I was preparing a compound by co-precipitation method. After reaching ph-11, I continue adding ammonia to it, but ph of the solution starts decreasing. Can anybody please explain this phenomenon?
Dear Kundan
The answer is 'YES' the PH of solution be reversible.
Ammonia is a weak base, which means that only a part of NH3 will be ionized and not all. Weak bases will have a higher H+ concentration because they are less completely protonated than stronger bases and therefore, more hydrogen ions remain in the solution. So the more water you add to dilute the NH3 the more "free" protons will be in the solution-->[H+] increases-->pH decreases, but only to a certain point, obviously, diluting a base won’t make it acidic.
More simple reason (though not based on pH): let’s say we have a basic solution and we start diluting it. After a certain amount of H2O the excess concentration of OH- created by the base will be so tiny that we can simply ignore it. So in theory an infinitely diluted solution will ALWAYS BE NEUTRAL (pH=7 because in pure water pH=7), that means in an acidic solution diluting makes the pH to increase, in a basic solution diluting makes the pH to decrease.
[Side note because some other answerer said that the pH change will be 2 because pH=-log[H+], this is NOT TRUE in this case:
NH3 + H2O NH4+ + OH-
So this is a reversible reaction with a reaction constant of 1.75*10^-5 in the direction of forming NH4+.
This is the so called base dissociation constant: Kb=[NH4+][OH-]/[NH3] (we don't need the water's concentration because it is constant). So we dont calculate the pH/pOH of weak bases/acids with the "general" formula. I'll write down one example that can be applied here:
Kb=[OH-]^2/[NH3]-[OH-], you can use this ONLY IF: Kb>10^-9 and Kb*[NH3]>10^-12 and [NH3]>10^-6. You can try to calculate 2 pHs one with [NH3]=0.1 and the other with [NH3]=10^-3 (100times diluted),
Regards,
Prem Baboo
sir, adding ammonia to water should create OH- ions rather than H+ ions. If, I am wrong please clarify me.
Dear Kundan
You are right ,But I am explaining simply PH in terms of OH - & H + ions.
Regards,
Prem Baboo
@ MIchael pach: sir i was increasing the ph of the solution by adding ammonia, upto ph 11 it increases but on further addition of ammonia the ph starts decreasing. Please explain
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