Transference 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. 

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. 

Some empirical assumptions have been made to avoid the difficult and time-consuming experiments (Hittorf or moving boundary method) to determine transference numbers. 

To determine transference numbers of liquid electrolytes, generally classical methods are used. These classical methods, meaning moving boundary [436], Hittorf s method, and combined data from emf [437], involve extensive experiments. 

In the study a mathematical model was developed to calculate the values of heat and moisture transfer through the porous textiles applied with PCMs. These values were compared with the porous textiles without PCMs [7]. The distribution of temperatme, liquid water content and moisture concentration were numerically computed on the basis of a finite volume difference model. To validate the mathematical model, the surface temperature of the fabric with PCM was measured by a series of experiments. The diameters of the microeapsules were assumed to be identical and they were embedded in the porous textile structure. The mechanies of coupled heat and moisture transfer and thermal regulation by the PCM were discussed by considering the phase change proeess as a moving boundary problem... 

Weder s experiments were carried out with opposing body forces, and large current oscillations were found as long as the negative thermal densification was smaller than the diffusional densification. [Note that the Grashof numbers in Eq. (41) are based on absolute magnitudes of the density differences.] Local mass-transfer rates oscillated by 50%, and total currents by 4%. When the thermal densification dominated, the stagnation point moved to the other side of the cylinder, while the boundary layer, which separates in purely diffusional free convection, remained attached. 

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