Electrolyte swamping

A correct value of the molecular weight is obtained for the charged polymer by the van t Hoff equation, provided that a large excess of indifferent electrolyte is present. These high concentrations are described as swamping electrolyte conditions. 

If we assume that the activity coefficients in the left- and right-hand half-cells are the same (which would certainly be a very reasonable assumption if a swamping electrolyte was also in solution), then the activity coefficients would cancel to yield... 

Minimizing junction potentials with a swamping electrolyte... 

The second method of minimizing the junction potential is to employ a swamping electrolyte S. We saw in Section 4.1 how diffusion occurs in response to entropy effects, themselves due to differences in activity. Diffusion may be minimized by decreasing the differences in activity, achieved by adding a high concentration of ionic electrolyte to both half-cells. Such an addition increases their ionic strengths I, and decreases all activity coefficients y to quite a small value. 

If all values of y are small, then the differences between activities also decrease. Accordingly, after adding a swamping electrolyte, fewer ions diffuse and a smaller junction potential forms. 

The effects of migration may indeed by neglected if a liquid electrolyte, e.g. water, is employed which contains an excess of unreactive ionic salt (often termed a swamping electrolyte). By excess , we usually mean that the concentration of such an electrolyte is about 100 times greater than the concentration of the electroanalyte (if at all possible). 

Look up why KCl or KNO3 are the most common choices of swamping electrolytes. (Hint think in terms of transport numbers, t, and conductivities, A.)... 

If adding an inert electrolyte is undesirable, or if no swamping electrolyte is sufficiently inert or soluble, then the effects of migration can be lessened somewhat by performing the analysis at low field, for instance with an electrode having a very small potential. Such a practice is seldom useful, though, from considerations of mass transport (see below). 

In summary, if a solution is quiet and contains a swamping electrolyte, then diffusion is the sole means for an electroactive species to approach the electrode. 

Migration in the absence of a swamping electrolyte is somewhat more effective than is diffusion, but migration can be ignored if a swamping electrolyte is added to the solution. Diffusion still occurs even if the solution is stirred, but convection is so much more efficient that we can ignore the diffusion completely. 

Mass transport comprises three different modes , i.e. convection, migration and diffusion. Convection (stirring) is the most efficient form of mass transport. Migration can be minimized by adding a swamping electrolyte to the solution. Diffusion occurs even in the absence of migration and convection. 

As junction potentials can have such a devastating effect on electroanalytical data, we need next to consider some means of minimizing them. There are two general methods that can be used - placing a salt bridge in the circuit (as alluded to above) or adding a swamping electrolyte to the solution. 

To learn that addition of a swamping electrolyte to a still solution of analyte ensures that the principal form of mass transport is diffusion. 

In Chapter 3, we looked at the way the activity coefficients can be more or less equalized if there is a swamping electrolyte in solution (see Section 3.4.4, SAQ 3.9 and Figure 3.8). By the nature of the species studied in a chronoamperometric experiment, (a) a swamping electrolyte is added to the solution in order to minimize migration effects, and... 

All of the electroanalytical techniques described in this present chapter have made use of the general relationship, faradaic current a analyte concentration , according to Faraday s laws. It is therefore important that such non-faradaic currents be minimized. First, the resistance of the solution can be minimized by adding an inert electrolyte to the solution in swamping concentration. (Adding a swamping electrolyte also decreases the extent of mass transport by migration.)... 

Swamping electrolyte An ionic salt added to a solution of analyte in order to minimize migration effects and increase the ionic conductivity. 

Mixed Micelles. The CMC values -for the two pure sur-factants and well de-fined mixtures thereo-f are shown in Figure 2. The experiments were run at a high added salt level (swamping electrolyte) so the counterion contributed by the dissolved sur-factant is negligible. Predicted mixture CMC values -for ideal mixing -from Equation 1 are also shown. Ideal solution theory describes mixed micelle -formation very well, as is usually the case -for similarly structured sur-factant mixtures (12.19.21—2A) ... 

Mixed Admicelles. The total sur-factant adsorption o-f the two pure sur-factants and mixtures thereo-f on alumina are shown in Figure 3. The mixtures are at constant surFactant ratio in the Feed or initial solution, but not necessarily in the Final equilibrium solution. The concentration on the abscissa is the equilibrium concentration. The individual surFactant adsorption isotherms For the pure surFactants and in the mixtures are shown in Figures 4 and 5. The experiments were run at the same swamping electrolyte concentration as were the CMC data. 

Charged colloids behave as if they were uncharged under swamping electrolyte conditions. 

These effects can be overcome by employing swamping electrolyte concentrations. Any potentials which might develop are then readily dissipated by a very small displacement of a large number of counterions. 

Potentiometric methods have eliminated the problems that beset earlier studies, due to the high electrolyte concentrations required for ideal electrode behavior. Following the so-called constant ionic medium principle [91], a large excess of an indifferent (or inert or swamping) electrolyte is added, so that the activity coefficients of the species can be considered constant when their concentration (very low compared to that of the indifferent electrolyte) are changed over a wide range. 

When both the swamping electrolyte and the IPR concentrations are varied, the retention is correctly described by a bivariate expression [59], The relationship predicts that the capacity factor of a solute oppositely (similarly) charged to the IPR decreases (increases) with increasing ionic strength, because of the lower net electrostatic attractive (repulsive) potential. The predictions are experimentally verified [59]. Again, it is epistemologically interesting to observe that the retention equation, in the absence of the IPR in the eluent, reduces to that developed and tested in RP-HPLC... 

Having executed the appropriate measurement, an Interpretational step is needed to obtain f and/or K" or, for that matter, Du. As with electrophoresis, this conversion is straightforward only under idealized conditions, in the present case for capillaries. Then one of the equations of sec. 4.3 can be used. With capillaries it is also simple to obtain K° from (4.5.10] by first measuring K in swamping electrolyte (K 2K /a). This gives the cell constant, which may then be used to obtain K and at lower using the bulk relationship between and c. .. 

Swamping Electrolyte An excess of indifferent electrolyte that severely compresses electric double layers and minimizes the influence of electric charges borne by large molecules or dispersed colloidal species. 

Swamping Electrolyte An excess of indifferent electrolyte that severely... 

Ligands also differ in their ability to form stable complexes. The ligands phosphate, hydroxide, carbonate, and so forth, are potent complex formers while the perchlorate, CIO4", and nitrate, NO3", ions show very little tendency to form complexes. It is for this reason that nitrate or perchlorate salts are used as swamping electrolytes in experiments where it is desirable to have a constant ionic strength. ... 

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