In the Laboratory edited by
Secondary School Chemistry
Diana S. Mason University of North Texas
The Extraction and Isolation of Saltpeter from Nitered Soil
Denton, TX 76203-5070
Erica K. Jacobsen University of Wisconsin–Madison Madison, WI 53706
A Curriculum Alignment Project for a First-Year High-School Chemistry Course
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Brett Criswell† Department of Chemistry, Central Columbia High School, Bloomsburg, PA 17815;
[email protected] One of the most recurrent criticisms of high school science curricula, particularly those based on existing textbooks, is that they present students with an endless set of facts that often show minimal connectivity with each other. Having kept this flaw in standard curricula in the back of my mind as I tried to construct something more effective for my own classroom, I was intrigued by an article appearing in this Journal several years ago (1). The article, “Curriculum Alignment Projects: Towards Developing a Need to Know”, presents an approach to curriculum design that could be considered truly constructivist in its format: At the outset of some block in the curriculum, students are presented with an overarching project that will guide all of the class work over the course of that block. Successful completion of the project requires students to synthesize a number of concepts that make up the block’s learning objectives. The constructivist aspect of this system is that students uncover those concepts at their own pace and in their own sequence. The instructor provides information as requested by the students through standard class presentations and more formal laboratory work. Having been duly inspired by this innovative approach to building course content, I took an existing framework of introductory chemistry course material and began looking for several such projects that would help to integrate the concepts within that framework. One area that was problematic in this respect was the topic of aqueous solution chemistry. This included such diverse content as basic properties of solutions, representing solution concentration, separation techniques, reaction chemistry, equation writing, and qualitative analysis. Just when I was about to accept the fact that I would not be able to find a relevant project into which to coalesce those concepts, another Journal article provided the inspiration for what I believe is a strong candidate. How that article, titled “Some History of Nitrates” (2), was parlayed into such a project and a brief overview of the project will be discussed in the remainder of this article. Project and Overview The article above, as well as a much older article in this Journal (3), offer historical perspectives on the isolation and use of saltpeter with its major application, of course, being the oxidant in black powder.1 One of the sections of the more recent article discusses the extraction of saltpeter from soil. † Current address: Department of Curriculum and Instruction, The Pennsylvania State University, University Park, PA 16802.
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It was this section that gave me the idea for the project to be used in my introductory chemistry class. As conducted during the spring of 2004, the project began by providing each lab group in my classes with an 800mL beaker containing soil that had been artificially enriched with calcium nitrate (> 10%), that is, artificially nitered. This is done in an attempt to simulate nitrates obtained from soils where it is produced by certain bacteria. At that time, students are informed that the main form of nitrate in the soil is the calcium salt. Unfortunately, for reasons that they must discover through their own research, calcium nitrate is not useful in black powder; only potassium nitrate is. This will require them to undertake a chemical conversion to transform the calcium nitrate into potassium nitrate. However, before that work can be undertaken, it is necessary for them to isolate the calcium nitrate from the soil and purify it. All of the particulars concerning how to conduct the isolation, preliminary purification of calcium nitrate, chemical conversion, and final purification of the potassium nitrate will come from procedures developed by their own devices. This general overview of the project is provided on day one along with a packet that outlines the different parts of the project. The packet is broken down into sections to help the students organize the different components of the work they will need to do to achieve the final goal of isolating (and testing) relatively pure crystals of potassium nitrate. Each section contains two subsections: (i) a goal section that defines the experimental objective of the section’s work and gives the students some leading idea to guide that work and (ii) a followup work section that summarizes the information the students should be recording in the lab report, which will be the main source of their final grade. That latter subsection also gives the students some focus questions to make sure that they are understanding the underlying chemical principles and often presents them with extra credit research questions that challenge them to go beyond the basics. The execution of the different components of the project requires a time frame of approximately one month at the end of the introductory chemistry course in which it is conducted. The students are provided some in-class time specifically designated for this project’s work at regular intervals, but they are encouraged to also work on it while conducting more formal experiments. Approaching it this way has made the project more feasible from a time-management perspective because, while some of the necessary project work, such as filtration and evaporation, can be time-consuming (and monotonous), the students can easily carry out these activities simultaneously
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In the Laboratory
with other lab work. One problem, though, with such a selfpaced approach is that some groups will wait to see what other groups are doing as the work proceeds. The rubric for the project discourages this by giving them points for independently developing the procedures necessary in each section. Of course, various groups will try to negotiate hints from the instructor concerning how they should proceed at certain points. If this project is properly integrated into the curriculum, students should, eventually, have all of the necessary chemical principles at their disposal for its successful completion. It is the instructor’s responsibility to maintain the integrity of the problem-solving nature of the project by only giving hints with regards to more esoteric procedural details (such as the use of activated charcoal to remove organic impurities if water is used in the original isolation of calcium nitrate). Another pedagogical aspect of the implementation of this project is the opportunity for the instructor to tie together several concepts in a single “experiment”. Students can be pressed to apply and unify such ideas as solubility tables, concentration, extraction techniques, precipitate formation, crystal growth, qualitative analysis, stoichiometry, and oxidation–reduction reactions. The latter two concepts can be incorporated if the project is taken to its ultimate conclusion: Each group’s final product (hopefully, purified potassium nitrate) is combined into a class sample that is then combined with the appropriate quantities of charcoal and sulfur (as calculated by the class) and tested (by a specially selected student) for its burn characteristics (evenness and completeness of combustion). Hazards If activated charcoal is added to remove organic impurities, standard protocol is to add a scoop-full of it to the solution while hot; students should be instructed of the possibility of spattering from this addition and be prepared for that event. The 1 M potassium carbonate solution that will be added to convert the calcium nitrate into potassium nitrate is basic and therefore needs to be handled with great care. The preparation of the black powder mixture from the calculated amounts of ingredients should be undertaken by the instructor because of the potential risks of creating this composition. If a black powder sample is prepared and tested, it should be placed on an appropriate ceramic surface and ignited with a sufficiently long fuse behind a safety shield. (Instructors may decide not to have a student conduct this test.) Conclusion The 2003–2004 school year marked the “pilot run” of this project. Four different chemistry classes saw it through to completion. It was a very rewarding experience to watch students tackle the problem at hand and proceed through the experimental details. Students exhibited a genuine excitement concerning the work, partly, I believe, as a result of the challenge of it and partly as a result of knowing what the end product would be if they were successful. There was a high level of similarity in how the different groups proceeded through the work, either as a result of the common classroom experiences the students had or because they took cues from the work being done by each other. Minor variations occurred during the extraction step with all groups eventu242
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ally settling on the use of water but with various modifications related to volume and temperature of the water. All groups chose evaporation as the method for concentrating the initial extract, but there were deviations from group to group in such things as whether they chose to let it evaporate on its own or sped up the process with a hot plate or Bunsen burner. Likewise, nearly every group settled on the use of potassium carbonate as the appropriate reagent with only one or two groups choosing potassium hydroxide. The largest variation came in those groups that decided to take on the challenge of removing the organic contaminants. While the majority of groups who did this came upon the use of activated charcoal through Internet research they conducted, a few groups tried to work through this by the use of chemical reasoning and asked to be given a base to neutralize the humic acid (which they were told in the reading was the main contaminant). Since this was something beyond the scope of their experience, I intervened to inform them that this neutralization would just convert it into a different contaminant and would not really remove the material from the extract. There were some marvelous moments of students showing insights into how the chemical principles that had been covered at one time or another in the course could be brought to bear on this problem. Reading the final reports that were presented provided a further window into the thought processes going on and how they reasoned through some of roadblocks. In the end, about three-quarters of the groups presented a product that gave a positive flame test for potassium and a positive strip test2 for nitrate. More impressively, three of the four classes produced black powder samples that burned effectively—a rewarding experience for all!3 W
Supplemental Material
The student version of the project handout, including background, goals, and followup work for all of the components of the project, and the rubric used to evaluate students are available in this issue of JCE Online. Notes 1. Black powder refers to gunpowder, which is formed from a chemical mixture of 75% potassium nitrate (saltpeter), 10% sulfur and 15% charcoal. 2. Nitrate test strips can be obtained from Hach Chemical Company. http://www.hach.com/ (accessed Nov 2005). 3. Although I was unable to verify this, I believe that the groups that were unsuccessful suffered from an incomplete conversion of the calcium nitrate into the potassium nitrate. This likely resulted from not checking for complete precipitation of the calcium (as CaCO3), which groups were guided to do when they got to this point in the work. Support for this hypothesis came from the fact that those groups that were unsuccessful had trouble drying their final nitrate product and calcium nitrate (which would have been left over in the case of incomplete precipitation) is known to be hygroscopic.
Literature Cited 1. Pinkerton, D. J. Chem. Educ. 2001, 78, 198. 2. Barnum, D. J. Chem. Educ. 2003, 80, 1393. 3. Ziemke, P. J. Chem. Educ. 1952, 29, 466.
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