Diet soda usually has acid as one of its ingredients ... that's how carbonated sodas of any kind create and maintain their fizz in bottles and cans. (Fountain sodas are carbonated in a different...
The pH ranges from 2.47-3.35 and our mouths about 6.2 to 7.0. At a PH of 5.2 to 5.5 or below the acid begins to dissolve the hard enamel of our teeth. Phosphoric and citric acids are found in soda