If you rinse your slides very, very well before placing them in chromic acid, 
the solution will last a long time.  If it is turning dark, it is because 
alcohol is being carried over into the solution and reducing it.  But as long 
as it remains the bright orange, I think it can be used for a long time.  It 
will gradually become diluted with water, and will have to be replaced at 
some point in time.

Freida Carson