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We carried out an experiment to investigate the effect of temperature on the acid carbonate equilibrium in carbonated water. We tested 5 different temperatures (room temp, 2 above and 2 below) by heating carbonated water in a closed test tube. After leaving the test tube for 10 minutes in the water bath, we opened it and quickly recorded the temperature and pH of the solution using probes.

We got the results that as temperature increases the pH decreased (so the solution got more acidic) and vice versa, however published results show that the pH should have increased for an increase in temperature. I understand this is because the increase in temperature decreases the solubility of carbon dioxide, decreasing the amount of carbonic acid and hydronium ions in solution and therefore increasing the pH.

The entire system should have shifted as per the equation:$2CO_{2(g)}+3H_2O_{(l)}\rightleftharpoons CO_{2(aq)}+2H_3O^{+}_{(aq)}+CO^{2-} _{3(aq)}$

I originally thought that the opposite trend in my data was due to the endothermic nature of the ionisation of carbonic acid making the system overall endothermic, but published data indicates that the system is overall exothermic.

It would be great if anyone had any ideas about how to explain the opposite shift in equilibrium that our results show.

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The experimental design is unfortunately not robust. Certainly, the pH of pure water decreases with temperature (slowly) but the biggest source of error is most like the pH probe. If you are using a pH-meter that does not have a built in temperature compensation, the pH readings are not reliable at any temperature except the one where it was calibrated. Secondly, pH meters operate in a limited temperature range and the pH calibration buffers are not meant for different temperature.

Secondly, pH probes have a response time, i.e., it take a certain amount of time to produce a stable reading, usually a minute or less. Since you mentioned "quickly recorded", this is another source of error.

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  • $\begingroup$ Would the issues with the pH probes have made the results skewed enough to go completely in reverse from what was expected? Because we tested three trials for each temperatures and they all matched up, and the overall data made what would have been the correct trend except going in the opposite direction $\endgroup$ Dec 9, 2022 at 20:10
  • $\begingroup$ High temperature does reduce pH because Kw of water is a function of temperature. You have to do controls as well. Measure pH of pure water exactly the same way. $\endgroup$
    – AChem
    Dec 9, 2022 at 21:20
  • $\begingroup$ If the data are reproducible, you did measure something; the question is What? The easy possibilities are that the system is not equilibrated, reversing the order might detect that, the closed tube is bothersome. Try to find a way to work at constant pressure. The prepared 4 and 7 buffers have the temperature pH on the bottle; calibrate at the temperature and do not use the ATC probe. Record the millivolt values to ensure the probe is responding properly. Use a combo electrode with a flowing junction not a gel filled. you disagreed with the lit. find out why! $\endgroup$
    – jimchmst
    Dec 10, 2022 at 4:47

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