3.11
Composition of Polyprotic Acid Solutions as a Function of pH
Polyprotic acids of the type H2M constitute two ionizable protons. As a result, on titration with a base, they exhibit two equivalence points in the t…
On titration with sodium hydroxide, polyprotic acids like maleic acid with two ionizable protons demonstrate two equivalence points on the titration curve.
During the titration, the different species in the solution are H2M, HM−, and M2−, whose relative concentration is represented by alpha-knot, alpha-one, and alpha-two, respectively.
The change in concentration of the species is obtained by plotting the alpha values as a function of the base volume.
Initially, the alpha-knot is 0.7, and alpha-one is 0.3, suggesting 70% H2M and 30% HM−. As the base is added, pH increases with alpha-knot decreasing to nearly zero at the first equivalence point. Simultaneously, the fraction of HM− increases and approaches unity.
On further addition of the base, while the alpha-one decreases to near zero, the fraction of M2−, alpha-two, increases to around unity at the second equivalence point.
The fraction of different species depends only on the pH and is independent of the total concentration of the solution.
Q1: What are the three species present during polyprotic acid titration?
During titration of polyprotic acids like maleic acid with sodium hydroxide, three species coexist: H2M (fully protonated), HM− (singly deprotonated), and M2− (fully deprotonated). Their relative concentrations change as pH increases, with each species dominating at different stages of the titration curve.
Q2: How do alpha values represent species composition in polyprotic acid titrations?
Alpha values (α0, α1, α2) represent the fraction of each species as a decimal. Initially, α0 = 0.7 means 70% H2M and α1 = 0.3 means 30% HM−. As base is added, these fractions shift, with each alpha approaching unity at its corresponding equivalence point, tracking solution composition during acid base titrations.
Q3: Why do polyprotic acids produce two equivalence points?
Polyprotic acids have two ionizable protons, so they release protons in two distinct steps. Each deprotonation event produces an equivalence point on the titration curve. At the first equivalence point, H2M converts to HM−; at the second, HM− converts to M2−.
Q4: What happens to alpha-one at the first equivalence point?
At the first equivalence point, alpha-one (α1), representing the fraction of HM−, reaches its maximum and approaches unity (approximately 1.0). Simultaneously, alpha-knot (α0) decreases to nearly zero, indicating complete conversion of H2M to the intermediate HM− species during the titration process.
Q5: How does pH affect the distribution of polyprotic acid species?
The fraction of each species depends exclusively on pH and is independent of total acid concentration. As pH increases during titration, alpha values shift predictably: H2M dominates at low pH, HM− at intermediate pH, and M2− at high pH, following the acid's ionization constants.
Q6: What occurs to alpha-two between the two equivalence points?
Between the first and second equivalence points, alpha-two (α2), representing M2− fraction, increases progressively. At the second equivalence point, α2 approaches unity while α1 decreases to near zero, indicating nearly complete conversion of HM− to the fully deprotonated M2− form.
Q7: How does the alpha plot reveal changes in polyprotic acid composition?
Plotting alpha values against base volume creates a visual map of species distribution throughout titration of polyprotic acids with a strong base. The curves show how each species rises and falls, with crossover points indicating where species concentrations are equal and equivalence points marking maximum conversion.