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 Smsitiw 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.
520
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
