16.15
Acid-Base Titration Curves
A titration curve is a plot of some solution property versus the amount of added titrant. For acid-base titrations, solution pH is a useful property t…
Acid-base titration can be performed to determine an unknown concentration of an acid using a known concentration of a base or vice versa.
When titrating an acid with a base, the titrant is gradually added to the acid, raising the pH. The pH change is plotted against the volume of the base to create a titration or pH curve.
The titration curves are S-shaped, though differences exist depending on whether the acid is strong or weak.
The pH at the start of the titration will be acidic but higher for a weak acid, assuming equal initial concentrations of acid.
The equivalence point is the point on the titration curve at which the moles of base equals the moles of acid in solution.
In the case of a strong acid, the equivalence point is reached when a strong base neutralizes all the hydronium ions produced by the strong acid; therefore, it always occurs at pH 7.
In contrast, a weak acid’s equivalence point occurs at a pH greater than 7 and is reached when all the weak acid is converted into its conjugate base.
Near the equivalence point, there is a sudden jump in the pH of the solution. However, in a weak acid titration, this rapid pH shift is not as steep as in a strong acid titration.
The pH after the equivalence point on a titration curve for both weak and strong acids gradually increases due to excess strong base.
Other features of a weak acid titration curve, but not a strong one, are the buffer region and the half-equivalence point.
When a strong base is added, a weak acid produces its conjugate base creating a buffer. The half-equivalence point lies within this buffer region.
This point is when half of the volume of base needed to reach the equivalence point has been added. Here, the concentration of weak acid equals its conjugate base, and the pH of the solution equals the pKa.
The titration curves of a strong or a weak base with a strong acid as the titrant is an inverted S-shaped, where the pH starts high and decreases with the addition of the strong acid.
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Q1: What is the equivalence point in an acid-base titration?
The equivalence point is where moles of added base equal moles of acid in solution. For strong acids titrated with strong bases, this occurs at pH 7. For weak acids, the equivalence point occurs at pH greater than 7 because the conjugate base produced hydrolyzes, making the solution basic. This point marks a drastic pH change on the titration curve.
Q2: Why do strong acid and weak acid titration curves differ?
Strong acid titration curves show a steeper pH jump at the equivalence point compared to weak acid curves. Weak acid titrations feature a buffer region and half-equivalence point where pH equals pKa, absent in strong acid titrations. Both curves are S-shaped, but weak acid curves start at higher initial pH values when concentrations are equal.
Q3: What happens in the buffer region during a weak acid titration?
When a strong base is added to a weak acid, it produces the conjugate base, creating a buffer solution. The buffer region occurs before the equivalence point and contains both unreacted weak acid and its conjugate base. At the half-equivalence point within this region, the concentration of weak acid equals its conjugate base, and pH equals the pKa.
Q4: How does pH change before reaching the equivalence point?
Before the equivalence point, pH increases gradually as titrant is added and the acid is consumed. The solution contains both unreacted acid and its conjugate base reaction product. This gradual increase contrasts with the sudden pH jump occurring at the equivalence point, where the solution transitions from acidic to neutral or basic.
Q5: What determines pH after the equivalence point in a titration?
After the equivalence point, pH is determined by the amount of excess strong base titrant added. Both strong acid and weak acid titrations behave similarly at this stage because excess base dominates the solution composition. The pH gradually increases as more titrant is added, creating the upper portion of the S-shaped curve.
Q6: Why is the initial pH different for strong and weak acids?
At equal concentrations, weak acids have higher initial pH values than strong acids because weak acids only partially ionize, producing fewer hydronium ions. Strong acids completely ionize, generating more hydronium ions and lower pH. This difference in initial pH is reflected at the start of their respective titration curves.
Q7: How are titration curves used to determine unknown acid concentrations?
Titration curves plot pH against titrant volume, allowing identification of the equivalence point where moles of base equals moles of acid. By knowing the base concentration and volume at equivalence point, you can calculate the unknown acid concentration using stoichiometry. The curve's shape also reveals whether the acid is strong or weak.