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If I start out with 100 mM acetic acid, what substance (to a final concentration of 1 M) could I add to change the pH considerably? I was thinking of NaCl or ethanol or acetone. Substances added should not be acids or bases, i.e. a 1 M solution of them should have a pH close to neutral. I'm asking because I am trying to create a demonstration on how pH calculations are just approximations under non-ideal conditions.

The acetic acid should not undergo an chemical reaction (other than acid/base). I would use a pH meter to monitor the pH. I would have a control where I increase the volume to match the experiment where I add a substance that influences the pH (or dissolve the substance in 100 mM acetic acid to keep the acetic acid concentration constant).

I am looking for changes in pH based on non-ideal behavior. The concentration of acetic acid should remain the same, and there should not be any neutralization reaction. In other words, the calculate pH should stay at 2.87 (assuming ideal conditions) while the measured pH would change.

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    $\begingroup$ This is an interesting question. The easiest way is to the change the properties of the solvent. Add something like DMSO. $\endgroup$ – Zhe Jan 4 at 14:20
  • $\begingroup$ If your initial solvent is water, the easiest to increase the concentration of aceitic acid and, therefore, increase the pH would be to somehow decrease the volume of water. Possibly look for a way to evaporate water. Distilling the solution might be tricky since the boiling point of acetic acid and water are close (118 celsius). Another way might be to use a dehydrating agent to eliminate water. The best dehydrating agent I can think of are all strong acids (sulfuric and phosphoric). However, hot aluminum oxide might also work (if it doesn't have any side reactions with aceitic acid). $\endgroup$ – Ethiopius Jan 4 at 14:41
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    $\begingroup$ @andselisk I appreciate the thoughtful comments, you are helping me to write a better question. Adding a second phase would be fine (there is a second phase already - air - and it might get me into trouble if CO2 starts dissolving in my solution). $\endgroup$ – Karsten Theis Jan 4 at 15:57
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    $\begingroup$ Here's another idea: what about adding or removing heat (i.e. showing the temperature dependency of kw)? It's not the exact scenario you ask for, but, in terms of showing that pH is based on ideal conditions, it should make the students question the validity of the kw = 1E-14 assumption. $\endgroup$ – Van Jan 4 at 20:12
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    $\begingroup$ You question sounds like you really misunderstood a lot of the science about pH. The pH is DEFINED according to the concentration of H+/OH-, so if there is a discrepancy between your prediction and reality, then your prediction is wrong. Also "substances added should not be acids or bases" does not make sense because all substances are acids or bases when compared to another substance. $\endgroup$ – SteffX Jan 5 at 13:17
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You could use an alcohol or acetone but they are slightly acidic (though much less so than acetic acid; also alcohols react to form esters). Now effects from acidic nature can't completely avoided as even a molecule as non-acidic as methane still has a pka of 50. I think an aliphatic ether such as tetrahydrofuran (THF) or diethylether would be the best substance to add as the ethers are only very slightly basic and are miscible with water.

More simply:

what substance could I add to change the pH considerably?

Why does it have to be a substance? The $K_\mathrm w = 10^{14}$ at $\pu{25^\circ C}$. If you add heat the $K_\mathrm w$ will increase and thus the pH will increase.

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Another option is addition of the neutral concentrated solution of some salt.

It will affect activity coefficients of the conjugated acids/bases.

E.g., if a phosphate buffer is used, $\ce{HPO4^2-}$ would be affected more then $\ce{H2PO4-}$.

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