The "Top-Off" Bottle My experience is that itudents, in diluting iolutions in volumetric flasks. oftrn choose to add the watrr trom a polycthylenr wash buttle and thus frequently overshoot the mark. Even when supplied ~ i t an h wedruppw and small henker, few sctrdentstakcthetimeandeff~,rrtuuuethem,or, iftheydo. therlranlinrssc~fthe beakeranddropprr may he in question. Then too, the beaker is sometimes in use for another solution. In our second-semester general chemistry laboratory course, every experiment involves quantitative dilutions. Unknowns are usually dispensed into volumetric flasks, which the student must then dilute to the mark before sampling. Overdilution of solutions represents a source of error and after-the-fact concern for students, since their grade depends in part on the accuracy of results, and a source of aggravation far the instructor, whose careful labor in preparation of the unknown has now been somewhat in vain and who must field questions like "Now, what do I do"? Some recourses include (1)viewing overdilution as a blunder for which student results and grades will suffer, (2) reissuing unknowns, or (3)making corrections for overdilutions (usually requiring an explanation from the instructor). The best solution, however, is to simply emphasize doing it right in the first place. I have found that accidental overdilutions seldom occur if each student is provided with an inexpensive small dropper bottle with screw eap/dropper (such as Fisher 02-983A)and instructed to fill it with distilled water only and use it for the final too-off of volumetric flasks. The "too-off' bottle is dedicated to this single operation. The bottle is always handy and ready'for use, its contents are free of codtamination, and the dropper facilitates slow, dropwise additions. Wayne L. Felty The Pennsylvania State University Wiikes-Barre Campus Lehman. PA 18627
182
Journal
of Chemical Education
