Some Considerations on the Electrolysis of Water from Sodium Hydroxide Solutions

[+] Author and Article Information
Edward A. Fletcher

Department of Mechanical Engineering, University of Minnesota, 111 Church Street S.E., Minneapolis, Minnesota 55455E-mail: fletcher@tc.umn.edu

J. Sol. Energy Eng 123(2), 143-146 (Dec 01, 2000) (4 pages) doi:10.1115/1.1351173 History: Received July 01, 2000; Revised December 01, 2000
Copyright © 2001 by ASME
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Grahic Jump Location
The variation of the saturation pressure of a saturated solution of a non-volatile solute whose solubility in water increases with temperature is superimposed on a conventional phase diagram of water. The line between the solid and gas phases of pure water gives the variation of the saturation pressure of ice with temperature; the line between liquid and gas phases of pure water gives the variation of the saturation pressure of pure liquid water with temperature; the line between the solid and liquid phases of pure water gives the variation of the melting point with pressure. In this example the solute is soluble in ice. The saturation pressures of all saturated solutions are lower than the saturation pressure of pure water at all temperatures. The saturation pressure of a saturated solution first increases as the temperature is raised from a low value because the effect of temperature on saturation pressure is the dominating factor. If the solubility of the solute increases rapidly with temperature however, the depression of the saturation pressure by the colligative effect of the solute may become the dominating factor. If that happens the saturation pressure of the saturated solution goes through a maximum. If that maximum saturation pressure is less than one bar, the solution will not have a normal boiling point. Glasstone pointed out that that would probably be the case with sodium and potassium hydroxides. It is interesting to note that if the maximum occurs at a pressure above one bar the system will have two normal boiling points: one as the temperature is raised from below, the lower normal boiling point, another when the temperature is reduced from above, the higher normal boiling point.
Grahic Jump Location
Variation of log10 of the saturation pressure (bars) of aqueous NaOH with mole fraction of NaOH and with the temperature, K. The temperature ranges from 293–623 K (y-axis) and the mole-fraction NaOH ranges from 0–1. (x-axis). The saturation pressure ranges from 10−6–103 bars (z-axis). The surface gives the base 10 logarithms of the saturation pressures on the z-axis. Isobars are shown on the surface. The boundaries of the variously shaded regions are keyed to the legend on the z-axis. For example, the 1 bar isobar is the lower boundary of the white strip that emerges from the y=0 plane at log(P)=0. Surface regions that are below this isobar represent conditions at which aqueous NaOH solutions will not boil at 1 bar. For example, even at 623 K, solutions which contain mole-fractions of NaOH greater than 0.902 will have sub-atmospheric saturation pressures. The figure can be used, e.g., to show that, to obtain high-pressure pipeline hydrogen and oxygen, e.g. at 100 bars, at 623 K an appropriate NaOH mole fraction would be about 0.22; a very dilute solution would have to be electrolyzed at a temperature below about 560 K. Limitations on the literature values of the saturation pressures of NaOH at higher temperatures, the simplifications I made selecting values for the saturation pressures and the limitations of graphical representation suggest that Fig. 2 should be used prudently, for scoping purposes, but the trends suggest that Glasstone’s prediction is probably correct.
Grahic Jump Location
Variation of the mole-fraction NaOH in an aqueous solution at which the specific conductance is at a maximum with temperature. Over the range 273.15–373.15 K the concentration for maximum specific conductance increases linearly with temperature. It is given by molfrkmax=−0.1761+8.6414×10−4 T. Over this temperature range, the maximum specific conductance ranges from 0.2 to 1.4 ohm−1 cm−1. The mole fraction of NaOH at 373.15 K is 0.15. One has good reason to believe that the specific conductance will be substantially higher at the higher temperatures and concentrations at which quasielectrolysis would be employed.




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