Hello!
An initial pH of 4,0 and an equivalence point at pH=9,1 corresponds to a titration curve for a weak acid to which strong base is added
An example would be the titration of Acetic Acid (CH₃COOH) with Sodium Hydroxide (NaOH)
CH₃COOH + NaOH → CH₃COONa + H₂O
The initial pH will be acid because only the Acetic Acid is present. In the equivalence point, there are no CH₃COOH nor NaOH present, and the only species responsible for pH is CH₃COONa. This is a weak base, as evidenced by the following reaction:
CH₃COO⁻ + H₂O ⇄ CH₃COOH + OH⁻
So, in the equivalence point, the pH will be basic. These observations help us conclude that this titration is of a weak acid with a strong base.
Have a nice day!
