the microscale laboratory The Microscale Organic Laboratory A Very Simple Method of Filtration and Recrystallization A. Laporterie Laboratoire des Organom6talliques URA 477, Universite Paul-Sabatier Toulouse, 31062. France In organic chemistry laboratory courses, many experiments can be carried out on a small scale.' This helps to economize on chemical substances, and it helps students to develop meticulous work habits. Reactions are generally perfo&ned in small. threaded vialsorinpund glass vials. Reactions can even be carried out in test tubes2 to wluch a condenser can be adapted. When the reaction products are crystallized, it is important to perform the filtration, washing, and crystallization without loss. Thus, it would be best if these three steps of purification were carried out without transferring the crystals. The following very simple and inexpensive system may be used. (See the figure.) Filtration
The assembly is represented in left part of the figure. A sinele-bore rubber stopper is fitted to the reaction vial and hy a hypoder& syringe needle, which reaches the bottom of the vial. A second single-bore mhher stopper is connected top to bottom to the reaction vial stoppe; by a short length of glass tubing. On the reaction vial stopper is placed a thin glasswool plug, which is protected by a filter paper that is folded to envelop the stopper and to fit to the reaction tube. This assembly is inverted and fitted on a smaU filtering flask. The liquid is then filtered off while the solid materi1 Mava. D. W.: Pike. R. M.: Butcher. Lab~. S. S. - Micmscaie Oroanic ,-. -~ " orat& 2nd ed.;'~ohn Wiley 8 Sons: New YO,;. 1989. ? KO b, 6.E : Fieo K. W.: Schatz. P. F The Microscale Laooralory A One-StepSynrhess ofCinnamrc Acid uang Malonrc Acid: The Verley-DobnerModification ofthe Knoevenagel Condensation;J. Chem. Ed.. 1990, 67(12)A304.
als are deposited on the filter and on the walls of the tube (see right part of figure). Washing
The filtering flask is disconnected and the assembly is arranged as in left part of the figure. With a syringe, the washing solvent is introduced into the vial, via the needle. After shaking, the liquid is filtered off as before. This process may be repeated several times. Recrystallization
The "filtering stopper" is carefully disconnected while the crystals remaining on the fdter are pushed down into the tube with a microspatula. Then a condenser may be fitted. and the ~roductis recrvstallized from a suitable hot solvent. In a similar way, filtration is accomplished using a new filterine -.olue. The cnrstals are dried bv water-~umo suction for a few minutes.
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A 39-Drop pH Scale Robert G. Sllberrnan
SUNY at Cortland Coriland, NY 13045 Most microscale lab manuals include at least one experiment involvine o nsolutions with different ., the ~ r e ~ a r a t i of but known pH's. The soiutiona are usually prepared by serial dilution of 0.1 M or 0.01 M solutions of strone acid or base. This method has pedagogical value because students fullv ex~eriencethe concept that a chanee of 1DHunit can be achieved by a 10-fold dilution. It has two disadvantaees. however. The DH'Sof solutions near the middle of the scale are seldom cldse to the comct DHdue to countine errors and variations in d r o size. ~ Preparing a solution k t h a pH in the middle range is tedious because many dropwise dilutions must be made.
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A Two-Solutlon System Some years ago Carmodyl introduced a two-solution buffer system that allows for the preparation of a solution of any pH of 2-12. Carmody gives instructions for preparing buffers accurate to 0.01 pH unit in his paper. With some modification, this system can be used to mepare solutions with pH's of ~ 1 2with ; an accuracy of k . 1 pH unit, using a total of 39 drops from two solutionx. It is seldom n e c e s k to get better accuracy than this in introductory microscale experiments. The proportions are The 39-Drop Table pH Units (a.1 units)
No. of Drops Solution A
No. of Drops Solution B
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Experimental set-up(1etI)and arrangement during filtration (right) A42
Journal of Chemical Education
12 3 Note: The salution for pH 6 rsq!dires a lot61 of 40 dmps lo be in the M.1 uncenainty range. The 24 dmps of Aand 15dmps of Bproduce asaiution with a pH of 5.8-5.9.)
