RESUMO
The vapor pressure of tungsten was measured by the Langmuir method in the temperature range 2600 to 3100 K using a vacuum microbalance. Four series of data gave concordant results and three of the four series gave second and third law heats of sublimation in excellent agreement. A vapor pressure equation representing the data is log P(atm) = -45385/T + 7.871, based on our mean third law heat and tabulated entropies at 2800 K. The mean third law heat of sublimation at 298.15 K is 205.52 ± 1.1 kcal mol-1 (859.90 ± 4.6 kJ mol-1) where the uncertainty is an overall estimated error. Rates of vaporization are about 1/2 those previously accepted for tungsten.
RESUMO
The vapor pressure of platinum was measured by the Langmuir method in the temperature range 1700-2000 K using a vacuum microbalance. Eight series of data gave concordant results and an average third-law heat of sublimation of 564.49 kJ mol-1 with an estimated overall uncertainty of 2.1 kJ mol-1 (134.92 ±0.5 kcal mol-1). Three out of eight second-law heats agreed with the third-law heats within one standard error but there was a tendency for second-law heats to be low. This was attributed to small systematic errors in the measurements. A vapor-pressure equation representing the data is log P(atm) = -29020/T+7.502, based on our third-law heat and tabulated entropies evaluated at 1800 K. Our data agree well with several previous Langmuir determinations but significantly decrease the error in the heat previously accepted. Inability to obtain saturation pressures at lower temperatures, previously reported in the literature, was confirmed. It was shown that microgram quantities of carbon are capable of blocking the sublimation reaction.
