Instead of attempting to add refrigerant yourself, take your Impala to a qualified mechanic or an auto AC specialist. They have the necessary equipment and expertise to:
* Identify the leak: Before adding refrigerant, they need to find and repair any leaks in the system. Adding refrigerant to a system with a leak is pointless and wasteful.
* Vacuum the system: Air and moisture in the system can contaminate the refrigerant and damage the compressor. They'll evacuate the system to remove these contaminants.
* Add the correct amount of refrigerant: The amount of refrigerant needed varies depending on your specific vehicle model. They'll use gauges to ensure the correct amount is added.
Trying to DIY this repair is strongly discouraged. A professional will do the job safely and correctly, saving you money in the long run by preventing costly damage.