Safe pH range (acid vs base) for food
According to this neat chart on Wikipedia: http://en.wikipedia.org/wiki/File:PH_Scale.svg

Lemon juice is around 2, and baking soda around 9. Our digestive juices are around a 1, so it's probably safe to go a bit less than 2, but you wouldn't enjoy it. Baking soda is 9, and milk of magnesia is 10...both of which are safe to consume in small quantities, but I wouldn't want to serve a dish in that range.

So...I'd probably stick with 2-9 as an acceptable safe range. As far as taste goes, we're not really wired to enjoy the taste of alkaline. Between 2 and 7 (acid to neutral) is the tasty range.