CHEM C2507y Spring 2000

Chemical analysis is a branch of analytical chemistry. The purpose of chemical analysis is to establish the composition of naturally occurring or artificially manufactured substances (called the analyte). This is usually done in two distinct steps. First, qualitative analysis is used to identify the sample components. This is followed with quantitative analysis, by which the relative amounts of these components are determined. In this experiment, you will perform mainly qualitative analysis of inorganic, organic, or organometallic substances.

Chemical analysis remains a prominent field throughout the history of chemistry for an obvious reason: we are curious to find out what different matters are composed of. During the last few decades, especially, not many branches of any science have undergone so much change in the equipment and procedures used as has the field of chemical analysis. This revolution also has impacted on how the principles of chemistry are taught today. The revolution in analysis also has had wide influence on technology in general because of the far greater accuracy with which chemical determinations can be made. While the impact of vastly improved chemical analysis has been felt by essentially all phases of science, dramatically more precise data have been of notable significance in the area of pollutants and pharmaceuticals.

As mentioned before, the subject of chemical analysis was divided into two readily understood areas. Qualitative analysis concerned simply with the identification of the components in a mixture (or the constituents of a compound), sometimes accompanied by observations (rough estimates) of whether certain ingredients may be present in major or trace proportions. Quantitative analysis concerned with the amounts (to varying degrees of precision) of all or frequently of only some specific ingredients of a mixture or compound. Quantitative analysis involves gravimetric analysis (i.e., weight of sample, precipitates, etc., is the underlying basis of calculation) and volumetric (titrimetric) analysis (i.e., solutions of known concentration are reacted in some fashion with the sample to determine the concentration of the unknown.)

A chemist (or analyst) who has an analyte in hand, and needs to know what it is, cannot possibly be aware of all the reported data for comparison with the properties of the unknown. Thus, a systematic approach is essential. This approach must first exclude as many structural possibilities as possible; then, reduce the number of possible structures to just a very few (say, three or four) possibilities; and finally, establish and confirm one structure. The approach you will use in this experiment will be very similar. You will first determine whether you have a mixture of components in your analyte. Once you have isolated the pure components, collect data on as many of the physical properties of a component as possible. Then, you will establish a short list of possible identities of your component. Finally, you will confirm the exact identity by performing confirmation testing.

Last, but not least, you may be aware that chemists are often confronted with either of the following extreme situations, each of which significantly reduces the merits of chemical analysis:

1. Determination of the identity of a compound that has no prior history. This is often the case for a natural products chemist who must study a very small amount of sample isolated from a plant or an animal. A similar situation applies to the forensic chemist who analyzes very small samples related to a lawsuit or crime.

2. The industrial chemist or college laboratory chemist who must analyze a sample that contains a major expected product and minor products, all of which could be expected from a given set of reagents and conditions. It is entirely possible that such a sample with a well-documented history will allow one to have a properly preconceived notion as to how the analysis should be conducted. Such a notion may not be desirable.

Shriner, R. L., et al. The Systematic Identification of Organic Compounds: A Laboratory Manual. 6th ed. John Wiley & Sons: New York, 1980.

Williamson, K. L. Macroscale and Microscale Organic Experiments. 2nd ed. Heath Publishing: Lexington, 1994.

A. LABORATORY SAFETY

Basic Safety

All existing laboratory safety protocols in this course will be strictly enforced (refer to the Safety Tour Check-list). Violation of any safety protocol may result in dismissal from the laboratory. Safety has two aspects: prevention of accident and response to emergency. The best safety rule is to use your common sense. Do not use the laboratory without permission. Treat your classmates and chemicals in the laboratory courteously. Ask your instructor whenever you do not know how to prevent an accident. Notify your instructor immediately when there is an accident (including broken glassware, major chemical spill, and bodily injury).

Material Safety Data Sheet (MSDS)

Federal law requires that manufacturers and distributors of chemicals provide users with Material Safety Data Sheets (MSDS). MSDS is a fairly concise technical document that give information on any particular chemical among the over 10,000 frequently encountered chemicals in research laboratories and industries. The information includes contact address and phone number of the chemical supplier, chemical names, physical and chemical properties, physical hazards (such as flammability, reactivity, explosibility, etc.), toxicity data and health hazards, storage and handling procedures, emergency and first-aid procedures, and disposal and transportation information. Today, the MSDS for a chemical can be conveniently located by submitting the name of the chemical to a MSDS searchable database on the internet. Many of such databases exist, and they can be found through the safety links of our course website.

Chemical Waste Disposal

No chemicals can go down the sink, period. Aqueous, organic, and solid wastes should be disposed properly in clearly labeled waste containers.

B. PLANNING BEFORE THE LABORATORY PERIOD

A Plan of Action is required. To create this Plan of Action, use the materials and ideas covered during the April 6 conference hour and in this handout. Sufficient preparation is particularly important since unexpected events often occur during chemical analysis and they require proper and prompt response from you. Apply the scientific method (hypothesis testing).

Use your laboratory notebook to take notes, record observations, and organize data. Keep measurements of mass, temperature, and also any qualitative and quantitative information at key steps of your analysis process. You will never know what pieces of information are the most pivotal ones that will take you to the end of the analysis.

Although you will be given a slightly larger than usual quantity of sample to work with, always remember to save samples at various stages of your analysis. Exercise common sense to decide how much sample you want to use for a particular procedure.

Finally, there is only limited time and limited resources allowed in our laboratory. Time and tasks management becomes very important under this condition. Your ability to collaborate effectively with your group members will be put to test in this experiment.

C. SUGGESTED EXPERIMENTAL PROCEDURES

Step 1: Visual Examination of the sample

Write down the color, odor, number of phases, and physical characteristics of the sample received. This is a surprisingly powerful step because many clues (on what the unknown can possibly be and thus allowing you to eliminate many obviously not possible identities of the unknown) can often be drawn simply from a careful visual examination coupled with your knowledge on the prior history of the sample. Although one should never make a conclusion prematurely at this stage, the choice of analyses that you will perform subsequently to this step will depend greatly on your knowledge of the sample from this step alone.

Step 2: Purity Check on the sample and Separation of Components if the sample is a mixture

If your sample contains multiple phases (solid and liquid), separate them with gravity or suction filtration. For the solid phase (or a solid sample), determine its melting point and test its solubility in several solvents while taking notice of the polarity of each solvent. Perform thin-layer chromatography and/or column and thin-layer chromatography to check for the purity of the solid sample and to determine whether the sample is a mixture of multiple components. Separate the components and perform further chromatographic testing to determine the purity of each component collected. In some cases, gas chromatography and high performance liquid chromatography may also be used for the same purpose. For the liquid phase (or a liquid sample), use gas chromatography (if the liquid is volatile) and/or high performance liquid chromatography to check for purity and to identify if it is a mixture. If the liquid is colored, you may be able to use column and thin-layer chromatography. Study the miscibility of each liquid component with various solvents. Separation of components may require fractional distillation.

Step 3: Physical/Chemical Properties Determination on each separated component of the sample

If a compound is suspected to be acidic, attempt to determine its Ka and the number of replaceable hydrogen atoms in the molecule. Phenol test may be helpful. Further chemical testing such as the preparation of a derivative from an analyte is desirable, but such a procedure is beyond the scope of this course. You may also obtain preliminary visible (for colored compound) and/or infrared spectra for your compound. However, these spectra may not be used for identification at this stage. Only recommendations for the existence of certain structural features in your compound can be drawn.

Step 4: Making a Short List of possible identities for each component in the sample

Step 5: Confirmation Testing with standards

Obtain standards (i.e., known chemicals) and compare their physical/chemical properties with your unknown. Possible confirmation tests are: melting point, Ka, Rf value, solubility, visible spectrum, infrared spectrum, retention time on a chromatogram, and other relevant tests.

After the conference hour on April 6, you should have constructed a chemical analysis flow chart as a possible Plan of Action for this experiment. You shall meet with your group members in a group meeting to share the information on each individual flow chart in attempt to create a group flow chart that the group will use (and submit to your instructor) as the actual Plan of Action.

At the beginning of the laboratory period, your group will receive a small glass vial containing sample and a sheet of paper on which you will find a list of compounds, each of which can possibly be the identity of the unknown sample (or components in a mixture). This list will include most chemicals used and chemical waste produced in previous experiments in this course. Your sample may contain anywhere from one to three components. Your group will then proceed with the Plan of Action until only a few possible identities for each component in the sample is narrowed down (step 4 above). Your group must show your instructor the short list(s) because your instructor will ascertain that one of the compounds on each list is the true identity of the unknown sample (component). Finally, you will perform confirmation testing and conclusively identify the unknown. You will expect to report your findings in a laboratory report. Guidelines for writing the laboratory report can be found in section VI of this handout.

Instructor

Your instructor will oversee all laboratory activities and ensure that all safety protocols are followed properly but will not interfere with your experiment unless it is deemed necessary.

ChemPreps (304 Havemeyer Hall)

ChemPreps has an inventory of glasswares, chemicals, and accessories. Staffs in Chempreps will assist you with your need if you cannot find a certain item in the laboratory. You are advised to ask your instructor before walking over to the ChemPreps window. Your instructor will clarify the exact item you need, tell you whether an item you need is in the laboratory or if not, is it necessary and worthy to get it from ChemPreps.

Chemistry Library (4th floor Chandler Hall)

The Chemistry Library has a remarkably large collection of chemical literature from textbooks, manuals, journal titles, to reference sets. The CRC Handbook of Chemistry and Physics and the Merck Index are available there. There are computer terminals available for you to search conveniently for chemical literature electronically. The Reserve section of the library contains all the primary references used to prepare this handout; these references are available to you on a two-hour loan basis. For further assistance in using the Chemistry Library, consult the librarian.

Internet

You may access the internet and the course website through computer terminals in the Chemistry Library.

The laboratory report for this experiment will be due, as usual, a week after the experiment is performed. The following guidelines may assist your group in writing the report:

Abstract - State the goal of this experiment, its rationale, the identity of your unknown sample, and summarize results and methods used in your experiment.

Introduction - Explain (and include) your chemical analysis flow chart.

Experimental Methods - Should be very short. Do not repeat the Introduction.

Results - All tables and figures must be properly captioned.

Discussion - Discuss how your hypotheses are tested at each step of the analysis process. State the reason your identification is conclusive. Offer alternative identities of your unknown and explain why they cannot be the true identity.

References - Use the proper citation.