Only if they are really, really old, or you don’t maintain their purity.
Like a two part nitrate test kit. dont, ever put the wrong cap back on the wrong bottle. I think it will pollute the reagents. make em go bad from a little contamination with their counter reagent. open, use and close the dropper bottles one at a time. same with using a syringe, rinse with RO water between drawing from reagent bottles. Otherwise they should: