professors is to give those who choose nonscience profeswould have us believe. It is perfectly good applied mathesions (1) an understanding of the interplay of the scientific matics to make a correspondence between the elements of community with the world and (2) a n appreciation of the a mathematical system and quantities, i.e., number with associated unit, so long as there is a well-defined way to important scientific discoveries. This course fulfilled these goals. A complete syllabus and list of reading materials is manipulate the units. This method is called the quantity available on request. calculus. The advantage is that we do not have to keep track of the units se~aratelv.This method is widelv used Angellna A. Scimone and needs no defense. ~ a & e m a t i c sdoes not presiribe a Department of Natural & Physical Sciences unique way of making the correspondence between "reality" Caldwell College and a mathematical system. Caldwell NJ 07006 For certain functions like log, sin, cos, which are defined in mathematics only for num&icalar&rnents, it becomes necessary to make the arguments not quantities but the magnitudes. Since the magnitude depends on the unit, we Writing Lewis Structures need to remind ourselves what the units are. Following Lodwig, one could keep this in one's mind, or remind ourTo the Editoc selves in the equation itself, e.g., sin (Thad)which tells us An article by Packer and Woodgate in the June 1991 to use the numerical value of the angle T expressed in rad, issue of this Journal decries the lack of a user-friendly apor In (plpe) which reminds us to take the natural logaproach to writing Lewis structures. The article refers to rithm of the numerical value of the pressure exoressed in (usually) atm or bar. This latter metkod is less e r n ~ r - ~ r o n e others who had written on the matter in the past three years. than the former and is the method recommended bv the My hook, Electron Movement; A Guide to Students of OrIUPAC. ganic Chemistry (Saunders College Publishing), was first If by unit-factorization Lodwig means the manipulation published 17 years ago and has remained in print until the of units a s algebraic entities, I subscribe to, but do not worsecond edition, renamed Pushing Electrons, became availship, it: It is valid applied mathematics, and I see nothing able in 1992. mad about it. I have not spoken to "mathematicians" about In its first chapter, this workbook provides students with these matters, and, in any case, I would hesitate to speak a reliable set of rules (quite similar to those proposed by for all of them. Packer and Woodgate) for writing Lewis structures and asAs for the lies about the real definition of pH, it is importsigning formal charge. The chapter provides 66 proant to draw a distinction between a nottom-/definition and grammed problems in writing Lewis structures of moleanomrational definition. Without a notlonal definition the cules, ions, and (in the new edition) free radicals. The coniept of pH would be close to meaningless. second and third chapters provide similar treatments for writing resonance structures and mechanisms, respecSebastian G. Canagaratna tively. Ohio Northern University The book is limited to examples in organic chemistry and Ada, OH 45810 is. therefore. not suitable for teachine eeneral chemistrv. 1t'does not cbver electron-deficientm&&ules or oxidation t however. that there was available a numbers. I ~ o i nout. gentle approach to teaching Lewis structures well before Nonmajors Course on the Notion of Scientific Progress the computer jocks coined the term user-friendly. Dear Editoc
In a n effort to increase the scientific literacy of our nonscience students. a course entitled "The Notion of Proeress: Scientific ~isco"eries of the 20th Century" wFs as part of the Scholars Program a t Caldwell College. While the traditional science course for nonscience maiors a t most colleees makes extensive use of a text. the assigned readings f i r this course consisted of journal'articles from various sources includine this Journal. Scientific . of the topics were American, C. & E. News, a n d ~ a k r eAU contemporary and varied in their relevance to soeiety as well as in their scientific scope. These topics included nuclear physics, genetics and biochemical engineering, evolution with regard to new drugs and pesticides, polymer chemistry, and adhesives. Environmental concerns, such as the greenhouse effect, ozone depletion, acid rain, and recycling, were also discussed as consequencesof the above technological advances. Background material was taught a t the beginning of each section and the ensuing discussion considered the scientist's envisionment of the discovery versus its ultimate use, and whether such a use should be deemed "progress". Furthermore, the crucial interplay of the patent system, prevailing economics, and the scientist was stressed. This nontraditional approach allowed the students to view science on a more humanistic level. The appreciation for such an approach was supported by the favorable evaluations from the students. Two practical goals of science
Daniel Weeks Director of Studies and Senior Lecturer Department of Chemistry Northwestern Universtiy Evanston, IL 60208
Proper Glove Box Etiquette To the Editor:
Recentlv Roper et al. (1)described an efficient and economical procedure for replacing the air inside a glove box by an inert gas. They stated that Shriver and Drezdzon (2) distinguish two limiting cases for this process. However, Shriver and Drezdzon clearly distinguish three limiting cases. The third case, which has beeGignored by Roper e i al., is one where the incoming gas leaves the box through the outlet in a kind of a "short circuit" without mixing with the gas inside the box. If the density of the inert gas differs greatly from the density of air then, by appropriate placement of the inlet and the outlet of the gas, it may he possible to obtain an efficiency for the removal of air that is better than what would be expected from a model with perfect mixine. However. in practice. the air has the tendencv to rema$ "trappediin dead spaces with little mixing suck as the corners of the box. Therefore. the efficiencv for the removal of air in most cases is c&siderahly smaller than Volume 70 Number 6 June 1993
519
what would be expected from the model with perfect mixing. This is in contradiction with what has been indicated by Roper et al. who mentioned that the actual purging efficiencies lie somewhere in between the limit for perfect mixing and nerfect displacement. Consider, for example, my experimental findings for a box with a volume of 630 L which was flushed with nitrogen, Nz, at a rate of roughly 2.5 Us. After approximately 9.5 box volumes had been used the air should have been diluted by a factor ofe-'' = 0.000075. However, the wncentrations of oxygen and water vapor had been reduced by a factor of 0.0015 and 0.024, respectively This means the concentration of oxygen was higher by a factor of 20 than what would be expected from the model for perfect mixing. The water vapor is in equilibrium with water adsorbed on the walls and comparisons with predictions by any models ignoring this fact are inappropriate. The rate of removal of water is much faster if the inside surfaces of the walls of the box were exposed to a moist atmosphere for a short while only. The data given above refer to a steel box with a safety glass window that had never been in operation before. By purging most of the air from the box by inflating balloons or trash bags inside the box it is possible to wme close to the most efficient limiting case, the case with perfect replacement ( I ) . Roper et al.hli&ted that they managed to consistently produce atmospheres suitable for tgeir needs using three box volumes of inert gas only. Sherfey (3)described a similar procedure where, however, a single large balloon of the kind used to carry metereological instruments to the upper atmosphere had been used. The instruments needed to monitor concentrations of impurities in the glove box are frequently more expensive to buy and maintain than the box itself. For this reason many oeonle. esveciallv a t universities, prefer to use empirical tests & t h chemicals to assay theatmosphere inside the box, or they . may- rely on intuition and on crude theoretical models to estimatethe concentrations of impurities. For glove boxes that stand idle for extended periods of time andlor that are operated by largely inexperienced staff, it is important that they can be put into operation quickly, for example after a mishap has caused a contamination of the atmosphere in the box. It turns out that under such circumstances the users'needs are served best bv a n evacuable glove box. With this type of glove box it i s possible to obtain quickly an adequate atmosphere inside the box with not much more than a box volume of inert gas. A man&turer of glove boxes near the place where I live indicated that for evacuable boxes made of acrylic glass (Plexiglas) he charws about 50% more than for standard boxes of similar size: Since the walls of the evacuable boxes are considerably thicker than the standard boxes they have the added advantage that the rate of diffusion of the impurities through the walls is reduced.
2. Shliver. D. F.;h e d m n , M. A. Th. Manipulation @Air Semitiw Compounds. 2nd ed.;Wiley: N e w York,1886; pp 4741. 3. Sherfeg 3. M. Ind E l y Chom. 1854,46.43&440
Peter Senn
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Literature Cited 1. Roper, G. C.; Khs, G.; Mukerjee, S. L.; Haff, C. D. J. Cham. E d l v 1881, E8.508.
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Journal of Chemical Education
lngenieurschule Zijrich Lagerstrasse 45 8004 Zurich. Switzerland
To the Editor:
In response to the letter by Senn, we would like to emphasize that the purpose of our note was to describe a simple, inexpensive, practical, and fun way to improve efficiency of glove box regeneration. He seems to take our note as more than that and seriously attacks it. Most of his arguments are correct, however the following points should be made: Wc dld not mnsider that settmg up thc purge ~n such a n ~nrfficlent way as to *shortc~rcult"thegaa flow was a Ilmiting case. We assume that most people using glove boxes have read Shriver and Drezdzon or similar references and have enough mmmon sense not to short circuit the gas flow.We did not consider that passibility as a limitingcase for gas mixing but as a mistake in setting up the experiment .......
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We were nut aware ofthc use by Sherfey n f a metrrological balloon, however ri, such balloons are quite cxpen~iveand (ii)they would probably only be usable for a near empty glove box. In our case we have a dual length glove box about 10 R long that houses an analytical balance, solu-
tion calorimeter. and small vacuum line. The cornhination of trashbags and party ballwns allows us to fill in around these items without removing them. The statement regarding evacuable glove baxes warrants comment. We also have a heavy duty evacuable glove box, however we use it only for special applications. Since it is not equipped with a recirculating purifier its performance is actuallv inferior to the larzer but less heavilv built svstem witha purifier. The reason for this is that the leak& rate through the gaskets, seals, and gloves are about the same for the two systems. Since there is no cleanup mechanism for the static system it degrades quickly with time. It is certainly not the case, as implied by Senn, that thicker walls deerease significantly the rate of diffusioninto the glove box. The rate of diffusion of oxygen through stainless steel and glass is low enough that for all practical purposes it doesn't matter if it is 114-in. thick or 4-R thick. Virtually all leakage in occurs thmugh the gloves and gaskets and seals.
Our note was not meant to detract the reader from the abundunt literature on glove box technique but to provide a simple suggestion on how to save time and money in regeneration of the glove box atmosphere. It works for us. Gerald C. Romr
DicWnson college Carlisle, PA 17013
