A NEW METHOD of DISPENSING QUALITATIVE UNKNOWNS CARL E. OTTO University
T
of Maine, Orono, Maine
HE work of dispensing qualitative unknowns may be divided into three parts, making the mixtures, dividing these into individual portions, and keeping record of the sample given each student. Some teachers do all, some none of this while the
student is waiting. A common method of dispensing the unknowns is to pour directly from a previously prepared stock bottle of mixture into the student's bottle or test-tube while he waits and record a t that time the mixture given. Some fill the individual bottles from
the stock bottles before the laboratory period begins. Swann' describes the filling of many small, serially numbered bottles from the stock bottles and keeping track of the mixture in each small bottle. Furman2 tags the bottles with the composition hidden by a wax seal and, when the bottle is issued, writes the student's name on the removed tag and files it for record. Taylor3marks the bottles with a coded number system that discloses the composition to one who knows the system. When the students of Swann or Taylor report their analyses, the bottles they return a t the same time give the clues to the correct analyses. These systems obviate keeping record of the sample given to the student, and this is occasionally a help, for it avoids the possibility of a student turning in a report for an unknown which the instructor bas no record of issuing. Dispensing from the stock bottle while the student waits is subject to the criticism that all students do not receive the same amount of unknown. Crafty students will bring a vessel of large cross-section because the instructor unconsciously pours a greater volume into it than into a more slender one. It is better, therefore, to have the samples previously divided into individual portions. There is also the more serious objection that, as long as ordinary bottles are used for the stock bottles or the individual portions, there is danger from contamination from laboratory fumes, from drip from upset bottles, or dust from spilt chemicals on higher shelves, and, perhaps, from the use of old contaminated corks. These difficulties have been overcome by issuing unknowns in small hermetically sealed ampoules which are used once and then thrown away. Since they are of uniform size, all portions are of approximately the same volume. Since they are sealed immediately and cleaned by the student before opening, the solution remains uncontaminated. Unknowns containing substances which react with air, volatilize, or attack cork, remain unchanged. The unknown mixtures are made up from stock solutions, which have been kept free of contamina-
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1 SWANN. W. 0.. "A method of dispensing un- qualitative knawns," j . CHRM.EDUC., 8, 350-2 (1981). FURWAN, N. H., "The technic of conducting the introductory college course in qualitative analysis," ibid., 5, 946-56 (1928). 8 T ~ n o nA. . L.. "A new code system for dis~ensinpqualitative anal& unkn&ns," ihid., 9, l k l l 4 (1932):
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tion by capping the bottles, and placed in fine tipped buret. From this the solution is run into the ampoules, the tips of which are immediately closed by fusing in the flame of a blast lamp. Two people can mix the solutions, and a 1 and seal three hundred ampoules (twenty each of fifteen mixtures) in about two and onehalf hours. In consideration of the minutes saved a t the time of issuance to the student, the avoidance of the mess of pouring solutions a t that time, and the prevention of contamination, the time is well spent. The ampoules are cheap enough to use once and discard. Alloys and other non-hygroscopic solids are portioned out in gelatin capsules, such as are used in medicine, and these, like the ampoules, permit equal quantities to be dispensed easily and quickly to each student. As for keeping a code book on the sample issued to a particular student, the labeling method of Swann, Furman, Taylor, or any other system can be used. In general, the simpler it is, the better, if it is safe from decoding by the student. Swann's method involves an intermediate record in getting back to the record of the composition. Furman's method requires the keeping of the separate tags and finding the right one when an analysis is reported. If Taylor's students ever read his article, they can quickly learn how to ascertain the number of ions in their unknowns and, with a few returned corrected reports, his key element and the whole secret of his code is out. With this in mind the author hesitates to disclose his method in full. The compositions are kept in a code book and numbered. A series of numbers, different for every portion of a given composition, is placed on each ampoule and this by an easy mental process is converted to the number in the code book. From this the correct analysis is obtained. SUMMARY
Unknown solutions hermetically sealed in ampoules of just the size for an individual student's analysis assure the instructor that the student receives them without contamination. A coded labeling system does away with keeping a record of which sample was issued to a student.
