RELIABILITY of QUALITATIVE ANALYSIS PROCEDURES in the HANDS of STUDENTS" CARL E. OTTO AND EDNA BISHOP OTTO University of Maine, Orono, Maine
The accuracy with which students detect the various ions has been determined by compiling the records of the reports of six hundred nine students. It i s shown that, while by the usual procedures some ions arefound a large proportion of the times they are given, others are detected relatively poorly. The record of ions incorrectly reported present i s also given. Possible sources of error are discussed. Where procedures have been changed and used for a suficient number of times, the results for the different methods hawe been studied separately. The records show that while the procedures may be reliable i n the hands of experienced analysts, there are weak spots i n them for the beginning students. Numerous failures cause some students to lose faith in qualitative procedures i n general and resort to guessing.
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second for the silver and copper groups, his third for the silver, copper, and tin groups, and so on. Thus, the metals of the silver group were present oftener and tested for oftener than those of successive groups. The concentration of each ion in the unknown solutions was 10 mg./cc. ACCURACY OF FINDING IONS
A correct analysis involves two factors, positive tests for those ions which are present and negative tests for those which are absent. These factors will be discussed successively. In the unknown solutions compiled here 13,395 cations were given and 11,162 or 83.3 per cent. were reported correctly. The accuracy varied, however, from 56.1 per cent. for zinc to 98.7 per cent. for mercurous ion, as is shown in Table 1. Comparison is also made in this table with studies by Gilbertson ( 1 ) and Heisig (2) and a column showing
HE FACT that the detection of different ions presents different degrees of difficulty to students of qualitative analysis is well known to all teachers and many students of that subject. The existing procedures are, beyond doubt, reliable for an experienced analyst, but the numerous failures that beginners encounter cause some of them to lose faith and conclude that good guessing is better than accurate and careful work. In order to determine the accuracy with which students detect the various ions, records of their reports for the past ten years a t the University of Maine have been compiled and studied. During this time 3707 unknown solutions and 2845 unknown solids were analyzed by six hundred nine students who were taking their second year of college chemistry. About half of these were chemistry or chemical engineering majors and most of the rest were premedical students for whom i t was not an absolute requirement. Since the number taking the course each year was comparatively small the time of collecting the data had to be fairly long to get reliable results. The conventional scheme of analysis with minor changes from year to year was used. Each student analyzed his first unknown for the silver group only, his the combined weighted data of all three is included. It will be noted that the general trend of results is * Presented before the Division of Chemical Education at similar but that certain differences stand out. The the ninety-fourth meeting of the A. C. S., Rochester, New York, percentage of correct results tends to be higher and Sentember 6. 1937.
the spread between the high and low narrower in Gilbertson's study than in Heisig's or the present authors'. Basis for explaining tbis difference is lacking. The major difference in the schemes of analysis is in the case of Group I, which Gilbertson combined with Group 11, as indicated by Gelbach (3). It will be noted that the general trend of results is reversed in this case and decidedly higher results are obtained in this and Heisig's study for mercury. The silver percentage is also slightly higher. Hence, the procedure separating the silver group seems desirable. Another marked difference from the general trend is the better results for calcium in this study. The percentage is as good as that of a certain group of Heisig's students who used sodium carbonate for the second carbonate precipitation of group IV and much better than that of his students as a whole. The senior author's students used ammonium carbonate for both precipitations. Gilbertson proposes to compensate for the differences in difficulty by using a weighted grading system. This seems desirable but difficult to devise. The data also indicate the desirability of modifying the scheme of analysis to improve the accuracy for certain ions, a t least for those which are low in all the compilations. Heisig has used his data in this manner and shown that improvement can be made. I t is planned to pursue this study further in this laboratory. The lowest metal in the combined data is tin. It is also low on each list. Besides the possibility mentioned by Heisig ( 2 ) that it becomes reduced to metallic tin by the iron there is also the possibility that after reduction to stannous i t reoxidizes to stannic before the students add the mercuric chloride. This possibility has always been stressed in the author's classes and the result is perhaps reflected in the better average obtained there. A test in which reduction was unnecessary should improve the detection of this metal. Next in the list is zinc. During the first five years of the study, zinc was separated from chromium by precipitation as the basic carbonate. It was found thirty-six per cent. of the time. During the last five years, chromium was removed as barium chromate and the zinc obtained as the sulfide. It was then found sixty-seven per cent. of the time. These results show definitely that the latter separation, though still poor, is the better. The post-precipitation of zinc on copper sulfide mentioned by Caldwell and Moyer (4) may also reduce the amount of zinc precipitating in Group I11 and thus reduce the probability of finding it. Magnesium, precipitated as the double phosphate with ammonium, tends to remain in supersaturated solution and tbis may be the reason why it is often missed by students who are in a burry. The precipitation of pentavalent arsenic by hydrogen sulfide is extremely slow under ordinary conditions. In spite of cautions about this property many students get their first precipitation of arsenic when they are boiling the hydrogen sulfide out of the filtrate obtained when the Group I1 precipitate is filtered off. When tbis boiling is not done, either because the remaining
groups are not to be tested for or because the student thinks he will take a short cut to the precipitation of Group 111, arsenic is frequently missed and causes further trouble with the finding of other metals. Barium is probably missed due to oxidation of some of the hydrogen sulfide and consequent precipitation as barium sulfate along with the precipitates of Groups I1 and 111 or with the ammonium salts that crystallize out when concentrating the Group 111 filtrate. I t gives much less difficulty (missed once in twentythree times given) in unknowns to be analyzed for Group IV directly in which no hydrogen sulfide has been used. Aluminum may be missed because the solution from which it is to be precipitated is not made completely ammoniacal. Frequently the author has found students passing up a precipitate of aluminum hydroxide because they did not stir enough after the addition of the ammonium hydroxide, were judging the alkalinity by the odor of the top layer of the solution, and did not look closely enough to see the almost colorless precipitate in the neutral zone. The substitution of the aluminon for the cobalt nitrate confirmatory test for aluminum, contrary to expectations, has not materially changed the percentage of times this metal was found correctly. Separate compilations for the periods (the first and last five years) when each test was used, show the insignificant change, and a reduction at that, from seventy-three per cent. to seventytwo per cent. Calcium was found fairly well by the senior author's students, but is low in the combined list. The reason for this is unknown, for no novel procedure was used. Ammonium carbonate was used for both carbonate precipitations in Group IV. As will be shown later the detection of calcium in solid salts by the senior author's students is not remarkably high. The confirmatory test for cadmium is often obscured by black sulfides of metals which should have been, but were not, previously removed. The supplementary procedure recommended by many authors, as for instance Curtman ( 5 ) , does not always clear up this trouble. It may be that metals of Group 111 have not been thoroughly washed from the Group I1 precipitate. The missing of cadmium may also be due to incorrect acidity preventing its precipitation in the Group I1 precipitate. Finally sodium should be mentioned. Although its combined average is fair its percentages in two of the studies were low. In this laboratory it bas been confirmed both by flame test and by precipitation as the pyroantimoniate. Students knew the flame test was not wholly dependable because i t could be given by impurities inadvertently admitted. The precipitation test was often obscured by the precipitation of other metals which had not been correctly and completely separated. Therefore, they frequently resorted to guessing. These facts also account for some of the many "extra" reports of sodium. It may be noted that all the colored ions are well
of calculating percentages does not give a true picture of the situation for strontium reported incorrectly thirty-seven times (the author's data) out of a possible 1061 is definitely worse than mercuric ion reported incorrectly fifty-six times out of a possible 2642. But, it will be noted again that the general trends are similar. The negative, as well as the positive results for mercury are better in this study, giving further evidence of the desirability of the separate silver group. Sodium is found oftener when it is not present than other metals. This is probably due to the indecisiveness of the tests as mentioned before and to the great danger of contamination by this metal. This may also explain the numerous extra reports of iron for a speck of iron rust falling into the student's solution would be REPORTING OF EXTRA IONS sufficient to give him a beautiful test for iron. Also the The percentage of "incorrect finds" has been calcu- incomplete reduction of manganese or the incomplete lated by subtracting the number of times an ion was separation of cobalt or nickel would, with potassium given from the number of times i t was tested for and ferrocyanide, give tests more or less resembling that of dividing this into the number of times it was reported iron. The extra reports of potassium are likely due to when it was not present. Thus, it was possible for a incomplete volatilization of the ammonium salts which would give precipitates very similar to that of potassium. The extra reporting of ammonium which is considerably worse in the present study than in the others may be explained by the extreme sensitivity of the litmus or Nessler's reagent tests and the likelihood of ammonia fumes in the laboratory air. These may Silver Mercurous get into the stock unknown solutions, deposit around copper the stopper and contaminate the student's portion as Barium Chromium it is poured out, or affect the test at the time the Cadmium student is making it since nearby students might be Cobalt Lead evaporating solutions containing ammonium salts to Manganese dryness a t the same time. While they are told tovolatilMercuric Bismuth ize these salts in the hoods, there is insufficient space strontium Antimony there for the students to use them also for the longer Nickel time needed for the preceding evaporation. ThereTin Arlenic fore, they carry out this first operation a t their desks Ammonium and often do not notice, while working a t something zinc ca1eivm else, that evaporation is finished and ignition begun. Alumioum Even when liberated in the hood, fumes are sometimes Iron Magnesium seen emerging from cracks in the upper part of the Potarsivm Sodium hood. It has been noticed frequently that calcium is Totals reported when strontium is missed. The similarity of their flame tests doubtless contributes to the error when metal of the silver group to be present in any of the strontium is not found and not eliminated before 3707 unknowns included in this study. Silver was testing for calcium. Magnesium is doubtless reported present eight hundred seventy-seven times and there- incorrectly so many times because practically all fore i t was absent and might have been reported in- metals will precipitate as phosphate in alkaline solucorrectly 2830 times. It was actually reported extra tion and if they have not been removed a t the proper fifteen times or 0.53 per cent. of the times it might places they will give a precipitate (not crystalline, it is have been. Summing them all up, 2020 ions were true) which the student mistakes for magnesium. incorrectly reported present out of the 44,084 times Aluminum is often reported extra when zinc is missed they were absent. The results are recorded in Table 2. and is probably due to the use of insufficient ammonium Direct comparison with Heisig's data can be made hydroxide to redissolve the zinc hydroxide. The lack and the combination of it with the present is shown in of ammonium hydroxide may also leave aluminum inthe last column. Only indirect comparison with completely precipitated and this may give false evidence Gilbertson's results can be made since he treated his of the presence of zinc. Better tests for all these ions negative results in a different manner. The percent- are needed. It may be noted that except for iron no ages he gives are those of the times an ion was re- colored ions are included in this list that are found ported extra compared to the total extra. This method extra so frequently.
detected. Considering that except for certain mixtures of cobalt and nickel their colors are clearly visible in concentrations of 10 mg./cc. it is strange that they are missed a t all. (Perhaps some students, finding only colorless ions in a colored unknown, think, if they think a t all, that the instructor is trying to fool them by adding a dye.) The colored ion with the poorest percentage is chromium and this metal was found much better in this (and in Gilbertson's) study than in Heisig's. This may be due to the senior author's continual stressing of the necessity of decomposing all sodium peroxide before acidifying, preceding the precipitation of aluminum.
ANALYSIS OP SOLID UNKNOWNS
After analyzing a number of solutions for their metal content, the students were given solid unknowns, usually one alloy, two or three single salts, and two mixtures, in which to detect both cations and anions. Some of the salts were soluble in water, some in acids, and some required more strenuous treatment. The six hundred nine students, whose records were studied, analyzed four hundred fifty alloys and 3395 salts and mixtures. The correctness of their reports is portrayed in Tables 3 and 4. Gilbertson also made a study of reports on a limited number of anions. It was not stated whether these were given as solutions or solids, and his data are not incorporated or combined with the authors'. The metals in Table 3 are in the same order as in Table 1, in order to compare more clearly the accuracy of finding the ions in the solution and in the solid state. TABLE 3
to student, to the fact that many of the unknowns were single salts and the students, knowing that only a small number of ions was probable in these, chose more carefully the ions for which they got the best tests, or to some other unknown factor. The accuracy of finding anions in the solid salts and mixtures is shown in Table 4. These are arranged in the order of the percentage found correctly. It is seen that several of the anions, including some of the
permanganate Farieyanide Sulfate Phosphate Ar~nite Femoeyanide
CENT.OP CATIONS CORRBC~Y AND INCOP~RBCTLY R E P O R ~ EIND SOLID Nitrite ualrliowns (SALTS AND ALLOYS) Iodide Sulfide Chloride Arrenate Bromide 7 0.18 Mercuraur Tartrate 10 0.26 Silver Carbonate 33 0.98 copper Thiocyanate 63 1.77 Nickel Oralate 33 0.89 Cobalt Silicate 71 1.99 Iron Borate 72 2.05 Lead Cyanide 62 1.67 Antimony Nitrate 14 0.37 Mercuric Chlorate 32 0.88 Mangane~e Antimonite 22 0.60 strontium Phaspbite 70 2.12 Ammooium Acetate 165 5.44 Potassium su1fite 27 0.74 Chromium Fluoride 163 5.00 Sodium Iodate 39 1.08 Bismuth Totals 25 0.69 Cadmium 75 2.14 Calcivm 120 3.50 Aluminum 43 1.19 Barium more common ones, are very 26 0.69 Arsenic results are representative of 75 2.11 Magnesium PEP.
zinc
Tin Totals
79 31 -
2.28 0.88
1357 Average 1.58
It is seen that the percentage found correctly is generally lower, in spite of the fact that more material was available to the student and that he has had more practice with the scheme of analysis. The order of the percentages is roughly the same, but mercurous has fallen frdm its leading position. A thought may occur that the student in dissolving the solid may oxidize it and find mercuric, and i t is true that mercuric is frequently reported when mercurous has been given. The small number of cases may also have some bearing on the accuracy shown. It may be noted that tin and zinc are again near the bottom of the list, but magnesium and calcium have become much worse. In the incorrect reporting of ions that were not given, sodium and potassium are again the worst, and indeed the order of all the metals is very much the same. The numerical values are, however, much smaller, due, perhaps, to less chance of contamination before issuing
17 1 12 0 8 105 86 8 9 40 13 41 121 30 8 42 42 8 71 69 24 13 37 58 0 7 22 52 25 3 972
-
0.50 0.03 0.37 0 0.24 4.0 2.9 0.24 0.27 1.19 0.40 1.26 4.3 0.90 0.24 1.29 1.43 0.24 2.3 2.1 0.73 0.39 1.22 1.72 0 0.21 0.67 1.57 0.76 0.09
Average 1.00
poorly detected. If these all classes of this caliber, the tests for fluoride, sulfite, acetate, chlorate, nitrate, cyanide, and borate, need to be made decidedly more sure. and others could stand imvrovement. Gilbertson's study, which showed less spread than the authors', also indicated the need for improved methods. To correlate the accuracy of reports with the procedure used, Table 5 shows the results by years for those anions given a sufficient number of times for this treatment to be significant. In the first four years, the senior author's own text in mimeographed formwas used, in 1930 Scott's "Qualitative Chemical Analysis" (6), TABLE 5 PERCBNI. OP ANIONSFOUND COXXEETLY POX I N D N ~ U ~ T Y .B I B S Yeor
Cmbonale Chloridc
Nitrore
Oxnlole PhorPholc S d f n l c
and from 1931 to 1935, inclusive, Curtman's "Qualita- presence of oxalate in the precipitate by its decolorizing of acidified potassium permanganate solution. While tive Chemical Analysis" (cf. reference 5). Carbonate shows a decided improvement in 1931 and there is variation in the accuracy of finding oxalate, remains fair thereafter. Yet, the only change of pro- this is not correlated with the use of any one textbook. The molybdate test was used throughout as the concedure was the use of lime water in 1930 and barium hydroxide solution other years to absorb the gas firmatory test for phosphate and the only difference liberated with HCl. The reason for the improvement is that can be found to account for the low accuracy in 1930 is in the preliminary test for the barium chloride therefore obscure. The detection of chloride is also better in the last group of anions, since in that year, after precipitation five years. When present alone, all methods of detec- of certain anions with calcium chloride in an acetate tion were the same. When mixed with simple or com- buffered solution, phosphate and certain other anions plex cyanides or other halides, the methods were were precipitated with barium hydroxide solution and different. In 1926-29 the chromyl chloride test was in the other years it was precipitated with the group used. In 1930 the cyanides were removed by oxidation as a whole, with barium chloride and calcium chloride with HN03, or by heating the silver precipitate to in a solution made just alkaline with ammonium redness, and the halogens, after resolution of the resi- hydroxide. It seems possible that in 1930 phosphate due, by successive oxidations with ferric salts and might have precipitated before i t was expected to in potassium permanganate. In 1931-35 the cyanides the preliminary test and thus escaped detection. The procedure for detecting sulfates, that is, barium (except CNS) were precipitated with cobalt nitrate, iodide, and bromide oxidized with hydrogen peroxide chloride in acid solutions, was the same for all the years and nitric acid, and thiocyanate removed by ignition. and although the accuracy shown is somewhat better in the period 1931-35, it is generally quite good throughChloride was tested for then with silver nitrate. Nitrate is far better in 1930 than before or after. out and hence this test is satisfactory. The other anions were not given sufficiently often to The brown ring test, after removal of interfering ions in various ways, was used in that year. Reduction to hear separate yearly treatment. They were not ammonia with zinc and sodium hydroxide, supple- decidedly worse in one period than in another, and mented by other tests to eliminate nitrite, was used hence those with low accuracy need other means of before that and the evolution of NO* with copper and detection than those used in the three texts. 18 N HB04 (after interfering anions had been removed) CONCLUSIONS after that. The superiority of the brown ring test is A study of the results obtained by students in thus demonstrated. Oxalate was precipitated with calcium sulfate solu- qualitative analysis shows the desirability of changing tion in the period 192&29 and also in 1930, and with the procedure in order to improve both the positive and calcium chloride thereafter. All schemes confirmed the negative tests for certain ions. LITERATURE CITED
(1) GILBERTSON, L. I., "A study of student results in qualitative analvsis." J. CXEM.E ~ u c .13,483 , (1936). (2) HEISIG,G. B., "The reliability of qualitative analytical procedures in the hands of beginning students,"ibid., 3,177 (1926). (3) GELBACH, R. W..''Modified methods for cation group I," ibid., 10, 621 (1933).
(4) CALDWELL, J. R. AND H. V. MOYER, "The use of crotonaldehyde to reduce the postprecipitation of zinc on copper sulphide," 3. Am. Chem. Soc., 59,90 (1937). uQualitative chemical analysis," The (5) CURTMAN, L, Maanillan Company, New York City, 1931, p. 317, note 3" *".
( 6 ) SCOTT, W. W., "Qualitative chemical analysis," 5th ed., D.
van Nostrand Co., New York City, 1928,264 pp.
