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The vapour pressures and melting points of sodium and potassium

1927; Royal Society; Volume: 113; Issue: 765 Linguagem: Inglês

10.1098/rspa.1927.0005

2053-9150

W. Edmondson, Alfred Charles Egerton,

Thermodynamic properties of mixtures

Accurate determination of the vapour pressure of sodium and potassium is of importance for several reasons. Sowerby and Barratt (1) have determined for the alkali metals the number of atoms required for the absorption of the first line of the combination series compared with that of the line of the principal series, basing the resulting ratio (1.2.10 4 ) on the vapour-pressure formulæ provided by one of us from the existing measurements. The present measurements were not sufficiently advanced to include them in the formulæ then given, but such change as should now be made improves the agreement in the ratios for sodium and potassium obtained by Sowerby and Barratt. Special interest lay also in these determinations, because the “chemical constant ” calculated from the vapour-pressure measurements did not appear to agree with the “theoretical” value from C = C 0 + 1.5 log M, where C 0 = —1.589 is a universal constant, and M the atomic weight of the element (2). Ladenburg and Minkowski (3) found higher values which were considered to be outside the error of experiment, a result which has been discussed by Simon (4) and has led to modification of the views held as to the validity in all cases of the Sackur formula. It has been suggested by Schottky (5) that an allowance for difference in the quantum weights of the lowest quantum state of the solid and of the vapour should be made. On the experimental side the justification for such modification is inconclusive, and further measurements of the vapour pressures of sodium and potassium seemed desirable.