Laboratory Modules from Dr. Mary Campbell Dept. of Chemistry Mt. Holyoke College South Hadley, MA 01075-1496

The study of chromatography is important in many areas of chemistry. In this experiment, a fairly standard treatment of paper chromatography has acquired some "human interest" by including some forensic aspects of this analytical tool. Since it is so straightforward, it can be used in liberal arts courses or in general chemistry. In addition, it can be used as one of the first experiments in the organic chemistry course if it is desired to introduce chromatography at that point, but some parts can be used later as a student project an analytical chemistry course. It is possible to take advantage of any locally available form of chromatographic analysis as a variation or a supplement to this module.Most of the actual experimental work can be done by one student in one laboratory period, but small "research projects" can be added to give a longer experiment.

Several possible variations exist for this experiment, and students can do any or all of them. The first stage can be to establish the efficacy of the method. The separation of ink components depends both on the solvent and the paper. As a result the first part of the experiment is to try different kinds of paper and different solvents to determine which conditions give the best separation and are best able to distinguish among various different pens.

The main preparation for this experiment is to obtain boxes of pens from different suppliers. As time goes on, it is possible to acquire a stockpile of dated pens that can be used for the "questioned document," which can be generated easily. One could, for example, have several pens and write a "ransom note." Students then determine which pen of the various possibilities was used to write the note. In subsequent years, with a supply of dated pens, it becomes possible to devise experiments based on back-dating in similar fashion. With the "questioned documents" students need to extract the ink from the relevant part of the document with the solvent they are to use and to spot the extracted ink onto the paper with a capillary.

†Professor of Chemistry, Mt. Holyoke College: author to which correspondence should be addressed.

The main point of this experiment is the composition of compounds, specifically oxides. Students find this a topic of interest especially when they can contrast the constant composition of the magnesium oxide with that of the non-stoichiometric 1-2-3 superconductor. This experiment teaches quantitative techniques, including titration if all parts are done. The determination of the composition of magnesium oxide is determined by careful weighing of the product of reaction of magnesium and oxygen from the air, while the titration part of the experiment is used to determine the composition of the superconductor. Most students will have done an acid-base titration in high school, so it is well to do one based on an oxidation-reduction reaction. The statistical analysis of data is optional, but better students find it an enjoyable challenge to their quantitative skills. The general chemistry course for science majors is an appropriate place for this experiment, usually fairly early in the semester; with both parts, it should take three laboratory periods with students working in teams of four as outlined in the experimental and data analysis sections. If the full statistical analysis of data is included, the expanded experiment may be more appropriate for an honors section of general chemistry or for an analytical chemistry course.

The materials required for this experiment are simple; the analysis of results is the part that is most challenging. For the metal oxide section, students simply heat magnesium ribbon in a crucible, but they have to think about what they are doing. The oxide mixture that is heated to produce the superconductor are commercially available, but it is also possible to purchase the components and to prepare the mixture on site. The option exists to have all, most, some, or a few of the students do the heating process that gives rise to the superconducting properties or it can be done as a demonstration. In any case, the crystal structure of the superconductor can and should be pointed out as the key point in the transformation of the mixture into superconducting material.

†Professor of Chemistry, Mt. Holyoke College: author to which correspondence should be addressed.

This experiment puts an environmental slant on the study of equilibrium. Since it deals with solubility product constants and with acid-base equilibria, it is best placed in a majors' course in general chemistry at the time when equilibrium, acids and bases are covered. It can also be used in an environmental chemistry course. Since the two carbonate compounds involved in this experiment are important components of stone buildings and monuments in their limestone, marble and dolomite forms, students themselves will probably think of the effects of acid rain and the damage seen in large cities with heavy auto traffic. The equipment and reagent needs are minimal; safety and cleanup are not a problem. There is a distinct requirement for thinking carefully about a deceptively simple experiment. Students are used to bring given solubility product constants and doing calculations based on them. Estimating the solubility product constant by mixing reagents of known concentration and observing the presence or absence of precipitates requires thinking about the problem in a new way. In addition, this experiment is not simply about precipitation, or about acids and bases, but about both topics simultaneously. The solubility product constants for calcium and magnesium carbonates are close enough to require careful control of experimental conditions, especially pH. The fact that the carbonate ion is a strong base implies that its concentration depends on pH, requiring close attention to acid-base dissociation.

One of the main features of this experiment is that it combines the study of two kinds of equilibria that are discussed prominently in general chemistry, reinforcing the concept that several equilibria can be operating at the same time. It is particularly important in the lab setup for this experiment to ensure that pH meters are functioning properly. In practice this means that the electrodes should be checked carefully before, during, and after lab.

†Professor of Chemistry, Mt. Holyoke College: author to which correspondence should be addressed.

Contact Professor Mary Campbell with feedback on her lab modules at mcampbel@mhc.mtholyoke.edu