When you wrote "unused" did you mean that the meat had not been IN it yet?
If you made the brine fresh and there has been no meat in it, I suppose there is no problem with that, and should be fine.
If the brine has had ANY sort of muscle meat in it, to include a spoon or mixer that has been in contact with the meat, then no. Discard that and make fresh.
Food-safety is critical.