What is a critical action to take when replacing a chlorine gas cylinder?

When replacing a chlorine gas cylinder, always replacing the lead gasket is critical to ensure a safe and effective seal during operation. The lead gasket helps to prevent the escape of chlorine gas, which can be hazardous to health and safety. Each time a cylinder is replaced, using a new gasket ensures that there is no wear or damage from previous use that could compromise the integrity of the seal. This practice minimizes the risk of leaks, which is essential in handling hazardous materials like chlorine.

Other options, while they might sound plausible, do not align with safe practices for handling chlorine cylinders. For example, reusing a lead gasket could lead to undetected damage or deterioration that would allow gas to escape. Verifying if the cylinder is empty is important but does not directly pertain to the sealing process, and while storing cylinders properly is critical, it is not specifically related to the act of replacing the gasket during the replacement of a cylinder. Thus, the action of always replacing the lead gasket stands out as the most critical safety measure.