一塩基酸/塩基滴定中の溶液組成

During titration of acetic acid with sodium hydroxide, the weak acid is gradually converted to its conjugate base, sodium acetate, forming a buffer solution. The alpha values track this composition change: initially, alpha-0 (acetic acid) is 0.987 and alpha-1 (acetate ions) is 0.013. As titration proceeds, alpha-0 decreases while alpha-1 increases, reaching equal concentrations at the half-equivalence point where pH equals pKa.

Alpha values represent the relative equilibrium concentration or fractional composition of species in a buffer solution during titration. Alpha-0 (α0) indicates the fraction of undissociated weak acid, while alpha-1 (α1) indicates the fraction of conjugate base. These values are expressed as decimals or percentages, allowing visualization of how solution composition changes throughout the titration process.

At the half-equivalence point, the pH of the solution equals the pKa of the weak acid. At this critical point, the concentrations of acetic acid and acetate ions are equal, with alpha-0 and alpha-1 both equal to 0.5. This relationship between pH and pKa makes the half-equivalence point essential for understanding buffer behavior and predicting solution pH during titration.

At the equivalence point, the titration is complete and nearly all acetic acid has been converted to acetate ions. The alpha-0 value drops to nearly zero, while alpha-1 approaches unity (1.0), indicating the solution is predominantly sodium acetate. This conjugate base solution exhibits basic properties due to hydrolysis of acetate ions.

As sodium hydroxide is added to acetic acid, a buffer forms because both the weak acid (acetic acid) and its conjugate base (acetate ions) are present in solution. The buffer composition changes continuously: initially dominated by acetic acid with trace acetate, then becoming increasingly acetate-rich. This coexistence of weak acid and conjugate base maintains relatively stable pH across much of the titration curve.

Before any sodium hydroxide is added, the alpha-0 value is 0.987, meaning 98.7% of the acetic acid remains undissociated. Only 1.3% of the acetic acid has dissociated into acetate ions and hydrogen ions. This initial composition reflects the weak nature of acetic acid and its limited tendency to ionize in aqueous solution.

As titration proceeds, alpha-0 (acetic acid fraction) continuously decreases from 0.987 toward zero, while alpha-1 (acetate fraction) increases from 0.013 toward unity. The rate of change accelerates near the equivalence point. This inverse relationship between the two alpha values reflects the conversion of weak acid to conjugate base and is fundamental to understanding buffer capacity and solution composition changes.