It is classed as an acid on the PH scale, which categorizes substances to PH neutral. It will be alkaline to anything that is more acidic than it, on the PH scale. As stomach (hydrochloric) acid, which would be diluted to various extents depending on stomach content, is higher than phosphoric acid, it means that phosphoric acid is alkaline to hydrochloric (but still technically an acid). And I'm not saying coca-cola is good by any means, but if one were to drink it, they would seriously extend the health of their teeth by neutralizing the acid with something like baking soda or milk of magnesium. Sorry to get so nerdy. I don't get that. Generally acids are reactive as they are hydrogen donating and bases are hydrogen acceptors. Unless one acid was exceptionally weak and the other exceptionally strong, mixing them wouldn't necessarily mean they would react. Like I said, sorry to get nerdy.