Communication pubs.acs.org/jchemeduc
Yielding Unexpected Results: Precipitation of Ba3(PO4)2 and Implications for Teaching Solubility Principles in the General Chemistry Curriculum Jeffery L. Hazen and David A. Cleary* Department of Chemistry, Gonzaga University, Spokane, Washington 99258, United States ABSTRACT: Precipitation of barium phosphate from aqueous solutions of a barium salt and a phosphate salt forms the basis for a number of conclusions drawn in general chemistry. For example, the formation of a solid white precipitate is offered as evidence that barium phosphate is insoluble. Furthermore, analysis of the supernatant is used to illustrate the concept of limiting reagent. In this paper, we present X-ray diffraction and gravimetric evidence that nabaphite is the species formed when aqueous solutions of barium and phosphate are mixed. In addition, our evidence suggests that nabaphite converts to barium apatite before barium phosphate is ultimately observed. This barium phosphate chemistry has implications when teaching solubility principles in general chemistry. KEYWORDS: First-Year Undergraduate/General, Curriculum, Inorganic Chemistry, Misconceptions/Discrepant Events, Aqueous Solution Chemistry, Precipitation/Solubility
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INTRODUCTION The precipitation of barium phosphate from aqueous solutions of barium chloride and sodium phosphate,
brought to volume with deionized water. Using a clean, dry graduated cylinder, approximately equal volumes of each solution (e.g., 39.0 and 40.0 mL) were added, with rinsing, to a large beaker. The resulting mixture (white fluffy powder suspended in aqueous solution) was heated over a Bunsen burner for 30 min. The temperature was kept between 75 and 90 °C. After cooling, a sample of supernatant was recovered for qualitative testing. The solid was vacuum filtered onto a weighed piece of filter paper (Whatman 5, 55 mm) and rinsed with deionized water. The filter paper containing the sample was stored in a drawer for 1 week and then reweighed to determine the mass of product. For the purpose of examining this laboratory exercise, we performed, in addition to the typical student preparation described above, several trials with the following modifications: 1. No heating of the initially prepared precipitated material. 2. Boiling the initially prepared precipitated material for 30 min. 3. Heating the initially prepared precipitated material for several hours at 105 °C in a sealed Teflon digestion bomb. Modification number 1 is contrary to the 30 min of heating students are told to perform. The reason given for the 30 min of heating is to digest the initial sample allowing for larger particles of barium phosphate to form. Since the recovery of the product depends on porous filter paper, larger particle size (at least larger than the pore size of the filter paper) would be important for recovery of the product. We had no difficulty recovering the solid product produced without heating the sample. In fact, we had a yield well in excess of 100% even after several days of allowing the sample to dry in open air. Modification number 2 is also contrary to the instructions given to the students. Boiling is typically avoided when
3BaCl 2(aq) + 2Na3PO4 (aq) → Ba3(PO4 )2 (s) + 6NaCl(aq)
(1)
is a popular reaction used in general chemistry laboratory classes for the purpose of examining limiting reagents, percentage yields, and solubilities.1−4 Students prepare known molarities of the starting solutions, mix equal volumes, and recover the insoluble product. Analyzing the supernatant qualitatively allows them to confirm which starting solution contained the limiting reagent. This information along with the mass of the insoluble product allows them to determine the percentage yield. We have several hundred students complete this exercise each year. While often the percentage yields are good, we have had enough variation, both above and below the theoretical yield, that we decided to examine our procedure for the purpose of producing a smaller spread in percentage yields across the entire course. In the course of this examination, we have discovered that this reaction does not proceed as presented to the students. In this paper, we present our results and discuss the implications with respect to general chemistry laboratory instruction.
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PROCEDURE We followed the standard published procedure for this laboratory exercise.1 In a typical procedure, barium chloride dihydrate (0.502 g) was weighed into a small beaker. Sodium phosphate dodecahydrate (1.021 g) was weighed into a second small beaker. Approximately 25 mL of deionized water was added to each beaker, and the resulting mixtures were stirred until all of the solids dissolved. Each solution was transferred with rinsing to a volumetric flask (50.00 mL), and the flask was © XXXX American Chemical Society and Division of Chemical Education, Inc.
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dx.doi.org/10.1021/ed400741k | J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
precipitates are present because precipitates tend to cause bumping in a heated solution. Also, the physical agitation caused by boiling could reduce particle size. When we boiled our solutions, we observed yields of
