My book states that nitrogen-containing rings are generally referred to as bases, and that under acidic conditions they can each bind an $\ce{H+}$. However, does this increase or decrease the concentration of $\ce{OH-}$ ions in an aqueous solution and why?


Brønsted (or Brønsted–Lowry) acid-base theory says a base will accept a proton from water. You can't take a H+ ion away from H2O without 'leaving' an OH-. The pKa of piperidine is 11.22, which means it is a fairly strong base. Apparently, you are asking what happens when you add a base to an aqueous acidic solution. The pH goes up. Meaning the pOH goes down. This is just another way of saying that [OH-] increases.


What one needs to understand first when dealing with acids and bases is that:

A substance can be acidic, if its constituent molecules have the capability to either donate an $\ce{H+ (like HCl)}$ or accept an $\ce{OH- (like H3BO3)}$ from water ,ie effectively have an electron deficient center


A base is a substance whose constituent molecules can donate an $\ce{OH- (like NaOH)}$ or abstract $\ce{H+ (like RO- ) }$ from water,ie effectively have an electron rich center,Now when Nitrogen is bonded to other atoms

[ up to 3 other atoms which are not electron deficient themselves (like an $\ce{NO2 or -CF3 group}$), or capable of conjugating the lone pair of nitrogen into themselves(like in anilene) ,above which it wont be able to show basic character longer as it cant form any more than 4 bonds ]

It still retains its lone pair,therby making itself an elecrton rich species; Hence retaining its ability to donate the lone pair to an electron deficient centre or abstract an $\ce{H+}$ from water,rendering a basic medium.

Note that nowhere in any of these processes has there been the involvement of an $\ce{OH-}$ group. Although technically, while in aqueous medium compounds with nitrogen abstract a proton from water and release an $\ce{OH-}$ therby increasing the $\ce{[OH-]}$


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