3.6
一塩基酸/塩基滴定中の溶液組成
弱酸を強塩基で滴定すると、水と酸の共役塩基が示す。たとえば、酢酸を水酸化ナトリウムで滴定すると、水と酢酸ナトリウムが示す。酢酸と酢酸ナトリウムの溶液は緩衝液を構成し、滴定のさまざまな段階での相対濃度は、弱酸とその共役塩基のパーセンテージを表すα値で示されます。
α0 と α1 の値は、それぞれ酢酸と…
酢酸を水酸化ナトリウムで滴定すると、酢酸ナトリウムと水が生成され、緩衝液が得られることを思い出してください。
滴定のさまざまなポイントにおける緩衝液中の異なる種の相対濃度は、アルファ値によって示されます。
曲線は酢酸と水酸化ナトリウムの滴定を表し、α-ノットとα-ONとラベル付けされた直線は、それぞれ酢酸イオンと酢酸イオンの相対平衡濃度を示しています。
最初に、0.987のαノット値は、酢酸の98.7%が解離しておらず、残りの1.3%が酢酸イオンを構成していることを示唆しています。
交差点または半等価点では、pHはpKaに等しくなります。したがって、溶液は等濃度の酢酸イオンと酢酸イオンを有する。
滴定が進むと、酢酸α-ノットの濃度は当量点でほぼゼロに減少します。あるいは、酢酸イオンであるα-ONの濃度が増加し、統一に近づきます。
Q1: What happens to acetic acid and acetate ions during titration with sodium hydroxide?
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.
Q2: What do alpha values represent in an acid-base titration?
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.
Q3: Why is the half-equivalence point significant in weak acid titrations?
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.
Q4: What is the composition of the solution at the equivalence point?
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.
Q5: How does the buffer solution form during the titration of acetic acid?
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.
Q6: What percentage of acetic acid remains undissociated before titration begins?
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.
Q7: How do alpha values change as the titration progresses toward equivalence?
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.