Changes in pH

[sajib_mostofa]

Anyone care to explain why the pH of a basic solution decreases if it is diluted?

[Martoman]

you're adding H+ ions to it from the water. It will eventually settle at around 7 (for 25 degrees) if you pour enough in.

[sajib_mostofa]

you're adding H+ ions to it from the water. It will eventually settle at around 7 (for 25 degrees) if you pour enough in.

But if we use that same logic when we're diluting an acid, then we'd expect the pH to decrease, which doesn't happen.

[toshibaj]

When you dilute a basic solution the concentrations of all subtances (except water) will decrease. After equilibrium is re-established, the concentration of OH- ions will have decreased overall. Assuming it is 25 degrees, that means [H+] has increased, and thus pH has decreased.

Think of it using this equation (or similar):    NH₃ + H₂O ====> OH^- + NH₄⁺

If its an acidic solution its the opposite. [H+] will decrease overall, meaning pH will increase.

   NH₄⁺ + H₂O ====> H₃O⁺ + NH₃

[masonnnn]

i was pretty confident with ph before reading this, i'm confused now.

[Martoman]

lollll its not that difficult. If you dilute acid your gonna reach around ph 7 with sufficent addition. Same the other way. Think of it this way.

- You are adding H and OH proportionally, in equal amounts.

- If your solution has a high acid concentration and you dilute it alot, then the high acid concentration becomes negligeable with reference to the amount of water you put in. The concentration will depend mostly on the amount of H and OH in water. Which is equal in pure water. So it goes to 7.


Similarly with high OH. You add sufficient water and the concentration of OH becomes negligeable. The concentration that will matter is the equal proportions of H and OH. So again, it will approach and reach 7.

[masonnnn]

you're adding H+ ions to it from the water. It will eventually settle at around 7 (for 25 degrees) if you pour enough in.

But if we use that same logic when we're diluting an acid, then we'd expect the pH to decrease, which doesn't happen.

^
that confused the hell through me but yeah martoman thanks, that makes total sense and yeah, pretty simple when said straight out aha.

[Mao]

Martoman, I disagree with your logic.

Consider this: I have 1L of pH = 12 solution (0.01mol OH-, 10^-12 mol of H+)

I dilute it by a factor of 10, i.e. adding 9L of pure water at pH = 7 (9L * 10^-7 mol of H+ and OH-)

The new ion count is 0.0100001 mol OH-, 0.0000001000001 mol of H+
We know the new concentration of pH will be ~11 (at 10L, 0.01mol of OH-, 10^-11 mol of H+), from this you can see that the amount of OH- virtually have not changed, even though 0.0000000999992 mol of H+ reacted.

Thus we conclude, the addition of H+ in water hasn't changed much. The fact that there is more volume has caused the concentration to decrease, but not the addition of H+.

[Martoman]

At least my second explanation is in line with this  

[Greggler]

yeah ive always figured that there are three situations to know/understand

Strong acid - adding water, what happens?
Weak acid - adding water, what happens? and why is the change in pH less than that in a strong acid?
Base - adding water, what happens?

With acids i believe that adding water simply increases the volume, hence decreases conc. hence increases pH. Since a weak acid ionises with more water, pH change is less. Then with bases, i've just figured it the same... add water, hence diluting the OH ion, and therefore reducing pH.

Is this logic correct?

[Mao]

Spot on Greggler.

[chansthename]

while we are at it, I have done questions where they are asking whether the strong acid or weak acid changes pH more when it is diluted.

Can I have a very concise explanation that is simple, (something I can really remember)

Thanks