In the Classroom edited by
Resources for Student Assessment
Thomas A. Holme Iowa State University Ames, IA 50011
Fluorine Compounds and Dental Health: Applications of General Chemistry Topics Gabriel Pinto Grupo de Innovación Educativa de Didáctica de la Química, E.T.S. de Ingenieros Industriales, Universidad Politécnica de Madrid, 28006 Madrid, Spain;
[email protected] It is widely accepted that students better appreciate chemistry concepts if these concepts are applied to everyday life. The idea of using real-world problems and questions for science education is common, because they can engage students in deeper cognitive processes (1). Nevertheless, problems of this kind are not found usually in textbooks. Thus science teachers are always looking for examples of how chemical phenomena affect our lives (2–5) and for novel problems to stimulate students’ interest (6–8). This work is part of a research program intended to help first-year-undergraduate and high-school instructors include connections between students’ daily experiences and chemical principles taught in the classroom (9–11). Here I suggest questions that teachers can use to relate general chemistry to dental fluoride applications, which students may find interesting. These questions are proposed to address various topics including stoichiometry, concentration units, resonance in polyatomic ions, bond order, bond length, geometry of polyatomic ions, and treatment of water. Fluoride used in dental applications is available from two major sources: products containing fluoride in their formulation (topical) and fluorides that are ingested from treated water and other sources (systemic) (12). Systemic fluorides are those that are ingested and become incorporated into tooth structure as it forms, whereas topical fluorides remain largely near the tooth surface. Topical fluorides include dentifrice (toothpaste), mouth rinses, and professionally applied fluoride therapies (13). Fluorine-containing compounds used in dentifrice include sodium fluoride, NaF, stannous fluoride, SnF2, and sodium monofluorophosphate, Na2PO3F. Sources of systemic fluorides include treated water (14), dietary fluoride supplements (15) in the form of tablets, drops, or lozenges, and fluoride present in food and beverages. Water fluoridation adjusts the fluoride content of fluoride-deficient water to the recommended level for optimal dental health (0.7–1.2 mg fluoride per liter). This is usually done with one of the three chemicals (16): sodium fluoride, NaF, sodium hexafluorosilicate, Na2SiF6, and fluorosilicic acid (also named hexafluorosilicic acid or hydrofluorosilicic acid), H2SiF6. More information about the use of fluorine compounds in dental applications is available in the online material. There is a controversy about water fluoridation (17): while consumption of fluoride from water presents very little risk of adverse effects in adults, consumption of relatively large quantities of fluorinated water appears to increase the development of dental fluorosis (normally characterized by the appearance of chalky-white lines or opaque-white patches in tooth enamel) in children (18). As an example, in New Zealand, referendums are becoming the norm for determining public opinion in cities on whether to fluorinate the water (19). That is a good example
of how people in modern societies need scientific knowledge to make important decisions that may affect their lives. Questions The following questions cover various topics of general chemistry that are related to fluorine compounds for dental care. The author considers these questions as a starting point: the reader is encouraged to develop additional questions (for example, related on physiological importance of the elements) that will encourage students to apply chemical concepts to the fluorine compounds in dental applications. Question 1 The information given on dentifrice packages (or in Internet sourcebooks; ref 20) indicates that a quantity of a fluorine compound or compounds is equivalent to a certain quantity of elemental fluorine, as indicated in the third column of the Table 1. Confirm the values listed in Table 1 from the stoichiometry calculations. Table 1. Fluorine Equivalences Data for Selected Dentifrices Brand
Fluoridating Agenta
Equivalence in F Element (ppm) Manufacturera Stoichiometryb
A
0.325% NaF
1477
1471
B
0.32% NaF
1450
1447
C
0.22% NaF
1000
995
D
0.177% NaF
805
801
E
0.11% NaF
500
498
G
0.055% NaF
250
249
H
1.9231% Na2PO3F
2500
2538
I
1.9% Na2PO3F
2500
2508
J
1.89% Na2PO3F
2500
2495
K
0.76% Na2PO3F
1000
1003
L
1.0230% Na2PO3F and 0.0335% NaF
1500
1502
M
0.445% Na2PO3F and 0.2% NaF
1495
1492
N
0.190 % Na2PO3F and 0.055 % NaF
500
500
aData given by the manufacturer on the packaging or in medicine handbooks. bData obtained by stoichiometry considerations. The data are given with the same number of significant figures as those provided by the manufacturer.
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 2 February 2009 • Journal of Chemical Education
185
In the Classroom
Based on the information supplied on the packaging of three dentifrices (H, I, and J from Table 1), the manufacturers indicate that there is the same equivalence in F element, that is, 2500 ppm, with an apparent different quantity of sodium monofluorophosphate. Explain the discrepancy by checking these equivalences and considering significant figures. Question 2 The main drawback for using fluorosilicic acid for fluoridating water is that it is a comparatively dilute source of fluoride; it is typically sold as a 20–40% aqueous solution. Calculate how much fluorine, in percent by mass, is contained in a typical commercially available solution of fluorosilicic acid (40% w/w), and compare it with the fluorine content (in % w/w) in solid sodium fluoride and sodium hexafluorosilicate. Question 3 In which compound, Na2SiF6 or Na2PO3F, can the bonding of the polyatomic anions can be explained in terms of resonance structures? Calculate formal charges for the atoms that form these anions and discuss these values in terms of electronegativities of involved atoms. Question 4 Estimate the bond lengths, in angstroms, for the anions in Question 3, taking into account the resonance structures. Consider the average bond lengths of P=O (1.47 ± 0.02 Å), P–O (1.66 ± 0.03 Å), P–F (1.56 ± 0.02 Å), and Si–F (1.65 ± 0.05 Å) given in the Handbook of Chemistry and Physics (21). Question 5 Predict the geometry of the polyatomic anions of the salts mentioned in Question 3 in accordance with the VSEPR theory.
fluorine compounds, only brands H and L give adequate number of significant figures in the equivalence of F element. In other cases only two or three significant figures could be expressed and we conclude that all brands included in Table 1 match the listed values of F content. Taking into account the number of significant figures given by manufacturers for the fluorine compounds, as referred before, the corresponding equivalences in F element are: Brand H (2538.3 ppm F), Brand I (2.5 × 103 ppm F), and Brand J (2.50 × 103 ppm F). This is an example of the importance of significative figures for chemical calculations. Answer 2 As molar masses of H 2 SiF 6 , NaF, and Na 2 SiF 6 are 144.11 g/mol, 41.99 g/mol, and 188.07 g/mol, respectively, the solution of acid contains 32% fluorine by mass, whereas sodium fluoride contains 45% and sodium hexafluorosilicate contains 61%. This is the reason that long-distance transportation costs of solid chemicals are more attractive than liquids. Although H2SiF6 is the least expensive source for fluorinating water, its main drawback is that it is a comparatively dilute source of fluoride. This is a good opportunity to discuss the importance of economy and other questions in the selection of a chemical for a specific function.
F F
O
gF
= 1471
gF 10 6 g dentifrice
O O
= 1471 ppm F
Equivalence of F element for dentifrices B to G can be calculated analogously. The stoichiometric calculations for dentifrices H to K required the molar mass of Na2PO3F (143.95 g∙mol). For dentifrices L to N both molar masses must be taken into account. A comparison of the cited fluorine content together with that found from mole calculations for the other dentifrices in shown in Table 1. In most cases the calculated values are close to those indicated by the manufacturers, with differences ranging from ‒0.5% to +1.5%. An important consideration is that, according to the number of significant figures given by different manufacturers for the 186
F
2ź
P
O
F
0. 325 g NaF 1 mol NaF 1 mol F 19. 00 g F 100 g dentifrice 41. 99 g NaF 1 mol NaF mol F g dentifrice
F
F
O
Answer 1 Students should be familiar with the chemical formula NaF. The formula of sodium monofluorophosphate can be found in texts, handbooks, and Internet sources. In the dentifrice A of Table 1, taken as an example, the stoichiometric calculus involves the expression:
= 0.001471
Si2ź
Figure 1. Chemical structure of SiF62– showing the formal charge.
Answers to Questions
0. 325% NaF =
F
O
P
O O
O
P
O O
O
P
F
F
F
O
O
O
P F
O
O
P F
O
O
P
O
O
F
Figure 2. Resonant structures of the PO3F2– anion showing the formal charges.
Journal of Chemical Education • Vol. 86 No. 2 February 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
In the Classroom
Answer 3 There is only one chemical structure for hexafluorosilicate anion, SiF62–, as shown in Figure 1. However, the monofluorophosphate anion, PO3F2–, exhibits resonance as shown in Figure 2. Students may be surprised to find out that Si and P in many of these structures has a negative formal charge considering that the fluorine is the most electronegative element. This is a good opportunity to discuss that actual charges lie somewhere between formal charge and oxidation number (two different models, ionic versus covalent) (22). Answer 4 For SiF62–, as shown in Figure 1, bond orders are 1.00. The structure of the monofluorophosphate anion can be drawn as resonance forms shown in Figure 2. The combinations of the resonance forms provide a P–F bond order of 1.00 and in the first row an average P–O bond order of 2.00, in the second row an average P–O bond order of 1.66, and for the last row a P–O bond order of 1.33. Taking into account those bond orders and the provided data of bond lengths we conclude that Si–F distances in the SiF62– are around 1.65 Å. This value is consistent with the value of 1.694 Å tabulated for this anion (21). In the monofluorophosphate anion we predict a P–F distance of 1.56 Å and an average P–O distance of 1.47 Å, 1.53 Å, and 1.60 Å for the three rows, respectively. Weil et al. (23) measured a mean distance of 1.510 Å for the P–O bonds and one longer P–F bond of 1.575 Å for this anion. According to the experimental P–O bond distance, the first resonance form appears to contribute to the observed structure. The multiple resonance forms could lead to a discussion of what factors should be taken into account when considering which resonance forms are most reasonable (24). Answer 5 According to VSEPR theory and considering that Si and P are the central atoms in the SiF62– and PO3F2– (it can be considered as an inorganic derivate of oxohalogen phosphoric acid) anions, the geometry of hexafluorosilicate ion is octahedral and for the monofluorophosphate anion is tetrahedral. These results are consistent with geometries found experimentally (23, 25). Summary By using the context of over-the-counter fluorine compounds for dental applications as topical (dentifrice) or systemic (fluoridated drinking water) use, core chemical calculations and questions can be made more interesting and challenging. Such contexts enable students to realize the relevance of chemistry outside the classroom and to understand the importance of the subject.
ments on the manuscript. The author would like to gratefully recognize the support provided by the Universidad Politécnica de Madrid under the Projects IE070535020 and IE08053505. Also I would like to thank the reviewers and especially Thomas A. Holme for helpful suggestions. I dedicate this work to my daughters (Elena Lucía and Elisa María), because, thanks to them, remembering the importance of dental care gave me the motivation to develop this exercise. Literature Cited
1. 2. 3. 4.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
Chin, C.; Brown, D. E. J. Res. Sci. Teach. 2000, 37, 109–138. Cunningham, K. J. Chem. Educ. 2007, 84, 430–433. Salter, C.; Langhus, D. L. J. Chem. Educ. 2007, 84, 1124–1128. Nentwig, P. M.; Demuth, R.; Parchmann, I.; Gräsel, C.; Ralle, B. J. Chem. Educ. 2007, 84, 1439–1444. Jacobsen, E. K. J. Chem. Educ. 2007, 84, 1747. Smart, J. L. Chem. Eng. Educ. 2003, 37, 142–153. Smart, J. L. Chem. Eng. Educ. 2007, 41, 115–120. McKay, M.; McGrath, B. Int. J. Eng. Educ. 2007, 23, 36–42. Pinto, G. J. Chem. Educ. 2005, 82, 1321–1324. Pinto, G. J. Chem. Educ. 2005, 82, 1509–1512. Oliver-Hoyo, M. T.; Pinto, G. J. Chem. Educ. 2008, 85, 218–220. Rakita, P. E. J. Chem. Educ. 2004, 81, 677–680. Foster, M. S. Protecting Our Children’s Teeth; Insight Books (Plenum Press): New York, 1992. Tate, W. H.; Chan, J. T. Gen. Dent. 1994, 42, 362–366. Adair, S. M. J. Public. Health Dent. 1999, 59, 252–258. Urbansky, E. T. Chem. Rev. 2002, 102, 2837–2854. Fluoride Facts. http://fluoridealert.org/fluoride-facts.htm (accessed Oct 2008). Pendrys, D. G. J. Am. Dent. Assoc. 2000, 131, 746–755. Hydrofluorosilicic Acid and Water Fluoridation. http://www.nzic. org.nz/ChemProcesses/production/1C.pdf (accessed Oct 2008). Dental Health Foundation of Ireland. http://www.dentalhealth. ie/dentalhealth/ (accessed Oct 2008). Handbook of Chemistry and Physics, 85th. ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2005. Pinto, G.; Rohrig, B. J. Chem. Educ. 2003, 80, 41–44. Weil, M.; Puchberger, M.; Fuglein, E.; Baran, E. J.; Vannahme, J.; Jakobsen, H. J.; Skibsted, J. Inorg. Chem. 2007, 46, 801–808. Moore, J. W.; Stanitski, C. L.; Jurs, P. C. Chemistry: The Molecular Science, 1st ed.; Brooks/Cole: Belmont, CA, 2002; p 342 Fonari, M. S.; Kravtsov, V. Ch.; Simonov, Y. A.; Ganin, E. V.; Gemboldt, V. O.; Lipowski, J. J. Incl. Phenom. Macro. 2004, 39, 85–89.
Supporting JCE Online Material
http://www.jce.divched.org/Journal/Issues/2009/Feb/abs185.html Abstract and keywords
Acknowledgments
Full text (PDF) with links to cited URLs and JCE articles
Maria T. Oliver-Hoyo, Chemistry Department, North Carolina State University at Raleigh, and Arrietta Clauss, The Journal of Chemical Education, are thanked for helpful com-
Supplement Information about the use of fluorine compounds in dental applications (as topical or as systemic use)
© Division of Chemical Education • www.JCE.DivCHED.org • Vol. 86 No. 2 February 2009 • Journal of Chemical Education
187
