Determination of Quinine in Tonic Water
In this experiment, we will make use of a recording spectrofluorimeter to study the luminescence
characteristics of the bicyclic alkaloid quinine, which is commonly used as a fluorescence
standard. Quinine is also used for medicinal purposes, most notably in the treatment of malaria
and also a common ingredient in tonic water.
The molecular structure of quinine is shown above with FW = 324.41 g/mole.
Experimental objectives are designed to introduce students to the basic operation and advantages
of fluorescence spectroscopy as an analytical technique.
1. Record the excitation and emission spectra of quinine.
2. Produce a standard calibration curve for quinine.
3. Determine the pH, ionic strength and temperature effects of quinine emission.
4. Determine the concentration of the analyte, quinine in tonic water.
5. Application of appropriate statistical analysis for data reporting.
Equipment
UV/Vis Spectrophotometer, Spectrofluorimeter, 1 1–L volumetric flask, six 25–ml volumetric
flasks, and a micropipet.
Reagents & Solutions
Prepare the following reagents: sulfuric acid, quinine sulfate dehydrate (FW=782.96),
1. 1 liter of 0.05 M H2SO4 (verify 2.75 mL of sulfuric acid diluted to 1.0 L) and 2000 ppm
NaCl in 0.05 M H2SO4
a. Remember to always add concentrated acids to water for diluted solutions.
b. Wear protective gloves, coat and goggles!
c. Prepare this solution in the hood.
2. 0.05 M sodium phosphate buffer, pH 7.4
3. 0.05 M sodium carbonate buffer, pH 9.6
4. Prepare 500 mL of 10 ppm quinine in 0.05 M H2SO4 (verify the use of 6.1 mg of quinine
sulfate dihydrate).
Standard Curve of Quinine Determination
5. Prepare 25 ml each of 1.00, 0.500, 0.250, 0.100, 0.0500, 0.0250 and 0.0010 ppm quinine
in 0.05 M H2SO4.
6. Measure the UV–Vis absorption spectrum of the 1.00 ppm quinine and verify that the
excitation wavelength is 350 nm.
7. Prepare the sample solution by diluting 1.0 μL of tonic water with 0.05 M H2SO4 to
100.00 mL in a volumetric flask. Determine the required dilution factor for analysis.
8. Set the excitation monochromator to 350 nm, and the slit widths to 5 nm.
9. Measure the fluorescence spectrum of each standard using the overlay mode.
10. Record the tonic water sample spectrum separately.
a. What is the concentration of quinine in tonic water?
b. Report quinine concentration with the confidence interval at a 95% confidence
level.
Effect of Ionic strength and Temperature on Quinine Emission
1. Re–measure the emission spectrum of 0.100 ppm quinine in 0.05 M H2SO4.
2. Increase the block temperature to 40.0 °C and rescan in overlay mode.
3. Prepare 25 ml of 0.10 ppm quinine in 0.05 M H2SO4 with 1000 ppm NaCl and measure
its emission.
a. Compare the spectra and describe the effect of ionic strength and temperature on
the emission spectrum of quinine.
Effect of pH on Quinine Emission
4. Prepare 100 mL of 0.1 ppm quinine in distilled H20 from the 10 ppm quinine in 0.05 M
H2SO4 and measure its emission.
5. Prepare 25 mL of 0.1 ppm quinine in 0.05 M sodium phosphate buffer, pH 7.4 and
measure its emission.
6. Prepare 25 mL of and 0.1 ppm quinine in 0.05 M sodium carbonate buffer, pH 9.6 and measure its emission.
a. Describe the effect of pH on the emission spectrum of quinine.
Waste: Discard all waste in the sink except concentrated H2SO4 (inorganic)
Questions:
1. What is the difference between an excitation and an emission spectrum?
2. What is concentration meant by inner filter effects?
3. What is the linear dynamic range for your work?
4. Compare the excitation and emission spectra you obtained. Comment on the similarities between the excitation and absorption spectra of quinine. Why aren’t these spectra identical
