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I tried an experiment at home where I had a $\ce{CO2}$ tank pumping $\ce{CO2}$ into a solution of $\pu{28 g}$ sodium hydroxide in $\pu{200 ml}$ water. The initial $\mathrm{pH}$ was $11.24.$

I expected it to form sodium carbonate and bicarbonate, but instead, the $\mathrm{pH}$ went up to $11.5.$ I should point out that my $\mathrm{pH}$ sensor needed a few minutes of cleaning in distilled water to prevent inaccuracy because the sodium hydroxide particles stuck to it.

Is this supposed to happen, or is my $\mathrm{pH}$ sensor inaccurate? Is there another way to do it? I also found some videos online and in those videos, the $\mathrm{pH}$ went acidic instead of the $\mathrm{pH}$ of sodium carbonate which is between $8$ and $10.$

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  • $\begingroup$ The initial pH should be >14, and after adding CO2, it should drop. NaOH is very soluble in water, so it is strange that you talk of sodium hydroxide particles. $\endgroup$
    – Karsten
    Oct 26, 2021 at 1:32

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pH measurements at extreme pHs are not accurate. It is not a surprise that you find unpredicted changes in pH after bubbling carbon dioxide because the sensor is not meant for such a high concentration of NaOH. There are well known sodium errors associated with glass electrodes. The sensor is not faulty, it is not designed to be used in concentrated alkalies! The electrode starts to respond to Na ions instead.

If you are talking of "particles", sodium carbonate can precipitate in strong caustic solutions, so you can indeed see particulates of sodium carbonate depending on the concentration.

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  • $\begingroup$ Thanks for your help but do you recommend I use a better pH sensor or would that not work as well? $\endgroup$
    – Aakarsh
    Oct 26, 2021 at 2:28
  • $\begingroup$ No pH sensor will work at such a high concentration. What are you trying to do? What is your experiment about? $\endgroup$
    – AChem
    Oct 26, 2021 at 2:29
  • $\begingroup$ I am trying to add co2 to sodium hydroxide to form sodium carbonate for a science experiment. If a pH meter can't measure that high, should I just make carbonic acid, use the pH sensor, and then add sodium hydroxide and not measure it? $\endgroup$
    – Aakarsh
    Oct 26, 2021 at 2:39
  • $\begingroup$ You cannot make carbonic acid. It does not exist. If you explain more what you are trying to demonstrate in your experiment, one can think of a solution. Is this a student project or you are teaching ? $\endgroup$
    – AChem
    Oct 26, 2021 at 2:43
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    $\begingroup$ Thanks for the help! I redid it with ph strips and the ph went from 14 to 11! $\endgroup$
    – Aakarsh
    Oct 30, 2021 at 19:17
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Well, if you start with a pH of 11.24 and it jumps to 11.5, that would be more basic, not more acidic. Using some pH calculators, the pH of your initial NaOH solution should have been 14.54, NaOH being very basic. After pumping excess CO2 through it, the pH of your Na2CO3 solution should have turned out to be 10.25. For this situation, I would say you either have a faulty pH sensor, or there is some other source of error that would change the results so drastically. Most likely the former.

This is completely up to you, but if you are planning on getting a new pH sensor, I recommend Sensorex. They have a few options, including one for about \$50, or a more sensitive one for around $100. https://sensorex.com/laboratory-ph-electrodes/?hsCtaTracking=6c3ce343-9554-4fba-8b74-8d232f941f6f%7Cf9b4d806-dea3-4aff-a8e9-7df730ead108

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