A METHOD OF DISPENSING QUALITATIVE UNKNOWNS* WILLIAM
0.SWAN. SOUTHWESTERN UNIVERSITY, MEMPRIS, TENNESSEE
A simple yet flexible system has been dmised for easily and uniformly prefiaring and issuing unknoums to a large class. Each student i s forced to make a defLnite and fGnal decision based ufion his oum work. Dishonesty on the part of the student and bias on the @rt of the assistant cannot become inwolved in this system.
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A successful qualitative analysis course demands that careful consideration be given to the preparation and dispensing of the unknowns. The introduction of qualitative analysis in the spring semester of the first year of college chemistry has changed the general attitude of both the class and the instructor. In a second-year course the students are usually only those who are interested in science and are really attempting to become acquainted with this form of analysis. It makes very little difference to them if their unknowns are knowns. I n the first-year courses those attempting the analysis are too frequently there because i t is a part of the required curriculum or the lesser of "academic evils." Any method to get through, any dodge or excuse is too frequently their goal. Being a beginning course the class is placed in the hands of the younger and less experienced instructors. They are usually too busy with their own work to consider the general welfare of the course and too often they too are looking for the easiest escape. A number of methods of handling unknowns has been tried and from experience with them the following method has been derived. To illustrate this routine method, which we have found convenient to use, let us select the silver group and explain the handling of it in detail. The same methods, of course, will apply to the preparation of any type of unknown in any group or in unknowns involving any number of groups. It is possible to prepare seven different combinations of unknowns with the three elements in this the silver group. On a piece of scratch paper is prepared a list of these combinations, or of those desired to be used, together with the number of unknowns to be made of each. Thus one may prepare the same number of samples of each combination or vary the number as desired. This group being the first studied by the class, they would thus all bear the letter A on the label of each unknown. To supply a class of sixty, a complete set of bottles for these unknowns are labeled from A-1 through A-130; thus providing two unknowns for each student. The extra ten form a convenient margin for accidents, repetitions, etc. The bottles to be filled with one type of unknown, let us say the silver and lead ions, are selected a t random. These are arranged in a row and in each is placed the proper amount of silver solution, then the lead solution.
* Presented before the Division of Chemical Education at the 80th meeting of the American Chemical Society,Cincinnati, Ohio, Sept. 8-12, 1930. 350
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They are now diluted, corked, and recorded. The combination of silver and lead on the preparation list is then crossed off,and a new combination is selected for the next set of unknowns. This is followed by a third set and so on until all seven sets are prepared, or as many of them as are desired. We have found it convenient, in keeping records, to use two booklets such as most schools use for their examinations. More permanent books may be used if desired. Similar lines on the like pages in each book are numbered. In one book, "the key book," is recorded the constituents present in the unknowns. The other book, "the report book," is spaced for the following records on each unknown: the unknown numher, the name of the student to whom it is issued, the student's report with room for correction by the instructor, and a space for the grade. In our laboratory this latter book is kept a t the supply window where the unknowns are obtained by the student. When a student desires an unknown, his name is placed beside the numher of the unknown issued to him. His report, when made, is recorded opposite his name and numher. This of course prevents more than one student reporting on the same unknown. At stated intervals, with us the middle of each laboratory period, the unknowns are graded by the professor in charge. A page of the "key book" is placed upon the similar page of the "report book" in such a manner that corresponding lines join and a t the same time the names of the students are covered. This will bring the student's report and the correct analysis side by side. Thus i t is possible for the instructor to quickly and unbiasedly grade. For the student's information each report is corrected. The student consults the "report book" for his grade. These grades may be transferred to a record book if desired. Some of the obvious advantages of this system are: 1. The unifoormity of fioreNration and issue of the unknowns is apparent. Like unknowns may be mixed in a beaker and each bottle filled from the common solution. The student known to be "weak" is too often given easy unknowns while a good student is given a difficult one. While i t is an advantage for the instructor to encourage the "weak" and test the "strong" with proper types of unknowns, yet i t is usually a student assistant, often a fraternity brother or friend, upon whom the task of issuing the unknowns falls. Since the assistant who made the unknowns and the professor with the "key book" are the only ones who know anything about the unknowns, the human element is removed. 2. The ease of preparation of a large number of unknowns minimizing the chance of errors becomes more evident as one becomes familiar with the system. One may incorrectly list opposite the numher, forget to record a set of numbers, or make other mechanical errors, but most, if not aU of these, will become apparent before the whole set is completed. Not
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only are the errors noticed but those bottles involved are evident as well. We have found that this method has reduced the usual mechanical errors to almost nil. In recording or marking the bottles of the unknown one does not have a long set of numbers to deal with, as in the system suggested by Taylor a t the Atlanta meeting of the American Chemical Society, nor tags to lose or become hopelessly mixed ( 1 ) . Using only three numerals together with a letter to represent the type of unknown, twenty-six sets of unknowns with 999 unknowns to the set can be prepared. 3. No biased aid can be obtained by the student either wilfully or otherwise from the instructor, for our "key book" is unavailable except at times of grading. The habit of shopping for the easy grading instructor or the instructor who wiU permit the "I think I got a test for" or "a sort of test for" reports is prohibited. Since all unknowns are graded by one individual and only at stated intervals, the student must make a definite decision from his own work. In the laboratory the instructors are a t liberty to give their own opinions since they know nothing about any of the unknowns except what the student has showed them. 4. The honesty of the student is not inerolered, in that it is impossible for any one other than the individual to whom the unknown is issued to make the report. The unknown, once reported for a grade, cannot be reported by other individuals as is possible under many systems. 5. There is no system to be handed down from class to class nor the method of analyzing the unknown from its number as in Taylor's method. 6. The above method is flexible enough to meet local conditions and still be carried on successfully. Literature Cited "The Technic of Conducting the Introductory College Course in Quali(1) FURMAN, tative Analysis," J. CHEM.Eouc., 5,950 (Aug., 1928).
