Section 4.17 Membranes Dialysis

Directions are provided for constructing and characterizing an ammonium ion-selective electrode. The electrode is then modified to respond to urea by adding a few milligrams of urease and covering with a section of dialysis membrane. Directions for determining urea in serum also are provided. 

Since membrane fording could quickly render the system inefficient, very careful and thorough feedwater pretreatment similar to that described in the section on RO, is required. Some pretreatment needs, and operational problems of scaling are diminished in the electro dialysis reversal (EDR) process, in which the electric current flow direction is periodically (eg, 3—4 times/h) reversed, with simultaneous switching of the water-flow connections. This also reverses the salt concentration buildup at the membrane and electrode surfaces, and prevents concentrations that cause the precipitation of salts and scale deposition. A schematic and photograph of a typical ED plant ate shown in Eigure 16. 

M. Paleologou, R. M. Berry, and B. I. Fleming, "Bipolar Membrane Electro dialysis A New Solution to the Problems of Chemical Imbalance iu Kraft Mills," 78th Finnual Meeting, Technical Section, Canadian Tulp and Taper Association, Preprints A,Jan. 28—29, 1992, pp. KM—KSl. 

Fig. 3. A commercial dialysis faciUty showiag the dialysis section of a German brewery where alcohol is removed from beer. Technical dialysis modules contain up to 50,000 capillaries and around 23 m (250 ft ) of membrane surface area. Typical plants might contain between 50 and 100 modules.

Fig. 5. Scanning electron micrographs of hoUow fiber dialysis membranes. Membranes in left panels are prepared from regenerated cellulose (Cuprophan) and those on the right from a copolymer of polyacrylonitrile. The ceUulosic materials are hydrogels and the synthetic thermoplastic forms a microreticulated open cell foam with a tight skin on the inner wall. Pictures at top are membrane cross sections those below are of the wall region. Dimensions as indicated.

Flow injection analysis (FIA) (Ruzicka and Hansen), since 1975 In continuous flow, stopped flow or with merging zones (FIA scanning or intermittent pumping) Adapted voltammetric electrodes Membranes for Partial dialysis Membrane amperometry (Clark) Differential techniques (Donnan) Computerization, including microprocessors Special measuring requirements in plant control (to avoid voltage leakage, etc., Section 5.5)... 

Fluorescence resonance energy transfer has also been used for ionic strength measurements.(95) Fluorescein labeled dextran (donor) and polyethyleneimine-Texas Red (acceptor) were placed behind a dialysis membrane. The polymer association is ionic strength dependent and the ratio of intensities (F o/Fw) was used as the measured parameter. Since both the donor and acceptor are fluorescent, this kind of sensor may allow expand the sensitive ionic strength range by shifts in observation wavelength, as was discussed for pH probe Carboxy SNAFL-2 (see Section 10.3). 

The dialysis membrane employed is usually hydrophilic and isolates two aqueous solutions in a static or dynamic regime depending on the particular purpose. While these sensors are formally similar to those discussed in the previous section, it is molecules or ions that are separated (by virtue of a concentration gradient), the process being aided both by the dynamic character of the acceptor solution and the reaction involved, which removes the species transferred across the membrane. 

Operationally, dialysis (cf. Section 8.2) utilizes differences in the diffusion rates of various substances across a membrane between two liquid phases. The diffusivities of substances in the membrane and liquid phases (particularly the former) decrease with increasing molecular sizes of the diffusing substances. Thus, with any hemodialyzer, the rates of removal of uremic toxins from the blood will decrease with increasing molecular size, although sharp separation at a particular molecular weight is difficult. In contrast, proteins (e.g., albumin) should be retained in the patient s blood. In the human kidney, small amounts of albumin present in the glomerular filtrate are reabsorbed in the proximal tubule. 

Conditions sometimes exist that may make separations by distillation difficult or impractical or may require special techniques. Natural products such as petroleum or products derived from vegetable or animal matter are mixtures of very many chemically unidentified substances. Thermal instability sometimes is a problem. In other cases, vapor-liquid phase equilibria are unfavorable. It is true that distillations have been practiced successfully in some natural product industries, notably petroleum, long before a scientific basis was established, but the designs based on empirical rules are being improved by modern calculation techniques. Even unfavorable vapor-liquid equilibria sometimes can be ameliorated by changes of operating conditions or by chemical additives. Still, it must be recognized that there may be superior separation techniques in some cases, for instance, crystallization, liquid-liquid extraction, supercritical extraction, foam fractionation, dialysis, reverse osmosis, membrane separation, and others. The special distillations exemplified in this section are petroleum, azeotropic, extractive, and molecular distillations. 

In this section the solution-diffusion model is used to describe transport in dialysis, reverse osmosis, gas permeation and pervaporation membranes. The resulting equations, linking the driving forces of pressure and concentration with flow, are then shown to be consistent with experimental observations. 

Now the major application of dialysis is the artificial kidney and, as described in Chapter 12, more than 100 million of these devices are used annually. Apart from this one important application, dialysis has essentially been abandoned as a separation technique, because it relies on diffusion, which is inherently unselec-tive and slow, to achieve a separation. Thus, most potential dialysis separations are better handled by ultrafiltration or electrodialysis, in both of which an outside force and more selective membranes provide better, faster separations. The only three exceptions—Donnan dialysis, diffusion dialysis and piezodialysis—are described in the following sections. 

In water studies it is standard practice to filter the sample soon after collection, usually through a 0.45p,m membrane disc (made of cellulose acetate, cellulose nitrate or polycarbonate). This process arbitrarily divides the sample components into soluble and insoluble fractions, but as shown in Table 2.3, the average size of different chemical species varies widely, and some differentiation between species can be obtained through using filter media of different pore sizes. For example, fully dissolved compounds can be separated from finer colloidal forms by using gel filtration and dialysis, and sub-division of the total content into fractions based on particle or molecular size (see Section 2.3) has been used for speciation of elements in waters. 

Figure 2.4 SEM micrograph of a cross-section of a hollow-fiber dialysis membrane (Polyflux, Cambro) with an anisotropic structure and macrovoids in the support layer (left), and details of the inner porous separation layer in two different magnifications (right reprinted from [12], with permission from Wiley-VCH, 2003).

Another common sample pre-concentration method is dialysis which serves to remove small molecules. For instance, affinity dialysis and pre-concentration of aflatoxins were achieved in a copolyester chip (see Figure 5.10). After affinity binding to the aflatoxin Bi antibody, various aflatoxins (Bj, B2, Gi> G2, G2J in a sample were retained, while the other small molecules passed through a PVDF dialysis membrane. Thereafter, the sample solution was exposed to a countercurrent flow of dry air, leading to water evaporation and analyte concentration. The concentrated and desalted sample was used in subsequent MS analysis [821], More details for MS analysis are described in Chapter 7, section 7.3. 

The ABC materials encourage transfer in numerous instances. Particularly good examples are found in the Challenge sections that conclude the laboratories. The Water unit, for example, includes a laboratory designed to help students understand how molecules move across membranes. The stated purpose of the laboratory is to help students determine the effect of concentration difference on the movement of water and solute across a membrane. The laboratory s stated objective is to enable students to predict the direction of material movement across a membrane based on the concentration of materials on both sides of the membrane. During the laboratory, students measure mass with a balance and work with dialysis bags. At the conclusion of the laboratory, students explore questions designed to help them transfer what they have learned to contexts outside the classroom ... 

Fig. 4. Cross-sectional view of a potentiometric gassensing enzyme electrode a, glass pH electrode b, internal filling soludon c, electrode outer jacket d, O-ring c, gas-selective membrane f, enzyme layer g, dialysis membrane.

Fignre 10-13. Electron micrograph of a cross-section through a membrane fiber used for hollow fiber dialysis. Note the similarity between this fiber and the.membrane shown in Figure 10-9. (Courtesy of Amicon Corporation, Lexington, Mass.)... 

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