Abstract
We present a method to monitor continuously the electrical conductivity of the solutions in both compartments of a diaphragm cell in which diffusion is occurring. This allows a much smaller initial concentration difference to be applied across the diaphragm rather than the usual larger initial concentration difference. Together with an improved filling procedure and a more reproducible stirring device, these improvements lead to the determination of differential diffusion coefficients with accuracies comparable to those of the precise optical interferometric methods. This is shown for aqueous solutions of NaCl, KC1, Na2SO4, and MgSO4 at 25 °C. By using aqueous solutions of HC1, the H+-exchange properties of the diaphragm and its influence on the measurements become detectable.
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Acknowledgments
I thank Drs. Donald G. Miller (deceased) and Joseph A. Rard for their aid in preparing this manuscript for publication. The delay in publishing our results is due to circumstances beyond the control of the authors.
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Appendix: Suggested Improvement
Appendix: Suggested Improvement
We have also tried to monitor continuously the concentration changes in the bottom and top half cells by an optical method, because a combination of both methods would permit measuring multicomponent diffusion coefficients without much difficulty. Although our use of the optical method has not yet reached the accuracy of the conductivity method (the standard deviation is greater by a factor of 4–5), we want to outline its construction. For details the reader is referred to Ref. [22].
The cell is situated in one half-arm of a laser-based Michelson interferometer which is installed on a vibration-damped disk. This apparatus is situated in a cellar or ground floor to insure good mechanical and thermal stability. It is completely enclosed by an air thermostat. Two optically-flat windows are mounted in the bottom half cells and two in the top half cells so that the light can pass through without any distortion. The interference fringes and their change with time are recorded by two photo-diodes. Figure 3 shows one sample output: one can see that at the start the interference patterns become constant. This marks the end of the filling process, which again is done by pumping. Then the diffusion experiment starts, and the time intervals between following fringes increase. One can also discern that there are deviations from a completely smooth and even progress of the fringes. This is due to small temperature variations (±0.02 K), which are nearly impossible to avoid with an air thermostat because of the size of the apparatus. These temperature changes can lead to about one changed fringe in 5–8 h, as evidenced by the reference beam that passes completely through air, when the number of fringes passed in the half cells is about 100. Results with aqueous KC1, NaCl, and MgSO4 had a relative standard deviation of ca. 0.7 %.
Typical record of the interference fringes and temperatures during a diffusion experiment in the Michelson interferometer
Unfortunately, we no longer have the possibility to continue this work and improve the apparatus, especially to make it smaller and more compact, thus reducing the temperature variations. However, we want to communicate our preliminary results, because they are better than the traditional integral diffusion coefficient diaphragm cell measurements.
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Breer, J., de Groot, K. & Schönert, H. Diffusion in the Diaphragm Cell: Continuous Monitoring of the Concentrations and Determination of the Differential Diffusion Coefficient. J Solution Chem 43, 71–82 (2014). https://doi.org/10.1007/s10953-013-0020-z
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DOI: https://doi.org/10.1007/s10953-013-0020-z