What does inserting an equal amount of air into a vial help prevent during fluid extraction?

Inserting an equal amount of air into a vial during fluid extraction is primarily intended to prevent vacuum formation inside the vial. When fluid is withdrawn from a sealed vial, a negative pressure or vacuum can develop if air is not allowed to enter the vial to balance the fluid that is being taken out. This vacuum can make it difficult to draw the desired volume of fluid, as the external atmospheric pressure is unable to push the fluid into the syringe smoothly. By injecting an equal volume of air, the pressure inside the vial is equalized, allowing for easier and more efficient extraction of the fluid.

Preventing vacuum formation is crucial not only for ease of extraction but also for maintaining the integrity of the vial and minimizing the risk of potential contamination or damage to the container. This practice ensures that the fluid can flow freely and consistently, making the compounding process safer and more effective.