In Search of the Future Instructional Laboratory Mlchael Schallies Padagogische Hochschule Heidelberg, Im Neuenheimer Feld 561, 0 6 9 0 0 Heidelberg, West Germany Jijrg Redeker Landwirtschaftliche Versuchsstation der BASF AG, D-6708 Limburgerhof, West Germany Comparing the gain of scientific knowledge to the consumption of materials and energy, one can recognize a change of growth rates during the last 15 years. Within a short period of time heginning in the late '70's the rapid development of microelectronics has led to an informational gain growing a t a steep exponential rate. A t the same time curves for material and energy consumption show decreasing rates. Extrapolating data into the future i t is predictable that our gain of knowledge could be achieved with less and less consumption of energy and material. With regard to laboratories thisalso means that in the future it will be oossihle to gain chemical information from less amounts of substance. As a consequence most of our conventional lahoratorv. e a.u i.~ m e nwill t he re~lacedhv miniaturized e a. u.b ment. If we also rake into account that microelectronics and other advanced technologies will further rerolutioniae work in the chemical laboratory and will eventually lead t o a rationalized robotized laboratory, i t was only logical to work on the development of miniaturized laboratory equipment (1,2).
-
Construction Principles for Minlaturlzed Laboratory
Glassware Miniaturized laboratory glassware developed on the premises above will mean fewer and easy-to-handle pieces, the use of screw-type connections and self-supporting apparatus with no need for clamping. Incidentally this will also make such apparatus ideally suitahle for the needs of the untrained beginner in any instructional laboratory. Advantages derived from use of microscale methods in instructional lahoratories were recognized early (3-6) and have recently received renewed attention in this Journal (7,8). As far as we are aware constructional principles have not yet been discussed. In contrast to conventional micro or semimicro laboratory glassware, which often consist of scaled-down macroapparatus, flat-bottomed cylindrical forms for reaction vessels seem to be most appropriate in our view. A discussion of the stahilitv and ~rooertiesof such vessels will show how advnntageouia slim.cyiindrical form is for most vurposes. The stability of a alass vessel of a radial syrnmetr; ®ards pressure i s shown in Figure 1. I t remains constant as the thickness of the walls is scaled down proportionally with miniaturization. If the walls are thinned underproportionally, the stability of miniaturized glass vessels toward oressure increases. This also a o ~ l i e to s the cvlinder walls or cylindrical vessels. In c o n t k t to spherically shaped vessels, the border region between the flat bottom and the cylinder wall is subject to mechanical instability. However, the stability of this region is enhanced as cylindrically shaped vessels are miniaturized. This fact is shown by the equations in Figure 2. One can thus deduce that their stability is squared as the radius is diminished by half. Comparing cylindrical vessels of the same volume hut different dimensions, i t can be further deduced that slim-shaped vessels with a high ratio of length to radius will have the greatest stability. This makes possible the evacuation of such vessels, such as for filtration under reduced pressure or distillation under reduced pressure (water jet pump vacuum). 74
Journal of Chemical Education
Figure 1. Tacgentlaltenslon within thewall of a hollow glasssphere(equatwia1 girdle) pressured by vapor. T = tangential tension; R = outer sphere radius: r = inner sphere r a d i k C =virtual cut.
Flgure 2. Bending strain at the transitmy edge of a cylinder wall to the flat . = bending mupie; Fb = bendlng force; 4 = bonom of reactlon vessel. C length of forcearm: L. = length of the edge; R = radius of the bonom: r = radius to the force center point; h = pan of the circle; Sa = bending strain.
Figure 3 shows some cylindrical vessels made of borosilicate glass (Wheaton Scientific "200") (for dimensions see the table) that have been tested in instructional laboratories (university and schools). They can easily be fitted to metal thermohlocks having cylindrical bore holes (see Fig. 4). A six-hole arraneement will allow for hieh densitv of installation if necessary, such as for a series of experiments. Variation in the heieht of the thermohlocks or the heiaht to which vessels areinserted into the tbermoblock will allow forvariation in the ratio of heated and cooled surfaces of reaction
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Figwe 3. Apparatuses for standard labaatwy operations. I, apparahls for heating reaction mixtures under refluxing conditions: 11, simple apparatus for refluxing mixtures or recrystallizationfrom boiling soluenh; Ill,filtration unit: a.b.c. reaction vessels: e. reflux condenser, air-cooled; d. reflux condenser, water-cooled; f. filtration unit consisting of teflon support and PTFE filtration membrane; vessel 1 contains liquidlsolid system: vessel 2 is receptacle for finrate: reduced pressure (waterlet pump) is applied via side arm of vessel 2.
vessels. The heat-transport capacity across a glass wall also deoends upon the ratio of height t o radius of cylindrical renction vissels. When closed the temnerature inside the ----vessels containing boiling solvent depends upon the ratio of heated and not-heated surfaces. Thus i t is oossible to heat reaction mixtures in closed systems by choosing a corresoondine ratio of heated and cooled surface areas in such a way thai condensation of reactants occurs in the cooler parts. This implies also that water coolers can be eliminated under the right conditions, which makes possible a simpler laboratory infrastructure. The mixing of components within miniaturized reaction vessels is achieved by physico-chemical or mechanical means. Molar diffusion is of importance as diffusion times are relatively short due to short maximal path lengths. Also, thermoconvection and heatine under reflux conditions will lead to intense mixing of components inside reaction vessels as liauid-steam-liouid . .ohase transfers occur with hiah volume specific rates. If the latter principles are not applicable. newlv desiened s ~ i nbars cause turbulent flows within reaction vessels even if they are position~deccentrically in the magnetic field of conventional magnetic stirs-this position is the result of a six-hole arrangement within the thermoblock. Reaction vessels are linked to other miniaturized components of an experimental setup by means of screwtype connections. If these are constructed so as to be stable against pressure, reduced pressure and tensile strength, selfsupporting apparatus can he set up that even make possihle the execution of ex~erimentsin closed svstems. Thus reactions with agressiv~chemicalscan be performed without the need for an exnensive laboratow infrastructure (ex., hood space). ~~
~
Figure 4. Subsidiary laboratwy equipment. I, working platform arm magnetic clip: 1, clip: 2. iron core: 3, circular magnetic m e : 4, machine screw; 5, fenmagnetic working platform: 6, U profile frame. 11, thermoblock top view: 1-6, apertures lor insertion of reaction vessels: 7, aperture lo hold electric bulb far illumination:. 8.. radial bwes to allow for observation d reaction vessels during experiments, Ill. Mermoblo~kdrawn in section.
~~
Components of a Baslc Kit for Standard Operations In Organic Chemldry Tvoe
,
Svmbal
1. reaction vessel 2. reaction vessel wilh sidearm 45O 3. reaction vessel wilh sidearm 90" 4. condenser 5. Yloint 6. Tjoint 7. cooler 8. Ropcwk 9. connection 10. connection 11. connection 12. adapter 13. adapter 14. filter disc (PTFE) 15. PTFE membraneb 16. hose oonnector 17. glass tubes 18. plast. tubing
Dimension
Swew Tvoe'
64
8.6 X 2.3 crn 8.6 X 2.3 cm
20-400 20-400113-425
s9
8.6 X 2.3 cm
20-400113-425
ca
16 X 6.0cm 6.0 X 1.5 cm 6.0 X 1.5 cm 10.4 X 1.3 cm 10.0 X 1.5 cm commerciala commerciala commerciala commercial' mmerclal' 2.0 cm diam. 1.9 cm dlam. commerciala 0.6 cm diam. 0.6 cm d i m .
20400 13-425113-425113-425 13-425113-425113-425 13-425113-425 13-425 13-425113-425 20-400120-400 20-400113-425 13-425 20-400
vo
Y
t co 0
xs xo xx as a0 to po ho
gl pt
13-425
Volume 65 Number 1 January 1968
75
the distillation in the opinion of the student seemed to have been performed successfully. Results obtained with "free solution experiments" will be discussed in greater detail elsewhere. Experimental Example 1: Synthesis of anilin Substances
tin
Figure 5. Tilting face at the edge of cubes.
niwobenzene hydrmhioric acid sodium hydroxide solution
Quality
input
Dimension
foil pa pro synlhesis
2.00 1.00 5.50 3.50
mL mL mL
37% 75%
Q
Execution. Instructional unit, chemistrvvoluntarverouo. Since miniaturized reaction vessels are easily toppled by accident when placed onto a laboratory bench, they have to be protected against this mishap by application of magnetic forces. This is a consequence of the fact that the force needed to tilt a body is diminished by the power of three as its linear dimensions are scaled down (Fig. 5). Therefore, we useferromagnetic working platforms which can be used in vertical or horizontal position (Fig. 4); i t is also possible to work on onnosite s bv .. sides a t the same time. Glass a.~.~ a r a tisu fixed magnetic clips that can he made to slide easily on the surface of the workine olatform. In this wav an uncomplicated setup of an experi&ntal apparatus is p&sible that allows for any leftlright or upldown preferences an experimenter might have for the connection of the different parts of the apparatus withone another. In this respect standard macroapparatus presents some great obstacles for untrained beginners in laboratory work that it takes a lot of practice to overcome. Examples for Experiments with a Baslc KH We have desiened a nrototwe for an exnerimental kit that allows the carryTng ouiof all siandard labbratory operations. I t consists of a small number of standardized Darts that can e b standard he combined in a multitude of ways. ~ x a m p ~ for setups are shown in Figure 3. They have been tested with: 1. ouoils of the ace mouo 12 to 13 wars (iunior hieh school): 2. ;he age group i6 o ; liyears (jun-ior h i i h school;; for example see experiment 1, standard operations: hearing under reflux, steam
distillation,extraction, distillation; 3. students at our laboratories (organic preparative chemistry), for example see experiment 2, standard operations: heating under
reflux, filtration, recrystallizetion;and 4. in industrial laboratories, standard operations: heating under
reflux, filtration, extraction. Dlscusslon Handling of the experimental apparatus presented no problems to experimenters irrespective of ape. The greatest advantage in our opinion-besibes the advantagesalready mentioned in the literature-lies in the pedagogical field: This new equipment made i t possible to let students work without explicit instructions by just giving them an experimental aim and lettine them find their own exnerimental setup and execution (typical example: "prepare ethene from ethanol and examine its orooerties"). In these exoeriments the students were filmed wkh a video-camera. 1; this way records of wrong experimental solutions were also obtained, for example, a distillation in a closed system where the students forgot about the opening to the atmosphere. The film demonst;atex nicely theadvantages alreadydiscussed theoretically above, since the high volumes-specific heat transfer in the miniaturized reaction vesselsdid not allow for a buildup of dangerously high pressures inside the apparatus. Instead the vapors were condensed in the cooler parts so that 78
Journal of Chemical Education
16-17 years old: Snfoil was placed into areaction ve&l wiih
side arm. Nitrobenzene and hvdrochloric acid were added with automatic pipet ( ~ ~ ~ e n Multipipette). d&f A condenser was attached (experimental setup corresponding to Fig. 3 I). The mixture was heated under reflux and stirring in the thermoblock. Temperature was regulated to -130 O C (thermoblock). After 10 min, a last portion of the acid (0.5 mL) was added through the condenser. Stirring was continued for another 5 min. 0h.wuation. The reactionstarted quite fast in a few cases so that the apparatus had to be lifted from the thermoblock in order to &ken the reaction rate. The Sn foil had dissolved completelv a t the end of the reaction time giving. a yellowish hiown solution. Cleanup. The reaction vessel was taken out of the thermoblock. Contents were dilutedarith 3mL ofwater. NaOH solution was then added with a Pasteur pipet. After the additionof the base, steam distillation was set up: A reaction vessel with side arm was three-fourths filled with water ("steam generator") and heated in the thermoblock. When the water was boiling the second reaction vessel with side arm containine the reaction ~ r o d u c t swas introduced into the same thermoblock. steamwas led into the second vessel via silicone tubing, and a glass tube was inserted into the second vessel well down to the bottom. The side arm of the second vessel was connected to a condenser. The contents of the reaction vessel were steam distilled for 10 min. The distillate was extracted twice with 2 mL of dicblorometbane. The oreanic ohases were united. One nellet of KOH was added,-and contents were distilled under normal pressure. The distillate was examined usine thin laver chromatoeraphy (precoated silica gel plates, mobile toluenelm&hanol = 9.511 mL). Obseruation during cleanup. Steam distillation went smoothlv with one eroup. The other mouo exverienced leakage thatin the endcaused the conteits of th;reaction vessel to be drawn into the steam generator hv reduced pressure. Extraction and distillation under normalpresrure pre3ented no experimental difficulties. Firsr the solvent was distilled off ar -40 'C. then the next traction from -80 OC onward was obtained bontainiug the anilin. The amount was only a few drops. On raising the temperature further the remnants in the reaction vessel turned brown, and distillation bad to be terminated. Chromatographic analysis of the product obtained by distillation showed two fractions one of which was identical in chromatographic behavior to authentic anilin. The other component had a higher Rfvalue, but was not further characterized. Total time required for the experiment including chromatography: 3 pe;iods (135 min). Apparatus: 1condenser, 2 reaction vessels with side arm 45'' 1 stopcock, 1 glass tube 6 mm diameter, 1 separatory funnel, 2 connections, 1 reaction vessel, 1 adapter, and 1 cooler.
Example 2: Synthesis of M-Dinitrobenzene substance^
Qualitv
sulfuric acid nitric acid niwobenzene water ethanol
ln~ut
Dimension
96 %
0.70
mL
fuming pro s y n l h e ~ i s dist.
0.50
mL mL
95%
0.50 8.00 3.00
mL mL
Execution. Nitration mixture is prepared inside a reaction vessel with side arm. I t is connected to a screw tube containing activated charcoal (absorption of nitrous gases). The nitrobenzene is slowly added through the side arm using a syringe while stirring. The temperature is slowly raised to 100 O C and held for 15 min. The reaction amaratus is taken out of the thermoblock and cooled down Groom temperature under a running water tap. Obseruation. Nitrous gases are evolved in small amount8 and are completely absorbed by the activated charcoal. On addition of the water at the end of the reaction period a yellowish precipitate forms. Cleanuo. The ~recinitate is ohtained bv filtration under . . reduced pressure (rxperiment~lsetup corresponding to Fig. 3 111). It is washed twice with water and recrystallized from
ethanol. On cooling the product crystallizes in long needles of slight yellow-green appearance. Experiment can easily be performed within 60 min. Apparatus: 1reaction vessel with side arm 4 5 O , 1 screw tube, 1connection, 1filter disc (PTFE), and 1PTFE paper. Acknowledgment
The authors would like to thank the Deutsche Forschungsgemeinschaft for the financial support granted. We also wish to express our thanks to Wheaton Scientific Inc. for supplying materials and providing the computer system for the evaluation of experiments and BASF AG for financial support for the preparation of experimental equipment. Literature Cited 1. Redeker, J.: Schslliea, M. R=ls N o t u w u s . C h m . 1987.36.2. 2. B6ttner.R.; Schellie%M.:Redeker, J. Pmria Natunuiss. Chem. 1981,36, 7. 3. Emieh, F. Mikroch~miseheaPlobtibum: erne Anleitungzur Avsliihrung der wichtigstan mikmchemiachen Handgrille, R ~ a k i i o m nund Bertimmungen mil Ausnahma der quonfifc&uon organixhan M i k r w ~ l ~ sBergmsnn: e; M-chen, 1924. 4. Gorbaeh, G. Mikrochrmisches Raktikum: Heidelberg, 1956 5. lieh, H.; Sch6nigcr. W. Anleifung rur Dorstdlung orgonischer Pflpomfe miL hleinm Springer, Wien, 1961. Substorumpngen, 2nd d.: 6. Msltrchawa,S. Chem.Sch. 1970.17(5),232.
springer:
7. Butcher, S. S.: Mayo. D. W.; Pike. R. M.: Fwte, C. M.: Hotham, J. R.; Page, D. S, rl Chom. Educ. 1985,62.147. 8. Mayo, D. W.:Butcher. S. S.; Pika, R. M.: Foote, C. M.: Hotham. J. R.;Page, D. S. J. Chem. Educ. 1985.62.149.
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