Indirect determination:

By determining the weight of the filler on the dried filter paper you can calculate the weight of aspirin in each tablet:

weight of acetylsalicylic acid (ie total weight of tablets - filler weight)

number of tablets

How does this compare to the manufacturer's statement?

Think about the possible reasons for any differences observed.

2. Direct determination:

Aspirin is acetylsalicylic acid and must be hydrolysed to salicylic acid before it can be assayed. Salicylic acid is a phenol and can be measured directly because it reacts with Trinder's reagent (Fe(NO3)3 in HCl) to give a coloured solution that can be determined in a spectrophotometer at 525nm.

This reaction can be used on the ethanol filtrates to estimate the amount of acetylsalicylic acid in a single tablet.

1. Dilute filtrates 1 and 2 with water to 1 in 100 (otherwise they will be offscale on the spectrophotometer).

2. Place 1ml of the diluted filtrates 1 and 2, the undiluted filtrates 3 and 4, and a blank of 1ml of water into separate labelled glass test tubes.

3. Add 3ml Trinder's reagent and incubate in the 80°C water bath for 15 minutes.

4. Cool the tubes (either in ice or under running cold water) and read the absorbance at 525nm. If your readings are too high or too low you might need to rethink your dilution factors.

5. Use the Beer-Lambert law to calculate the concentration of salicylic acid in your filtrates (don't forget the various dilutions you have made!). 525 = 949 L mol-1 cm-1

6. From these concentrations, use the RMM of acetylsalicylic acid to convert this to a total weight in your filtrates, and hence per tablet.

7. How does this compare to a) the claimed content and b) the indirect determination?