Question

An analysis is being done by capillary electrophoresis to quantify the amount of caffeine in a cola. A standard solution, containing 18.9 mg/mL of caffeine, and 5.92 mg/mL of benzoic acid, is analyzed by the CE and found to have absorbance values of 523 mAu and 194 mAU respectively. A 10.00 mL solution of the cola is taken for analysis, and to this volume, 2.50 mL of a standard benzoic acid solution (45.8 mg/mL). When the mixture is analyzed, the signals from the caffeine and benzoic acid were 492 mAU and 300 mAU, respectively. What is the concentration, in mg/mL, of caffeine in the cola?

Answer 1

since A= ebC, where A = absorbance and C= concentration and e= extinction coefficient

for Benzoic acid standard A1/A2= ebC1/ebC2

C1= concentration of Benzoic acid in the standard sample = 5.92 mg/ml

C2 =concentratin of benzoic acid in the second sample , amount of benzoic acid in 2.5ml of 45.8 ml = 45.8*2.5 =114.5 mg

The volume after mixing = 10+2.5=12.5 ml

Concentration of Benzoic acid in the second sample = 114.5/12.5= 9.16 mg/L

from C2/C1= A2/A1= 9.16/5.92= 300/194 =1.55

let x=concentratin of cocal cola in the sample from A3= ebC3 and A4= ebC4

from x/18.9 = 492/523, x= 17.8 mg/L