LABORATORY MANUAL
A315: Chemical Measurements Laboratory
Department of Chemistry Fall 2009
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A315: Chemical Measurements Laboratory Term:
Fall 2009
Instructor:
Randy J. Arnold email:
[email protected] office: Simon Hall 120C office hours: by appointment
Lecture
Sections 7723 (A315)
Pre-Lab Exams Laboratory
8590 (A315) 7724 (A315)
Time MWF 12:20-1:10 pm
Room C001
F, 12:20-1:10 pm
C001
M, 1:30-5:15 pm W, 1:30-5:15 pm
C133 C133
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A315: Chemical Measurements Laboratory Chemical Measurement Laboratory (A315) teaches chemists the techniques of chemical measurement and introduces them to the tools of measurement science. A315 is: 1) application of the proper tool and the correct procedure to the relevant measurement problem, 2) description of the measurement and its quality to those who did not perform the experiment, and 3) development of analytical intuition. The goal of A315 is to teach you to begin thinking like an experimental chemist. In this course, you will become familiar with important tools, techniques, and procedures of experimental chemistry. The experiments are not "cookbook" oriented, but are designed to test your understanding of concepts and procedures. Each experiment builds upon the chemical experience that you gained in previous lecture and lab courses. Each experiment will require constant evaluation on your part as you proceed in the laboratory. Each will require outside reading. Most importantly, each will require proper preparation before the laboratory session begins. A315 will require a great deal of your time. This arises because each of the experiments is new to you. You will use the general principles learned in A315 whether you find yourself in an industrial laboratory, or in graduate or professional school. The tools of A315 are the same tools used in advanced chemical and medical research. Along with the methods of research, considerable emphasis is placed on the ability to prepare laboratory reports and to master the skills of scientific reporting and explanation. In the past many students have used the book by Stephen Brewer, "Solving Problems in Analytical Chemistry”, John Wiley, NY, 1980. It is a practical guide to many of the experiments of A315, with sections on Beer's Law, gas chromatography, high performance liquid chromatography, and spectrophotometric measurements. It also provides a brief review of statistics and stoichiometry. Reviews of the instrumental aspects of the experiments are available in Peters, Hayes, and Hieftje (PHH), Willard, Merritt, Dean, and Settle (WMDS), and Skoog, Holler, and Nieman. Neither lecture nor lab reviews the principles of solution preparation, serial dilutions, or general statistics, all of which should be familiar to you from your previous courses. The following general analytical chemistry texts are on reserve in the Chemistry Library: 1) S. Brewer, Solving Problems in Analytical Chemistry, John Wiley, NY, 1980. 2) H.H. Willard, L.L. Merritt, Jr., J.A. Dean, and F.A. Settle, Jr., Instrumental Methods of Analysis, Seventh Edition, Van Nostrand, NY, 1988. 3) D.G. Peters, J.M. Hayes, and G.M. Hieftje, Chemical Separations and Measurements, Saunders, NY, 1974.
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4) H.H. Bauer, G.D. Christian, and J.E. O’Reilly, Eds., Instrumental Analysis, Allyn and Bacon, Boston, 1978. 5) D.A. Skoog, F.J. Holler, and T.A. Nieman, , Principles of Instrumental Analysis, Fifth Edition, Brooks/Cole, 1998.
LABORATORY LOGISTICS The laboratory is located in Room C133. This laboratory is equipped with all of the instrumentation used in A315, hoods, sinks, balances, and chemical storage. What is not available is an excess of space, considering the large number of students scheduled into the laboratory. Cooperation and patience are essential. As always, safety rules will be enforced. This means safety goggles, proper labeling of solutions, and clean benches and common areas. In addition to Room C133, the GC-MS instrument used for A315 is located in the Mass Spectrometry Laboratory, Room A411. Because of equipment expense and your safety, you will be allowed to work in the A315 laboratory only when an AI is present. Each experiment is designed to be finished in the time allotted for it, provided you come to the laboratory prepared for efficient work. To help ensure that this is the case, each experiment has associated with it a qualifying exam, which is scheduled the week before laboratory work on the new experiment is to begin. This pre-lab exam will be taken in conjunction with all of the other students who are scheduled for that experiment. You must pass the exam in order to begin work on a given experiment. In addition to the pre-lab exam, each student will submit a written procedure for the lab prior to the entering the lab. You will have lab partners for all of the experiments that you will perform. A list showing lab partners will be distributed during lecture. The lab schedule for the whole semester might be changed from that shown in the initial schedule. If there are necessary changes, a revised schedule will be distributed at the next lecture. Lab partners will change with experiments so that you have the opportunity to work with different people. In general, experiments are carried out by groups of two or three. This surfeit of personnel affords plenty of opportunity to plot data as they are obtained and to evaluate them, to label the data accurately, and to repeat experiments within the allotted time if necessary. It is permissible for you to join forces with your lab partner(s) to analyze your data. However, you must write up your lab reports yourself. Use this opportunity to learn to write about what you have done in a way that can be easily understood by others. If the AIs cannot follow your descriptions or arguments, this will be reflected in your course grade. The bench space and drawers are marked with the name of the experiment. Do not use equipment from any drawer other than that appropriate to your experiment. If something appears to be missing, ask the AI to provide a replacement. It is best to check the drawer contents at the beginning of each experiment to make sure that you have everything you need. It is prudent to check the drawer contents also after you complete the experiment to be sure that you have replaced everything. At random, drawers are pulled and their contents checked; shortages are reflected in your lab grade. Missing/broken glassware constitutes one of the major expenses of
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this lab and is the main reason behind the institution of a laboratory fee for this course. Do not leave glassware out or unlabelled between your lab sessions.
IMPORTANT NOTICE! You must have passed A318 in order to be in this course. If you have not had A318 or received a grade lower than C- in A318, see the course instructor immediately.
REQUIRED MATERIALS 1) A315 Laboratory Manual and lab handouts for Fall 2009 –on the course website 2) goggles (required in lab at all times!) 3) bound laboratory notebook with numbered pages (no loose leaf).
GENERAL LABORATORY POLICIES 1) All students are responsible for reading and understanding the information presented in this document. A315 course requirements apply equally to all students, regardless of whether they have read this material or not. 2) Your Associate Instructor is not authorized to waive any of the requirements in this document. 3) Instructors cannot give credit for work not done. Any student who does not meet the course requirements (such as submitting lab reports by the deadline) should consider withdrawing from the course prior to the last day to withdraw with a "W". 4) To maximize your point total in this class, attend every scheduled class meeting. You are responsible for obtaining information that is announced or handed out during any class period, including those you did not attend. 5) If you wish to report a grading error or a grade injustice, you must contact your AI no later than seven days after the item in question has been placed in the file cabinets. No re-grades and no redresses for missed points will occur after this time. If you contact your AI in the first seven days and the two of you cannot reach an equitable agreement, bring the issue to the instructor’s attention for discussion. 6) THERE ARE NO MAKEUP LABORATORIES AND NO DROPPED GRADES IN A315. If an absence is unavoidable (especially for university-related business), it is the responsibility of the student to contact the AI responsible for your section at least 24 hours prior to the absence. No redresses for a missed laboratory will be given if such contact is not made. If, in the judgment of the instructor, the circumstances warrant accommodation, students may be allowed to complete a laboratory prior to a scheduled absence. In cases of rare and serious circumstances (such as a death in the family or serious illness), you should contact an AI by email as soon as 5
possible. To accommodate such events will probably require documentation by someone in authority. Regardless of the circumstances, no accommodation for an absence will guarantee the possibility of full credit for a lab. Your AI is not authorized to grant a makeup experiment. 7) Late laboratory reports are not accepted, and no credit will be awarded for late reports. All laboratory reports will be submitted through turnitin.com and are due at the start of the laboratory session that meets one week after the last day of your previous lab. 8) The computers in C133 are to be used for course-related work only. Violations will be subject to a standard point deduction. 9) The contents of this syllabus including these and other policies are subject to change. All changes will be preceded by announcements in class or during the laboratory period.
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LABORATORY SAFETY Your safety in the laboratory is of utmost concern. The following rules must be observed. 1) Safety goggles must be worn at all times while you are in the laboratory. Even though you might not be doing something hazardous, a person a few meters away could make a big mistake. A point deduction will be applied to the experiment being performed for any student found in the lab without safety goggles. 2) No sandals, open-toed shoes, or bare feet are allowed in the lab because of chemical spills and glassware breakages. You may choose to wear a lab coat, but it is not required. Clothing must cover the shoulders and all skin below the armpits. Violators will not be allowed to enter the laboratory. In general, we recommend that you do not wear expensive clothing or jewelry into this or any other chemistry laboratory. 3) Know the location and proper use of the safety equipment in the lab. This includes fire extinguishers (and their type), fire blankets, safety showers, eye washes, and the first aid kit. The eyewash and shower area must be kept clear. During the first week in the lab, your AI will show you the location of all safety equipment. 4) All chemicals and solutions must be labeled. Proper disposal procedures for acids, bases, and heavy metals must be followed. 5) The instruments in A315 use high-voltage electricity, high gas pressures, open flames, and/or hazardous light emission. The instrument manuals explain the proper precautions to be observed. If you have questions, ask an AI! 6) The attached information gives proper guidelines for handling of high-pressure gas cylinders such as those used in the atomic spectrometry and GC-MS experiments. 7) No Cell Phones in the laboratory. 8) No eating, drinking, or smoking in the laboratory. 9) Because of safety and equipment expense, you will be allowed to work in the A315 laboratory only when an instructor is present. Any student found working in the lab alone is subject to dismissal from A315. The only exception to this rule will be for students who wish to read instrument manuals ahead of lab time. Also, this latter activity will be permitted only while another lab section is in session and only if the activity does not interfere with that section's activities.
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LABORATORY NOTEBOOKS Professional chemists keep professional notebooks. It is not a practice that exists without justification. In the "real world," lab notebooks come into play in patent disputes, in post-mortem examination of failed experiments, and in confirmation of proper procedure. In the "A315 world," your lab notebook serves the last two functions. Your notebook should be the central focus for your clear thinking and efficient experimental procedure. In preparing for lab, perform any calculations which will make the experiment go faster once you are in lab. Make these calculations in your lab notebook, so they will be easily available. If there are tables of data that need to be obtained, prepare the headings and spaces ahead of time in the notebook. This will remind you to take the data. If there are especially important pieces of data that you will need to get, emphasize that ahead of time in the notebook. In the lab, keep a log of what you did and why. You should not duplicate the written procedure of the lab manual. Rather, you should record: 1) The date, along with your name, the name(s) of your lab partner(s), and the name of the experiment. 2) How you prepared your solutions. 3) Relevant instrumental parameters. 4) Errors or difficulties encountered with the instruments. 5) Names of computer-generated data files and descriptions of their contents. 6) Questions you have about procedure or the reliability of the data. 7) Breaks in continuity -- when samples were stored, when the instrument was turned off and then restarted, and when the parameters of the instrument were changed. As the experiment progresses and while you are still in the laboratory, you should make some preliminary evaluation of how the experiment went — are there data you might have to reacquire? Do the plots of the data look good? A good idea is to create a plot in Excel and add data points as you acquire them. You can neither prepare new samples nor make any measurements on them once the lab is over. Your laboratory report should be written from the notebook. A good notebook will make lab report writing a cinch rather than a chore. If there are questions about your lab report, the AIs will ask to see your lab notebook. Typically, you will spend far too much time preparing the lab report if your notebook is not properly prepared. Since your lab notebook is your own, there is no officially approved notebook you should use. However, a laboratory notebook must: 1) be a bound notebook (no loose pages), 2) have numbered pages and a table of contents (leave space at the beginning, e.g., 5 pages, and update as necessary), 3) have entries in ink, 4) not have erasures or white out, 5) have consecutive entries, 6) have pages dated and signed, and 7) be in the lab when you are.
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The instructor or AIs will at random examine your notebook in the laboratory for completeness and accuracy. This examination will form part of your lab grade. In addition, as was mentioned above, it may be examined as part of the formal lab report grade, or whenever there are questions of data or procedure. Please read the following article by Anne Eisenberg on "Keeping a Laboratory Notebook."
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GRADES Grades in this course will be based on a total of 900 points received for: 1) two regular laboratory reports including associated pre-lab exams (25 points for each pre-lab exam and 100 points for each report; 250 total points), 2) two short laboratory reports including associated pre-lab exams (25 points for each pre-lab exam and 100 points for each report; 250 total points), 3) research article presentation (100 points) 4) 100 points assigned by AIs for lab preparation, attendance, attention to safety, lab notebook quality, efficiency of work, and ability to work with lab partners 5) class project (200 points) PRE-LAB QUALIFYING EXAMS For each experiment there is a short pre-laboratory qualifying exam (a qualifier) that is designed to test your preparedness for the experiment. These pre-lab qualifiers will be administered by the AIs, and students must complete the qualifier to gain entry into the lab. A qualifier is worth 25 of the total 125 points for each experiment. The qualifier will generally consist of a short quiz (5-6 short-answer questions). In some cases, the AI will wish to administer the pre-lab exam in the lab next to the equipment that you will use during the experiment. Nonetheless, plan to meet initially in room C001. Qualifiers are administered on Fridays during the lecture period. To pass the qualifier, you will have to have read the Laboratory Manual section for that experiment. Scattered throughout the experimental write-ups are parenthetical questions that are designed to test your understanding of the material. Expect these questions to be asked as part of the qualifier. Also, information from in-class presentations and any suggested reference material that the Laboratory Manual cites may be asked on the qualifier. You must pass the qualifier before you begin work on a new experiment. Typically, a score of 60% or higher is needed to pass. You will be notified by email by the AI if you did not pass a pre-lab qualifying exam. If the qualifier is not passed on the first attempt, a second chance to take the qualifier for a total of ten points will be given. If you do not pass this second opportunity, you will forfeit the experiment and all the points involved with it. Second-chance qualifiers are administered by appointment with the AI for that experiment.
LABORATORY REPORTS Professional reports are an integral part of professional work, and scientific writing is distinct from creative writing. There should be no ambiguity or nuances of meaning in scientific writing. The grading of laboratory reports constitutes the majority of your grade in A315, so you should attach some importance to the perfection of your skill in their preparation. The purpose of the laboratory report is to demonstrate precisely the quality of your data and your understanding of the experiment. The care with which the data are presented reflects the care with which it is taken. All lab reports for A315 must be typewritten! 14
The Bloomington campus of Indiana University has a large number of computer labs with extended hours of service and the most up-to-date systems. Furthermore, all students have access to turnitin.com from any internet-accessible computer. You do not need to own a computer for this course. Your AI will accept hard (printed) copy of selected portions of your report that cannot be easily submitted electronically (for example instrument print-outs). The studentwritten portions of lab reports will only be accepted in electronic format through turnitin.com.
REGULAR LABORATORY REPORTS In general, your lab report should follow the format established in the professional journals in chemistry. Good examples are papers found in the journal Analytical Chemistry, which is found in the Chemistry Library. The general outline is title, author, introduction, experimental section, results and discussion, conclusions, literature cited, answers to parenthetical questions, and then an appendix containing the figures and graphs. The experiments vary in the amount of discussion appropriate, or in the number of tables and figures required. Remember to include all of what was asked for in the lab manual preferably in the order requested, to label the information clearly, and to discuss the results scientifically. Regular lab reports must be typed doublespaced in a 12-point font and submitted electronically through turnitin.com. 1) Title page. The report should have a cover page. This page should include the title of the experiment, your name, your section, and the date of submission. Additionally, it should list the names of your partners, and the code numbers for unknowns analyzed in the experiment. The title page is located in the front of the report. Your name should also appear on each page of the report. 2) Introduction. You can assume that the reader is generally familiar with the objectives and methods of the experiment. The purpose of the introduction is simply to remind the reader what the experiment is about. Since the work you will be doing repeats that which has previously been carried out by chemical researchers (see the references section at the end of each experiment), you will want to introduce the experiment in light of that previous work. You should introduce here also the theory and any fundamental equations that you'll use in your calculations in the latter sections. 3) Experimental Section*. A lab report must describe experimental procedures and materials in detail sufficient for replication of the experiment by a competent researcher. Do not repeat the verbiage from the laboratory manual in this section, as this is available and should be listed as a reference. You should take care to note any deviations from the procedure in the manual, writing with the same detail as the lab manual itself. You need also to record the sources of the chemicals used and the models of instruments used in taking the measurements. These are reported in the literature because chemicals from different suppliers may have slightly different compositions that on occasion influence the results obtained. Similarly, different instruments operate in slightly different ways. For example, an IR spectrum from one brand or model of instrument may have subtle differences from that obtained with another. The instrument parameters should also be reported in this section, as well as the details of instrument calibration. 15
*The experimental section of your lab report must be submitted in advance of entering the laboratory to begin the experiment. Late submissions will not be accepted. For completion, the experimental section may be included in the final report, but is not required in the final report. 4) Results and Discussion (R&D). This will be the longest part of the lab report and the most important. You should not start writing this portion of the lab report until you have assembled all of the data and figures in the proper order and constructed all of the figures that are called for in the lab report. Having pondered the data, you should write a general outline of this section, and then flesh out with details in several drafts. In the literature, or in grading of A315 lab reports, the reader will look for the results in the tables and figures before reading the R&D section. Thus, the text should not repeat verbatim data presented in the figures or tables, but the text should clearly refer to them, comment on their significance, and why they did or did not correspond to previous results reported in the literature. One of the goals of the R&D section is to convince the reader that your results and your conclusions should be seriously considered. Adding material that does not contribute to this objective, which belabors the obvious or is more effectively presented in tables or graphs, detracts from the quality of the report. Padding is painfully obvious and, in A315, will result in a lower grade. In the "real world," this alone will be sufficient reason to reject a paper submitted for publication or will not impress your supervisor who must then extract the relevant results of your work. Perhaps the best attitude from which to write this section of the report is to assume that you are performing this experiment prior to starting a larger project. Before investing a great deal of time, effort, and expense in the larger project, the reader will search for a logical, objective analysis of your initial results. If the experiment failed, it is to your credit to recognize this and to say so explicitly. You are obliged to suggest reasons why this failure might have occurred and actions that you might take to prevent its recurrence. To help a reader to assess the significance of your data and their relevance to your report, it is essential to include an error analysis with your data. This analysis should be as quantitative as possible. You should evaluate the sources of uncertainty in your measurements and perform the appropriate propagation-of-errors analysis. With a measurement and its associated uncertainty, you can confidently compare your result with the expected or theoretical behavior. In cases of statistically significant deviations (for example, a curvature of a theoretically linear Beer's Law plot, or the non-intersection of a line with the origin of the graph), you should attempt to assign the sources of the error and quantitatively rank their significance. It is poor scientific method to report the result of a measurement without the associated uncertainty. For example, Ca2+ determined in water might be reported as 20±5 ppm, but should not be reported as 20 ppm. There are numerous texts that cover statistical analysis of data and propagation of errors analyses (PHH has an extensive discussion.). A brief summary is given in this lab manual. Tables of data should be referred to in the R&D section but contained in the Appendix. Figures should be number consecutively as they are referenced in the R&D section but should likewise appear in the Appendix. In some instances, it will be more appropriate to submit Tables and 16
Figures in hard copy as opposed to electronically, but if feasible, electronic submission is preferred. 5) Conclusion. Included here should be a summary of the experimental results. You should reiterate the identity of your unknown or the value of the measurement that you obtained. You should in a sentence or two explain whether your results are consistent with literature or theory and, if not, why. 6) Literature Cited. This is a list of references that have been specifically cited in the text of the lab report. Use the standard, complete format such as that followed in Analytical Chemistry. References in the text should be numbered consecutively in the text. Examples: 1) A. Eisenberg, J. Chem. Educ., 59, 1045 (1982). 2) 315 Laboratory Manual, Department of Chemistry, Indiana University, Fall 2005, pp. 27-35. 3) F. W. McLafferty, Ed., Tandem Mass Spectrometry, John Wiley, New York, 1983. 4) D. G. Peters, J. M. Hayes, and G. M. Hieftje, Chemical Separation and Measurements, Saunders, Philadelphia, 1974. 7) Appendix. This section should contain the tables and figures. Make sure that your name is on all of the tables and figures that you turn in with your lab report. One partner should submit the original instrument output, and photocopies are acceptable for the other report(s). Tables should be self-contained. The reader should be able to make use of them without having to refer to the textual material. The table should have a number, a title, and a caption that specify the nature of the included data. In most cases, these should be submitted electronically. Figures should be numbered consecutively and have a self-explanatory caption. For the figures, a) the axes should be clearly labeled with titles and units, b) the scales should be marked with appropriate divisions, c) experimental points and associated error bars should be distinct, and d) if appropriate, the least-squares best fit line should be drawn through the points with the m, b, and n values given in the figure caption. Do not fall prey to the "connect-the-dots" syndrome.
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HOW TO ORGANIZE YOUR WRITING EFFORT 1) Experimental. Write this section first and submit it prior to beginning your lab work. Include all details that you do not want to repeat in the R & D section because they would disrupt the flow of the discussion. Use sub-sections for clarity, e.g., apparatus, reagents. 2) Prepare the figures and tables. Decide the presentation format that will emphasize the points you want to make. Organize the order of the figures and tables so they tell the story most clearly, and use this order as your outline for writing the Results & Discussion section. 3) Results & Discussion. Begin with the first figure or table that you selected in the "outline" mentioned above. Describe in the text of your report exactly what the experiment was, and then introduce the data in writing. The data can be described first qualitatively, and then quantitatively. Remind the reader of any theoretical relationship that might be useful to explain the data you have just mentioned. Refer to this relationship with a title (e.g. Beer's law), if one exists, and refer to any equation that was entered in the Introduction. Next describe qualitatively and quantitatively the quality of the fit, and explain any major discrepancies. Draw any major conclusions from the agreement or disagreement with the theoretical fit. Go on to the next figure or table and repeat this procedure. Continue until you have finished with all figures and tables. NOTE: As you write this section, keep notes of items to include in the Introduction such as theoretical relationships and in the Conclusion such as important findings. Similarly, you might wish to compile a list of experimental details that were modified and should be added to the Experimental section. 4) Conclusions. Using the outline that you prepared during your writing of the R & D section, try to summarize the objective and the experimental findings as succinctly as possible. Then emphasize all the major conclusions on your list. 5) Introduction. This should, interestingly, be one of the last things that you write. First introduce the experiment and briefly describe its objectives. Introduce in order any mathematical or theoretical relationships that you will need in the R & D section (obtained from the notes that you compiled when you wrote the R & D section). Make sure you define all variables that are used in these relationships and include a literature citation where the relation was obtained or can be found. References can be indicated by parentheses (9) or brackets [9]. 6) Literature Cited. As stated earlier, the references should appear after the Conclusion section, listed in the order in which they were used in the text. To help keep the references in order, it is often useful to leave blank parentheses or brackets in the text until the report is finished. Then you can go back and fill in the brackets once the final reference order is known. This procedure avoids time-consuming renumbering of citations when a new one is discovered. Even better, you may wish to use a program such as EndNote, which interfaces seamlessly with most wordprocessing programs and keeps track of reference order for you.
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SHORT LABORATORY REPORTS 1) Experimental. Write and submit this section prior to beginning your lab work. Include all details that you do not want to repeat in the executive summary because they would disrupt the flow of the discussion. Use sub-sections for clarity, e.g., apparatus, reagents. 2) Executive Summary. The purpose of the executive summary is to succinctly present your experimental findings. Similar to the regular lab report, you can assume that the reader is generally familiar with the objectives and methods of the experiment. The summary should begin with a brief introduction to tell the reader what the experiment is about. Prior to writing the executive summary, you should first prepare the tables and figures that are called for in the lab manual and assemble them in the proper order. Thus, the text should not repeat data presented in the figures or tables. The text in the summary should clearly refer to them, comment on their significance, and why they did or did not correspond to previous results reported in the literature. Where necessary, e.g., a calculation, refer to the notebook pages appended to the summary. The executive summary is limited to one page of text that is single-spaced, has 1” margins, and is typed in a 12 point font – it must be submitted electronically through turnitin.com. 2) Literature Cited. This is a list of references that have been specifically cited in the text of the summary. Use the standard, complete format such as that followed in Analytical Chemistry. References in the text should be numbered consecutively in the text. 3) Appendix. This section should contain the tables, figures, and photocopies of all notebook pages associated with the experiment. Make sure that your name is on all of the tables, figures, and notebook pages that you turn in with your lab report. One partner should submit the original instrument output, and photocopies are acceptable for the other report(s). Tables should be self-contained. The reader should be able to make use of them without having to refer to the text. The table should have a number, a title, and a caption that specify the nature of the included data. Figures should be numbered consecutively and have a self-explanatory caption. For the figures, a) the axes should be clearly labeled with titles and units, b) the scales should be marked with appropriate divisions, c) experimental points and error bars should be distinct, and d) if appropriate, the least-squares best fit line should be drawn through the points with the m, b, and n values given in the figure caption. Photocopies of all Notebook Pages associated with the experiment must be submitted in addition to the short lab report. If material in the notebook is referenced in the executive summary, the information should be highlighted on the photocopies for easy identification. The notebook pages must be signed and dated by the AI present for the lab period.
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LAB REPORT GRADING Electronic reports must be submitted by 11 am and hard-copy materials must be turned into the AIs at the beginning of the lab period one week after the completion of an experiment. You should keep a copy of your lab report for safekeeping. The point assignments can be found at the end of each experiment. Use these tables as a guide for including relevant information in your report. Also, to expedite grading and to minimize hunting for information within your report, present your data in the order listed in these tables. Points will be deducted for late, illegible, or incomplete reports. Also, reports will be downgraded for having faulty reasoning, lacking statistical evaluation, containing poor quality figures or tables, lacking comparison with literature or theory, having no conclusions, using significant figures improperly, having poor organization, excessive padding, using poor grammar, containing misspellings, and plagiarism. Every year we make a few minor changes in the lab manual. Because of these changes, it is easy for an AI or turnitin.com to spot plagiarism when grading a lab report. If a person is caught copying from someone else's report (from the present or an earlier year), his or her grade for that experiment will automatically be zero. This minimum penalty is sufficient to bring your overall course grade down by more than a whole letter grade. There is also the possibility of academic probation. Lab reports will be returned to you during the laboratory period one week after the due date. Each student has a file in the lab where the reports are kept and are available for in-lab inspection. The reports must remain in these files until final grades are assigned. At the end of the semester, the entire file will be returned to the student upon request. Grading of lab reports is a time-consuming business, so an AI may count off points or put question marks on a page without any comment or with only brief explanation. You should see that AI as soon as possible after you get your lab report back and ask what the notations mean. This action will help you prepare a better report next time and will help you learn the things you did not understand in the report. Afterward, if you still don't understand the grading of the material, please see the instructor.
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INSTRUMENT CARE The experiments in A315 utilize a number of instruments and tools with which you are not familiar. Several of the instruments cost more than you will make in your first year as a professional in chemistry. All of them are of fundamental importance in chemical research, and this first exposure to them will likely determine how efficiently you can use these methods in the future. The instruments are designed to be used in a specified manner which is laid out in the instruction manuals. The instruments are not fragile— rather, they are fairly robust and perform quite well considering both the intense use and abuse by A315 and the other courses that use them. Nevertheless, if not treated properly, they will fail. Each experimental write-up in the manual contains apparatus notes that describe the hardware, and you should be familiar with this material before beginning an experiment. The qualifiers will draw on this material. There are also detailed operator's manuals in the lab for each instrument that you may examine. Do not remove these from the lab, since other operators will need to refer to the same manual. Experience has shown that many instrument “malfunctions” can be traced to improper operation or to incorrectly prepared samples. In many cases, you are performing analyses at the ppm level or below, and seemingly small errors in weighing and dilution can be magnified into large errors in analysis. The analytical balance is the most critical instrument in the laboratory—accurate weights of standards all follow from the correct use of the balance. It is imperative that the balance be used properly, and that it be kept clean and calibrated. Failure to keep the analytical balances clean will result in a grade penalty for ALL A315 students present in the lab when it occurs – if you find a mess at any of the balances, don’t wait for someone else to clean it up – do it yourself (unless you fear it may be harmful to do so).
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CHEMICAL WASTE DISPOSAL An experiment is not complete until you have dealt with the chemical waste that the experiment has generated. Proper chemical waste disposal is essential. The Environmental Protection Agency (EPA) has laid out guidelines for how to collect, store, transport, and dispose of many different types of chemical waste. There are only three categories of chemical waste in A315: 1) Aqueous 2) Heavy Metals 3) Organic Each experimental procedure contains specific information concerning the disposal of the various solvents and samples that you will prepare. If you are not sure how to dispose of a sample, ask an AI. Proper disposal of waste becomes especially important in view of the fact that compared to aqueous waste, heavy metal waste is much more expensive to safely discard. When only 1 mL of heavy metal waste is inadvertently added to a bottle containing 4 L of aqueous waste, the entire bottle is, by EPA definition, converted to heavy metal waste. A single misplaced milliliter can be very expensive!
CHEMICAL WASTE Experiment AA CE
Organic
Ferrocyanide and methyl viologen
CV FTIR GC–MS HPLC
Aqueous
Hexane Hexane DMSO
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Heavy Metal Lanthanum
PREPARING STANDARD SOLUTIONS Students frequently find the following guidelines on preparing standard solutions helpful. This material is contained in most common textbooks on quantitative analysis. Units of concentration 1. Molarity (molar): Concentrations are expressed in units of molarity (M), which is the number of moles of solute per liter of solution. For example, sodium hydroxide has a molecular weight of 40.00 g/mol; hence, 100 mL of a 0.5 M solution of sodium hydroxide contains 0.05 mole or 2 g of sodium hydroxide. 2. Normality (normal): Concentrations are expressed in units of normality (N), which is the number of chemical equivalents of solute present per liter of solution. For solutes such as sodium hydroxide that contain one equivalent per mole, normality and molarity are identical. For solutes such as sulfuric acid that contain more than one equivalent per mole, normality and molarity are not the same. For example, a 0.1 M solution of H2SO4 (which contains two moles of protons per mole of H2SO4) is 0.2 N. Similarly, a 0.1 M solution of H3PO4 is 0.3 N. 3. Percentage (by mass and volume): Concentrations expressed in percent solute by mass or volume are in no way linked to the concept of the mole and therefore have little significance for the analytical chemist. However, percent concentrations are still commonly used in industry as well as in medicine and pharmacy, so you should be familiar with the following expressions: •
In industry, the most common expression of concentration for solutions of solid solutes is percent by mass, which is also referred to as percent by weight and often abbreviated as (w/w). Simply put, percent by mass refers to the ratio of the mass of solute to the mass of the solution as a whole. For example, household bleach is 5.25% sodium hypochlorite by mass, which means that 100 g of bleach contains 5.25 g NaOCl. A 100 mL sample of bleach contains more than 5.25 g NaOCl because it weighs more than 100 g!
•
Concentrations of solutions prepared with liquid solutes are often expressed in units of percent by volume. This percentage refers to the ratio of the volume of solute to the volume of the solution as a whole and is often abbreviated as (v/v). Thus, an aqueous solution containing 25% methanol by volume may be prepared by dissolving 25 mL methanol in enough water to make the final volume 100 mL. Read that statement again, because mixing 25 mL of methanol with 75 mL of water does not result in a 25% solution! Can you explain why?
•
In medicine and pharmacy, concentrations are frequently expressed in percent by mass/volume. This expression refers to the ratio of a given mass of solute (in grams) to the volume of solvent (in mL) and is often abbreviated as (w/v). Note that the volume of solvent is not necessarily equal to the volume of the final solution. Thus, a solution prepared by dissolving 0.9 g of sodium chloride in 100 mL of water is 0.9% NaCl (w/v).
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4. Parts-per-million (ppm) and parts-per-billion (ppb): Concentrations are expressed as the weight ratio of solute to solvent, where ppm corresponds to a ratio of 1 in 106 and ppb corresponds to a ratio of 1 in 109. For room-temperature aqueous solutions, the approximation is frequently made that the density of the solution is exactly 1 g/mL. In that case, one liter of water weighs exactly one million milligrams, which means that: 1 ppm =
1 mg 1 µg = L mL
1 ppb =
1 µg 1 ng = L mL
Similarly,
A subtle distinction arises in considering how to use ppm and ppb to express concentrations of elements in solution. An aqueous solution labeled “1000 ppm sodium” actually contains 1000 mg of sodium ions per liter of solution. This is because the ppm unit is associated only with sodium, which exists exclusively as Na+ in aqueous solutions. Fortunately, the weight of 1 mole of sodium ions is essentially identical to the weight of 1 mole of sodium atoms and the solution therefore contains: 1000 ppm sodium =
1000 mg Na + L
1 g Na + 1 mol Na + 4.35 × 10 Š2 mol Na + 1000 mg Na + x x = L L 1000 mg Na + 22.99 g Na +
Note that the concentration above is expressed in moles of sodium ions per liter, which says nothing about the identity or concentration of the counter ion that we know must also be present. In other words, the person who made this solution did not begin by reaching for a bottle of sodium ions—he or she began by weighing out a sample of a given sodium salt. Let’s say that the salt used to prepare this solution was sodium chloride and, for simplicity, that the volume of the solution is exactly one liter. That would require: 1L x
1 mol NaCl 58.44 g NaCl 4.35 × 10 Š2 mol Na + x x = 2.54 g NaCl 1 mol Na+ L mol NaCl
Hence, one liter of water containing 2.54 g sodium chloride is “1000 ppm sodium”.
Verify for yourself that 100 mL of water containing 9.0 mg sodium bromide (NaBr, 102.90 g/mol) is 20 ppm sodium.
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SERIAL DILUTION The example above for ppm/ppb concentrations highlights a problem in preparing standard solutions, namely that preparing very dilute solutions would seem to require weighing out very small quantities of solute. This is often not possible given the precision of most analytical balances, and even if it were possible, it would not be especially advisable since the error in such a measurement would be relatively large. The correct way to prepare such solutions is by the method of serial dilution. This simply means that one starts by preparing a concentrated solution (a “stock” solution) for which the amount (weight) of solute can be measured accurately. Then, more dilute solutions are obtained by volumetric dilution of the concentrated solution with the appropriate solvent by using volumetric glassware such as volumetric flasks and pipettes. The relative precision of the volumetric glassware is much greater than that of the analytical balances, particularly when the balances are used to weigh very small amounts of material. Hence, even when the uncertainty associated with several volumetric dilution steps is propagated; it is still much smaller than the uncertainty that would be obtained by weighing a very small quantity of material and preparing a dilute solution in a single step. It is almost always advisable to use the serial dilution method to prepare dilute standard solutions over a range of concentrations. In some cases, it will be advisable to weigh out a small amount of material to prepare a dilute standard solution rather than using the serial dilution method described above. These include instances where the material of interest is: • • • • •
Available in only small quantities Unusually expensive Difficult to synthesize Especially toxic Reactive (unstable)
In these cases, preparing large volumes of concentrated solutions wastes material and creates large quantities of toxic waste that must be disposed of. Your decision about how much solution to prepare is ultimately a compromise between a need for high precision and accuracy and a desire to limit the amount of material used. Consult the AI if you are uncertain as to how much of a given standard solution you should prepare for a given experiment.
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Example of serial dilution: A student prepared a stock solution of iron nitrate in water by dissolving 0.7402 g of ferric nitrate nonahydrate (Fe(NO3)3 • 9 H2O, 403.99 g/mol) in exactly 1 L of distilled water. The concentration of iron in the stock solution was therefore: ⎛ 1 mol Fe 3+ ⎜0.7402 g × 403.99 g ⎝
55.847 g Fe 3+ 1 mol Fe 3+ 1.000 L ×
×
1000 mg ⎞ ⎟ g ⎠
102.3 mg Fe 3+ L
=
= 102.3 ppm Fe 3+
The student then prepared a more dilute sample (solution A) by adding 5.00 mL of the stock solution to an empty 100-mL volumetric flask and diluting to the mark with distilled water. In similar fashion, an even more dilute sample (solution B) was obtained by adding 1.00 mL of solution A to an empty 100-mL volumetric flask and again diluting to the mark with distilled water. To find the concentration of iron in solution B, the student first calculated the concentration of iron in solution A: 102.3 ppm Fe 3+ ×
5.00 mL = 5.12 ppm Fe 3+ 100.0 mL
Solution B therefore contained: 5.12 ppm Fe 3+ ×
1.00 mL = 0.0512 ppm Fe 3+ = 51.2 ppb Fe 3+ 100.0 mL
STATISTICAL TREATMENT OF DATA When more than one observation is made of some quantity, it is important to find the best single value to represent the data obtained. Furthermore, it is important to provide some measure of uncertainty that is associated with the data. Below are just some of the formulas that will aid you in reporting your data. A more complete discussion of statistical analyses can be found in any instrumental analysis text. z
∑x Number of observations = n
Average ( x ) = z
∑ (x Standard deviation ( s x ) =
i =a
− x)
2
i
n −1
Relative standard deviation (RSD) =
sx ×100% x
(Sometimes incorrectly called the Coefficient of Variation or CV)
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i= a
n
i
⎛ ts ⎞ Confidence limits for an average: x ± ⎜ x ⎟ ⎝ n⎠ (Where t comes from the table below)
The t Distribution (n – 1)
90% Confidence
95% Confidence
99% Confidence
99.9% Confidence
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 20 30 60
6.314 2.920 2.353 2.132 2.015 1.943 1.895 1.860 1.833 1.812 1.796 1.782 1.771 1.761 1.753 1.725 1.697 1.671
12.706 4.303 3.182 2.776 2.571 2.447 2.365 2.306 2.262 2.228 2.201 2.179 2.160 2.145 2.131 2.086 2.042 2.000
63.657 9.925 5.841 4.604 4.032 3.707 3.499 3.355 3.250 3.169 3.106 3.055 3.012 2.997 2.947 2.845 2.750 2.660
636.62 31.598 12.924 8.610 6.869 5.959 5.408 5.041 4.781 4.587 4.437 4.318 4.221 4.140 4.073 3.850 3.646 3.460
Example: A driver suspected of DUI submitted a blood sample that was assayed in triplicate for the alcohol content. In units of percent by volume, the results were 0.084, 0.089, and 0.079. Calculate (a) the RSD and (b) the 95% confidence limit for the mean.
sx = 0.0050
x = 0.084
RSD =
C.L. =
0.0050 × 100% = 5.6% 0.084
(4.303)(0.0050) = 0.012 3
Thus, at 95% confidence the percentage of alcohol was 0.084 ± 0.012 with an RSD of 5.6%.
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Linear Regression Linear regression can be performed with the aid of Microsoft Excel or an equivalent program. If required, y-error bars should also be added by means of Excel. No x-error bars will be necessary. All calibration curves should have the equation of the line as well as the square of the correlation coefficient (R2) displayed on the chart. In some cases, it is appropriate to specify a y-intercept for a linear regression. When a y-intercept is specified, it is usually set to zero, which has the effect of reducing the equation from y = mx + b to y = mx. One example includes the construction of this calibration curve for a series of solutions containing a colored iron complex by means of UV-Visible spectrophotometry. In this case, setting b = 0 has a negligible effect on the correlation coefficient and is therefore justified. Absorbances of Standard Fe(SCN)2+ Solutions 0.45 0.4 0.35
y = 7270.9x R² = 0.9983
Absorbance
0.3 0.25 0.2 0.15 0.1 0.05 0 -1E-05
6E-20
1E-05
2E-05
3E-05
[Fe(SCN)2+] (Mol/L)
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4E-05
5E-05
6E-05
TEST FOR OUTLIERS Occasionally an experimental result is obtained in which one duplicated measurement appears unusually large or small in comparison to the others. If you encounter such a measurement and suspect that it is untrustworthy, apply one of these tests to the datum to determine if it is an outlier. If the experimental parameter is larger than the critical parameter(s) given, reject the result. Critical Values for Rejection Quotient T Tcrit (Reject if Texp > Tcrit) n
95% Confidence
97.5% Confidence
99% Confidence
3 4 5 6 7 8 9 10
1.15 1.46 1.67 1.82 1.94 2.03 2.11 2.18
1.15 1.48 1.71 1.89 2.02 2.13 2.21 2.29
1.15 1.49 1.75 1.94 2.10 2.22 2.52 2.41
The T–test
Texp =
xq − x sx
xq is the questionable result
Critical Values for Rejection Quotient Q The Q–test
Qexp =
xq − xn w
xq is the questionable result xn is the nearest neighbor w is the range of the results
Qcrit (Reject if Qexp > Qcrit) n
90% Confidence
96% Confidence
99% Confidence
3 4 5 6 7 8 9 10
0.94 0.76 0.64 0.56 0.51 0.47 0.44 0.41
0.98 0.85 0.73 0.64 0.59 0.54 0.51 0.48
0.99 0.93 0.82 0.74 0.68 0.63 0.60 0.57
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PROPAGATION OF ERROR For almost any method of analysis, the outcome is a function of not one but several experimental variables. Consequently, the final errors associated with chemical measurements are the result of the accumulation of the individual errors associated with the experimental variables. A specific set of guidelines exists for the evaluation of cumulative errors and is referred to as Propagation of Error, or POE. For any quantity x, which is a function of several variables:
x = f ( p,q,r...) then the error associated with x is: 2
2
⎛ ∂x ⎞ ⎛ ∂x ⎞ ⎛ ∂x ⎞ s x = ⎜⎜ ⎟⎟ s 2p + ⎜⎜ ⎟⎟ s q2 + ⎜ ⎟ s r2 + ... ⎝ ∂r ⎠ ⎝ ∂p ⎠ ⎝ ∂q ⎠ 2
Thus, the standard deviation of any given result is computed from a square root of the sum of squares. Identities of the squared terms depend on the relationship that x bears to each variable. Example: Joe Shmoe needed exactly 50.00 mL of water for an experiment in A315. Unfortunately, all the 50-mL volumetric pipettes in the lab were dirty, so Mr. Shmoe combined one aliquot of water from a 10-mL volumetric pipette with 2 aliquots of water from a 20-mL volumetric pipette. If the uncertainty of a 10-mL pipette is 0.02 mL and the uncertainty of a 20-mL pipette is 0.03 mL, calculate the uncertainty associated with the volume of Mr. Shmoe’s solution. Solution: In this case, the result (x) is a function of two variables (p and q), where p is the 10-mL pipette and q is the 20-mL pipette. Thus, x = p + 2q
for which the partial derivatives are: ⎛ ∂x ⎞ ⎛ ∂x ⎞ ⎜ ⎟ = 1 and ⎜⎜ ⎟⎟ = 2 ⎝ ∂p ⎠ ⎝ ∂q ⎠
Assuming that the given uncertainties correspond to s p and s q , we can solve for s x : 2
2
⎛ ∂x ⎞ ⎛ ∂x ⎞ s x = ⎜⎜ ⎟⎟ s 2p + ⎜⎜ ⎟⎟ s q2 = ⎝ ∂q ⎠ ⎝ ∂p ⎠
(1)2 (0.02)2 + (2)2 (0.03)2
= 0.06
The volume is therefore 50.00 mL ± 0.06 mL. Note that POE requires that all uncertainties be expressed in absolute terms. Had the uncertainties in the above example been expressed in relative terms (i.e., 20.00 mL ± 0.15%), it 30
would have been necessary to convert them into the absolute uncertainties first (0.15% of 20.00 mL is 0.03 mL, thus 20.00 mL ± 0.15% is equivalent to 20.00 mL ± 0.03 mL). A number of other useful relations for POE are given below (note that where they are omitted, coefficients are assumed to be one):
Calculation
Standard Deviation of the Result
x = p+q−r
sx = s 2p + sq2 + sr2
p×q x= r
⎛ s p ⎞ ⎛ sq ⎞ ⎛ s ⎞ sx = ⎜⎜ ⎟⎟ + ⎜⎜ ⎟⎟ + ⎜ r ⎟ x ⎝ p⎠ ⎝q⎠ ⎝r ⎠
2
2
2
⎛s ⎞ sx =c⎜ p⎟ x ⎝ p⎠
x = pc
sp
x = ln p
sx =
x = ep
sx = sp x
p
Uncertainties for Common Laboratory Equipment Analytical Balance Pipettes
2 × 10–4 g 1 mL 2 mL 5 mL 10 mL 25 mL 50 mL
Volumetric Flasks
0.006 mL 0.006 mL 0.01 mL 0.02 mL 0.03 mL 0.05 mL
Burette
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10 mL 0.02 mL 25 mL 0.03 mL 50 mL 0.05 mL 100 mL 0.08 mL 200 mL 0.10 mL 500 mL 0.15 mL 1000 mL 0.30 mL 0.05 mL
Example: A student prepared a stock solution of potassium ferricyanide (K3FeCN6, FW = 329.26 g/mol) by weighing out 1.0582 g at the balance, adding it to a 100-mL volumetric flask, and diluting to the mark with distilled water. A dilute solution was then prepared by adding exactly 1.00 mL of the stock solution (via 1-mL volumetric pipet) to a 50-mL volumetric flask and again diluting to the mark with distilled water. Assuming standard uncertainties (tabulated above), express the concentration of ferricyanide in each flask in molar units with error. Solution: The concentration in the stock solution (Cs) is the result of multiplication and division involving the mass of reagent (m), the formula weight (FW), and the volume of the flask (Vs): ⎛ ⎞ ⎜ 1.0582 g ⎟ ⎛ m ⎞ ⎜ g ⎟ ⎟ ⎟ ⎜ 329.26 ⎜ ⎝ FW ⎠ ⎝ mol ⎠ = 0.0321387... M Cs = = Vs 0.10000 L
Neglecting the uncertainty in the formula weight, the error in Cs is: 2 ⎛ sm ⎞ ⎛⎜ sVs = ⎜ ⎟ +⎜ Cs ⎝ m ⎠ ⎝ Vs
sC s
(
)
2
⎞ 0.0002 ⎞ ⎛ 0.08 ⎞ Š4 ⎟⎟ = ⎛⎜ ⎟ +⎜ ⎟ = 8.22... × 10 ⎝ 1.0582 ⎠ ⎝ 100.00 ⎠ ⎠ 2
(
2
)
sC s = 8.22... × 10 Š 4 (Cs ) = 8.22... × 10 Š 4 (0.0321387... M ) = 2.64 × 10 Š 5 ≈ 3 × 10 Š5 M
Noting that the first uncertain digit is the last significant digit, the concentration of the stock solution is 32.14 ± 0.03 mM. Similarly, the concentration of the dilute solution (Cd) is the product of multiplication and division with the concentration of the stock (Cs), the volume of the pipet (Vp), and the volume of the second flask (Vd): Cd = Cs × ⎛ sC = ⎜⎜ s Cd ⎝ Cs
sCd
⎞ ⎛ sV p ⎟ +⎜ ⎟ ⎜V ⎠ ⎝ p 2
2
Vp Vd
= 0.0321387... M ×
1.00 mL = 6.4277... × 10 Š4 M 50.00 mL
2 2 ⎞ ⎛ sVd ⎞ ⎛ 2.64 × 10 Š 5 ⎞ ⎛ 0.006 ⎞ ⎛ 0.05 ⎞ ⎟ +⎜ ⎟ = ⎜⎜ ⎟⎟ + ⎜ ⎟ +⎜ ⎟ = 0.006138... ⎟ ⎜V ⎟ ⎝ 0.0321387... ⎠ ⎝ 1.000 ⎠ ⎝ 50.00 ⎠ ⎠ ⎝ d ⎠ 2
2
(
)
sCd = (0.006138...)(C d ) = (0.006138...) 6.4277... × 10 Š 4 M = 3.94 × 10 Š 6 M ≈ 4 × 10 Š6 M
Therefore the concentration of ferricyanide in the 50-mL flask is 643 ± 4 µM.
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