In an ester hydrolysis reaction using a dilute base what is the pH change before and after the reaction occurs. In this reaction I want to react methyl ethanoate with dilute sodium hydroxide. I want to know if the pH of the solution will change after I add the ester to the base. If it doesn't, is there a reaction I can do with esters that will result in a pH change?

  • $\begingroup$ Assuming this is in water. The pH shouldn't change much because the pKa of alkoxides in water are pretty much the same as that of water itself. $\endgroup$ Feb 1, 2016 at 23:38
  • $\begingroup$ Okay, thanks for replying. Is there some reaction that you can do with esters that will allow a change in pH? $\endgroup$ Feb 1, 2016 at 23:53
  • 1
    $\begingroup$ The pH will definitely change... Write out the mechanism to see why. $\endgroup$
    – jerepierre
    Feb 1, 2016 at 23:59
  • $\begingroup$ Just any reaction where I can see a pH change that I can use with an ester. And how would I write out the mechanism? I am a beginner with organic chem, are there any websites you could recommend to be able to write out the mechanism? $\endgroup$ Feb 2, 2016 at 0:38
  • $\begingroup$ Looking at the mechanism properly. At the start when it is only dilute NaOH, it is pH basic. But when I add the ester, it will react and form an alcohol and carboxylic acid, which is acidic. So wouldn't the pH of the final solution be lower than the start? $\endgroup$ Feb 2, 2016 at 1:14

1 Answer 1


Alkaline hydrolysis of ester:

$\ce{RCOOR' + OH- -> RCOO- + R'OH}$

Almost all organic acids are weak acids:

$\ce{RCOO- + H2O <<=> RCOOH + OH-}$

Therefore, a lot of $\ce{OH-}$ is consumed during the esterification, with the product $\ce{RCOO-}$ generating little $\ce{OH-}$.

There is a net consumption of $\ce{OH-}$, making the solution more acidic, lowering the pH.


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