since at all times some water is dissociating as:

[math]

\ce{H2O <=> H3O+ + OH-}

[/math]

 

Could a substance [math]XA[/math] where [math]XA[/math] is a soluble gas or solid so that:

[math]

\ce{XA <=> X+ + A-}

[/math]

when combined with water. And that it then reacts as:

[Math]

\ce{X+ + OH- <=> XOH_{(g)} or_{(s)}}

[/math]

and where:

[math]

\ce{H3O+ + A- <=> H3OA}

[/math]

either does not take place or has an extremely small equillibrium constant so that the water's Ph goes down by using up the hydroxide without any additional acid?

If so what substance would [math]XA[/math] be and if not, why not?

So basically you want a super insoluble hydroxide salt of a compound that will soak up the tiny amount of free OH- in water thus causing the pH to drop? I think the closest I can think of is ammonium nitrate. NH4NO3 will dissolve readily in water whereby the NH4+ ion will pull an OH- ion from water leaving H+ NO3- in solution and giving off some ammonia gas. The pH of 0.1 molar ammonium nitrate solution is about 5.12.

What you basically are talking about is hydrolysis and this effect is very common.

 

Two examples:

 

Na2S is a basic salt. When it is dissolved in water, the sulfide ions, S(2-), combine with H(+) from dissociated water to form HS(-) and the OH(-) remains in solution. A 1 M solution of Na2S is very alkaline, it feels slippery, almost as slippery as a 1M solution of NaOH.

 

An other example is iron (III) chloride, FeCl3, which is very acidic, when dissolved in water. Fe(3+) ions tend to attract OH(-) ions, forming ions like [Fe(OH)](2+), leaving the H(+) ions in solution. A solution of FeCl3, hence, is quite acidic. [in reality things are quite more complex with hydrolysis of Fe(3+), but for understanding the concept, this answer should be sufficient].

Thank you woelen, that's it