Degasification

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

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

Additional recommended knowledge

Daily Sensitivity Test

Essential Laboratory Skills Guide

Daily Visual Balance Check

Pressure reduction

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 usually enhance the efficiency. But care must be exercised with volatile solvents not to depressurize too much.

Heating

Generally speaking, the higher the temperature of a solution becomes, the less a gas dissolves provided it doesn't react with the solvent. Consequently, heating a solution can expel the remaining gas. Ultrasonication and stirring at high heat 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.

Membrane degasification

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.

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

Substitution by inert gas

See also: sparging (chemistry)

Bubbling a solution with an inert gas substitutes the dissolved harmful, reactive gases such as oxygen and carbon dioxide. Nitrogen, argon, helium, and other inert gases are commonly used. To complete the substitution, the solution should be stirred vigorously and bubbled for a long time.

Addition of reductant

If oxygen should be removed, the addition of reductants is sometimes effective. For example, especially in the field of electrochemistry, ammonium sulfite is frequently used as a reductant because it reacts with oxygen to form sulfate ions. Although this method can be applied only to oxygen and involves the risk of reduction of the solute, the dissolved oxygen is almost totally eliminated.

Freeze-Pump-Thaw cycling

In this laboratory-scale technique, the fluid to be degassed is placed in a Schlenk flask and flash-frozen, usually with liquid nitrogen. Next a vacuum is applied, and the flask is sealed. A warm water bath is used to thaw the fluid, and upon thawing, bubbles of gas form and escape. The process is typically repeated three times.^[1]

References

1. ^ Mark Lonergan. Freeze-Pump-Thaw Degassing of Liquids. University of Oregon.