WEAK BASE VERSUS STRONG ACID TITRATION

Weak base versus strong acid titration is an example of acid base titration. The titration of weak base 0.1 M ammonia that titrated with strong acid 0.1 M hydrochloric acid is shown below

At the beginning of the titration ( before the titration is started) we only have ammonia 0.1 M 100 mL. ammonia is a weak base so we can calculate the hydrogen ion concentration with the formula

When the titration is started some of ammonia reacts with hydrochloric acid to yield ammonium chloride and water, so in the Erlenmeyer flask contains ammonium chloride, ammonia, and water this means we have buffer system. As the titration proceeds the pH slowly decrease and the ratio of [NH4+]/[NH3] changes, and in the midpoint of the titration [NH4+] = [NH3], and the pH is equal to pH= 14-pKb. We calculate the pH by pH = (pKw-pKb) + log (CB/CBH+)

At the equivalence point , in the solution contains NH4Cl, because it is a salt from strong acid with weak base it will hydrolyzes partially and the pH at the equivalence will be acid, and the pH will be depends on the concentration ofNH4Cl. The formula for calculate the pH is

When hydrochloric acid is added the free H+ suppresses the ionization, and the pH of the solution is determined only with concentration excess of H+. We calculate the pH by [H+] = [ excess H+].

The indicator for this titration must have a transition range within about pH 4 to 7, so the methyl red is meet s this requirement. If phenolphthalein had been use as the indicator it would have gradually lots its color between pH 10 and 8 before the equivalence point was reach.

note :
CB       = concentartion of weak base (NH4OH)
CBH+ = concentration of the salt ( NH4Cl)