Understanding Back Titration: Concept, Importance, and Examples

What is Back Titration?

Back titration is an indirect titration method used when direct titration is impractical. It involves reacting the analyte with an excess known amount of a standard reagent, then titrating the unreacted excess with another standard solution to determine how much of the first reagent was consumed.

Why is Back Titration Important?

Back titration is especially useful when:

• The reaction is slow or incomplete.

• The analyte is insoluble or poorly soluble in water.

• The endpoint in direct titration is difficult to detect.

Example: Determination of Aspirin in Tablets

Aspirin (acetylsalicylic acid) is weakly acidic and only partially soluble, making direct titration inaccurate. Here's how back titration works in this case:

1. Tablet Dissolution: A known amount of aspirin is reacted with excess NaOH (a strong base), which hydrolyses the aspirin into salicylic acid and sodium acetate.

2. Reaction:

   $${\text{C}}_9{\text{H}}_8{\text{O}}_4+\text{NaOH}\to {\text{C}}_7{\text{H}}_6{\text{O}}_3+{\text{CH}}_3\text{COONa}$$

3. Titration: The excess NaOH is then titrated with standard HCl.

4. Calculation: By subtracting the unreacted NaOH from the initial amount, the amount of NaOH that reacted with aspirin is determined, allowing calculation of the aspirin content.

Other Common Back Titration

Back titration is a powerful and essential analytical method when direct titration is not feasible. It ensures accuracy in analysing substances like aspirin, calcium carbonate, and antacids. Understanding and applying this technique is crucial for academic and industrial chemistry practices.