8.5
电导滴定：强酸-弱碱与弱酸-强碱滴定
当弱酸（如乙酸）用强碱（如氢氧化钠）进行滴定时，由于乙酸解离程度较弱，初始电导率相对较低。然而，随着氢氧化钠加入溶液中，其与乙酸反应生成高度电离的乙酸钠，导致电导率上升。当所有乙酸被完全中和后，过量加入的氢氧化钠会引入迁移速率较快的氢氧根离子，从而引起电导率更显著的上升。以电导率对碱液体积作图，可得…
考虑用强碱 NaOH 滴定弱酸乙酸。最初，由于乙酸解离程度较弱，其电导率较低。
随着 NaOH 的加入，生成了高度电离的醋酸钠，从而增加了电导率。
当所有酸被完全中和后，任何额外加入的碱都会引入高迁移率的 OH− 离子，导致电导率急剧上升。
电导率随碱液体积变化的曲线图显示两条相交的直线，其交点指示滴定终点。
相反，当用强酸 HCl 滴定弱碱 NH4OH 时，由于酸中快速移动的 H3O+ 离子被碱中较慢移动的 NH4+ 离子所取代，电导率最初会下降。
电导率持续下降直至酸被完全中和，此后由于碱的弱解离，电导率不再发生显著变化。
滴定终点通过确定电导图上两条直线交点的位置来确定。
View the full transcript and gain access to JoVE Core videos
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.