8.5
Titulaciones conductométricas: titulaciones ácido-base débil y ácido-base fuerte débil
Cuando un ácido débil como el ácido acético se titula frente a una base fuerte como el hidróxido de sodio, la conductancia inicial es relativamente ba…
Considera la titulación de un ácido débil, el ácido acético, frente a una base fuerte, NaOH. Inicialmente, la conductancia del ácido acético es baja debido a su débil disociación.
Al añadir NaOH, se produce el acetato de sodio altamente ionizado, aumentando la conductancia.
Una vez neutralizado todo el ácido, cualquier base adicional introduce iones OH− de rápido movimiento, lo que provoca un aumento brusco de la conductancia.
El gráfico de conductancia en función del volumen de la base muestra dos líneas que se intersectan, con el punto de intersección indicando el punto final.
Por el contrario, cuando un ácido fuerte, HCl, se titula contra una base débil, NH4OH, la conductancia disminuye inicialmente debido a la sustitución de iones H3O+ de rápido movimiento por los iones NH4+ de la base, que se desplazan más lentamente.
La conductancia disminuye hasta la finalización de la neutralización ácida, tras lo cual no hay un cambio significativo en la conductancia atribuido a la débil disociación de la base.
El punto final de la titulación se determina identificando el punto de intersección de dos rectas en el gráfico de conductancia.
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.