8.5
導電滴定:強酸弱塩基および弱酸強塩基滴定
酢酸のような弱酸を水酸化ナトリウムのような強塩基に対して滴定すると、初期の伝導率は酢酸の弱解離により比較的低くなります。しかし、水酸化ナトリウムが溶液に加えられると、酢酸と反応して高度にイオン化された酢酸ナトリウムが生成され、これが導電率の増加を引き起こします。すべての酢酸が中和されると、追加の水酸…
弱酸である酢酸を強塩基であるNaOHと滴定することを考えてみましょう。初期には、酢酸の伝導率は解離が弱いため低いです。
NaOHが加わると、高度にイオン化された酢酸ナトリウムが生成され、コンダクタンスが増加します。
すべての酸が中和されると、追加の塩基は高速移動するOH− イオンを導入し、伝導率が急激に増加します。
底面の体積に対する伝導率のプロットには、2本の交差線が示され、交点が終点を示しています。
逆に、強酸HClを弱塩基NH4OHに対して滴定すると、速い移動するH3O+ イオンが塩基の遅い移動するNH4+ イオンに置き換わるため、導電率は最初に低下します。
伝導率は酸中和が完了するまで減少し、その後は塩基の弱解離による伝導度の有意な変化はありません。
滴定の終点は、伝導図上の2本の線の交点を特定することで決定されます。
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Q1: Why does conductance increase when a weak acid is titrated with a strong base?
When acetic acid is titrated with NaOH, the initial conductance is low due to weak dissociation of acetic acid. As NaOH is added, highly ionized sodium acetate forms, significantly increasing conductance. After neutralization, excess OH− ions further boost conductance sharply due to their high mobility and fast movement through the solution.
Q2: What happens to conductance during a strong acid-weak base titration?
When HCl is titrated against NH4OH, conductance initially decreases because fast-moving H3O+ ions are replaced by slower-moving NH4+ ions from the weak base. This decline continues until neutralization is complete. After that, conductance remains relatively stable due to the weak dissociation of the base producing few additional mobile ions.
Q3: How is the endpoint determined in conductometric titrations?
The endpoint in conductometric titrations is identified by plotting conductance against the volume of titrant added. Two intersecting lines appear on the graph, and their intersection point indicates the equivalence point where the titration is complete. This graphical method provides a clear visual marker for determining when neutralization occurs.
Q4: Why do fast-moving and slow-moving ions affect conductance differently?
Ions have different mobilities based on their size and charge. Fast-moving ions like H3O+ and OH− contribute more to conductance than slower ions like NH4+. When fast ions are replaced by slower ones during titration, overall conductance decreases, creating the characteristic curve shape observed in strong acid-weak base systems.
Q5: What is the role of sodium acetate in weak acid-strong base titrations?
Sodium acetate is the product formed when acetic acid reacts with NaOH during titration. Unlike acetic acid, sodium acetate is highly ionized in solution, producing mobile ions that significantly increase conductance. This sharp increase in conductance after the equivalence point helps identify the titration endpoint clearly on the conductance plot.
Q6: How do conductometric titrations differ between strong acid-weak base and weak acid-strong base systems?
In weak acid-strong base titrations, conductance increases after the endpoint due to excess OH− ions. In strong acid-weak base titrations, conductance decreases until neutralization, then plateaus because the weak base produces few mobile ions. Both systems show two intersecting lines on conductance plots that mark the equivalence point.
Q7: Why is conductance initially low when titrating acetic acid?
Acetic acid is a weak acid that only partially dissociates into ions in solution. Since conductance depends on the concentration of mobile ions, the low degree of dissociation results in low initial conductance. This changes dramatically when the strong base NaOH is added and produces highly ionized sodium acetate with mobile ions.