I did an experiment for my chemistry class and did a control with sparkling water and the pH increased by a significant amount, but when done with sodas like cream soda, coke, or 7 Up, the pH change is so minimal that I can't say that there is a significant change in the pH level. Why do the pH level of these sodas to stay the same when they lose their fizz?
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1$\begingroup$ The sodas have other acids with pKa's lower than that of carbonic acid. $\endgroup$– MaxWJun 25, 2018 at 20:11
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In addition to fizz (carbonic acid), Coca-Cola contains phosphoric acid (for an additional tangy taste). According to this leaflet put out by Coca-Cola, a 250 mL can of Coca-Cola contains 43 mg of phosphorus. With some quick calculation, you can find that this translates to $\ce{5.5538 \times 10^{-3} M}$ $\ce{H3PO4}$, which would have a pH of approximately 2.44. Pretty acidic! The other sodas are sure to contain other acids as well.