SF04 THE DETERMINATION OF QUININE IN BEVERAGES

 

Solutions of quinine fluoresce strongly when excited by radiation at 350 nm. The relative intensity of the fluorescent peak at 450 nm provides a sensitive methode for the determination of quinine in bverages. Preliminary measurements are needed to define a concentration regio in which fluorescent intensity is either linear or nearly so. The unknown is then diluted as necessary to produce readings within this range.

(a) Sulfuric acid, 0.05 M. Add about 17 mL of 6 M H₂SO₄ to 2 L of distilled water.

(b) Quinine sulfate standard, 1 ppm. Weigh (to the nearest 0.5 mg) 0.100 g of quinine sulfate into a 1-L volumetrie flask, and dilute to the mark with 0.05 M H₂SO₄ (sufficiënt for 60 to 70 analyses). Transfer 10.00 mL of this solution to another 1-L volumetrie flask, and again dilute to the mark with 0.05 M H₂SO₄. This latter solution contains 1 ppm of quinine; it should be prepared daily and stored in the dark when not in use.

 

To find a suitable working range, measure the relative fluorescent intensity of the 1 ppm standard at 450 nm (or with a suitable filter that transmits in this range). Use a graduated cilinder to dilute 10 mL of the 1 ppm solution with 10 mL 0f 0.05 M H₂SO₄; again measure the relative fluorescence. Repaeat this dilution and measurement proces until the ralative intensity approches that of a blank consisting of 0.05 M H₂SO₄. Make a plot of the data, and select a suitable range for the analyses (that is, a regio within wich the plot is linear).

Use volumetrie glassware to prepare three or four standards that span the linear regio; measure the fluorescence intensity for each. Plot the data.

Obtain an unknown. Make suitable dilutions with 0.05 M H₂SO₄ to bring its fluorescence intensity within the calibration range.

Calculate the parts per million of quinine in the unknown.