Degassing

Degasification is the removal of dissolved gases from liquids, especially water or aqueous solutions. There are numerous possible methods for such removal of gases from liquids.

Gases are removed for various reasons. Chemists remove gases from solvents when the compounds they are working on are possibly air- or oxygen-sensitive, or when bubble formation at solid-liquid interfaces becomes a problem. The formation of gas bubbles when a liquid is frozen can also be undesirable, necessitating degassing. This process is called degasification.

The solubility of gas obeys Henry's law, that is, the amount of a dissolved gas in a liquid is proportional to its partial pressure. Therefore, placing a solution under reduced pressure makes the dissolved gas less soluble. Sonication and stirring under reduced pressure can usually enhance the efficiency. This technique is often referred to as Vacuum degasification. Specialized vacuum chambers, called vacuum degassers, are used to degas materials through pressure reduction.

Generally speaking, an aqueous solvent dissolves less gas at higher temperature, and vice versa for organic solvents (provided the solute and solvent do not react). Consequently, heating an aqueous solution can expel dissolved gas, whereas cooling an organic solution has the same effect. Ultrasonication and stirring during thermal regulation are also effective. This method needs no special apparatus and is easy to conduct. In some cases, however, the solvent and the solute decompose, react with each other, or evaporate at high temperature, and the rate of removal is less reproducible.

Gas-liquid separation membranes allow gas but not liquid to pass through. Flowing a solution inside a gas-liquid separation membrane and vacuating outside makes the dissolved gas go out through the membrane. This method has the advantage of being able to prevent redissolution of the gas, so it is used to produce very pure solvents. New applications are in Ink Jet systems where gas in the ink forms bubbles and degrades print quality, a de gas unit is placed prior to the print head to remove gas and prevent the buildup of bubbles keeping good jetting and print quality.

The above three methods are used to remove all dissolved gases. Below are methods for more selective removal.

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