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.
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, 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.
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.
Substitution by inert gas
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. Because helium is not very soluble in most liquids, this technique is also used to reduce the risk of bubble formation in HPLC systems.
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.
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. While this is a viable method for degassing a wide variety of organic solvents, some solvents are prone to significant expansion upon flash freezing. This expansion can cause practical problems by breaking the container. Some solvents known for such behavior are water and methanol.
Yeast uses sugar to produce alcohol and carbon dioxide. In wine making, carbon dioxide is an undesired by-product for most wines. If the wine is bottled quickly after fermentation, it is important to degas the wine first before bottling.
Wineries can often skip this step by aging their wines prior to bottling. Storing the wines in steel and wood barrels for months and sometimes years allow gases to be released from the juice and escape back into the air through air-locks.