The properties of UF6 summarized in this section have been taken primarily from a comprehensive report by DeWitt [D3].

Vapor pressure, triple point, and critical point. Table 5.10 gives the triple-point pressure and temperature of UF6 measured by Brickwedde et al. [B6], the critical pressure and temperature reported by Oliver et al. [01], and values of the vapor pressure at temperatures between —200°C and the critical point from the following sources:

1. Solid, below 0°C, Eq. (5.4) fitted by Llewellyn [LI] to his measurements between —15°C and the triple point:

log, o Ртогг = — 75.0 exp — 1.01 log10 T + 13.797 (T = r, °С + 273)

V (5.4)

2. Solid at 0°C, measured by Weinstock et al. [W2].

3. Solid, between 0°C and triple point, Eq. (5.5) fitted by Oliver et al. [01] to their measurements between 0°C and the triple point:

logro Pro* = 6.38353 + 0.0075377Г — , Гт^іб (5’5)

4. Liquid, between triple point and critical point, values quoted by DeWitt [D3] from a sixth-order polynomial fitted by Brooks and Wood [B7] to data of Oliver et al. [01]. These appear to be the most reliable and consistent data among the many cited by DeWitt. Values below — 15°C are of order-of-magnitude reliability only.

Density. Values of the density of liquid UF6 between the triple point and 160°C given in Table 5.10 are from measurements of Wechsler and Hoge [Wl]. The critical density is the estimate of Oliver et al. [01]. The solid density at 25°C is calculated from x-ray diffraction data. Solid UF6 formed when the liquid freezes or vapor condenses is much less dense because of contraction on solidification.

UF6 vapor is unassociated, but displays van der Waals-type departures from the ideal gas law. Weinstock et al. [W2] proposed Eq. (5.6) as an equation of state for UF6 vapor:

, , 3. _ 4.29 lp,, ,,

Pg (g/cm ) ^ _ 1(376(9оор/Гз) ( • )

where p is the pressure in atmospheres and T is in kelvins.

Thermodynamic properties. Table 5.11 gives thermodynamic properties of UF6 under its vapor pressure, selected from the comprehensive review of DeWitt [D3]. The original data sources were as follows:

Heat capacity and enthalpy of solid and liquid: measurements by Brickwedde et al. [B6].

Molal heat of vaporization: from the Gapeyron equation

dp dT

Change of vapor pressure with temperature: dp/dT from the measurements of Oliver et al. [01] summarized in Table 5.10.

Molal volume change AV: from

y_ 352 _ 352 Pg Pc

with pG from Weinstock’s Eq. (5.6) and Pc from Table 5.10.

Enthalpy of vapor: enthalpy of condensed phase plus heat of vaporization.

The value of 12,965 for the molal heat of vaporization at О К was found by Weinstock et al. [W2] to provide the best correlation of measurements of vapor pressure, density of each phase, and heat capacity of each phase through the Gapeyron equation.

Table 5.12 gives thermodynamic properties of UF6 in the ideal gas state at 1 atm pressure, denoted by the superscript (°). Values of C£, the free-energy function —(G° —Я£)/Г and the entropy S° are taken from DeWitt’s [D3] citation of calculations by Bigeleisen et al. [B3] from spectroscopic data. The enthalpy of UF6 in the ideal gas state relative to the solid at 298.15 К was evaluated from

Barm and Knacke [B1 ] give for the heat of formation of solid UF6 at 25°C

AHCi298 = -523,000 cal/g-mol (5.12)

Hence the heat of formation of UF6 in the ideal gas state at 25°C is

298 = -523,000 + 11,807 =-511,193 (5.13)

The free energy of formation of solid UF6 at 25° C from the elements, from the same source, is

ДССі298 = —492,252 cal/g-mol (5.14)