Introduction:
Coke has become so popular that the drink is replacing healthy beverages in people’s diets around the world. (1, 2) Phosphoric acid is one of the ingredients in the colas that should only be consumed in moderation. The food industry calls the acid used to lower the pH of a food or beverage an acidulant. (3) Phosphoric acid is a food additive under the category of acidulant. (4) It adds “a pleasant tartness to the beverage.” (5) The food industry uses phosphoric acid a lot, “It is added to foods as a preservative, acidifying agent, flavor enhancer, and clarifying agent.” (6)
An accurate measurement of phosphoric acid concentration in colas is important as the health risks to consumers are becoming well researched. Health problems associated with bone density have been reported in people who consume large amounts of cola; especially replacing milk with colas. The problem results from the phosphoric acid which binds with the calcium in the stomach. When that happens the calcium is not available for the bones to absorb. (7)
An absorption spectrometer which measures Ultraviolet-Visible (UV-VIS) wavelengths is used as a standard instrument for laboratories to measure solutions of aqueous analytes. Beer’s Law is essential in analyzing data from UV-Visible Spectrometers. (8)
Beer’s Law is the equation which is used for determination of absorpitivity or concentration by using the measured quantity of absorbance which is given as a printout of wavelength absorption when samples are analyzed by the UV-VIS spectrophotometer. The equation for Beer’s Law shows the relationship between the measurable components. (9)
Beer’s Law describes absorbance, A as
A = εbc
Where, ε is the molar absorbance (L Mol-1cm-1)
b is the path length (which is often 1 cm-1) and
c is the concentration (moles/L).
An alternate method for making working standard solutions has been published in the literature. Researchers report that “colored soft drinks absorb below 650 nm whereas the absorbance measurement for the analysis of phosphoric acid is made ast830nm.” Researchers studied several different colored soft drinks, some contained phosphoric acid and some did not. After analyzing the data the researchers reported that the analysis for the concentration of phosphoric acid with a UV-VIS spectrometer can be done without using a 20-fold dilution for a working standard but by instead using “a final volume of 50 ml rather than 5 mL.” The researchers concluded that stopping after the dilution to 50 mL works fine for the analysis.(10)
For the degasification of the colas researchers suggest sonification of the samples for 15 minutes. They report this as the most efficient and reliable method. They noted that some colas do not completely degas in 24 hours. (11)
The molar absorpitivity of the solutions is graphed against the wavelength to find out how much “aqueous analyte” is in each sample. (12)
The objective of this experiment was to measure the concentration of phosphoric acid in unknown colas samples using UV-Vis absorption spectroscopy.
Experimental
The concentration of phosphoric acid in cola was measured with a Cary-Bio UV-Visible Spectrometer following standard procedures outlined in the Plan of Analysis.
The stock standard solution for the experiment was prepared using KH2PO4 which was diluted with deionized water in a three step process in order to prepare the needed dilution; from 1000 ppm to 50 ppm to 4 ppm. A standard curve was prepared from a stock standard solution of 1000 ppm phosphoric acid diluted through a series of dilutions to 4 ppm (working standard). (13)
In addition a reducing solution comprised of ammonium molybdate, ascorbic acid and sulfuric acid was prepared to be added in the working standards and the unknown samples in order to remove in interference of color in the determination of phosphoric acid. A reducing solution was prepared so that the HPO4-1 will be available for measurement with no color interference. The reducing solution (a mixture) was diluted with deionized water in a 250 mL volumetric flask. The cola sample needs to be allowed to sit opened to the air for 24 hours because the solution cannot be carbonated for the experiment. After 24 hours it was prepared in a dilution (2.5 ml of cola with deionized water to the mark in a 50 mL volumetric flask). (14)
Nine test tubes were prepared including a blank, Pepsi solution, cream soda solution and the energy drink solution the other test tubes contained known concentrations of the working standard solution, 2 mL of the reducing solution and the appropriate amount of deionized water. The test tubes were placed into a 50°C warm water bath for 45 minutes to allow the right conditions for reduction so that color would not interfere with the measurement of the phosphoric acid. Each of the test tubes with solution was analyzed using the Cary 100 Bio UV-VIS Spectrophotometer. (10) The resulting spectrographs were analyzed and the results are reported below.
Table 1 presents the amount used of working standard, reducing solution and deionized water for the preparation of the blank (tube 1) and the five standards (tubes 2-6) as well as the unknown samples.
Results and Discussion
Table 2 presents the phosphoric acid concentrations of the five working standards and their respective measured absorbance. The working standard concentrations ranged from 0.4 to 1.6 ppm of phosphoric acid.
Figure 1. Phosphoric acid standard curve for UV-Vis spectrometer.
The standard curve is plotted in Figure 1 (phosphoric acid concentration versus absorbance) and the linear regression coefficient of determination, R2 was 1 suggesting an excellent linearity of the data.
y = 4.1377x – 1.3853, where x is phosphoric acid concentration & y, absorbance.
In order to achieve this standard curve, the second standard of 0.8 ppm was eliminated from the calculation because it incorporated errors and deviated significantly from the linear nature of the curve. Ideally this standard should have been repeated. Because three points are adequate to determine a standard curve the graph gives the necessary information without redoing part of the preparation. By reverting the axis of the graph in the excel spreadsheet the following regression equation of the standard curve is obtained.
Concentration of H3PO4 (moles/L) = 0.242*(Absorbance) + 0.335
Table 3 presents the measured absorbance of the unknown samples (Pepsi, Cream soda and Energy soda). The concentrations of phosphoric acid in each sample were calculated using the above equation.
However, the cola samples were diluted at a 20 to 1 so that must be taken into account. Therefore the final concentrations were 7.274 mg/L for the Pepsi, 6.706 mg/L for the Cream soda and 6.789 mg/L for the Energy soda as listed in Table 4.
Conclusion
In this laboratory experiment, the phosphoric acid concentration was determined in three unknown soft drink samples containing Pepsi, Cream soda and an Energy soda. The concentrations of phosphoric acid determined in the samples were similar but the Pepsi sample measured the largest concentration at 7.274 mg/L. Cream soda and the Energy soda samples measured approximately the same amount of phosphoric acid, 6.706 mg/L and 6.789 mg/L consecutively.
The standard curve was very good with an R2 equal to 1.0 using three points determined from measurements of working standards on the UV-VIS spectrophotometer. However, in order to achieve such linearity in the standard curve, one of the standards was eliminated from the calculation because it deviated significantly from the linearity. This deviation was due to a source of error in the experiment and ideally the standard should have been prepared again.
The source of the error could have been from a mistake made during the dilution process. The unknown samples were diluted at a final ratio of twenty to one in a three step procedure so there was opportunity for error.
Another source of error could be from glassware that is not thoroughly clean or from the mishandling of the test tubes as they were added to the water bath or when they were taken out of the water bath.
Fortunately three points were measured that worked well to make a standard curve with a slope (R2) equal to 1. The resulting standard curve was useful for the analysis. It was not necessary to prepare another working standard solution.
References
1. Coke and Pepsi face-off an even match Coca-Cola $2.75 for 1.5 litres vs. Pepsi
$2.15 for 1.5 litres. (2011). New Zealand Herald. Independent Print Ltd.
http://www.highbeam.com/doc/1G1-252949471.html (accessed Nov. 19, 2011)
2. Riaz, M. N.& Chaudry, M. M. 2004. Halal Food Production. Boca Raton, FL: CRC
p. 201.
3. Kysar, D. A. (2005). Sustainable Development and Private Global Governance. Texas Law
Review. 83 (7) p. 2109+. www.questia.com (accessed Nov. 18, 2011).
4. Food additives. Understanding food additives. Chapt. 2 pg. 9.
http://www.understandingfoodadditives.org/pages/Ch2p9-1.htm
(accessed Nov. 18, 2011)
5.Coke Cola website
http://questions.coca-cola.com/ (accessed Nov. 26, 2011)
6. Phosphoric Acid. Chromic Toxicity Summary. A-145 – A- 151.
oehha.ca.gov/air/chronic_rels/pdf/7664382.pdf (accessed Nov. 19, 2011)
7. Shaw G. (2005). Soft Drinks Linked to Osteoporosis. National Health Review. 95, 17
8. Pappas, D. UV-VIS absorption measurements of aqueous analytes. Texas Tech.
Univ. Chemistry 4114- Instrumental Analysis Lab, n.d.
http://webpages.acs.ttu.edu/dpappas/page4/Lab%20packet.pdf (accessed Nov 2011).
9. Ibid.
10.Rodgers, J. and Koether, M. (2005). Analysis of phosphoric acid content in popular
carbonated drinks. J. of Chem. Ed. 82, 10, p. 1471.
11.Ibid.
12. Skoog, D.; Holler, E.; Crouch, S. Principles of Instrumental Analysis. 6th
Edition. Thompson Higher Education, Belmont, CA. 2007.
13.Potts, G. (2011). Plan of Analysis. Lab Experiment hand-out for measuring the
concentration of phosphoric acid in colas.
14.Ibid.
