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JOURNAL OF CHEMICAL EDUCATION
JANUARY, 1928
QUALITATIVE ANALYSIS IN HIGH-SCHOOL CHEMISTRY BRUCEH. GUILD, IRON MOUNTAIN HIGHSCHOOL, IRONMOUNTAIN, MICHIGAN
The value of qualitative analysis as a cultural study was admirably presented in an article in THISJOURNAL for March, 1927, by Mr. Edgar J. Witzemannl and all who read this article should preface it by a careful reading of the above. One of the fundamental objects of high-school chemistry is to give the pupil a type of mental training that is not brought out in any other subject, namely, to develop the power of reasoning and to apply it to actual and practical matters. I t is a known fact that qualitative analysis gives this type of training in fullest measure but the ordmary course in the subject is usually not heard of or encountered unless a student desires to make a more or less complete study of chemistry after he enters college or university. This group seems a pitifully small minority to have the opportunity of enjoying the benefits of such a mind-training and reasonsharpening experience. It is the purpose of this short article to show how the subject can be used in the general high-school chemistry course and to relate some of the experiences and methods used by the writer in presenting the subject. True, some texts have been prepared for a course to be given as an additional subject but few students have the time or desire to go farther than the one-year course and for most schools it is unprofitable to offer such a course. The writer attempted to present a bird's-eye view of the subject in two weeks and the results were very gratifying and interesting. The best time for this work to come is toward the end of the course when the study of the metals has been completed. The work consisted in: 1. Studying reactions of the metals and dividing them into groups. 2. Preparing charts and trying the tests. 3. Analyzing unknowns. Twelve metals were selected and divided into four groups, Group 1, metals precipitated as chlorides, H ~ ' , Pb, and Ag. Group 2, metals precipitated as sulfides, Zn, Fe"', and Cu. Group 3, metals precipitated as carbonates, Ca, Mg, and Al. Group 4, metals not precipitated by ordinary reagents, Na, K, and NH.. The twelve metals chosen were common ones that had been studied so that the students were more or less familiar with their chemical properties. The first laboratory period the students worked out their chart for the first two groups from information found in their texts and former experiments and additional information from the instructor. They then tried the tests and did one simple unknown. The following period the same 1 THIS JOURNAL. 4,355-8 (March, 1927).
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thing was done for the last two groups. Much better work is done by the students if they work out their charts. The charts were then printed on cardboard outside of class and with many of the students, interest was aroused t o such a pitch that some of the charts were veritable works of art. The charts were then corrected and the two laboratory periods of the following week were given t o the solving of unknowns and here the writer was very much interested in just bow much the students would be able to do. Each student was required to do, first, two simple unknowns consisting of single salts and after this one of a mixture of two salts and, finally, one of a mixture of three salts. The unknowns were given out as dry salts (either the nitrates or chlorides of the metals) in little packets. All four were numbered and given out a t once. If a student reported incorrectly he must repeat his analysis and do another for penalty. This rule was made to eliminate guess-work and comparison. The results were highly satisfactory. I n two classes of fifty-two pupils all but six completed their work in the allotted time and all of the six finished by coming in after school. This was not compulsory, but they all wished to complete their work and regarded it as an interesting game of wits and skill rather than an irksome task. Several boys made themselves a nuisance by getting their work done before the time was up and many students asked if they might do extra work in spare periods, so interested were they in the subject. Qualitative analysis courses for the high-school student may be varied considerably, more metals may be used and the acid ions may also be studied, depending upon the length of time and the amount of work that the teacher wants t o spend on the subject. The following is an example of a scheme used by the writer in a four-period or two-week course. Metal
Group test
Individual test
Add 10 cc. HC1 White pet. HgCI, PbCI2, AgCl Filter ppt. and save filtrate
Pb Pour 10ec. hot HzOrepeatedly thmugh ppt. PbCL dissolves in hot H20, hut crystallizes out when cool Hg Pour 5 cc. NH,OH over precipitate and catch filtrate. If ppt. turns black, Hg Ap To filtrate add HCI: white ~nt.-Aa
Group 2
To filtrate from above add
Zn ZnS is the only white sulfide. Do not
Zn
15 cc. HIS sol. White ppt. ZnS
Fe"'
Black ppt. Fe& CuS
confuse with colloidal sulfur. C o ~ m a t o l y test. Heat on charcoal with blow pipe. Yellow color while hot, white, while coldZn Fe To a portion of the group 1filtrate add 5 cc. NH,OH, reddish brown ppt.-Fe Cu White pet. sol. in excess NKOH; bright blue color with above test-Cu
Group 1 Pb HgL
Ax
Cu
Group 3 Ca Mg A1
Group 4 Na
K NHa
To filtrate from above add Ca To portion of filtrate from group 2 add 10 cc. Na2C08 5 cc. (NH4)zC20,-white ppt.-Ca Mg To portion of filtrate add 5 cc. N~HsPOI; White ppt. CaCO. M~COI.Alx(C0s)~ white ppt.-Mg Al To portion of filtrate add 5 cc. NHIOH. gelatinous ppt.-Al. Confirmatory test. Filter and transfer ppt. to charcoal block. Moisten with Co(N01)s sol. and heat with blowpipe; blue color-A1
No ppt. with above reagents
Na To fresh portion of sample give flame test; yellow flam-Na
K Flame test-violet
flame invisible through cobalt glass NH4 To fresh portion of sample add 5 cc. NaOH and warm; ammonia smell-NH,
This chart is far from being flawless and a few directions and additions are necessary. The hydrogen sulfide solution had to be made alkaline with ammonium hydroxide to cause the zinc to precipitate. This in turn would cause aluminum in group 3 to precipitate. This can be explained to the students and the difference between zinc sulfide and aluminum hydroxide is easily discernible. Other minor troubles were encountered but none caused difficulty. As was stated before this is only a bird's-eye view of the subject and our bird cannot be a buzzard when we consider the fact that we are dealing with high-school students and the scheme has been worked out chiefly from their own information. If simplicity were sacrificed for detailed accuracy perhaps the harm would be greater than the good in this case. Probably entirely diierent and better schemes than this one can be devised but this article is not written with that point in view but only to show the value and practicability of qualitative analysis to the highschool student. In no part of the course did the students become so keenly interested, work so diligently, or derive so much benefit as in this work and the writer feels sure that his experiences will be duplicated by any other teachers of high-school chemistry who will give their students a taste of qualitative analysis.
