Key Features of Acid-Base Titration
- Picture yourself in a laboratory, holding a burette filled with a base and slowly releasing it into an acidic solution.
- Each drop causes a subtle change in pH, and you’re closely watching for that pivotal moment when the acid and base perfectly neutralize each other.
This is the equivalence point, a cornerstone concept in acid-base titrations.
The Equivalence Point: Stoichiometric Perfection
Definition
Equivalence point
The equivalence point in a titration is the precise moment when the amount of base added is chemically equivalent to the amount of acid initially present.
At this stage, the acid and base have reacted in exact stoichiometric proportions as dictated by the balanced chemical equation for the neutralization reaction.
Example
Consider the reaction between hydrochloric acid (HCl) and sodium hydroxide (NaOH):
- One mole of HCl reacts with one mole of NaOH.
- At the equivalence point, all the HCl has reacted with NaOH, leaving only water and the neutral salt, NaCl.
Does the pH Always Equal 7 at the Equivalence Point?
The pH at the equivalence point depends on the strengths of the acid and base involved:
- Strong Acid + Strong Base: The equivalence point occurs at pH 7 because the resulting solution contains only water and a neutral salt.
- Strong Acid + Weak Base: The equivalence point is acidic (pH< 7) because the conjugate acid of the weak base hydrolyzes in water, releasing H
- Weak Acid + Strong Base: The equivalence point is basic (pH >7) because the conjugate base of the weak acid hydrolyzes in water, releasing OH
- Weak Acid + Weak Base: The equivalence point can be near neutral, but the exact pH depends on the relative strengths of the acid and base.
Tip
- The equivalence point is distinct from the end point, which is the stage where an indicator changes color during titration.
- A suitable indicator is chosen so that its end point closely aligns with the equivalence point.
Note
For SL, only pH curves for strong acids and bases need to be studied. pH curves involving weak acids and bases are covered in more detail in R3.1.13 (Higher Level Only) later on.
How pH Changes During a Titration
The pH variation during titration depends on the acid and base combination. Let’s examine the general trends for different pairings of strong and weak acids and bases.
Strong Acid and Strong Base
- When titrating a strong acid (e.g., HCl) with a strong base (e.g., NaOH), the pH starts very low (around 1 for a 0.1 M solution of HCl).
- As the base is added, the pH rises gradually at first but increases sharply as you approach the equivalence point.
- After the equivalence point, the pH levels off at a high value (around 13 for excess NaOH).
- At the equivalence point, the pH is exactly 7 because the only species in the solution are water and a neutral salt.
pH curve for the addition of a strong base to a strong acid:
pH curve for the addition of a strong acid to a strong base:
Reflection and Review
Theory of Knowledge
- How does the precision of titration compare to other concentration-determining methods, such as spectroscopy?
- In what contexts might one method be preferred over the other?
