In the Laboratory
Determination of Phosphorus in Cola Drinks Diego Lozano-Calero and Pilar Martín-Palomeque Servicio de Bioquímica, Hospital La Paz, Paseo de la Castellana, 261, 28046 Madrid, Spain Silvia Madueño-Loriguillo Departamento de Química Inorgánica, Universidad de Málaga, Campus de Teatinos, 29071 Málaga, Spain Laboratory experiments can improve student interest in science. However, the contrary effect could occur if they are not well designed and seem tedious, too laborious, and disconnected from daily life. Cola beverages are one of the most widely consumed drinks and are most popular among students. Much attention is being paid to possible consequences of excessive consumption for human health. Intensive efforts are being made to assess the erosive potential for teeth because of the beverages’ acidity (1, 2); adverse effects secondary to high caffeine intake (e.g., hypertension, allergic reactions, gastrointestinal disturbances) (3–5); and adverse effects on calcium metabolism due to their high phosphoric acid content, which combined with low dietary calcium intake could increase the risk of suffering from bone diseases (6–9). We propose here the quantification of the phosphorus content in this kind of drinks by a different procedure from that previously described by Murphy in this Journal (10). We think this laboratory experiment will seem very interesting to students.
ter. Place 39 mL of the ammonium molybdate solution, 60 mL of the ascorbic acid solution and 125 mL of diluted sulfuric acid into a 250-mL volumetric flask. Fill to mark and mix.
Theory The method consists of the formation of a complex of phosphate ion with a molybdate compound in acid solution: 7 H3PO4 + 12 (NH4)6Mo7O24 ?4H2O →
Figure 1. Plot of absorbance vs. concentration of the standard solutions. Table 1. Sample and Calibration Curve Preparation
+
{
Calibration Curve
7 (NH4)3[PO 4(MoO3)12 ] + 51 NH4 + 51 OH + 33 H2O The colorless hexavalent molybdenum phosphate complex is reduced to a blue pentavalent form by ascorbic acid in acid medium. The intensity of the blue color is measured spectrophotometrically at 830 nm (11). Obviously, the sample cannot be colored; therefore we used a clear cola, Crystal Pepsi®, acquired in the market. Experimental Procedure
Preparation of Solutions Stock Standard Solution. Weigh out 1.9412 g of dry KH2PO4, reagent grade, and dissolve in deionized water in a 1-L volumetric flask. Dilute to mark. The phosphorus content of this solution expressed as P2O5 is 1000 ppm. Working Standard Solution. From the stock standard solution place 2.5 mL into a 50-mL volumetric flask and dilute to mark. From there deliver 4 mL into a 50mL volumetric flask and fill to mark. The final resulting solution is 4 ppm P2O5. Reducing Solution. Dissolve 1.00 g of ammonium molybdate in 50 mL of deionized water and 1.76 g of ascorbic acid in 100 mL of deionized water. Dilute 17 mL of concentrated sulfuric acid in 200 mL of deionized wa-
Volume from working standard solution (mL) Volume from diluted sample (mL)
0
0.5 1
1.5 2
Sample 2.5 0.25
Reducing solution (mL)
2
2
2
2
Deionized water (mL)
3
2.5 2
2
1.5 1
0.5
2.75
Final volume (mL)
5
5
5
5
5
Final concentration expressed as P2O5 (ppm)
0
0.4 0.8 1.2 1.6 2
5
2
2
5
Table 2. Absorbance Obtained for Sample and Standards Solution
A830
Standards, ppm 0.4
0.255
0.8
0.588
1.2
0.866
1.6
1.151
2.0
1.436
Sample, no. 1
0.565
2
0.571
3
0.563
Vol. 73 No. 12 December 1996 • Journal of Chemical Education
1173
In the Laboratory
Sample Preparation and Measurement Let the sample stand at atmospheric pressure for 24 h before analysis in order to eliminate gases. Then transfer 2.5 mL of the sample into a 50-mL volumetric flask. Fill to the mark with deionized water. Dispense the volume of each solution into test tubes as indicated in Table 1. Warm the tubes for 45 min at 50 °C in a water bath. Read the absorbance against a blank at 830 nm. Make at least three determinations of the sample. Results Our results are shown in Table 2. The linear calibration curve was obtained for the range 0.4–2 ppm of P2O5 and was fitted by least squares (y = 0.72x – 0.02, r = .999; see Fig. 1). The concentration of phosphorus present in the test solution was calculated by the equation of the calibration curve. Taking into account the dilutions made on the original sample and the transfor-
1174
mation from P2O5 to P content, the phosphorus present in the sample was calculated by the following equation: ppm of P in sample = 20 × 20 × X ppm P2O5 × (62 mg P/142 mg P2O5) We found that Crystal Pepsi® contains an average 142 ppm phosphorus. Literature Cited 1. Lussi, A.; Jaggi, T.; Scharer, S. Caries Res. 1993, 27, 387–393. 2. Grenby, T. H.; Phillips, A.; Desai, T.; Mistry, M. Br. J. Nutr. 1989, 62, 451–464. 3. Cabllero, T.; García, A. C.; Pascual, C.; Díaz, J. M.; Ojeda, A. J. Invest. Allergol. Clin. Immunol. 1993, 3, 160–162. 4. Brikett, N. J.; Logan, A. G. J. Hypertens. 1988, 6(4 Suppl.), S620–S622. 5. Hadas, H. I; Hiller, N.; Guberman, D.; Am. J. Gastroenterol. 1993, 88, 127–129. 6. Wyshak, G.; Frisch, R. E. J. Adolescent Health 1994, 15, 210–216. 7. Hernandez, A; Stampfer, M. J.; Ravnikar, V. A.; Willet, W. C.; Schiff, I.; Francis, M.; Longscope, C.; Mckinlay, S. M. Epidemiology 1993, 42, 128–134. 8. Smith, S.; Swain, J.; Brown, E. M.; Wyshak, G.; Albright, T.; Ravnikar, V. A.; Schiff, I. Arch. Intern. Med. 1989, 149, 2517–2519. 9. Spencer, H.; Kramer, L.; Osis, D. J. Nutr. 1988, 118, 657–660. 10. Murphy, J. J. Chem. Educ. 1983, 60, 420–421. 11. Charlot, G. Chemie Analytique Quantitative, Masson: Paris, 1974; p 478.
Journal of Chemical Education • Vol. 73 No. 12 December 1996
