THE USE OF ANION GROUP TESTS IN TEACHING QUALITATIVE ANALYSIS* Anion group tests may be used to emfihasize oxidation-reduction, solubility produd, common ion, complex ions, and the ionization of polybasic acids in stages. Also, the group tests serve as a convenient introduction to the study of the anions and in some cases may be used to shorten the procedure necessary to identify the anions.
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A well-known memory course is based on association of newly acquired ideas or facts with those already firmly fixed in the mind. Oxidation-reduction, solubility product, common ion, complex ions, and primary, secondary, and tertiary ionization are mere words to a beginner in chemistry and remain such until chemical reactions involving these are carried out and form a mental picture around which may be built a framework of theoretical principles. Elementary qualitative analysis affords an excellent opportunity for teaching the fundamental theories on which chemistry is based, and most textbook writers avail themselves of this opportunity. Thus, in the separation of the acid-insoluble and acid-soluble sulfides, in the solution of the alkaline earth carbonates in acetic acid, and numerous other reactions, the solubility product principles is emphasized. At the proper place in the procedures, attention is called to reactions involving each of the above words. However, but few writers of qualitative texts make use of group tests for anions as fully as can be done advantageously, both from the standpoint of theory involved and the simplification of laboratory tests. It is the purpose of this paper to call attention to a way in which certain of these group tests may be used for the above purposes. The group tests given below were chosen on account of their simplicity, their reliability in student bands, and for the chemical principles involved.
Groups of Anions' Group 1.-Anions decolorizing dilute permanganic acidZ (reducing constituents): arsenite, cyanide, ferricyanide, iodide, nitrite, sulfide, sulfite, thiocyanate, and thiosulfate, without warming. Bromide, oxalate, and tartrate give the test when warmed.
* Presented before the Division of Chemical Education a t the 81st meeting of the A. C. S., Indianapolis, Indiana, March 31, 1931. ' In this article, the following anions are considered, although not all of them give positive group tests: acetate, arsenate, arsenite, borate, bromide, carbonate, chlorate, chloride, chromate, cyanide, ferricyanide, ferrocyanide, fluoride, fluosilicate, hypochlorite, iodide, nitrate, nitrite, oxalate, phosphate, sulfate, sulfide, sulfite, tartrate, thiocyanate, and thiosulfate. Bieskey (I), using the same reagents as are used in this paper, classified nitrate and chlorate as oxidizing constituents. However, a nega1858
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Group 2.-Anions liberating iodine from very dilute hydriodicS acid (oxidizing constituents): arsenate, chromate, ferricyanide, hypochlorite, and nitrite. Group 3.-Anions with silver nitrate forming precipitates insoluble in dilute nitric acid: bromide, chloride, cyanide, ferricyanide, ferrocyanide, iodide, sulfide, thiocyanate, and thiosulfate (the latter, on decomposition of its silver salt). Group 4.-Anions with silver nitrate forming precipitates insoluble in 4N ammonium hydroxide: ferrocyanide, iodide, and sulfide. Group 5.-Anions with silver nitrate forming precipitates soluble in 2N ammonium hydroxide, but insoluble in dilute nitric acid: bromide, chloride, cyanide, ferricyanide, thiocyanate, and thiosulfate. Group 6.-Anions forming stable acids and with silver nitrate forming precipitates in neutral solution, but not in dilute nitric acid or dilute ammonium hydroxide solution: arsenate, arsenite, chromate, oxalate. and phosphate. For a group test to be of value in the identification of anions, i t must be given by every member of the group, even in very low concentrations, and must not be given by other substances. In this laboratory, it has been found that these group tests in most cases are more sensitive than individual tests for the anions (see Table I), and anions not in the groups, with five exceptions4do not interfere when present in concentrations as high as tivc test for oxidizing constituents does not mean that these ions are absent, or even not present in fairly high concentrations. This is true particularly of nitrate, for under the experimental conditions used in this laboratory (detailed instructions will be sent gratis to any one requesting them) 25 mg. of nitrate per cc. give a negative test. By using a higher concentration of hydriodic acid, the tests for nitrate and chlorate, as well as arsenate, become more delicate. Also, the possibility of obtaining a positive test by atmospheric oxidation of or by decomposition of hydriodic acid becomes much greater. 'The permanganate solution was 0.05N in sulfuric acid and contained enough permanganate that dilution of a sample with an equal volume of water left a distinct color. 250 cc. of the pwmanganate solution were reduced by 0.38 cc. of 0.1050N ferrous sulfate solution. Equal quantities of anion solution and permanganate solution were used in the tests. The iodide solution used was 0.25N in potassium iodide and 0.05N in sulfuric acid and contained 1 per cent of starch. 3 cc. of this solution were used with 5 cc, of the anion solution. Prolonged heating, low concentration of permanganate, and high concentration of acid or ferricyanide favor the interference of the latter in Group 1. However, if it is present in high enough concentration to ave a test, usually its color will be strong enough to indicate its presence. Bromide and thiocyanate interfere in Group 4, if present in much greater concentrations than 14 and 16 milligrams, respectively, per cc. Oxalate precipitates in Group 5, if the concentration is appreciably greater than 10 milligrams per cc. Chlorate oxidizes hydriodic acid to free iodine under the experimental conditions, if chlorate is present in concentrations as great as one milligram per cc.
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25 mg. per cc. As is evident from the table, in four out of forty positive tests, it is necessary to have from 0.12 to 0.20 mg. of anion per cc., but usually the amount needed for a test is between 0.001 and 0.05 mg. per cc. Frequently, by the use of the anion group tests, many anions are eliminated, so the time required to analyze the solution is shortened materially. TABLE I Group Tests* (Amount necessary to give posibivc test expressed i n mg. of anion
per cc.
of solution)
Groups
Arsenate Arsenite Bromide Chloride Chromate Cyanide Ferricyanide Ferrocyanide Hypochlorite Iodide Nitrite Oxalate Phosphate Sulfide Sulfite Tartrate Thiocyanate Thiosulfate Anions not in groups: acetate, borate, carbonate, chlorate, fluoride. fluosilicate. nitrate, and sulfate.
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'The sensitivity of each test was worked out carefully by two students, Gerald Leslie and W. T. Forsee. " Heated. Heated and cooled. In case of doubt, let stand several minutes.
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Use of Group Tests to Illustrate Chemical Principles In an experiment involving color change, the student readily recognizes that a chemical change has taken place, and the more noticeable the color change, the more vividly is the experiment impressed on his mind. Actual treatment of an acid solution of potassium permanganate with each of the twelve reducing constituents, writing of equations for reactions taking place, and the testing of unknowns for reducing constituents teaches an alert student more about reducing constituents than the average student learns in the entire course in qualitative. Similarly, the formation of the deep blue of starch-iodine solution affords a mental picture which assists in remembering oxidation reactions.
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An unusual opportunity for the discussion of solubility product, common ion, complex ions, and the ionization of polyhasic acids in stages is given in the study of the groups of anions which precipitate with silver nitrate in acid solution, which precipitate in ammonia solution, which precipitate in acid but not in ammonia solution, and which precipitate in neutral but not in either acid or ammoniacal solution. The adjustment of the conditions in the solution, so as to test for each of these groups of silver salts, the writing of the equations for the reactions involved, and the explanation of the observed phenomena, in the light of the words a t the beginning of the paragraph, all serve to impress important chemical principles on the mind of the student. Last, but not least. the anion group tests help the student get acquainted with the anions and their reactions. Almost every one of the metals forming the cations studied in elementary qualitative analysis is familiar to the student, either as a free element, or in some compound associated with his every-day life. The same is not true of the anions. Such words as ferrocyanide, ferricyanide, thiosulfate, and tartrate usually mean very little to the qualitative student when he starts his anion procedures. The lack of system in these procedures usually adds to his bewilderment. Likely the carrying out of any reactions of the anions would serve to introduce the student to these more or less unfamiliar radicals, hut it is our opinion that this can he done more easily with a few group reactions than with a much larger number of individual reactions. After the student becomes acquainted with the anions, and some of their group reactions, it becomes easier for him to learn specific tests for each anion, particularly if these tests are based on some anion group tests, as many are. Literature Cited (I)
BIRSKRY,Chem. Rundschau Mettleuropia Balkan, 2, 106-7, 119-23 (1925)
