Measurement of Densities and Estimation of Critical Properties of the Alkali Metals

The vapor and liquid densities of the alkali metals rubidium, cesium, sodium, and potassium were measured from room temperature up to near the critical point by a method which utilizes radioactive isotopes of the alkali metals. Critical temperature and critical density were then estimated by several methods of correlation including those of (1) Rowlinson, (2) Cailletet and Mathias, and (3) Kordes, and by a generalized correlation of reduced density versus reduced temperature. Critical temperatures of the alkali metals estimated by these correlating methods were as follows: 2573°±350°K for sodium, 2223°±600°K for potassium, 2093°±25°K for rubidium, and 2057°±40°K for cesium. Corresponding critical densities were: 0.206±0.041 g/cc for sodium, 0.194±0.37 g/cc for potassium, 0.346±0.009 g/cc for rubidium, and 0.428±0.012 g/cc for cesium. The critical temperature and critical density of lithium were estimated using the liquid-density data for lithium reported by Tepper and the generalized correlation of reduced density with reduced temperature. The estimated critical temperature of lithium was 3223°±600°K and the estimated critical density 0.120±0.033 g/cc. The technique used in these measurements involved charging a metal capsule (molybdenum or molybdenum—tungsten alloy) with a calculated amount of alkali metal, sealing the capsule, and irradiating it in a thermal neutron flux to obtain a radioactive isotope of the metal being tested. The irradiated capsule was then placed in an insulated inductively heated high-temperature cell made of molybdenum—tungsten alloy. Gamma radiation emanating from the vapor and liquid regions in the capsule passed through the thin capsule wall (30 mil) and telescoping holes in the high-temperature cell to a sodium iodide crystal outside the furnace. The measured radiation was proportional to the density of the vapor or liquid. While this method of measurement was demonstrated on alkali metals, the technique is general and can be applied in many cases to measure vapor and liquid densities at high temperatures (up to 220°K) for metals, alloys, fused salts, and other materials.