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D Solvent Considerations
the heat of solution is endothermic, the solubility increases when you heat the
solvent. For example, the solubility of He in H
temperature, but the solubility of He in benzene increases with an increase in
temperature.
Effects of partial pressure
The mass of gas dissolved in a given volume of solvent is proportional to the
partial pressure of the gas in the vapor phase of the solvent. If you decrease
the partial pressure of the gas, the amount of that gas in solution also
decreases.
Vacuum degassing
The in-line vacuum degasser operates on the principle of Henry's Law to
remove dissolved gases from the solvent. According to Henry's Law, the mole
fraction of a gas dissolved in a liquid is proportional to the partial pressure of
that gas in the vapor phase above the liquid. If the partial pressure of a gas on
the surface of the liquid is reduced, for example, by evacuation, then a
proportional amount of that gas comes out of solution. Refer to the Vacuum
Degassing discussion below and to
Vacuum degassing can change the composition of mixed solvents.
Tip:
Vacuum degassing considerations
The longer a solvent is exposed to a vacuum, the more dissolved gases are
removed. Two factors affect the amount of time the solvent is exposed to the
vacuum:
•
Flow rate – At low flow rates, most of the dissolved gas is removed as the
solvent passes through the vacuum chamber. At higher flow rates, lesser
amounts of gas per unit volume of solvent are removed.
•
Surface area of the degassing membrane – The length of the degassing
membrane is fixed in each vacuum chamber. To increase the length of
membrane, you can connect two or more vacuum chambers in series.
Wavelength selection
The tables in this section provide UV cutoff values for these items:
•
Common solvents
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July 29, 2013, 715003794 Rev. B
O decreases with an increase in
2
"Degasser considerations" on page
30.
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