Changes in pH when adding substances other than acids or bases 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.
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.