L.A.K. Staveley

The densities of liquid argon, krypton xenon, oxygen, nitrogen, carbon monoxide methane, and carbon tetrafluoride along the orthobaric liquid curve

Abstract The densities along the orthobaric liquid curve of the following liquefied gases have been measured with a precision of a few parts in 104: argon (86 to 118 K); krypton (118 to 164 K); xenon (164 to 219 K); oxygen (80 to 121 K); nitrogen (78 to 105 K); carbon monoxide (78 to 111 K); methane (92 to 151 K); carbon tetrafluoride (91 to 185 K). The measurements were made on a sample of about 9 cm3 in a glass pyknometer, which was incorporated in a cryostat assembly similar to that used in conventional lowtemperature adiabatic calorimetry. The results have been considered from the point of view of the law of corresponding states. When the volume is plotted as a fraction of the critical volume Vc against temperature expressed as a fraction of the critical temperature Tc, the results for liquid krypton, xenon, and nitrogen conform to a common curve. By changing the “best” values of Vc for argon and oxygen by about 0.5 per cent (which is probably within the uncertainty limits of these values of Vc), the points for these two liquids could also be made to fall on the same curve. But there are indications of small divergences from this curve for methane and carbon monoxide, even if the values of Vc adopted for these two substances are altered. Over the temperature ranges studied, the so-called rectilinear diameter for all eight substances examined is in fact a very slight curve, but extrapolation to the critical temperature of a least-squares straight line gives values of the critical volume within about one per cent of the literature values.

Experimental Study of the Equation of State of Liquid Krypton

The gas expansion method has been used to measure the density of liquid krypton at 11 temperatures from 120 to 220°K and at pressures up to 3680 atm. The results have been fitted to the Strobridge equation, which has been used to estimate, at regular intervals of pressure and temperature, the following properties: density, isothermal compressibility, thermal expansion coefficient, thermal pressure coefficient, configurational internal energy, and entropy relative to the saturated liquid. The equation of state results, together with estimated values of the third virial coefficient and published values of vapor pressure, second virial coefficient, and sound velocity in the liquid phase, have been used to estimate the following properties of the saturated liquid: enthalpy of vaporization, configurational internal energy, isothermal compressibility, thermal expansion coefficient, thermal pressure coefficient, adiabatic compressibility, and specific heats. The linear dependence of configurational internal ener...

An experimental study of the equation of state of liquid xenon

The gas-expansion method has been used to measure the density of liquid xenon at 17 temperatures from 165.00 to 289.74K and at pressures up to 3815 atm. The 530 experimental points have been fitted to the Strobridge equation, which has been used to estimate, at regular intervals of pressure and temperature, the following properties: density; isothermal compressibility; thermal expansivity; thermal pressure coefficient; configurational internal energy; and entropy relative to the saturated liquid. Within the range of the experiments, the configurational internal energy is very nearly a linear function of density. The experimental results, together with estimated third virial coefficients and published values of vapour pressure, second virial coefficient, and sound velocity in the liquid phase, have been used to estimate the following properties of the saturated liquid on the liquid-vapour coexistence curve: enthalpy of vaporization; configurational internal energy; isothermal compressibility; thermal expansivity; thermal pressure coefficient; adiabatic compressibility; and heat capacity. The results have been use, together with published pressure, temperature data for the melting curve, to estimate the following properties of the saturated liquid on the liquid-solid coexistence curve: density; isothermal compressibility; thermal expansivity and thermal pressure coefficient.

A heat capacity study of the movement of the ammonium ion in ammonium stannichloride and stannibromide by comparison of the heat capacities of the ammonium, rubidium and potassium salts

Abstract The heat capacity of the following six salts has been measured from ∼20°K to ∼300°K: ammonium, rubidium and potassium stannichlorides, ammonium, rubidium and potassium stannibromides. By comparison of the heat capacities of the ammonium and rubidium salts (which are isomorphous), that part of the heat capacity due to the torsional oscillations of the ammonium ions has been separated, and for both the stannichloride and the stannibromide is found to be decreasing as room temperature is approached from below, reaching at 298° K a value roughly halfway between that for free rotation and for classical torsional oscillations. This implies that the ammonium ions are restricted rotators prevented from freely rotating by comparatively low energy barriers. The bearing of this on the charge distribution in the stannihalide ions has been considered. Gradual transitions have been found in ammonium stannibromide and potassium stannichloride. Whereas the latter is apparently a simple λ-type transition with an entropy change of ∼1·1 3 e.u., that in the stannibromide has an entropy change of ∼4·5 e.u. and appears to be a transition of some complexity. The possibility is considered that the transition in potassium stannichloride might be due to the availability to the anion of two alternative orientations.

The heat capacity and entropy of calcite and aragonite, and their interpretation

Abstract Heat capacity measurements have been made on calcite from 10 to 303 K and on aragonite from 22 to 291 K. Assuming the two polymorphs to be perfectly ordered at 0 K, the molar entropy of calcite at 25 °C exceeds that of aragonite by (0.89±0.05) cal K −1 mol −1 . This entropy difference ΔS is effectively established at quite a low temperature (about 80 K). The information which can be derived from this value of ΔS about the pressure-temperature relation for the equilibrium between calcite and aragonite is in very fair agreement with the results of recent direct experimental studies of this equilibrium. An attempt has been made to calculate the heat capacity of calcite, making use of spectroscopic information available for this crystal about the lattice vibrational frequencies. A key question briefly discussed is the division of the vibrational frequency spectrum into optical and acoustic branches and the assignment of an appropriate Debye temperature to the acoustic branch. An important point, due to Professor M. Blackman, is that the value of this temperature depends on the size of the elementary unit chosen for consideration of the vibrational frequencies. At best, the agreement between the observed and calculated C p values is only moderate, and may represent more or less the limit of what can be achieved for a crystal as complicated as calcite by an approach that uses only Debye and Einstein functions.

Experimental Study of the Equation of State of Liquid Argon

The density of liquid argon has been measured at 10 temperatures from 100.9° to 143.1°K and at pressures up to 680 atm. The results have been expressed as an equation of state in the form of a double Chebyshev expansion. This equation has been used to estimate, at regular intervals of temperature and pressure, the following properties: molar volume, isothermal compressibility, coefficient of thermal expansion, thermal pressure coefficient, and configurational internal energy. Comparison is made between the present results and previous work on liquid argon, both at the saturation vapor pressure and at higher pressures.

Excess enthalpies of the liquid mixtures nitrogen + oxygen, nitrogen + argon, argon + ethane, and methane + carbon tetrafluoride

Details are given of an improved calorimeter for the measurement of the excess enthalpies of condensed gases. The calorimeter also allows the measurement of the equilibrium vapour pressure of the mixture in the calorimeter, thus leading to a simultaneous determination of the excess Gibbs function. Results of HE measurements are given for N2 + O2 at 80.5 K, N2 + Ar at 84.5 K, Ar + C2H6 at 90.2 K, and CH4 + CF4 at 98.1 K. Values of GE are given for N2 + Ar and Ar + C2H6. A brief comparison is made in the case of CH4 + CF4 with the predictions of perturbation theory.

A comparative study of the electrical conductivity of solid ammonium chloride and other ammonium salts

Abstract Measurements have been made of the conductivity of single crystals of pure and doped ammonium chloride and of single crystals of caesium chloride and potassium chloride. Less detailed studies have been made of ammonium and potassium bromides and iodides, tetramethylammonium iodide, ammonium and potassium stannichlorides, and ammonium and rubidium fluophosphates. The conductivity of ammonium chloride is significantly greater than that of a comparable alkali chloride and reasons are given for believing that a conductivity mechanism operates in the ammonium salt which is not possible for the alkali metal salts. It is suggested that this mechanism is a three-stage process, involving (1) a proton switch from an ammonium ion to a chloride ion adjacent to a vacancy, which produces an ammonia molecule and a hydrogen chloride molecule, (2) the migration of one of these molecules into the vacancy and (3) a reverse proton switch to reform a pair of ions. Part of the evidence for this mechanism comes from a consideration of the thermodynamic properties of ammonium chloride, which are consistent with the view that even at room temperature and below the formation of molecules within the lattice takes place to a significant extent.

Triple-points of low melting substances and their use in cryogenic work

Abstract Some practical uses of the constancy of the triple-point temperature ( T t ) of a pure substance in cryogenic work are described. Simple techniques for attaining constant and known temperatures are discussed, as is the degree of impurity which can be tolerated while meeting desired limits of uncertainty of, and variation in, temperature. A comprehensive survey of low-melting elements and compounds has been carried out, covering 190 substances from neon to carbon tetrachloride, which span T t values from 24.5 to 250.5 K. These substances have been divided into three groups according to their availability, the state of purity in which they can be purchased or alternatively the ease with which they can be purified, and the reliability with which T t is known in the IPTS-68. Group 1 includes substances of particular importance for which T t is accurately known and which are readily available in a high state of purity. Group 2 lists substances which are fairly readily available and for which at least one reasonably reliable measurement of T t has been made, while Group 3 comprises all the remaining substances, for which at least one apparently careful determination of T t has been published.

Excess enthalpies and Gibbs free energies for nitrogen + methane at temperatures below the critical point of nitrogen

Abstract Mixing calorimetry has been used to determine the excess enthalpy HE as a function of composition for liquid mixtures of nitrogen + methane at 91.5 and 105.0 K. By use of a separate static apparatus, total vapour pressure as a function of composition was determined for this same liquid system at 90.68 K. Barkers method has been used to convert the results into excess Gibbs free energies GE as a function of mole fraction x. An intercomparison is made of available GE data for nitrogen + methane at temperatures below the critical point of nitrogen (126.2 K). G E T at x = 0.5 is plotted as a function of reciprocal temperature with slopes taken from the experimental HE values. Good agreement is found between the present results and a number of other recent studies.