The pH at the equivalence point of titration of $\ce{Na2CO3}$ solution with $\ce{HCl}$ is around 3.7, as shown in this titration curve:

enter image description here
At this point, the following reaction is completed:
$$\ce{NaHCO3 + HCl -> NaCl + H2O + CO2}$$ But considering the fact that $\ce{NaCl}$ is a neutral salt, shouldn't the pH be closer to 7? The $\ce{H2CO3}$ formed also decomposes to form $\ce{H2O + CO2}$, and shouldn't affect the pH either. Why then is the equivalence point at such a low pH value?

  • $\begingroup$ Contrary to what you say, CO2 in water does affect the pH. You need a little bit more HCl to obtain the equivalence point. $\endgroup$ – Maurice Nov 27 '20 at 14:34
  • $\begingroup$ @Maurice, Wouldn't the equivalence point be the point where the solution has been completely neutralized, and thus would only contain $\ce{NaCl + H2CO3}$? Also doesn't the $\ce{CO2}$ formed escape, and not cause a significant decrease in pH? $\endgroup$ – jerrythealpaca Nov 27 '20 at 15:40
  • $\begingroup$ The equivalence point is not the point where the solution contains $\ce{NaCl}$ and that's all. No. The equivalent point is when the solution contains $\ce{NaCl}$ plus the small amount of $\ce{CO2}$ which remains dissolved. And this amount makes the solution a bit acidic. So the pH at the equivalence point is not $7$, but a bit less. $\endgroup$ – Maurice Nov 27 '20 at 17:25
  • $\begingroup$ @Maurice, You are mixing the concept of "end-point" and "equivalence point". At the end-point there is slight excess of the acid or base. At the equivalence pt, stoichiometric amounts have reacted. $\endgroup$ – M. Farooq Nov 28 '20 at 0:45
  • $\begingroup$ @M. Farooq. OK. You are right. I should have rewritten my text. But this does not change too much my message. $\endgroup$ – Maurice Nov 28 '20 at 9:56

When the $Na_2CO_3$ is completely neutralized by the $HCl$, the solution will be saturated with $CO_2$, so the pH will be lower than 7 - think of carbonated beverages. Although the $CO_2$ bubbles out, not all of it bubbles out - not even if stirred. If you had stopped the titration at pH = 7, the pH reduction from 11 would have been partly from $HCl$ and partly from $H_2CO_3$.

  • $\begingroup$ Okay, so the low pH at the equivalence point can be attributed to just the dissolved $\ce{CO2}$, correct? $\endgroup$ – jerrythealpaca Nov 27 '20 at 16:02
  • $\begingroup$ Yes. You might expect that CO2 is all expelled, but some just loves to stay in solution. $\endgroup$ – James Gaidis Nov 28 '20 at 2:13

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