Identifying aspirin (acetylsalicylic acid) in a tablet using a series of chemical reactions can demonstrate the functional properties and reactivity of certain minerals and compounds in the presence of other reagents. The procedure you've described involves using sodium hydroxide (NaOH), hydrochloric acid (HCl), and ferric chloride (FeCl₃), and we can break down the functions and expected reactions step by step.
Step 1: Reaction with Sodium Hydroxide (NaOH)
Function: Sodium hydroxide is a strong base that can cause hydrolysis of the acetylsalicylic acid in aspirin to produce salicylic acid.
Reaction: When aspirin is mixed with NaOH, the ester bond in acetylsalicylic acid may be hydrolyzed, forming salicylic acid and sodium acetate:
\[ \text{C}_9\text{H}_8\text{O}_4 + \text{NaOH} \rightarrow \text{C}_7\text{H}_6\text{O}_3 + \text{CH}_3\text{COONa} \]
- Aspirin (acetylsalicylic acid) is converted into salicylic acid.
- Sodium acetate is also produced.
Salicylic acid can be detected through its characteristic properties, such as its ability to form complexes with ferric ions.
Step 2: Reaction with Hydrochloric Acid (HCl)
Function: Hydrochloric acid acts to neutralize any remaining sodium hydroxide, thus restoring the acidic environment.
Reaction: Upon adding HCl, the reaction will neutralize the sodium hydroxide in solution, converting the sodium acetate back into acetic acid:
\[ \text{CH}_3\text{COONa} + \text{HCl} \rightarrow \text{CH}_3\text{COOH} + \text{NaCl} \]
There are no direct changes to the salicylic acid at this step, but it ensures that the reaction environment is appropriately acidic for the next step.
Step 3: Addition of Ferric Chloride (FeCl₃)
Function: Ferric chloride is used to test for the presence of phenolic compounds, specifically salicylic acid in this case.
Reaction: When salicylic acid is mixed with FeCl₃, it forms a complex, which is typically a reddish-violet color due to the formation of the phenolate ion:
\[ \text{C}_7\text{H}_6\text{O}_3 + \text{FeCl}_3 \rightarrow \text{C}_7\text{H}_5\text{O}_3\text{Fe}^{3+} + 3\text{Cl}^- \]
This color change indicates the presence of salicylic acid, confirming that aerobically, the aspirin in the tablet has been successfully identified through the reaction chain.
Summary
- NaOH hydrolyzes aspirin into salicylic acid and sodium acetate.
- HCl neutralizes the leftover base, ensuring the solution is acidic.
- FeCl₃ tests for the presence of salicylic acid by forming a colored complex, indicating a positive test for aspirin.
This series of reactions demonstrates the chemical properties of aspirin and its hydrolysis products, showcasing the utility of these reagents in qualitative analysis.