The chemical and other thermal constants of sodium and potassium

Abstract

Sodium and Potassium. 533(3) The vapour pressure of sodium has been measured by the same method between 220° and 300° C., and the results are expressed by \ogV = -557A '2?. + 9-7255 -0-6794 log T, with which other observations are in accord.(4) A type of thermostat control was employed, obviating difficulties at the mercury contact surface.(5) The action of sodium on glass was indicated to be mainly due to metal absorbed within the glass at the temperatures of the experiments.A Royal Society grant covered part of the expenses of this work, and it is a pleasure to have this opportunity of expressing thanks. Our thanks are also due to Mr. C. F. Moore for assistance. The Chemical and other Thermal Constants o f Sodium andPotassium.By W. E dmondson, B.A., B.Sc., and A. E cierton, F.R.S. (Received September 21, 1926.)The vapour-pressure equation = -r can be integrated when the variadt RT2 tion with temperature of X, the latent heat of vaporisation, is known : this variation is given by where X0 is the latent heat of vaporisation at absolute zero, f m the latent heat of fusion, and the remaining Cp terms refer to the atomic heats at constant pressure of the solid, liquid and vapour. In the case of most metals there is considerable uncertainty as to the specific heat of the liquid and particularly in the cases of sodium and potassium. In this circumstance it is considered better to extrapolate the vapour-pressure equation to the melting point and to compute the integration constant C at that temperature from the equationwhere p is the vapour pressure in atmospheres at the melting point Tm.on May 12, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from 534The Chemical Constant of Potassium.Three estimates of the value of the constant are given in Table I. W. Edmondson and A. Egerton. The considerations leading to each estimate follow. 1.The vapour pressure at the melting point (63-6° C.) was found by extra polating the linear equation adopted to express the vapour pressure of potas sium.* This involves an extrapolation of less than 40° below the lowest observed point (100° C.). The equation agrees with observations in the other directions to 250° C. and is also in good agreement with Kroner's measurements still higher. XT," = 4-575 T2 -= 4507 • 0 X 4 • 575 = 2The specific heat of the solid has been determined by several investigators. The results are not in good agreement. In converting Cp to Cv, the LindemannMagnus relation Cp = Cv + aT1*5 was used. The value of a, found from Cp -Cv = 0-6 at 20° C., calculated from compressibility data (Richards'), and the thermal expansion is 1-17 . 10 4. A later value given by Eastman. Williams and Young (35) would make a -1 * 0 .10" 4. The empirical value * V. supra, p. 526. 535 a = l-9 .1 ( T 4 used by Ladenburg and Minkowski (3) fits the specific heat measurements (Ref. 29,30,31,32,33,23) better, the C" curve being given by Cp = F ( ) + 1 • 9 . 10-4 T3'2. The specific heat measurements do not extend to sufficiently low temperatures to establish the value of the frequency w...