Question

Exactly 1.00-mL aliquots of a solution containing phenobarbital were measured into 50.00-mL volumetric flasks and made basic with KOH. The following volumes of a standard solution of phenobarbital containing 122 ppm of phenobarbital were then introduced into each flask and the mixture was diluted to volume: 0.00, 5.00, 10.00, 15.00, and 20.00 mL. The fluorescence of each of these solutions was measured with a fluorometer, which gave values of 42.0, 212.8, 383.6, 554.4, and 725.2, respectively. What is the concentration (in ppm) of phenobarbital in the unknown?

Answer 1

By the addition of every 5ml known 122ppm solution raises the fluorescence intensity by a value of 170.8

5ml of known intensity - unknown intensity = 212.8-42 = 170.8

This 171.8 value increment was observed in every 5ml addition of known 122ppm solution.

Therefore, 122mg in 1000ml (1ppm solution)

Amount of phenobarbital in 5ml = 0.61mg

So, 171.8 intensity is corresponding to 0.61mg

42 intensity is corresponding to ?mg

Amount of phenobarbital = 0.15mg in 1ml

In 1000ml = 1000*0.15 = 150mg

The concentration of solution unknown = 150ppm