A STUDY of STUDENT RESULTS in QUALITATIVE ANALYSIS L. I. GILBERTSON The State College of Washington, Pullman, Washington
A study of student results in the qualitative laboratory shows the need for ewaluation of each sample anulyeed. All ions are not identified with the same ease. Consideration should be given to the fad that certain ions are more
likely to be reported when not present than others, and that some ions interfere with the identification of other ions. The results of 18.678 cation and 4073 anion tests are summarized.
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A
NUMBER of schemes have been devised by which student results in the qualitative laboratory may be evaluated. Any scheme for the equitable evaluation of student results in qualitative analysis should meet the following requirements. (1) Credit should be given for the separation and identification of ions in ratio to the difficulty of detection of each particular ion. (2) Credit should be given for the ability to establish the fact that certain ions or groups of ions are not present in the sample; or suitable penalties should be assessed for the reporting of the presence of ions which are not in the sample. (3) Consideration should be given the fact that some ions are more difficult to identify when in the presence of certain other ions or when present in small amounts. This study was made in an effort to obtain a basis for the apportionment of credit in the evaluation of ~ualitativesam~leswith soecial reference to reauirem a t s 1 and 2. The source of data for the study was the laboratory records of some 1300 students. These records were kept continuously over a period of seven years, during which time the nature of the work done and the type of samples analyzed were not appreciably changed. The scheme of analysis used was conventional with the exception of the omission of the hydrochloric acid group as indicated by Gelbach.' The record of each student was kept in such a manner that it was possible to know the content of each sample, the ions reported correctly, and the ions reported but not present in the sample. The tables following were compiled from those records. CATIONS Table 1 indicates the relative difficulty experienced by the students in the separation and identification of each cation. A sufficient number of cases was studied, 18,678 cations being present in the samples studied over the seven-year period. The cation appearing least often in the samples was silver (302 times), and potassium appeared the most often (1710 times). Of the total number of cations present (18,678), 86% (or 16,929) were correctly reported. In the analysis of these samples 2232 cations were incorrectly . reported G~macn,"Modified method for cation group I." J. &EM.
E~uc..10, 621 (Oct.. 1933).
as present. Of these, the ion reported most often was sodium (12% of the 2232 cases) whereas silver was incorrectly reported the fewest times (0.4% of the total). The results of such reports are shown in Table 2.
Copper Silver Nickel iron
Cobalt Man.anesr ~ e ~ d Antimony Chromium Bismuth
sodium
Mercury Potassium Aluminum Ammonium Arsenic Strontium Cadmium Tin Zinc Magnepium
Barium Calcium
430 560 1078 429 938 1027 749 811
358 462 874 340 740 805 582 622
83 83
81 79 79 78 78 77
Silver COPPer
Chromium Nickel Manganese Tin Mercury Cadmium Calcium Cobalt Antimony Lead
Ammonium strontium Iron Amenie Aluminum
zinc
Magnesium potarsium Sodium
It is quite evident from the data in Table 1 that all cations are not detected with the same ease.
ANIONS
A similar study was made on 4707 anion cases. Of these 4073, or about 86%, were correctly identified. At the same time there were 326 anions reported hut not present in the samples. The results are tabnlated in Table 3. TABLE 3
Anion
Pei Ccnt, of C o ~ e rFound
Per Cenl. of Total Reported but Nor Prrrrnl
Chlmide Sulfate Iodide Carbonate
Chromate Arsenite Phosphate
sume
Bromide Nitrite Sulfide Nitrate ArsenatF Borate
CONCLUSION
From the foregoing data i t is evident that neither cations nor anions are all identified with the same ease by the student in the laboratory. The application of
these data to the evaluation of student results is not nearly as simple as it might seem. Remembering that consideration should be given to the difficulty of identification, the ability to establish the absence of other ions, and the fact that the presence of some ions interferes with the identification of others, i t would seem that the most logical scheme would be to evaluate each sample separately. Thus it would be possible to consider the data in Table 1 and to provide suitable reward for the identification of the more difficult ions. In the same manner, suitable penalties might be assessed for failure to find colored ions (when present in reasonable amounts) or for the reporting of ions not present. It is not the purpose of this report to present a scheme for the evaluation of student results but merely to show the need of something better than the pro rata grading system so often used. It will be necessary for the instructor to evaluate the individual samples considering not only the ions present but the quantity of each and the scheme of analysis used. Inasmuch as the scheme of analysis used in this study is that commonly used, and the students not specialized, the data in the accompanying tables might well serve as a guide in sample evaluation in cases in which such data are not available.
