[KenWittlief]

well, ok maybe this is a matter of semantics

If you have invented a process or created a system that uses someone elses commercially available product, then yes, you can patent your system or process

but you dont hold a patent on their product. You hold a patent on your process or chemical formula.

even still, there has to be more involved than simply buying someone else product and using it by itself for some specific application - there has to some level of innovation on your part to get a patent. Your chemical formula cannot have only one ingredient: someone else's product.