Nuffield Advanced Chemistry - I am doing my A-level coursework on the mass of aspirin in different tablets. Colorimetry is one method which I have used and I am really stuck trying to calculate the mass of aspirin in my sample. I made up concentrations of 0.005, 0.004, 0.003, 0.002 and 0.001 mol dm<sup>-3</sup> in 50 ml flasks. and set up a calibration curve with absorbance on the Y axis and concentration on the X. I hydrolysed 2 samples of aspirin (tablets) and tested them both on the colorimeter. Both came out as absorbance of 2, which seems wrong as they should differ. But now I just don't know where to go with my calculations. My hydrolysed aspirin samples were made up in 250 ml volumetric flasks. However I removed 10 ml of this and then added iron(III)chloride solution to this 10 ml. I assumed that the calculations which I would need to do would be as follows: 1. Read across the graph to find a concentration, for example concentration of 0.005. 2. Multiply 0.005 by volume, which is 250ml/1000 3. Then multiply this by Mr of salicylic acid, which is 138, = 0.1725 4. Then I should x 0.1725 by 1000 to get mg. Aspirin tablets contain 300mg and this calculation is saying that contains 172.5 mg which is completely wrong- surely? I am aware I have just used an example concentration (0.005) because my hydrolysed samples did not actually work on the colorimeter, but I am not even sure if these calculations are correct?

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I am doing my A-level coursework on the mass of aspirin in different tablets. Colorimetry is one method which I have used and I am really stuck trying to calculate the mass of aspirin in my sample.

I made up concentrations of 0.005, 0.004, 0.003, 0.002 and 0.001 mol dm^-3 in 50 ml flasks. and set up a calibration curve with absorbance on the Y axis and concentration on the X. I hydrolysed 2 samples of aspirin (tablets) and tested them both on the colorimeter. Both came out as absorbance of 2, which seems wrong as they should differ. But now I just don't know where to go with my calculations.

My hydrolysed aspirin samples were made up in 250 ml volumetric flasks. However I removed 10 ml of this and then added iron(III)chloride solution to this 10 ml. I assumed that the calculations which I would need to do would be as follows:

1. Read across the graph to find a concentration, for example concentration of 0.005.

2. Multiply 0.005 by volume, which is 250ml/1000

3. Then multiply this by Mr of salicylic acid, which is 138, = 0.1725

4. Then I should x 0.1725 by 1000 to get mg.

Aspirin tablets contain 300mg and this calculation is saying that contains 172.5 mg which is completely wrong- surely? I am aware I have just used an example concentration (0.005) because my hydrolysed samples did not actually work on the colorimeter, but I am not even sure if these calculations are correct?

Corrie writes ..

I presume your calibration curve was made up using salicylic acid (2-hydroxybenzoic acid), not aspirin! An absorbance of 2 for your hydrolysed samples means they are absorbing virtually all the light and are too concentrated to get useful information from. The fact that both your samples gave the same absorbance reading strongly suggests that they are both just absorbing all the light, so the colorimeter cannot distinguish between them.

Ideally you should be aiming for an absorbance somewhere around 1, which is hopefully within the range covered by your calibration curve. So, assuming your calibration curve is OK, you need to dilute your hydrolysed aspirin sample to get a more reliable absorbance readings.

In your calculation the concentration of salicylic acid from the absorbance should give you the concentration of aspirin in your sample. Then you need to use the RMM of aspirin, not salicylic acid, to calculate the mass of aspirin!

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updated: 18 April 2007

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