W. E. Horris
and B. Krotochvil University of Alberta Edmonton, Alberta, Canada
A Practical Test in Basic Laboratory Operations
Experimental work in chemistry courses has the general aim of providing an understanding and appreciation of the activities of the present day chemist. In an analytical chemistry course the goal of laboratory work also includes emphasis on techniques for gathering reliable experimental data. In introducing students to rigorous and realistic experimental work in chemical measurements, it is best to begin with basic operations and assume no prior experience. This instruction should conclude with an objective assessment of each student's proficiency; for this purpose a practical test given just after instruction and practice in basic operations helps develop experimental competence. At this time the instructional value is much greater than when given near the end of a course. Problems with technique should be uncovered and corrected as soon as possible. Outline of the Practical Test
A series of test stations (Fig. 1) is set up in a way that allows each student to begin the test individually at an assigned time and proceed through it a t his own pace. For the test outlined here the average time required is about 40 min. It consists of a brief written quiz dealing with data evaluation, the functions of the parts of a balance, reading a vernier scale, significant figures, and so forth; the quiz is followed by the practical items described below. Fundamental to precise reading of dials, meters, slide rules, and so forth is interpolation between lines. Illustrative of this simple operation is reading the position of the meniscus in several burets. Short buret sections (Fig. 2) can be sealed 2/3 full of water and mounted in a holder. Best values to the nearest 0.01 ml can be established with high confidence by six or more independent readings by the instructors. Chemistry laboratories now almost always have electronic or mechanical calculating machmes available. The next test item (Fig. 1) assesses competence in their use. Several sets of calculations can be assigned on a rotating basis, so that no two students are likely to have the same set a t the same time. Each set should involve multiplication and division of four-digit numbers of the type encountered in analytical problems. Correct use of volumetric ware, critical in many experimental measurements, can be tested by asking each student to take an aliquot of a solution. Volumetric ware and a conical flask containing 3 M HzSOaare provided, along with the instruction: "Pipet 10 ml of the concentrated solution into the 250-ml volumetric flask provided. Dilute to volume. Pipet a 10-ml aliquot of the diluted solution into the 200-ml conical flask pro-
vided." The aliquots are collected and titrated by the instructor with 0.06 M NaOH to a methyl red end point. Use of 3 M HzS04 is convenient because it has little tendency to either evaporate or absorb water under usual conditions of relative humidity. Anot,her fundamental laboratory operation is weighing. Weighing three objects appears necessary and sufficient to adequately assess weighing technique. Aluminum, glass, and brass weights ranging from about 1to 30 g are suitable. Typical Results
The frequency and variety of errors committed by students learning to do careful experimental work are surprisingly large. For typical second- and third-year chemistry students, we have found that even after careful instruction and practice about 25% cannot read a vernier scale correctly; about 12% report one buret reading out of eight incorrectly by more than 1 ml and about 9% by 0.1-1 ml; about 25% rate a zero on the calculation item (graded on a liberal ecale in terms of correctness of the digits, decimal point, significant figures, and rounding); about 17% submit an aliquot whose titer deviates from the correct value by more than a part per hundred; and about 17% report one weight in three in error by 10 mg or more. The test brings into focus those areas requiring further attention.
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Balances
Figure 1. Ileftl A rchemotis layout for o practical test. indicate locations of personnel conducting the lest.
The numbers
Figure 2. (right1 Buret section for instruction in reading burolr. There may be made from a broken buret. The position of the menixus will remain unchanged indefinitely. Store in inverted position.
Volume 48, Number 8, August 1971
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543
Computer-Assisted Test Evaluation and Instruction
A computer program' has been written that provides each student uith a sheet of computer output listing the results he reported for each item, the correct results, his grade, a detailed analysis of errors, and advice for corrective action where needed. Typically a st,udent receives four to six statements that give either encouragement or constructive advice. For example some output statements for the aliquot item are Your technique with the pipet snd volumetric flask is excellent. The titer value for your aliquot was slightly high, probably because you did not rinse the pipet adequately with the dilute solution in the volumetric flask before it was used to take the aliquot. The titer value for your diquot was so far from the correct one that it must be rated 8. blunder. The value obtained probably resulted from insufficient mixing of the contents of the volumetric flask before the aliquot was withdrawn. Because of the large error it is not possible to assess your proficiency on m y of the detailed aspects of the operittions. Please ask your laboratmy instructor for assistance.
544
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Journal of Chemical Education
The computer program is also written to provide the instructor of each laborat,ory section with a detailed summary of the results and performance of his students. He can then be prepared to suggest corrective measures as required. Additional details of the material, logistics, and personnel required to test about 40 students per hour are available from the aut,hors. For small classes the entire test can he given readily by a single instmct,or; in this case about four or five students per hour can be accommodated. In summary, many students have difficulty with simple, fundamental laboratory operations. Early elimination of errors greatly increases student confidence, morale, and the quality of later work. The practical test outlined here appears to be an effective means of pinpointing faulty technique. Available from the authors in Fortran I V for an IBM
360/67 computer.
