You are sharing a workspace with others. It is just as much their space as it is yours. You cannot reasonably demand they stop talking inside that space.
What you can do, however, is admit that you are unable to work efficiently with distractions, and you would appreciate it if they were able to continue there conversation at either another time or place. You should word it in a way in that it is your inability to work while the conversation is going on that is at fault, not the fact that they are talking.
Most people are more accomidating to help someone than they are when they feel they are being attacked.
If they refuse to talk elsewhere, then you need to either accept your fate with them talking, be more forceful in your approach and risk making enemies of them, or seek a non-interpersonal resolution.