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During a lecture, my professor said that sublimation at atmospheric pressure (1 atm), as a purification/separation technique, can be performed when the sublimating substance has a triple point falling at pressures above 100 mmHg. Why is this assumption made? Shouldn't sublimation at atmospheric pressure only occur when the atmospheric pressure (1 atm) falls below the triple point? In fact, only under this condition can be found a temperature at which the vapor pressure of the solid equals atmospheric pressure.

I don't understand why we consider 100 mmHg as the limit. Furthermore, it has been said that the sublimation point of vanillin, at atmospheric pressure, falls at 48°C, so at these pressure and temperature values ​​we actually fall on the sublimation curve. Right?

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  • $\begingroup$ For options of formatting of the posted plain text, see as inspiration SE - help - formatting. $\endgroup$
    – Poutnik
    Jan 24, 2023 at 10:21
  • $\begingroup$ @Poutnik so, what you mean is that sublimation can happen at any point (of pressure and temperature) that falls in the area of ​​the solid in the phase diagram. As one moves away from the sublimation curve, the extent of the same decreases. Therefore, if we are above 100 mmHg, the sublimation will take place with an acceptable speed at 760 mmHg, even if it is not possible to find a temperature at which the vapor pressure of the solid equals the atmospheric pressure (760 mmHg), because we would trespass the liquid zone. $\endgroup$
    – Luckenberg
    Jan 24, 2023 at 11:01
  • $\begingroup$ @Poutnik The most we can do is raise the temperature slightly, to raise the vapor pressure. Right? $\endgroup$
    – Luckenberg
    Jan 24, 2023 at 11:01

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Water triple point is $\pu{+0.01 ^\circ C}$ at about $\pu{610 Pa} \ll \pu{100 kPa}$ and water does sublime at freezing temperature. But slowly. I assume $\pu{100 mmHg}$ is a practical threshold for sublimation to have a possibly sufficient rate. There is no need to fit atmospheric pressure. Liquid evaporation occurs with vapor pressure below external pressure too.

Phase diagrams are about equilibrium.

At temperatures below their melting point, solids sublimate until the vapor pressure reaches the solid-gas line. Higher temperature means faster sublimation with higher vapor pressure. If the triple point has too low a pressure, we can use only a low maximum vapor pressure which may not be practical for larger quantities.

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  • $\begingroup$ What do you mean with 'which may not be practical for larger quantities'? And so, is what I wrote in the comments above right? $\endgroup$
    – Luckenberg
    Jan 24, 2023 at 15:30
  • $\begingroup$ Try sublimation of 100 g of ice at -5 deg C ice to another place, cooled to- 24 deg C. $\endgroup$
    – Poutnik
    Jan 24, 2023 at 16:07
  • $\begingroup$ More or less correct. Sublimation and evaporation occur at any temperature of the solid or liquid, it is just a question of vapor tension at given temperature which is proportional to rate of phase conversion. $\endgroup$
    – Poutnik
    Jan 24, 2023 at 16:19
  • $\begingroup$ Great! Thanks! So, obviously, ice at -5 °C will sublimate greater. But, we are always at a pressure (760 mmHg) really far from pressures that individuate the sublimation curve, so It Is Always a really slow phenomenon $\endgroup$
    – Luckenberg
    Jan 24, 2023 at 21:06
  • $\begingroup$ The external pressure is not much relevant. $\endgroup$
    – Poutnik
    Jan 24, 2023 at 21:26

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