These two weak acids have similar disassociation constants in water. A low Ka indicates a relatively small number of acid molecules will form an anion stabilized by water. Two weak acids with similar disassociation constants will coexist in a solution without interacting. If two weak acids have very different Ka then there can be a reaction. In this case nothing would happen. For more generalized information search for mixture of weak acids.

Carbonic Acid Ka = 4.3 x 10-7 = [H3O+][HCo3-] / [H2C03]

Citric Acid Ka = 1.8 x 10-6 [H3C6H5O7] + 3 [H2O] <=> [C6H5O73-] + 3 [H3O+]

These two weak acids have similar disassociation constants in water. A low Ka indicates a relatively small number of acid molecules will form an anion stabilized by water. Two weak acids with similar disassociation constants will coexist in a solution without interacting. If two weak acids have very different Ka then there can be a reaction. In this case nothing would happen. For more generalized information search for mixture of weak acids.

Carbonic Acid

$$\ce{H2CO3 + H2O <=> HCO3- + H3O+}$$ $$K_a = 4.3 \times 10^{-7} = \frac{[\ce{H3O+}][\ce{HCO3^-}]}{[\ce{H2CO3}]}$$

Citric Acid

$$\ce{H3C6H5O7 + H2O <=> H2C6H5O7- + H3O+}$$ $$K_a = 1.8 \times 10^{-6} = \frac{[\ce{H3O+}][\ce{H2C6H5O7^-}]}{[\ce{H3C6H5O7}]}$$