A student determines the concentration chloride in an unknown sample by pipettin

Question

A student determines the concentration chloride in an unknown sample by pipetting 10 mL of the sample and 10 mL of 1.000 times 10^-5 M e solution to a 100 mL flask and diluting to volume with 1 M H_2SO_4. A series of six chloride standards were prepared using a 0.0100 M standard chloride solution, adding an increasing volume of chloride to 100 mL volumetric flasks, in addition to 10 mL of 1.000 times 10^-5 M quinine. Each solution was diluted to volume with 1 M H_2SO_4. Calculate the chloride concentration for the unknown using the collected fluorescence data and the Stern-Volmert graph below for the data.

Explanation / Answer

Quenching, a decrease in fluorescence intensity, of a fluorophore can be described by the Stern-Volmer equation

I0 / I = 1 + K [Q]

where I0 and I are the fluorescence intensity in the absence and presence of a quencher (Q), K is the quenching constant, and [Q] is the molar concentration of the quencher. If the value of K is known or has been determined, the concentration of the quencher [Q] can be determined by measuring I0 and I.

I0 = 140

I = 133.50

K is the slope of the line = 25.636

140/133.50 = 1+ 25.636 [Q]

0.0487 = 25.636 [Q]

[Q] = 0.0487 / 25.636 =0.0019 M

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