A Lost Centenary: Lassaigne's Test for Nitrogen The Identijkation of Nitrogen, Sulfur, and Halogens in Organic Compounds S. HORWOOD TUCKER The University, Glasgow, Scotland TUDENTS of chemistry have just missed an important centenary; for, 101 years ago (1843) Lassaigne (1, 2) discovered that organic matter containing nitrogeneous constituents on fusion with potassium gave potassium cyanide, which he identified by treating the alkaline cyanide solution with "sulfate ferroso-ferrique" followed by hydrochlori: acid solution, thereby producing a deep blue precipitate of ferric ferrocyanide (Prussian blue). This simple test has stood the greater test of time, with practically no major change, for over a century. Attempts were made a t an early date to "improve" it, hut with no success. Thus, Jacobsen (3), who seems to have introduced the use of sodium in place of potassium, tried to increase the sensitivity of the test by adding iron powder to the fusion mixture. Results indeed improved, but a kill-joy named Tauher (4) pointed out that improvement could be expected, since heated iron takes up nitrogen from the air! Tauber concluded that iron must therefore be excluded unless fusion is conducted in an inert atmosphere. The method was, however, reintroduced recently (1937) by Robertson (5), who added a small quantity of iron (primarily to remove sulfur) and claimed that under the conditions of his test the iron does not take up nitrogen from the air, but aids in forming ferrocyanide in the melt (instead of in the solution subsequently obtained). Since, however, he performed the fusion in the customary open tube, and took no precaution to exclude air, the method is open to question. It has been stated (6) that the Lassaigne test for nitrogen fails when the compounds lose nitrogen a t comparatively low temperatures, e. g., with diazo compounds (7). It was also said to fail with pyrrole derivatives (8). But Kehrer (9) who introduced an improved method of heating-namely, passing the vaporized compound over molten sodiun-found that pyrroles and other refractory compounds gave positive results. Miceli (10) following the same principle, passed the vapor of the compound over molten sodium supported by means of glass wool in the upper part of a test tube. Fischer's (8) objection to the Lassaigne test for nitrogen in pyrroles is not so much to its unreliahility with pyrroles as to its insensitiveness; but since he is referring to tests carried out with 3 mg. of substance his objection need not be taken too seriously. Doubtless, as he says, a micro-Dumas (Pregl) estimation for nitrogen which is quantitative, and so necessarily qualitative also, is preferable; but whilst every soldier carries
S
a marshal's baton in his knapsack, very few chemists possess a portable "micro-Dumas." I t has been found as a result of researches on qualitative tests for, and quantitative estimation of, small quantities of cyanides that the original method of Lassaigne can profitably be simplified: e. g., Berl and Delpy (11) merely added a ferrous sulfate solution to the alkaline cyanide solution, allowed the mixture to stand for 10 minutes, boiled for several minutes, and then acidified with hydrochloric acid. Viehoever and Johns (I.!?) worked similarly in determining small quantities of hydrocyanic acid. They showed that addition of ferric chloride is not only unnecessary, since atmospheric oxidation of the f m o u s sulfate readily produces all the ferric iron required, but that ferric chloride imparts a green shade to Prussian blue and is otherwise detrimental. Consequently, they also replaced hydrochloric acid, in the h a 1 acidification, by sulfuric or nitric acid, thus again avoiding vitiation of the color by yellow ferric chloride. They found that addition of potassium fluoride "hastened the formation of Prussian blue" and rendered heating unnecessary; but in the absence of fluoride beating was found to be beneficial. In carrying out the Lassaigne test several (13,14) adopt this modification. Vohl (1863) (15) appears to have been the first to use potassium for the detection of sulfur. He heated various oils, a t about their boiling points, with potassium, and tested for any resulting potassium sulfide with sodium nitmprusside solution. ' He also introduced the use of sodium in place of potassium. But nowhere does he mention Lassaigne or the latter's method of testing for nitrogen. It had been pointed out that sulfur masks the formation of cyanide by causing formation of sulfocyanide, but Graebe (7) demonstrated that this was counteracted if excess of alkali metal was used (NaCNS Na + Na2S 4- NaCN). As already mentioned Robertson (5) used iron powder to remove sulfur. Fisher (13) points out that "sometimes the sulfur in organic sulfonates cannot be detected by the sodium fusion." With regard to the halogens the simplest test is that due to Beilstein (16) : a halogen-containing compound when ignited on copper oxide imparts a green color to the flame. Unfortunately many halogen-free compounds, e. g., certain pyridines, quinolines, acids, acid amides, cyano compounds, etc., also give a green coloration. This complication is eliminated by Hayman (17) by vaporizing the compound underneuth the
+
heated copper oxide (a Monel metal tube was used). Only halogen-containing compounds under such conditions give the green coloration to the flame. Spica (18) utilized the sodium fusion test for nitrogen and sulfur as above, and also for the halogens. The sodium halides produced in the reaction are identified by the well-known methods of inorganic analysis; the methods of Mulliken (19) and of Shriner and Fuson (20) being simple and reliable. Castellana (21) introduced the first essential change in the Lassaigne test when he replaced potassium by a mixture of magnesium powder and anhydrous potassium carbonate-a preparation of potassium in sit% But it was pointed out by Flieringa (22) that this mixture when heated absorbs nitrogen from the air. He modified the method by incorporating hydrated sodium carbonate, in order to remove air, but rendered the test less sensitive thereby. Baker and Barkenbus (14) with the same objective-elimination of ai-tried znter alia, sodium acetate, naphthalene, and paraformaldehyde but finally found ether to be most suitable. They increased the sensitivity of the test by passing the vapor of the substance over a small amount (0.2 g.) of the heated mixture of magnesium and potassium carbonate previously moistened with ether and lying along the side of an inclined test tube stoppered with glass wool. Middleton (23) replaced the magnesium-potassium carbonate mixture by zinc dust and anhydrous sodium carbonate (with a limited number of substances, sucrose and sodium carbonate answers the purpose) without any camplication arising, since zinc does not combine with atmospheric nitrogen. Results are uniformly good, especially with compounds containing sulfur. The writer finds, however, that although the above magnesium or zinc and carbonate methods appear simple, a considerable amount of skill is required in practice. Thus in Middleton's test preliminary heating of the zinc and sodium carbonate portion often affects its projection from the tube; furthermore, on plunging the hot tube into water reaction is often violent. In Baker and Barkenbus' method the use of ether by students is apt to be dangerous, although the authors say that according to their experience it has not proved to be so. On the other hand our expmence in these laboratories has been that for stucknt purposes the original Lassaigne test, modified in most of its details, is simple, quick, safe, and reliable. It is realized that most of the modifications are not new (cf. 12,13,19, 20, 24, 25). THE SODIUM FUSION TEST FOR NITROGEN, SULFUR, AND HALOGENS
(See Notes A, B, and C) Place a cube (3 mm. = 0.03 g.) (Note D)of freshly cut sodium a t the bottom of a small glass tube (60 X 7 mm. test tube). Support the tube in a clamp or in a holed asbestos board, so as to have the hands free. Place the substance (0.02 g.) (Note E) on a spatula (or tip of a knife blade), and then arrange the Bunsen
burner, turned low, under the tube so that the sodium melts. Immediately drop the substance portionwise from the spatula directly on to the molten sodium. (If the substance is a liquid dip a "melting-point" capillary tube, open a t both ends, into it; insert the filled end into the reaction tube, then, by jerking, drop three or four droplets on the molten sodium.) Remove the tube from its support and heat a t first carefully then strongly, rotating the tube until all excess sodium has combined with the glass and the whole end of the tube is a red-hot mass. Plunge the red-hot tube into water (2 ml.) contained in a test tube (150 X 20 mm.) (Note F). Filter the hot solution (using preferably a small filter funnel of the Hiisch or sintered glass type and suction, for speed and to avoid dilution of the small bulk of solution by washings), boil the residue with more water (2 ml.), filter, and test the combined filtrate (X), which should be water-clear and alkaline, for the elements: Nitrogen. Pour a portion (1 to 2 ml.) of the solution (X) into a test tube containing powdered ferrous sulfate (0.1 g.) (Notes G, H); beat the mixture gently, with shaking, until the solution boils, then, without cooling, acidify with dilute sulfuric acid (Note I). A Prussiau blue precipitate or coloration proves that nitrogen is present. Sulfur. (a) A drop of the solution (X) placed on silver foil or a silver coin (cleaned of grease by ammonia or an organic solvent) will give a brown stain if sulfur is present. (b) Addition of sodium nitroprusside to a diluted drop of the solution (X) will give a purple coloration if sulfur is present. Halogens. (a) General test for presence of halogens: Beilstein test (Note I): Heat a piece of copper gauze (or a looped wire) until i t imparts no green coloration to the Bunsen flame. When the gauze is cold place a few crystals (or drops) of the substance upon i t and reinsert in the flame (if the wire is used it should be dipped wbilst hot into the substance, to ensure adherence of particles). A green coloration indicates that halogen may be present; but since certain compounds which do not contain halogen also respond as above i t is advisable as a confirmatory test to vaporize the substance on a copper-free spatula held a few centimeters underneath the oxidized copper gauze. A green colpration above the gauze is evidence of the presence of halogen. (b) Specific test for halogens: When one or more halogens may be present they may be identified thus: Acidify the solution (X) (2 ml. or more) with moderate excess of glacial acetic acid. If nitrogen or sulfur has been shown to be present, boil the solution (Note K ) , cool, and to a portion add a drop of carbon tetrachloride and solid sodium nitrite: if iodine is present i t will color the carbon tetrachloride reddish violet (Note L). If such is the case treat the rest of the acid solution with a small amount of sodium nitrite, just sufficient to liberate all the iodine; remove the iodine by extraction with carbon tetrachloride until the last extract is colorless. (To ensure that all iodide has been de-
composed add another small amount of nitrite, warn, and shake-the carbon tetrachloride should remain colorless.) Boil the acid solution until no more nitrous fumes are evolved, cool. To a portion add a small amount of lead dioxide (Note M), place a strip of fluorescein paper across the mouth of the test tube, and warm the solution. If bromine is present it will color the strip pink (eosin) (Note N). In such case add lead dioxide to the main bulk of the acid solution and boil until all bromine is driven off (fluorescein test). Filter, to remove unreacted lead dioxide, then add silver nitrate solution-a white or gray precipitate, insoluble in concentrated nitric acid, but soluble in ammonia solution proves the presence of chlorine. It may be worth while to mention a simple method which the writer finds reliable in the hands of students, for the identification of halogens when it is known that one halogen only may be present: Add silver nitrate in slight excess to the alkaline solution (X) (1to 2 ml.): a black precipitate (AgOH, etc.) will be produced (Note 0). Add excess of concentrated nitric acid, and warm the mixture. If nitrogen and sulfur are present in the substance hydrogen cyanide and hydrogen sulfide should be detectable by smell. Boil the mixture. If no precipitate survives, halogen is absent; but if a heavy, flocculent, white or yellow precipitate remains decant the supernatant liquid, add to the precipitate more concentrated nitric acid, and again boil the mixture (one to two minutes) until there is no longer any smell of .hydrogen cyanide or of hydrogen sulfide. If a precipitate remains it is silver halide (Note 0 ) and may be examined thus: decant the liquor from the silver halide, wash with water by decantation, and treat the precipitate with concentrated ammonia solution-if the precipitate is white and easily soluble in the ammonia solution then chlorine is present, if pale yellow and difficultly soluble then bromine is present, if yellow and insoluble then iodine is present. NOTES
( A ) Substances on fusion with sodium give NaCN if N is present, NalS if S is present. NaCl (NaBr, NsI) if CI (Br, I ) is present. I n th; nitrogen test, the NaCN solulion is treated with ferrous sulfateand so forms sodium ferrocyanide 6NaCN
+ FeSO, = Na,Fe(CN)s f NanS01
The ferrocyanide then reacts with ferric salt-inevitably produced by the action of air on the boiling alkaline ferrous salt solution-producing ferric ferrocyanide (Prussian blue) which is precipitated on addition of slight excess of dilute sulfuric acid (ferrous and ferric hydroxides simultaneously dissolving): 3Na,Fe(CN)a
+
2Fez(SO& = Fer[Fe(CN)sla
+
tact with water; but if the test is carried out as described there should be no free sodium. ( C ) Polynitro compounds explode on coming into contact with molten sodium; but if the technique of adding a few crystals a t a time,is followed, as described, even picric acid and 2:4:6-trinitrotoluene can be safely submitted to the test. Carbon tetrachloride and chloroform react without explosion (26). (D) The student will find it worth while to keep a t hand a 3mm. cube of rubber ( 6 . g.), for comparison of size when cutting the cube of sodium. ( E ) Students should weigh this amount (0.02 g.) until used to the appearance of the "bulk" of matted hairy flake, or prismatic crystals. The amount recommended (0.02 g.) may he considered a n upper limit. The writer has obtained excellent results with 2 to 5 mg. of substances. But, although the quantity of substance being tested may be reduced, the student is advised to adhere t o the other weights and measures given, without corresponding reductions. When more than one halogen is present, and a separation is therefore necessary, it is inadvisable to use less than 0.02 g. of substance. ( F ) The reaction is vigorous, but not violent, if care has been taken to fuse the excess sodium into the glass. I t is quite unnecessary to use more than 2 ml. of water. The larger quantities, almost invariably recommended, reduce the concentration of solutions being subsequently tested. (G) Powdered ferrous sulfate loosely heaped on a 1-cm. square weighs from 0.10 to 0.15 g. The writer finds that the amount of the Prussian blue precipitate is related to the amount of ferrous sulfate used; the apparently excessive amount recommended is necessary for production of a deep blue. gives a ( H,i If sulfur is mesent. addition of ferrous sulfate . black precillitnte of ferrwr sulfide. I n such care note evolution of hydrogert culfide when, I s t r r , dilute sulfuric arid is added. ar also i n lhc rest for halogens. when nirric a, id i < added (-+ H2S
.
If sulfur and nitrogen are bath present sodium thiocyanate. NaCNS, may be produced: in the test for nitrogen i t may give a red coloration with ferric ion but no Prussian blue since there will be no freecyanide ion. If the relative quantitiesof substance and sodium are as recommended sodium will be in excess and thiocyanate will he destroyed, if formed, according to the equation NaCNS
+ Na = N e S f NaCN
If necessary repeat the sodium fusion using relatively less of the substance being tested (see E). ( I ) Hydrochloric acid should not be used since the yellow color, due to ferric chloride formed, causes the Prussian blue to appear greenish. ( I ) Certain halogen-free compounds, when heated on copper oxide, impart a green coloration to the &me, e. g., certain pyridines, quinolines, acids, acid amides (urea), cyano compounds. etc.; but there is no coloration of the flame if these are vaporized underneath the heated copper oxide. (K) Hydrogen cyanide must he expelled since i t combines with halogens: HCN + I 1 = I C N
+HI
6NamSO4
Separate addition of ferric salt, e. g., the chloride, is unnecessary, and in fact detrimental. Addition of potassium fluoride may
be advantageous. ( B ) When performing this test the glass screen of the hood should always be kept between experimenter and experiment, since any unattacked Na may explode when it comes into con-
Lead dioxide in acetic acid solution gives lead tetraacetate which decomposes hydrogen bromide (and also hydrogen iodide) but has practically no effect under these experimental conditions
on hydrogen chloride. Persulfates are often recommended in place of the lead dioxide used above, but in the sulfuric acid solution in which they are used they react with hydrogen chloride. and accordingly, if the last is present in small amount it may be completely destroyed. ( N ) Fluorescein paper is prepared by dipping filter paper into a dilute solution of fluorescein in ethanol; it dries rapidly, and is then ready for use. Fluorescein (yellow ~ d i u msalt) reacts with bromine to give eosin (pink sodium salt).
(O) If nitrogen and sulfur, as well as halogens, are present in the substance being examined, the addition of silver nitrate to the alkaline solution ( X ) will give a black precipitate containing AgCN, AgnS, AgCl (AgBr, AgI), and AgOH. Boiling concentrated nitric acid dissolves AgCN (- AgNOa HCN). Ag.S (- H a S), and AgOH, but the silver halides are unattacked and remain as white or yellow precipitates. The method usually recommended for removal of hydrogen cyanide from the solution (X) is t o acidify with dilute nitric acid, and then boil the solution; hut this procedure can effect removal of hydrogen halides hy vaporization and oxidation. The writer has therefore introduced the method of fizing the halides (as AgCI, etc.) b y addition of silver nitrate previous to haling with concentrated nitric acid.
+
+
Among a large number of compounds tested the following have given positive results in tests for elements other than C, H, and 0: pyridine, diphenylamine, diazoacetic ester, pyrrole, picric acid, 2:4:6trinitrotoluene, S-benzylthiuronium chloride, potassium 3-nitro-4-chlorobenzene sulfonate, vitamin B1 (aneurin, thiamin), ethyl bromide, chloro- and bromobenzene, chloranil, trichloroacetonitrile, carbazole-3diazonium chloride, o-bromonitrobenzene, p-chloronitrobenzene, etc. In the test with vitamin Bl the amount used was 3 mg.
J. B., "The qualitative test for nitrogen in ROBERTSON. organic substances," J. S. African Chern. Inrt., 20, No. 1, 17 (1937) [Chem.Abs., 31, 2963 (1937)j. TSCHIRCH,A., AND A. B. STEVENS,"iiher den Japanlack (Ki-urushi)," Arch. Pharm., 243, 519 (1905): Chem. Zenlral., 1905,II, 1793. TSCHIR~H,A.,ANDA.B.STEVENS, "Uberdie Gummienzyme (Gummasen):speziellden Nachweis des Stickstoffes in ihnen," Pharm. Cenlr.-H, 46, 501 (1905): Cham. Zenlr., 1905, II, 408. TscHIncH, A,, AND H. C E D E ~ E R "Uher G, das Glycyrrhizin," Arch. Pharm., 245, 101 (1907); Chem. Zentr., 1907, I , 1797. GRAEBE,C., "Ueber den Nachweis des Stickstoffs in organischenVerbindungen,"Ber., 17,1178 (1884). FEIST, F. (E. Stenger), "Studien in der Pyrrolgruppe," Ber..35,1559(1902). TSCHIRCH,A.,ANDE.SCHERESCHEWSKI, "Uher Balata," Arch. Pharnz., 243, 363 (1905); Cham. Central., 1905, 11, 554. FISCHER,H., "Notiz zur Preglschen Mikro-Stickstoff-bestimmung,"B n . , 51, 1325 (1918). LEMBERG, R., "Chromoproteide der Rotalgen. 11. Spaltung mit Pepsin und Siiuren. Isoliemng eines Pyrrolfarhstoffs,"Ann., 477,236 (1930). KEHRER,E. A., "Ueber den Xachweis des Sticksto5s von Pyrrolverhindungen mittels des Lassaigne'schen Verfahrens." B w . , 35,2524 (1902). MICELI,A. S., "Improved sodium fusion technic for volatile or difficultly decomposable liquids." J. CHEM.EDUC.,13, 515 (1936). BERL,E., AND M. DELPY,"Uher die quantitative colorimetrische Bestimmung kleiner BlausPure-Mengen." Ber., 43,1430 (1910). VIEHOEVER, A., AND C. 0. JOHNS."On the determination of small quantities of hydrocyanic acid," 3. A m . Cham. Soc.. 37, 601 (1915). FISHER,H. L., "Laboratory Manual of Organic Chemistry," 4th Ed.. John Wiley & Sons. Inc., New York. 1938, p. 51. BAKER,R. H., AND C. BARRENBUS,"Detection of the elements in organic compounds,." Ind. Eng. Chem., Anal. Ed.,9,135 (1937). VOHL.Dinglen polytech. J., 168, 49 (1863); see ref. (2), p.
.
147
(16) BEILSTEIN,F.. "Ueber den Nachweis von Chlor, Brom, und Iod in organischen Suhstanzen," Ber., 5, 620 (1872). See also ref. (2), p. 166. (17) HAYMAN, D. F., "Modified Beilstein test for halogens in organic compounds," I d Eng. Chem., Anal. Ed., 11, 470 (1939). (18) SCHIPF,H., Ber., 13,205 (1880)-Ahstraft. (19) MULLIKEN,S. P.. "Identification of Pure Organic Compounds," John Wiley & Sons, Inc., New York. 1914, Vol. I, pp. 9-14. (20) SHFNER, R. L., AND R . C. FUSON,"Systematic Identhicatlon of Organic Com~ounds,"2nd Ed... -Tohn Wilev 81 Sons, Inc., NewYork,j940.
LITERATURE CITED
(I) LASSAIONE," M h o i r e sur un pr0~6d6simple pour constater la prbence de l'azote dans des qusntites m i n i m s de matiereorganique," Compt. rend., 16,387 (1843). (2) MEYER, H., "Analyse und Konstitutionsermittlung organischer Verhindungen," 6th Ed., Julius Springer, Wien, 1938, p. 144. (3) JACOBSEN,O., "Ueber die Oxydatiou der Parasulfamintoluylsiiure," Bcr., 12,2318 (1879). ( 4 ) TKUBER,E., "Ueber die Pnifung Schwefelhaltiger, organischer Suhstanzen auf Stickstoff," Ber., 32, 3150 (1899).
(23) M ~ D D L E ~ O H., N , "Tests for elements in organic compounds," Analyst. 60, 154 (1935). (24) KAMM,O., "Qualitative Organic Analysis," 2nd Ed., John Wiley & Sons, Inc., New York, 1932. (25) VOGEL,A. I., A Text-Book of Qualitative Chemical Aualysls. 2nd Ed., Longmans, Greenand Co., NewYork. 1941, p. 267. (26) Cf. MANN,F. G., AND B. C. SAUNDERS, "Introduction to Practical Organic Chemistry," Longmans, Green and Co.. New York, 1939, p. 94. STAUDINOER. H., "Erfahrungen iiber einige Explosionen." Z. angem. Chem., 35,658 (1922).
0 The Bureau of Audio-visual Aids, of Indiana University, Bloomington, Indiana, issues a regular bulletin. Number 5 of volume 5, which appeared recently, contains a report on the most widely used films. In the
field of chemistry, those which led were "The Molecular Theory of Matter," and "The Story of Dr. Carver." "Electrons," "Earth's Rocky Crust," and "Velocity of Chemical Reactions" followed in that order.
