In the Laboratory
Titrimetric Determination of Carbon Dioxide in a Heterogeneous Sample (“Pop Rocks”)
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Craig M. Davis* and Matthew C. Mauck Department of Chemistry, Xavier University, Cincinnati, OH 45207-4221;*
[email protected] A traditional exercise in quantitative analysis is the titration of mixtures of sodium hydroxide, sodium carbonate, and sodium bicarbonate (1–3). In particular, Crossno et al. (1) employed balloons filled with aqueous sodium hydroxide to analyze consumer products. Carbon dioxide, released through physical agitation or by acid treatment of a sample in an Erlenmeyer flask, passed into a balloon attached to the top of the flask where it was converted to an equivalent amount of carbonate. The resulting solution of sodium carbonate and sodium hydroxide was titrated with standardized HCl, first to the phenolphthalein endpoint and then to the methyl orange endpoint. At the first endpoint the sodium carbonate is converted to sodium bicarbonate and the excess sodium hydroxide is consumed. Thus, only sodium bicarbonate (an amount equivalent to the evolved carbon dioxide) was titrated by the HCl as the reaction proceeded to the second endpoint (1). The consumer products analyzed by Crossno et al. were homogenous mixtures (carbonated beverages) or intimately mixed, nearly homogeneous mixtures (antacid tablets). We decided to apply their protocol (with our modifications; see below) to a decisively heterogeneous mixture, the popular candy “Pop Rocks”. This exercise offers a practical, fun way to introduce the concept of sampling heterogeneous mixtures (4, 5).
Experimental Procedure
Pop Rocks
The patent claims the CO2 content is typically 0.5–15 cm3 per gram of candy, preferably 2–7 cm3 (6). We determined the CO2 content to be 2.42 cm3 (± 0.15 cm3) per gram of candy, in good agreement with the patent data. The popularity of the sample and the simplicity of the procedure make this exercise suitable for a wide variety of students: from nonscience majors in an introductory course to chemistry majors in a quantitative analysis course.
The manufacture (6) of Pop Rocks is described briefly. A mixture of sugars is dissolved in water and evaporated at 320 ⬚F (160 ⬚C) to reduce the moisture content to 3% by mass. This melt is cooled to 280 ⬚F (138 ⬚C) and, while being stirred vigorously, is pressurized with CO2 at 50 atm for five minutes. The pressure is maintained as the mixture is allowed to cool and solidify. Upon release of the pressure the solidified, gasified candy fractures. The resulting pieces resemble sharp-faced chunks of gravel. The patent claims a majority of the CO2 bubbles are greater than 225 nm in diameter, with a maximum size of 350 nm (6). The random fracturing of the candy causes the pieces to vary both in size and shape (the latter leading to changes in surface-area兾volume ratios). Consequently, the number of CO2 pockets per volume of candy in each piece varies in an uncontrollable manner, as do the pocket sizes. However, when we analyzed the volume of CO2 per gram of candy, the precision from packet-to-packet was a modest 6%, despite the candy’s heterogeneous nature. By comparison, Crossno et al. (1) reported the precision from can-to-can of carbonated beverages to be 9% and from tablet-to-tablet of antacid tables to be 3%. Apparently, the large number of bubbles per piece coupled with the large number of pieces per packet (several hundred) results in a nearly constant volume of CO2 per gram of candy in each package.
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We offer a modification of the experimental design of Crossno et al. (1). They placed a sample solution in an Erlenmeyer flask, then covered the top with a balloon filled with aqueous sodium hydroxide. We used two 250-mL round-bottomed flasks connected by a distillation head (see Figure 1). The sample solution is placed in one flask and the aqueous sodium hydroxide is placed in the other flask. A balloon is attached to the top of the distillation head simply to accommodate the evolving CO2 gas before it dissolves in solution. (This gives new life for your macroscale glassware after you convert your organic laboratories to microscale!) In a typical experiment, one flask is charged with three packages (about 30 grams) of Pop Rocks and the other flask with 100.00 mL of 0.100 M NaOH. Next, about 100 mL of deionized water are poured onto the Pop Rocks and the distillation head is placed as quickly as possible between the two flasks. The Pop Rocks solution is stirred overnight (or for one week). A 25.00mL aliquot of the NaOH solution is removed, and titrated with 0.05 M HCl (previously standardized to three significant figures). Results
Figure 1. Experimental set-up.
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu
In the Laboratory
Literature Cited
Hazards There are no significant hazards. Students should avoid contact with the dilute hydrochloric acid and sodium hydroxide solutions. Safety goggles should be worn when handling the chemicals. Acknowledgments The authors thank Susan Schrimpf Davis for the artwork and Andrew Jackson for conducting an additional trial. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
1. Crossno, S. K.; Kalbus, L. H.; Kalbus, G. E. J. Chem. Educ. 1996, 73, 175, and references therein. 2. Harris, D. C. Exploring Chemical Analysis; W. H. Freeman: New York, 1997; pp 392–393. 3. Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry, 7th ed.; Saunders College Publishing: Fort Worth, 1996; pp 257–260. 4. Ross, M. R. J. Chem. Educ. 2000, 77, 1015. 5. Hartman, J. R.; Bacon, D. W.; Wolsey, W. C. J. Chem. Educ. 2000, 77, 1017. 6. Kleiner; F.; Roy; P. K.; Kuchman; M. J. Process of Preparing Gasified Candy. U.S. Patent 4,289,794, September 15, 1981.
JChemEd.chem.wisc.edu • Vol. 80 No. 5 May 2003 • Journal of Chemical Education
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