Moving-boundary experiments

Figure 3. Anion transference number as a function of concentration for sodium in NH% 0 calculated from the e.m.f. data of Kraus assuming activity coefficients of unity A from moving boundary experiments of Dye et al. (12).

In following the movement of the boundary, no matter how it is formed, use is made of the difference in the refractive indices of the indicator and experimental solutions if the boundary is to be clearly visible, this difference should be appreciable. If the distance (1) moved in a given time and the area of cross section (a) of the tube are measured, and the equivalent concentration (c) of the experimental solution is known, it is only necessary to determine the number of coulombs (Q) passed for the transference number to be calculated by equation (13). The quantity of electricity passing during the course of a moving boundary experiment is generally too small to be measured accurately in a coulometer. It is the practice, therefore, to employ a current of known strength for a measured period of time the constancy of the current can be ensured by means of automatic devices which make use of the properties of vacuum tubes. 

In a moving boundary experiment with 0.1 N potassium chloride, using 0.065 N lithium chloride as indicator solution, Macinnes and Smith [/. Am, Chem, Soc.y 45, 2246 (1923)] passed a constant current of 0.005893 amp. through a tube of 0.1142 sq. cm. uniform cross section and observed the boundary to pass the various scale readings at the following times ... 

That such a mechanism really functions has been shown by Maclnnes and Cowperthwaite 22 and by Maclnnes and Longsworth.28 In these investigations it was found on stopping the current during a moving boundary experiment, that the initially sharp boundary disappeared when two colorless solutions were in contact, or could be seen, when... 

Figure 11.11 (a) A moving boundary experiment showing a schematic apparatus, (b) Movement of the boundary with time. 

Figure 11.12 Behaviour of potential and field at various times in a moving boundary experiment. Magnitude of field X is the gradient of the graph of i/r vs distance.

In a moving boundary experiment electrical neutrality requires that the two cations migrate down the tube at the same velocity and this, in turn, means that a sharp boundary is always maintained. In turn, this implies that the fields under which the ions move are different. 

Figure 6. Schematic illustration of Masson s moving boundary experiments.

There are a number of complications in the experimental measurement of the electrophoretic mobility of colloidal particles and its interpretation see Section V-6F. TTie experiment itself may involve a moving boundary type of apparatus, direct microscopic observation of the velocity of a particle in an applied field (the zeta-meter), or measurement of the conductivity of a colloidal suspension. 

Because it is more complicated to solve the moving boundary problem for the rotation of the screw, the barrel rotation models described above have been extensively adopted and investigated. In practice the screw is rotated and not the barrel. The barrel rotation theory has several limitations when describing the real extrusion process, so correct interpretation of the calculated results based on barrel rotation becomes necessary. Most screw design practitioners, with substantial previous design experience, make major adjustments in design specifications to obtain effective correiations. 

Fig. 2.7. The moving boundary method for determining transport numbers. AB and CD represent the frontiers between MX and NX at the beginning of the experiment and after time t respectively.

Electrophoresis experiments in glass tubes were reported as early as in the nineteenth century, but the first real breakthrough occurred in the first half of the twentieth century when the Swedish chemist Arne Tiselius applied free-solution electrophoresis—i.e moving boundary—to serum protein analysis, for which he later received the 1937 Nobel Prize [2], In less than two decades, just after the striking scientific discovery of the double-helical structure of DNA by Watson and Crick in 1953 [3] and the following unveiling of the genetic code, electrophoresis became a standard and indispensable tool in the field of modern... 

When carrying out a transference number measurement by the moving boundary method the bulk concentration of the indicator solution is chosen so as to comply with equation (15), as far as possible, using approximate transference numbers for the purpose of evaluating c. The experiment is then repeated with a somewhat different concentration of indicator solution until a constant value for the transference number is obtained this value is found to be independent of the applied potential and hence of the current strength. 

By measuring the conductance of several picrates in di-wopropyl ketone at different concentrations, it was shown by the method of Fuoss and Kraus 6 that up to concentration of 01 M there is no detectable triple ion formation. Thus concentrations high enough to satisfy condition (ii) are attainable without the formation of multiple ions. The results of semi-quantitative preliminary experiments indicated that tetraethylammonium and picrate ions had nearly the same mobility in di-wopropyl ketone. This was confirmed by measuring the transport number of the picrate ion by the moving-boimdary method. The conditions for the successful use of the moving-boundary method have been fully examined by Longsworth and Maclnnes.7 A simplified apparatus was used and is shown in fig. 3 camphor-sulphonate was found to be a suitable indicator ion. 

Early experiments in the development of isoelectric focusing, a high-resolution steady-state electrophoresis method, occurred in 1912, with an electrolytic cell that was used to isolate glutamic acid from a mixture of its salts.1 A simple U-shaped cell, such as that used for moving-boundary electrophoresis (Chapter 9), with two ion-permeable membranes equidistant from the center, created a central compartment that separated anodic and cathodic chambers, as shown in Figure 11.1. Redox reactions occurring in the anodic (Eq. 11.1) and cathodic (Eq. 11.2) electrolyte compartments generated H+ and OH ions in the respective chambers ... 

Figure 13.11. Results for a moving-boundary ultracentrifuge experiment using different optical detection systems and a double-sector cell. Part (a) is a graphical representation, (b) is the result of an uv photograph, (c) is a plot of absorbance versus distance (from b), id) is a photograph obtained with Schlieren optics, (e) is an interference diagram obtained using Rayleigh optics, and (f) is another interference diagram, obtained with Lebedev optics.

Fig. 2.1. Schematic representation of the four electrophoretic methods (A) zone electrophoresis (B) moving boundary electrophoresis (C) isotachophoresis and (D) isoelectric focusing, (a) The beginning of the experiment (b) separation of a mixture of the substances.

Phenomenological Fickian equations, disassociation rate, moving boundary problem, experiments [43]... 

Electrofocusing makes it possible to measure the pi of a protein with great accuracy. This fundamental characteristic of a protein can be demined with better controlled and more stable conditions in the surroundings than in early experiments, where isoelectric points were measured with the help of the moving boundary method. 

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