Strong Titrant–Strong Titrant Curves in Acid-Base Titrations

Strong Titrant–Strong Titrant Curves

Introduction to Titration Curves

Titration curves are graphical representations of the pH change during the addition of a titrant to an analyte. When both the titrant and the analyte are strong acids or bases, the resulting titration curve displays characteristic features that are important for understanding acid-base reactions.

• Strong titrant: A titrant that is a strong acid or strong base (e.g., HCl, KOH).

• Strong analyte: An analyte that is a strong acid or strong base.

• These titrations are used to determine the concentration of an unknown strong acid or base.

Key Features of Strong Acid–Strong Base Titration Curves

• Sharp Equivalence Point: The pH changes very rapidly near the equivalence point, resulting in a steep vertical region on the curve.

• Equivalence Point pH: For strong acid–strong base titrations, the equivalence point occurs at pH 7, because the resulting solution contains only water and a neutral salt.

• Initial and Final pH:

  • When titrating a strong acid with a strong base, the initial pH is low (acidic).

  • When titrating a strong base with a strong acid, the initial pH is high (basic).

Example Titration Curves

• Strong Acid (e.g., HCl) with Strong Base (e.g., KOH):

  • pH starts low, rises sharply at equivalence point, then levels off at a high pH.

• Strong Base (e.g., KOH) with Strong Acid (e.g., HCl):

  • pH starts high, drops sharply at equivalence point, then levels off at a low pH.

Important Equations

• Neutralization Reaction:

$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$

• Equivalence Point Calculation:

$M_1V_1 = M_2V_2$

Where $M_1$ and $V_1$ are the molarity and volume of the acid, and $M_2$ and $V_2$ are the molarity and volume of the base.

Example Problem

Question: Consider the titration of 100 mL of 0.500 M HBr solution with 120 mL of 0.450 M KOH solution. Which species would be in excess?

• Solution: Calculate moles of each reactant:

$\text{Moles of HBr} = 0.100~\text{L} \times 0.500~\text{mol/L} = 0.050~\text{mol}$ $\text{Moles of KOH} = 0.120~\text{L} \times 0.450~\text{mol/L} = 0.054~\text{mol}$

• KOH is in excess, so the solution will be basic after the reaction.

Practice Question

Which combination would give a pH = 7 at the equivalence point?

• Answer: HCl and NaOH (strong acid and strong base)

Summary Table: Strong Acid–Strong Base Titration Features

Type of Titration	Initial pH	Equivalence Point pH	Curve Shape
Strong Acid + Strong Base	Low	7	Sharp rise at equivalence
Strong Base + Strong Acid	High	7	Sharp drop at equivalence

Additional info: In real laboratory settings, indicators such as phenolphthalein or bromothymol blue are used to visually detect the equivalence point in strong acid–strong base titrations.