What is the effect of change in pH on precipitation? I was curious as to how pH actually affects precipitation.

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    $\begingroup$ Now that's a really, really broad question... You realize there are quite a few different types of compounds that can precipitate, right? $\endgroup$ – Ivan Neretin Feb 10 '17 at 15:09
  • $\begingroup$ What compounds are in your solution and what are you trying to / not to precipitate? $\endgroup$ – airhuff Feb 10 '17 at 15:56

It actually has multiple effects.

First, most of the precipitates contain ions that are either acids or bases. For example, if you look at CaCO3, and decrease pH, the carbonate ion might get protonated:

$$\ce{CO3^2- + 2H+ -> H2CO3 -> H2O + CO2 ^}$$

So what you will see is that the precipitate is dissolved. It can also go the other way. If you have calcium-bicarbonate solution, and increase the pH, it might get deprotonated and precipitate:

$$\ce{HCO3- -> H+ + CO3^2- ->[Ca^2+] CaCO3 v}$$

Note that most of the cations can have similar reactions, but complexation might also play an important role. All in all, things can get really complicated, and this is why it is an useful test in determining compounds.

Second, the surface charge might also play a role. For example proteins can be denaturated — precipitated — by acids, because their surface charge, hence their solvataion state changes with adding more acid to it, making the solution instable. This is a whole research area in the chemistry of colloids.

Lastly there is an effect on ionic strength and ion activities. This is the most complex of all, and little is known about the exact nature of this effect.


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