A tablet weighing 1000 mg was analysed for aspirin, caffeine and paracetamol con

Question

A tablet weighing 1000 mg was analysed for aspirin, caffeine and paracetamol content. The tablet was crushed and extracted by a solvent mixture (methanol 50%, water 50%). Then, 100 mg of phenacetin was added as an internal standard. The solution was made up to 100 mL and subsequently analysed by HPLC. A solution consisting of a mixture of standards was also analysed. The standard solution was prepared by dissolving 250 mg of each component in 100 mL of solvent in a volumetric flask. The following results were obtained from the HPLC analysis of: (a) the tablet and (b) the standard. Area /mV.s Sample ParacetamolCaffeine Aspirin Phenacetin 84,922 19,05053,480(internal standard Tablet 68,994 Standard contains 250 mg L-1 of each component 38,801 14,236 26,301 39,594 From these results, calculate the composition of the tablet in %(w/w) remembering that the tablet contains the three active components and a binder.

Explanation / Answer

Concentration of phenacetin in sample =100mg /100 ml = 1mg /ml = 1000 mg / l

Area corresponding to 250 mg/l phenacetin =39,594

So, we calculate the response factor F,

Area of analyte/ concentration of analyte = F ( area of standard/ concentratio of standard)

so we get, 84922/1000 = F( 39594/250)

or F= 0.536

so using the above formula we calculate unknown concentrations of paracetamol, caffeine and aspirin

i. Paracetamol................> 68994/ x = 0.536 (38801/250) = 829.36 mg/l

ii. Caffeine.........................> 19050/x = 0.536( 14236/250) = 624.14 mg/l

iii. Aspirin.............................> 53480/x= 0.536 (26301/250) = 948.4 mg/l

The total volume was 100 ml. So amount of each compound in 100 ml:

Paracetamol = 82.936 mg

Caffeine= 62.414 mg

Aspirin = 94.84 mg

So % composition in a 1000 mg tablet comes out as:

Paracetamol = 8.2936 %

Caffeine = 6.2414%

Aspirin = 9.484%

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