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Acid-base titration curve: An acid-base titration curve is a plot of pH vs the volume of titrant supplied.

HCl is a strong acid and ethylenediamine is a weak base. Two nitrogen atoms are present and thus ethylenediamine can take up two protons. Thus it will have two equivalence points. The graph is given below. The important characteristics of the graph are:

• Before neutralization, since the solution contains only base the pH of the solution should be greater than 7.
• After addition of HCl, the pH decreases. The almost linear part represents the equilibrium formed when ethylenediamine takes up one proton. The first neutralization point is marked at pH 9.93.
• The pH further decreases and we find another linear region. This region represents the equilibrium after ethylenediamine takes up the second proton.
• After complete neutralization, the solution contains only H⁺ions. Thus the pH further decreases with addition of more HCl.

Hence, the above picture depicts the qualitative curve for the titration of ethylenediamine.