I have a jar of Sodium Metabisulfite which has been opened and used from. A volume of about 4 oz. is left and looks to be in good shape. I would want to use it to sanitize some equipment. Should I expect it to be effective after purchasing it after 10 years?
Sodium metabisulphite, if kept dry and out of direct sunlight, should be entirely stable. If exposed to air (or rather, the humidity in air) it will degrade very slowly, depending on environmental conditions. An opened but reclosed jar kept in a cupboard or something similar should be quite alright to use. I've used sodmet that was that old and it was fine.
If in doubt, use 10% more to make up for any possible loss in SO2 over the years.