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I'm reading something about nickel affinity chromatography and it talks about using imidazole to affect pH. In which direction does it change the pH?

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Imidazole is an amphoteric molecule and can therefore act as an acid and a base.
imidazole (courtesy of wikipedia)
It's acidity is given with $\mathrm{p}K_a=14.5$ for the proton as $\ce{N~1}$. The basic site is the lone pair at $\ce{N~3}$. Its basicity is given through the following equilibrium: $$\ce{Im + H+ <=> IMH+}$$ with a $\mathrm{p}K_a=7.05$.
DOI: 10.1021/jo01066a039(source)
You can obtain more information about this nice little molecule on its wikipedia page.

So in conclusion, it very much depends on the solution with which you are working, which effect of imidazole takes over.

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