In the Laboratory
Determination of Sugar Content in Commercial Beverages by Density A Novel Experiment for General Chemistry Courses Susan K. Henderson, Carol A. Fenn, and John D. Domijan Department of Chemistry, Quinnipiac College, Hamden, CT 06518
A typical experiment performed early in the general chemistry laboratory sequence involves the determination of density. A number of experiments based on density determination of rubber stoppers, plastic chips, pennies, and antifreeze–water mixtures have been described (1–6 ). Experiments have also been published in which the density of salt solutions of known concentrations is determined to illustrate the linear relationship between salt concentration and density (7, 8). An alternative to these classical approaches is to use this linear relationship to ascertain the concentration of a major solute in a liquid sample. In the experiment described in this article, the density of solutions of known sugar concentration is measured and the data are used to determine the sugar content in commercial beverage samples. Students have an avid interest in laboratory investigations of the properties of products they routinely purchase. They are often surprised at the results they obtain in this experiment, which indicate the relatively high levels of sugar contained in the beverages they consume on a regular basis. Background The goal of introductory laboratory sessions is to focus on the experimental application of the principles of the topics discussed in lecture and to provide an opportunity for the development of specific laboratory skills. Introductory experiments that utilize chemicals that are inexpensive and readily available, pose no physical or health hazards, and produce no hazardous wastes are desirable. The experiment described in this article takes into consideration the above factors and has been successfully performed in our general chemistry laboratory courses by approximately 1500 students during the past five years. Experimental Details
Materials This experiment uses granulated 100% cane sugar purchased at a local supermarket and water for the preparation of standard solutions. Commercial beverages with high sugar content, such as fruit juices, iced tea, lemonade, thirstquencher drinks, or carbonated beverages, are used for sample materials. These items are readily available and inexpensive. They pose no chemical exposure hazards and no hazardous wastes are produced. All excess reagents, samples, and wastes from this experiment can be disposed down the drain. Common laboratory glassware, including 10-mL volumetric pipets, and balances with an accuracy of 0.01 g or greater are needed. Graphs are constructed using a relatively simple graphing software package (e.g., Graphical Analysis
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software, Vernier Software, Portland, OR) or are constructed manually.
Procedure Students prepare about 50 mL each of at least four standard aqueous sugar solutions in the range of 0 to 17 percent by mass sugar. The percent by mass sugar concentration of typical beverage samples will fall within this concentration range. Instructors can have students pool their results if additional data points are desired. Specific instructions for standard solution preparation may be provided, or students can be required to independently determine the amounts of sugar and water needed. The actual concentrations of the prepared sugar solutions are calculated on the basis of the weight of sugar and water used to prepare each solution. Care must be taken to ensure that all the sugar is dissolved and that the solutions are homogeneous prior to making any density determinations. All solutions and samples should be at room temperature. Three determinations are made of the density of each standard solution. Density values are calculated based on the measurement of the mass of 10.00 mL of each solution delivered by a volumetric pipet. Students are encouraged to practice their pipetting technique until they are able to obtain reproducible results. A graph is constructed by plotting the average density on the y-axis in units of g/mL versus the percent by mass sugar concentration on the x-axis. A linear relationship should be obtained in the specified concentration range. Students then determine the density of commercial beverage samples in the same manner as described for the standard solutions. Carbonated beverages must be degassed before pipetting. We have found that this can easily be accomplished by stirring the sample with a magnetic stirrer until the evolution of bubbles ceases. The percent-by-mass sugar concentrations of the commercial beverage samples are read directly from the prepared graph or calculated from the slope and intercept obtained from linear regression analysis of the standard curve data. Results Typical student data for average density values based on three determinations per solution are listed in Table 1 along with the percent by mass sugar concentration found in several commercial beverages based on this method. The graph corresponding to the density data for the standard sugar solutions and water is shown in Figure 1. Linear regression analysis of this data yields a slope of 0.00401 and a y-intercept of 0.995 g/mL. Typical per cent by mass sugar concen-
Journal of Chemical Education • Vol. 75 No. 9 September 1998 • JChemEd.chem.wisc.edu
In the Laboratory
trations found in commercial beverages are in the 8 to 12% range. Students’ results generally reflect their pipetting and solution preparation techniques and their attention to the proper use of significant figures. In this experiment students can evaluate their pipetting technique by examining the precision of the masses or the calculated densities obtained for successive determinations. They usually are able to obtain values within ± 0.002 g/mL for successive determinations of the density of a specific solution. If the graph exhibits poor linearity, it is typically related to errors in the preparation of the standard solutions. Such errors can include the loss of sugar upon transfer or nonhomogeneity of the solutions due to incomplete dissolution of the solute or insufficient mixing of the solution. The experiment can be completed in a three-hour laboratory session. The amount of time required to complete the experiment is dependent on the number of balances available, the number of standards utilized, and the number and type of samples (carbonated or noncarbonated) evaluated.
Figure 1. Calibration curve for determination of the sugar concentration of beverages.
Discussion This method is not specific for the determination of sugar; however, it is feasible for many commercial beverages because the only major constituents of these samples are water and sugar. Natural and artificial colorings, flavorings, electrolytes, and preservatives in these samples are minor constituents and therefore do not contribute significantly to the measured density values (9). This is demonstrated by comparing the density values of a sugar-containing product, its diet counterpart, and water. Typically, the diet beverage and water will have very similar density values. As indicated in Table 1, the density value determined for the diet beverage listed was the same as the value determined for water. These results are consistent with the assumption that other solutes do not contribute significantly to the density of these types of samples. Sucrose is used in this method for the preparation of standard solutions and results are reported simply as per cent by mass sugar concentration. The sugar present in the beverage sample may come from a variety of sources and may or may not be in the form of sucrose. Results from the analysis of beverages by this method are, however, consistent with product nutritional label statements of grams sugar per serving. If the instructor desires to evaluate the accuracy of students’ results, samples for analysis can be prepared from sugar solutions of known concentration and food coloring. Another approach is for the instructor to prepare solutions of known sugar concentration from powdered unsweetened soft drink mix and known quantities of sugar and water (e.g., KoolAid brand unsweetened soft drink mix). This experiment has been considered successful for use in the introductory laboratory sessions of our general chemistry laboratory sequence for several reasons. It provides an opportunity for students to make density measurements, to prepare solutions, to address the use of significant figures, and to gain experience in correct pipetting and weighing techniques. These skills are used in many subsequent laboratory experiments. The experiment also provides an opportunity to introduce graphing techniques and the use of calibration curves. In addition, student interest is established owing to the nature of the samples and the purpose of the experiment. All this is accomplished with the use of readily available, inexpensive, and nonhazardous materials. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
Samsa, R. A. J. Chem. Educ. 1993, 70, 149–150. Bruzan, R.; Baker, D. J. Chem. Educ. 1993, 70, 397–398. Sardella, D. J. J. Chem. Educ. 1992, 69, 933. Flowers, P. A. J. Chem. Educ. 1990, 67, 1068–1069. Richardson, W. S.; Teggins, J. E. J. Chem. Educ. 1988, 65, 1013–1014. McCullough, T. J. Chem. Educ. 1992, 69, 293. Hall, J. F. Experimental Chemistry, 3rd ed.; Heath: Lexington, MA, 1993; pp 41–49. Kerner, K. K. Chemical Investigations; Benjamin/Cummings: Redwood City, CA, 1986; pp 27–31. Whitney, E. N.; Rolfes, S. R. Understanding Nutrition, 6th ed.; West: Minneapolis, 1993; Appendix H.
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