Question

When someone perform an acid-alkaline titration between two strong acid and alkaline.
At the verge of end pt, why the pH value can be vary sharply?(drop from 9 to 5 when one more drop added) That's mean the concentration of H+ increase by 1000 times. Why cause this result?

Answer 1

When you are at the equivalence point the pH is about 7 (depending on the acid/base), which means the hydronium ion concentration is 10^-7.
If you add an acid or base it will move away from that concentration by orders of magnitude, which causes the sharp up or down (depending what is being tritrated). After a few drops have been added in, you are only doubling or adding to the ion concentration at the same magnitude instead of changing the order of magnitude.

Addition of a buffer solution might help maintain this pH at a roughly constant pH, approximately plus or minus 0.2

Answer 2

the amount of each drop of HCl( for tritrating NaOH) is constant
say the original moles of H+ ions in the NaOH solution is 10^-13 mol.
say a drop of HCl contains 10^-10 mol of H+ions, why the concentration of H+ in the NaOH will increase rapidly especially in pH 9-pH4 but not in the beginning of the experiment?