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I was conducting a titration of a saturated $\ce{Ca(OH)_2}$ solution against $\ce{HCl}$ using a pH meter (the kind with a glass electrode). At the beginning of the titration, the pH reading would drop a bit every time I added a drop of $\ce{HCl}$. However, near the equivalence point, when I added a drop of $\ce{HCl}$, the pH reading would drop rapidly, but then it would rise again at a steady pace for a few minutes. Why does the pH rise again? My teacher said that the pH meter's membrane is somehow becoming "saturated" with the solution, after being left in the solution for awhile. But what's actually going on in terms of the ions and the membrane?

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Your teacher gave you a poor answer.

You are observing three different things.

(1) The solution isn't instantly homogeneous when HCl is added. It takes a couple of seconds of stirring to mix it thoroughly.

(2) The calcium hydroxide solution initially has considerable buffer capacity. As the solution gets less basic then a small amount of HCl can change the pH by a relatively large amount. So a drop of HCl in 50 ml of 0.1 molar calcium hydroxide won't change the pH much, but a drop of HCl in a neutral solution at pH 7 would change the pH by quite a bit.

(3) The electrode and meter have a response time. So if you could move the pH electrode instantly from an acid solution to a basic solution it would take the electrode and meter themselves a few seconds to equilibrate. (The pH electrode has a hydrated silica layer that forms the boundary on the surface which takes time to equilibrate. The pH meter itself will have some RC time constant so that electrical noise is minimized.)

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  • $\begingroup$ I guess I wasn't clear about my question, but I'm mainly wondering what caused the pH to rise again. And this rising happened for a few minutes after the drop was added (and it continued until I lost patience waiting for the pH to become constant and decided to add another drop of HCl). I'm pretty sure the solution was homogeneous by then. $\endgroup$ – carbenoid Jan 16 '16 at 1:54
  • $\begingroup$ The solution mixing and the meter response time would be a few seconds at most. For changes over minutes two other thing come to mind. First you could be getting carbon dioxide form the air dissolved in the solution. Particularity problematic if you're using vigorous stirring with a mechanical stirrer. Second the pH electrode could be damaged so that it takes much too long a time to equilibrate. Such electrodes get flaky if you let the electrode dry. You should be able to test pH electrode response time by switching between buffer solutions. Expect a stable reading within 3 to 5 seconds. $\endgroup$ – MaxW Jan 16 '16 at 2:06
  • $\begingroup$ I was using a magnetic stirrer, not a mechanical stirrer, so the stirring shouldn't have been too vigorous. Also, the pH meter did have a fast response time when I calibrated it. It was also working fine before and after the equivalence point, so I'm guessing it has something to do with the pH at the equivalence point dropping very fast? $\endgroup$ – carbenoid Jan 16 '16 at 3:37
  • $\begingroup$ If the pH was drifting down slowly at a pH of about 7, then I'd strongly suspect carbon dioxide going to bicarbonate. $$ \ce{CO2(g) -> CO2(aq) }$$ $$\ce{ CO2(aq) +H2O -> HCO3^{-} + H^{+}}$$ $\endgroup$ – MaxW Jan 16 '16 at 4:08
  • $\begingroup$ The pH was rising up steadily at a pH of about 7, not drifting down. Are there any side reactions that could have caused that? $\endgroup$ – carbenoid Jan 16 '16 at 18:24

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