bleach is sodium hypochlorite, which is basically table salt with some oxygen mixed in. I wouldn't want to add either the sodium or the chlorine to my container medium.

Yeah, bad idea. First, why do you feel you need to sterilize the mix?

You can sterilize a bare pot if you want with a mild bleach solution, but not with mix in there. Just replace all or some of the mix.

Keep in mind if you had other plants growing in that mix before, they could've used up much or all of the nutrients in there, leaving nothing for the incoming tomato plants. Also keep in mind if the mix had a lot of peat in it originally and it has since dried out sufficiently, it could be hydrophobic and tough to keep wet.

It's not like adding table salt to water and pouring it on your plants. Both sodium chloride (table salt) and sodium hypochlorite dissociate into their ions in water, but dissolving sodium chloride in water has no effect on pH.

The hypochlorite ion (ClOâÂȉ») pulls a hydrogen from water molecules and forms hypochlorous acid. This is a weak acid, so it doesn't fully dissociate back into its ions when in water, resulting in a net increase in OHâÂȉ», and raising the pH of the solution (it becomes more basic). So it acts more like a dilute solution of sodium hydroxide (lye) than table salt.

In other words, both of them aren't good for your plants, but for different reasons: bases are oxidants, and that's something you generally don't want to expose your plants' roots to. A high concentration of dissolved salts will damage your plants for reasons probably having to do with osmotic pressure. This could be a problem with sodium hypochlorite as well, but I would guess that its oxidizing behavior is the greater risk.