorable partition coefficient. 

Then, batchwise concentration experiments were performed with the selected membranes (360 cm ) and conditions (Figure 22.5). In this experiment, impurities such as creatinine and sodium chloride were taken out from the system together with permeate, and anserine and carnosine were purified and concentrated. Initial and final volumes of the feed were 11.3 and 3.0 L, respectively. 

The phenomenological description of the excitability phenomenon given in Section 1.3 cannot claim to contain a final solution to the problem of the nature of transport systems of biological membranes responsible for nervous impuse generation. Where we stand, we can only conclude that the membrane as a whole is a nonlinear ion conductor whose properties are largely dependent upon the electrice field. For all that, the fact that the use of certain specific blocking compounds—tetrodotoxin and tetraethylammonium—allows the sodium and potassium ionic currents to be separated is alone sufficient to support the conception of selective transport systems located in the lipid matrix... 

Aquatech Systems, a business unit of Allied-Signal, Inc., has patented the SOXAL process, which is a process for regenerating the spent scrubbing solution of an alkaline sodium salt scrubber using electrodialysis cell stacks (electrolytic cells with ion-selective membranes) (Byszewski and Hurwitz, 1991). 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

Other potentiometric electrode systems are ion-selective electrodes such as fluoride, calcium, magnesium, sodium, potassium and chloride, selective gas electrodes based on membranes such as 02, C02, CO, NO, N02 and S02, and enzyme electrodes. These electrodes fall beyond the scope of this book and are not discussed further. 

OUABAIN A chemical of botanic origin that inhibits sodium-potassium-activated ATPase in cell membranes, thereby being toxic to the cell the selection of mutants that are resistant to the toxic effects of ouabain provides the basis of a mutation-detection system in mammalian cells. 

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