Originally Posted by StoneNYC
Selenium toning of negatives is another way to increase its contrast, a bit like N+1, as the intensification is more pronounced in highlights. The added benefit is that you don't increase grain, like longer dev time would. You can even do it selectively, here is a picture of John Sexton doing it to a portion of one of his negs.
However, if you scan, I suspect this won't be much use to you at all, you can just do Cmd-K in Lr.
As others said, this stuff lasts until exhausted, if mixed normally.