I\'m performing an experiment using photometric analysis to determine the acetyl
ID: 510960 • Letter: I
Question
I'm performing an experiment using photometric analysis to determine the acetylsalicylic acid content of a commercial aspirin tablet. In the procedure, it describes this step: (I assume this is relevant)
b. Zero the spectrophotometer using the iron (III) chloride solution as the blank.
Now, when confronted with this question:
What is the blank used in this experiment? Why is that used as the blank?
My guess is that the ferric chloride is the blank, but I'm not really sure what that means and why it is used that way.
Explanation / Answer
Determining the concentration of acetyl salicylic acid in aspirin sample by spectrophotometer:
The highly colored complex is formed between acetylsalicylic acid (aspirin) and iron(III) chloride enables us to determine colorimetrically the specific amount of acetylsalicylic acid present in a tablet. Since the intensity of the color formed is directly related to the amount of acetylsalicylic acid present, a series of solutions, each with a different concentration of acetylsalicylic acid, can be prepared and the absorbance of each solution measured. A calibration curve is then constructed and from this curve the amount of acetylsalicylic acid in a commercial aspirin product can be determined.
The complex ion is formed in two steps.
First the acetylsalicylic acid is reacted with sodium hydroxide to form the salicylate dianion. Then the addition of acidified iron(III) ion produces the violet tetraaquosalicylatroiron (III) complex.
Since, FeCl3 is pale yellow in colour it will show some absorbance. FeCl3 react with the acetylsaclicylic acid to give voilet coluored complex. Unreacted FeCl3 will show some absorbance. Thus a blank of FeCl3 is run so to deduct the absorbance shown by unreacted FeCl3 from the sample solutions.