I am using $\ce{CO2}$ for the reduction of pH of $\ce{NaOH}$ solution ($\pu{2.5g}\; \ce{NaOH} + \pu{250ml}\; \ce{H2O}$, with $\mathrm{pH}\; 13+$). I am introducing $\ce{CO2}$ with different dosages ($\ce{50cc, 100cc, 150cc, 200cc, 250cc, 300cc, 600cc, 900cc, 1200cc}$, and $\pu{1500cc}$) using syringe into $\pu{35 ml}$ solution of $\ce{NaOH}$ prepared as mentioned above. The $\mathrm{pH}$ first increases and then starts decreasing but stop at the same level of initial $\mathrm{pH}$ value of the solution (The amount of $\ce{CO2}$: $\pu{50cc}=\pu{0.03g}$). Trials run for $\pu{60 min}$ after that, the samples are preserved at $\pu{4 ^\circ C}$ and $\mathrm{pH}$ measured again after $\pu{24 h}$, but the value does not reduce.

Ideally, if everything works properly, the $\mathrm{pH}$ of the solution should decrease gradually. However, instead of $\mathrm{pH}$ reduction first it increases and then decrease at the same level, when we mix $\ce{CO2}$.

I will appreciate if someone share his or her experience or comments as to what is the possible cause of this unexpected trend and how to fix it.

  • $\begingroup$ You're trying to neutralize with gaseous CO2? Well, that's like least effective way that could be still used in practice. Also I don't see why you'd think it'll work like you added strong acid... $\endgroup$ – Mithoron Oct 10 '19 at 21:01
  • $\begingroup$ @Mithoron Well, I would at least expect pH does not increase... $\endgroup$ – Poutnik Oct 10 '19 at 21:28
  • $\begingroup$ @Poutnik And I would expect OP to point out clearly that pH rises instead of get lower. In that case he's both neutralizing it in wrong way and using wrong tool to measure pH. May be a matter of too low pH for glass electrode. $\endgroup$ – Mithoron Oct 10 '19 at 21:41
  • $\begingroup$ He did point it clearly, you may have missed it. But it is a well known issue most of glass electrodes do not behave properly at pH 12+. $\endgroup$ – Poutnik Oct 10 '19 at 21:43
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    $\begingroup$ The pH increase by 0.3 units is just an instrumental artifact of the glass electrode. $\endgroup$ – M. Farooq Oct 11 '19 at 2:10

There are a so-called acid error and an alkaline error with glass electrodes. Which means that the pH electrode shows a systematic bias when your solution is too acidic or too basic and if it contains lot of sodium ions. The value which you quote for +13 are unreliable. So if you are bubbling carbon dioxide into NaOH solution, you are indeed consuming NaOH and converting it into sodium carbonate. The Wikipedia article on glass electrode is poorly written however if you search the term "alkaline error of glass electrodes" you will hundreds of better results.

May be you should follow the pH changes spectrophotometrically using a proper indicator (dyes) in NaOH.

Also keep in mind that you shouldn't store NaOH in glass containers because glass starts to dissolve in NaOH (very slow process though)

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  • $\begingroup$ NaOH especially destroys the glass electrode. $\endgroup$ – Karl Oct 10 '19 at 23:44
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    $\begingroup$ @M. Farooq, your suggestion is appreciatable. $\endgroup$ – Engr Muhammad Aleem Oct 11 '19 at 1:26

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