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The Vapor Pressure of Solid Sodium and Solid Potassium Amalgams

1926; American Institute of Physics; Volume: 28; Issue: 1 Linguagem: Inglês

10.1103/physrev.28.208

1536-6065

Franklin E. Poindexter,

Chemical Thermodynamics and Molecular Structure

The vapor pressure of solid sodium and potassium amalgams.---A Buckley ionization gauge was used to measure the vapor pressure of eight sodium amalgams ranging in concentration from 1: 1 to 15: 1 mol ratio of Hg to Na and of 3 potassium amalgams whose concentrations were 5: 1, 10: 1 and 21: 1, Hg: K. The vapor pressure of each amalgam was measured at a number of temperatures which were determined by means of a platinum resistance thermometer. The pressures measured varied from approximately ${10}^{\ensuremath{-}8}$ mm to ${10}^{\ensuremath{-}3}$ mm over a maximum temperature range of from -56\ifmmode^\circ\else\textdegree\fi{} to 90\ifmmode^\circ\else\textdegree\fi{}C. The $\mathrm{log} p$ vs $\frac{1}{T}$ graphs for the different amalgams were found to be approximately straight lines. The difference between the free energy calculated by R. H. Gerke and the internal energy change as calculated in this work shows that the $logp$ vs $\frac{1}{T}$ graphs can be expected to be straight within the experimental error. The heats of reaction were calculated by means of the Van't Hoff reaction isochore, the pressures at different temperatures for these calculations being taken from the $logp$ vs $\frac{1}{T}$ graphs. These heats of reaction for the sodium amalgams varied from 15,000 cal. in the case of the amalgams rich in Hg to 14,000 cal. for the 2: 1 amalgam. This small heat difference over such a wide range of concentrations indicates that most of the heat of reaction between Na and Hg is liberated in the formation of the initial compounds. The heats of reaction for the potassium amalgams were calculated to be 25,500 cal.Duration of the efficiency of a sodium mercury vapor trap.---The vapor pressure of 1: 1 sodium amalgam was extrapolated to 20\ifmmode^\circ\else\textdegree\fi{}C and found to be 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ mm. The time necessary for sufficient mercury to diffuse through 100 cm of tubing having an internal diameter of 1 cm to form a 1: 1 amalgam with 10 g. of sodium was found to be 1554 days. That is, a sodium mercury vapor trap containing 10 g. of sodium should hold the mercury vapor beyond the trap down to ${10}^{\ensuremath{-}8}$ mm for 1500 days under the prescribed conditions.The relation between the free and internal energy changes of potassium and sodium.---The internal energy change of potassium as calculated from the $logp$ vs $\frac{1}{T}$ graphs are somewhat greater than the free energy change as calculated by R. H. Gerke. This indicates that the temperature gradient of the e m.f. of concentration cells should be small and negative. There seems to be no experimental data with which to check this prediction. For sodium Richards and Conant found that the temperature gradient of the e.m.f. of concentration cells was comparatively large and positive, thus indicating that the free energy change is considerably greater than the internal energy change. R. H. Gerke calculated the free energy change from electrochemical data for a 5: 1, Hg: Na sodium amalgam to be 18,300 cal. which is of the order of 3,000 cal. greater than the internal energy change as calculated from the $logp$ vs $\frac{1}{T}$ graphs. That is, the present work is in agreement with the above.