A system of qualitative microanalysis

A SYSTEM OF QUALITATIVE MICROANALYSIS*,t CARLJ. ENOELDER AND WILLIAM SCHLLER UNIVERSITY on P I T T S B ~PIrsBrmcn, ~H, PENNSYLVANIA

A scheme for the qwlitutive separation and detection of the commoner cations and anions has been worked out w i n g micro technic. For the most $art the schemes are based on the reactions used in the regular procedures but using drop tests and at most limiting the volume of solution to 1 cc. The microscope is not used. The course of instruction in qualitatiwe to sophomore chemists at the Uniwersity of Pittsburgh i s now micro chemical. The conservation in time and in chemicals, the e w e of manipulation, and the working conditions for student classes effected by this new technic are most satisfactory.

. . . .

The development of methods for the investigation of small amounts of materials has been of comparatively recent origin. Most of the work has been done in the field of chemical microscopy. Emich ( 1 ) in his book, "Lehrbuch der Mikrochemie," traces the beginnings of miaochemical analysis back to the year 1866, when some use was made of the microscope in chemical analysis. In 1877 Boricky wrote a treatise entitled, "Elements of a New Miaochemical Analysis of Minerals and Stones." Other works on the use of the microscope in chemical analysis appeared in 1881 when Reinsch described some microchemical reactions and Streng recommended miaochemical analysis as a valuable aid in the investigation of minerals, especiaUy in blowpipe analysis. Haushofer in 1885 issued a book on "Microscopic Reactions," which'laid a foundation for the tendency to draw all elements into the domain of fnicrochemical analysis, a tendency which was also expressed in the French work of Klement and Renard. One of the outstanding workers in the field of miaochemical analysis was Behrens (Z),who did much toward placing this phase of analysis on a sure footing. Among Behrens' contributions was the introduction of methods ior the separation of compounds from each other; these gave more surety to confirmatory tests. Along with the name of Behrens is associated that of Schoorl (3), who also introduced methods for the separation of compounds. Another miao scheme of separation is given by Kley (4). In the past few years miaoanalysis has been developed through the refinement of the existing m a a o methods. Thus in quantitative organic microanalysis Pregl (5) has succeeded in producing much smaller counterparts of the ordinary balance, combustion tube, buret, electrolysis device, *From a thesis submitted by William Schiller to the Graduate School of the University of Pittsburgh in partial fulfilment of the requirements for the degree of doctor of philosophy. Contribution No. 220 of the Department of Chemistry, University of Pittsburgh. t Presented before the Division of Chemical Education of the A. C. S. at the 83rd meeting, New Orleans, Louisiana, March 29, 1932. 1636

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etc., all of which has made possible the investigation of much smaller amounts of materials than heretofore. Another direction in which research in microanalysis has led is toward the utilization of organic reagents which give very sensitive tests with inorganic ions. These tests, which give rise to colored solutions or precipitates, are easily detected by the naked eye. They are carried out on filter papers, spot plates, or textile fibers. Not only is there a searching for organic reagents which yield these colored compounds, but also yield them in the presence of other ions. Gutzeit (6) has developed a scheme in which specific tests are obtained for individual ions in the presence of others. Feigl (7) has made an excellent compilation of exceedingly sensitive tests for the cations and anions. Chamot's (8) "Chemical Microscopy" covers the microscopic tests in an excellent manner. Micro methods are being slowly introduced into school laboratories. At Cairo, Egypt, Grey (9) has developed a course in general chemistry using micro methods, and has published a hook, "Practical Chemistry by Micro Methods." Recently an article has been published by Van Nieuwenburg (10). The scheme described in the article was developed and used in the laboratory of analytical chemistry of the Technical University a t Delft, Holland. According to present indications there will be an increased use of micro methods in the future. It is inevitable that these sensitive, time-saving, economical methods will eventually supplant the slower, more tedious methods which are now in existence. I t has been the aim of the authors of this paperko apply the methods and technic of microanalysis to the ordinary (macro) schemes of qualitative analysis with the object: first, of confining the operations to drops or, a t most, to one or two cubic centimeters of solution; second, of developing a micro scheme which could he placed in the hands of undergraduate students taking their first course in qualitative analysis; and third, by dispensing with the use of microscopes, of bringing the micro technic within practical pedagogic and economic reach of large laboratory classes of sophomore or even freshman students. Attention has been focused on conducting separations, washings, filtrations, and similar operations with usually not more than one cubic centimeter of solution and on performing final tests by means of drops. The procedures and methods worked out here can he employed by beginning students and require no previous special training. When one compares the relative bulkiness of the ordinary size of beakers, flasks, test tubes, and similar common apparatus with the tiny counterparts used in the micro work; the relatively large locker space required for storage with the small "shoe-box" compartments in which a student's micro outfit can be packed; the tedium, awkwardness, and loss of time necessitated in filtering several hundred cubic centimeters of solution from a large bulk of

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precipitate with the ease and rapidity of making a separation by means of the centrifuge; the fatigue and nervous strain of the student standing for hours at his work place with the comfort and relaxation of sitting at a table with all materials within reach; a laboratory atmosphere filled with hydrogen sulfide, sulfur dioxide, and similar noxious or objectionable fumes, with one practically free from such contaminations; the inevitable "sloppy" technic which the student acquires in spite of strict supervision with the neatness and care with which he must work at the micro methods-the advantages are predominantly on the side of the micro method. Instruction in theory, in fundamental chemistry, and in the general methods of separation and detection can be carried out just as effectively with the micro as with the macro method. What the student may lose by reason of not acquiring the full technic of the usual large-scale manipulations is probably compensated for by an appreciation of tlie care required in this new technic. The schemes developed have been worked out by first making a rather exhaustive study of the properties and reactions of the individual cations and anions, using the micro technic and the tests described by other investigators, and finally selecting those tests which were sufficientlysensitive for the purpose in mind. In general, the tests were confined to single drops, the test solutions containjng one milligram of the active 3 cc. 2 cc. cc. constituent per cc. The usual schemes 1.-CEN-WOE TUBES FIGURE ,' 1,2 -.of qualitative analysis were then modimze fied to provide for the small-scale precipitations, separations, and similar operations, and the procedures thus evolved were thoroughly tested at all points, gradually increasing thenumber of components in the mixtures analyzed. Finally, the .completed schemes were used by a small group of beginning students in qualitative analysis during the summer session of 1931, and again by the class of sophomoresspecializingin chemistry during the fall semester of 1931-32. The Technic and Apparatus of Qualitative Microanalysis The operations and apparatus of ordmary qualitative analysis have their counterpart in the new micro technic. Except for a small hand centrifuge and certain other simple apparatus, the appliances of microanalysis are in general miniatures of those used in ordinary analysis. Reagents and test solutions are conveniently kept in 30-cc. dropping bottles, provided with medicine droppers. Test tubes are of 2 cc. capacity and centrifuge tubes of 1, 2, and 3 cc. capacity, respectively (see Figure 1). Test tubes and centri-

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fuge tubes are conveniently kept in wooden blocks which are drilled to bold a suitable number of such tubes. Solid reagents are also kept in small stoppered test tubes in a similar block. The porcelain crucibles are of 1 cc. .. capacity. The wash bottle is made from a 50-cc. Erlenmeyer flask. A suction flask, shown in Figure 2, is useful. Capillary pipets are made from medicine droppers or glass tubing (see Figure 3). Many tests are carried out on the glass slides used in microscopic work, on ordinary spot plates and on drop reaction paper, the latter being Schleicher and Schiill's "Tupfreactionspapier, No. 601." The H2S generator is shown in Figure 4. A small hand centrifuge was mounted on the laboratory bench and surrounded for safety during use with a heavy wooden Frcms ~ . - + U C ~ ~ O NFLASK box. The small centrifuge tubes were Natural size provided with cork stoppers equipped with wire hooks and during centrifuging were placed inside the larger (15 cc.) tubes, regularly supplied with the centrifuge. The followingare the more important operations and manipulations. Precipitation.-Precipitations are carried out in centrifuge tubes, on spot plates, glass slides, and drop reaction paper. In the first method, a drop of the precipitating agent is added to a drop of solution in a centrifuge tube. The advantage of this procedure lies in the ease with which the precipitate may be observed after centrifuging, and the facility with which a separation can he camed out. In the spot plate method, a drop of the solution to be tested is brought into contact with a drop of the precipitating agent in the cavity of a spot plate. This method is very effective for detecting the formation of colored precipitates. If the precipitation is to be conducted on a glass slide, a drop of the solution and one of the (Rubbrr Bulb precipitant are placed L ) side bv side on the slide and the two are mixed F I G ~3.-MICRO E CAPILLA~Y Pmer by means of a platinum 11. ~ i . ,x wire. When heating is necessary, the drops are placed as near to one corner of the slide as possible and the heat from a micro burner cautiously applied. Filtration.-The operations of filtration and washing are conducted

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SEFTE~ER 1932 ,

either in a centrifuge tube or on a glass slide, depending upon the method by which the precipitate was produced. In the former case, after centrifuging, the supernatant liquid is withdrawn by means of the capillary pipet. If washing of the precipitate is required a few drops of wash water are added, the precipitate stirred with a platinum wire, the mixture again centrifuged and the supernatant liquid drawn off. When filtering and washing on a

FIGURE 4.-H,S GENERATOR I/. Size

glass slide, the liquid surrounding the precipitate is withdrawn by placing a small wad ,of cotton near the edge of the soluton and pushing this into the drop with the cotton in the opening of a micro capillary pipet and applying suction by releasing the pressure on the compressed rubber bulb. Occasionally, when close examination of precipitates is required, filtration by means of the suction flask is resorted to. A piece of filter paper of proper diameter is punched out with a cork borer and placed in the funnel.

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The solution to be filtered is then placed in the funnel and suction applied by the mouth through a rubber tube attached to the flask. Evaporation.-Evaporations are best conducted by transferring the solution to the 1-cc. crucible and heating with a micro burner (Figure 5). Considerable care is necessary in this operation to prevent large losses due to spattering. Occasionally liquids are evaporated from glass slides or watch glasses. $ i h m Mie Evolution of Gases.-The apparatus of Figure 6 is used for this purpose. A drop of solution to be tested is placed in the test tube, a drop of reagent added and the F,,,,, 5.-,41cR,, BURNER funnel inserted. Filter paper moistened Size with the test reagent is placed in the mouth of the funnel and the tube gently warmed, causing the evolved gas to come in contact with the paper. Development of Color.-With certain of the reactions with organic reagents, the colors are brought out by allowing hydrochloric acid vapor or ammonia fumes to come into contact with the reaction mixture on drop reaction paper. This is done by simply holding the reaction paper in the fumes evolved by heating in the small porcelain crucible a few drops of concentrated HCl or NHIOH.

L

Notes on the Cation Scheme The "unknowns" consisted of 1 cc. of solution containing not over 1 mg. of any single cation. Group unknowns as well as general mixtures covering all groups are submitted for &innel analysis. Group I included silver, lead, and mercurous mercury and the group precipitation, separations, and & J + , . identifications were made by the usual reactions. In Group 11, which included the usual cations, the separa-M&b tion into two divisions was found most satisfactoty with sodium instead of ammonium polysulfide. Iron, manganese, chromium, and aluminum were then removed with excess NH40H and nickel, cobalt, and zinc FIGURE6.-GAS subsequently precipitated from the filtrate with H2S. E v o ~ u ~ rAPPA. o~ RATUS The alkaline earths were removed with (NH&C03, Size leaving magnesium and the alkali metals to be tested for separately. In addition to, and in some cases in place of, the usual identifying and confirmatory tests, some unusual reagents and methods were employed,

i

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the reactions being mainly adopted from the work of other investigators. These tests are inserted here in tabular form. TABLE I Identifying Testa--Cations Reagenl

Mslhlhod

CsCI. K I 1-Cinchonine HGH.0,. Pb(GtO,),, KNO,, CsCI, TINOS Benzoinoxime, NHS Diphenylcarbazide CsCI, KI Cacotheline Gold chloride

Red precipitatr Red precipitate

Glass slide Drop reaction paper Drop reaction paper Spot plate Drop reaction paper Drop reaction paper

Black precipitate Green coloration Violet wloration Red precipitate Violet coloration Violet to black wloration Briaht . blue flame Blue precipitate Black precipitate

Granulated Zn, HCI 2NHCNS.Hg(CNSh CuSO,, 2NH4CNS.Hg(CNS)z NaOH. Tartaric acid, Benzidine

Volatile hydride Spot plate Spot plate

Benzidine. HGHa02 para-Nitrobenzene-azoresorcinol 8-Hydroxyquinoliue

Drop reaction paper

Zinc uranyl acetate

Spot plate

Spot plate Spot plate '

RcruD

Spot plate Centrifuge tube

Centrifuge tube

Blueprecipitateor wloration Blue ring Blue precipitate Green to yellow prccipitate Yellow precipitate

Notes on the &on Analysis No systematic scheme of separation has been worked out for the anion analysis, the tests being applied specifically for each anion. The order of anion testing follows that given in Engelder's "Elementary Qualitative TABLE 11 Identifying Tests-Anions Rapml

CuS NaOH, p-Aminodimethylaniline ZnSO,, Sodium nitroprusside Ba(0Hh (NH&MoO,, benzidine (NHI)~MoO*SnCL BaClr, HdNOdn (NHdrMoOa, benzidine (NH&Mo04, benzidine Reduced fuchsin Diphenylamine

Mzthod

Drop reaction paper

Rcsull

Decolorization

Glass slide Methylene blue Filter paper Red coloration Pt. loop White. BaCO? Drop reaction paper Blue wlor Drop reaction paper Blue color Ydom ppt. on white, Centrifuge tube BaSO, Porcelain crucible Blue color Porcelain crucible Blue color Drop reaction paper Red color Spot plate Blue wlor

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Analysis." Suitable procedures have been worked out for a considerable number of casesof interference. At the bottom of p. 1642 are tabulated the unusual tests which have been found to give good results on the micro scale. Results Obtained with the Micro Method During the summer session of 1931, facilities were provided for four beginning students in qualitative analysis to use the schemes developed. The results, condensed in Table 111, show a highly satisfying degree of accuracy. Each student analyzed seven unknowns covering cation and anion mixtures. The time for an analysis in general is much less; in some cases a 5- or 6-component sample was reported within one hour's time. The micro course was again subjected to trial by the group of fourteen sophomores specializing in chemistry during the fall semester. Altogether eight different samples were analyzed by each student, each sample containing an average of 5 components. The results here, especially on the anion unknowns and salt mixtures, were particularly gratifying, from the standpoints of both accuracy and speed. TABLE III Students' Individual Scores Sludrnl

E. W. J. H. G . H. N. B.

Total

Told A7umbcr of conponcnrs in Unknown

33 37 38

Number of Componcnfr carrcaly Reporled

24 31 , 36 30

-

36

-

144

121

*

N u d a r of com9rmwrs ~ncarrrnly RcPorlcd

P w c c d n g c of

9

73 84 95 83 84

6 2

6

23

components carredly Repartcd

Conclusions The use of the micro technic by students in a laboratory course of inorganic qualitative analysis has shown the following advantages: 1. A considerable saving of time. The work was accomplished in one-fifth to one-half of the time taken for the regular analyses. 2. A saving in the cost of the course. Less apparatus is required and only small amounts of reagents were used. 3. The improvement of working conditions in the laboratory. The student remained seated during most of the work. This made possible a calmer view of experiments and made the taking of notes easier. Noxious fumes were practically eliminated from the laboratory.

* * * * * * Note: The authors invite inquiries and discussion of this new method of instruction. Complete schemes, notes, lists of reagents, etc., as supplied to students are available in mimeographed form.

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Literature Cited Enrrcn, "Lehrhuch der Mikrochemie," Verlag von J. F. Bergmann, 1911, pp. 43-64. BEHRENS,H., "A Manual of Microchemical Analysis," The Macmillan Co., London and New York, 1894,246 pp. SCEOORL,"Analyse der Silber Gruppe," Z. anal. Chem., 47, 209-24 (1908); "Die Gruppe der Sauren Sulfide (Arsen, Antimon, Zinn)," ibid., 47, 367-89 (1908); "Die Gruppe der basischen Sulfide," ibid., 47, 729-54 (1908); "Analyse der Eisen Gruppe," ibid., 48, 209-31 (1909); "Die Gruppe der Erdalkalimettale (Baryum, Strontium, Kalzium)," ibid., 48, 401-15 (1909); "Die Restgruppe," ibid., 48, 593-611 (1909); ",Die unlljslichen Suhstanzen." ibid.,48, 665-78 (1909). KLEY, "Behrens-Kley Mikrochemische Analyse," Leopold Voss, Leipzig und Hamburg, 1915, pp. 1-254. PnecL, "Quantitative Organic Microanalysis," J. and A. Churchill, London, 1924, 186 pp. GUTZEIT,GREGOIRE, "Sur une methode d'analyse qualitative rapide. Methodes d'analyse rapide 'a la touche' dcs cations et anions les plus usuels," Helu. C h i m Ada, 12, 82940 (1929). FEIGL,"Qualitative Analyse mit Hilfe von Tiipfel-Reaktionen," Akademische Verlagsgescllschaft. Leipsig, 1931. C ~ M OAND T MASON,"Elementary Chemical Microscopy," John Wiley & Sons, Inc.. New York City, 1931. GREY, "Practical Chemistry by Micro-Methods." Cambridge, W. Heffer and Sons, Ltd., 1925, 124 pp. NIE~WENBURG, "Systematical Qualitative Analysis by Means of Modern Drop Reactions," Mikrochemie,3, 199-220 (1931).

A system of qualitative microanalysis

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