A Review of Techniques in the Lassaigne Sodium-Fusion

R. P. Gowerland I. P. RhodesReading, BerkshireEnglandA Review of Techniques in the LassaigneSodium-FusionThe sodium-fusion test forms an essentialpart of most qualitative organic analyses sinceit represents a rapid method for the detection of thepresence of various elcments in the substance tested.The elements usually tested for are nitrogen, sulfur,and the halogens-others being tested for usually onlyif their presence is suspected from other evidence.Since its original use by Lassaigne in 1843 (1, 2) andsubsequent modification by Jacobsen in 1879 (S), inwhich sodium replaced potassium as the active reagent,the test has altered very little, and basically is still inuse today.However, despite the fact that in essence this is avery simple tcst, and quantitative work has proved itssensitivity (4), poor results are often obtained bystudents at the bench. This fairly general lack ofdexterity in using this test is also aggravated by thefact that most t.exts on practical organic chemistrypresent details of only two, or perhaps three, of thepossible techniques of applying this test, it usually beingtrue that under different circumstances differenttechniques yield the best results. Thus, the presentarticle is intended to collect together the availablemethods with some indication of the most suitablecircumstances in which each one may be best applied.It should be noted that although alternative methodsof detecting elements in organic compounds have beendeveloped, such as Middleton's test (5-7) and a recentignit,ion method using oxygen developed by D. C.Ayres (a), these will not be dealt with in this article.A review article dealing with the history and some usesof t,he Lassaigne test entitled "A Lost Century, Lassaigne'sTest for Nitrogen" was published by Tuckerin 1943 (9).Fusion of Sodium and Organic CompoundIn all the techniques to be described for fusing metallicsodium with an organic compound, it is generallybest to commence with very gentle heating, a pointoften not sufficiently stressed in the textbooks. Afterany reaction has begun, the ignition tube should be removedfrom the Bunsen flame until the reaction has almostceased, vhen further heat may be applied. Theextent of heating should be gradually increased untilno further change appears to be taking place in theignit,ion tube. The tube should thcn be raised to redheatfor a few moments to complete reaction and tovolatilize any unreacted organic material. The redhottube is then plunged into a boiling-tube, beaker, orevaporating basin containing water, care being takento shield the mouth of the vessel with a wire gauze.Boiling of the water-extract is often advised in textbooksbut appears unnecessary although the filtering of'Present address: 23 Pon Close, Reading, Berkshire, England.hot solutions is generally faster than for cold solutions.Methods of Fusion1) The most common method is to place the organicsample and sodium at the bottom of an ignition tubeand then to bring about reaction by gentle warming(5, 6, 10-1 2).This technique has the advantage that it allows observationof whether the compound has "activehydrogen"since, if easily replaceable hydrogen ispresent, direct reaction with sodium occurs in the cold.The major disadvantage is that with very volatilematerials, the organic substance may very easily belost from the ignition tube before reaction with sodiumcan occur.This still remains the most usual technique used.2) A simple modification of (1) in which the sampleand sodium are warmed in the ignition tube on a microburner only to the extent of allowing the organic materialto gently reflux within the tube.When reaction begins the tube is removed from theburner and the reaction is allowed to subside.This method is applicable to volatile materials orthose which are rather inert. However, for volatilematerials very great care is required and other morecertain methods are available.These methods will now be considered.3) The organic sample and sodium are placed in thebottom of an ignition tube and covered by anhydroussodium carbonate to approximately half-fill the tube.More sodium is placed on top of the sodium carbonatetopped by a little more sodium carbonate. The contentsof the tube is tamped down and this is heatedover a macro-burner beginning at the top of the tubeand working down. In this method, thc sodiumcarbonate tends to hold the sample in contact with thesodium long enough for reaction to occur. The sodiumhalf-way up the tube acts as a "scavenger" talung upany vapors of unreacted sample that may permeate upthrough the layer of sodium carbonate.A variation of this method is to replace the sodiumcarbonate by glass-wool (1 0).4) The organic sample is placed at the bottom of thetube and a piece of sodium placed about a quarter of theway from the top of the tube. The sodium is thenheated over a micro-burner until it melts and the headof molten sodium is allowed to roll down the inside wallof the ignition tube onto the sample at the bottom.The heat contained in the molten sodium is generallyenough to cause reaction but if reaction does not occur,the method (2) above may be applied to the contentsof the tube.5) In another method, the sample and sodium arearranged in the ignition tube as in (4). The sodium is606 / Journol of Chemical Education

then gently heated, with the tube held almost horizontally,and the vapors from the sample are allowed to passover the piece of heated sodium (13).6) A final method is to place the sodium only in thebottom of the ignition-tube and then to gently heat ituntil the sodium melts. Small quantities of organicmaterial are then added and heating continued aftercessation of any initial reaction (6, 7, 14).The main advantage of this method is that a fairlylarge quantity of sample can he added (it is easy toestimate visually how much sodium remains) thusgiving a fairly high concentration of nitrogen, etc.,if present, with subsequent ease of detection.The main disadvantage of the method is that withliquids cracking of the tube can occur.Note. It appears that, particularly for highlysubstituted nitro-compounds with a low carbon content,better results are obtained by leaving a little liquidparaffin or solvent naphtha (in which sodium is stored)on the sodium and this supplies sufficient carbon to forma reasonable concentration of cyanide ions (15).Notes on Testing for Nitrogen and HalogenThe test for sulfur with sodium nitroprusside is notdealt with since this is a very sensitive test and seldomleads to erroneous results-unless solutions become contaminatedwith sulfur in which case a positive result isobtained when sulfur is in fact absent!Test for NitrogenThis usually involves boiling an alkaline sample ofthe water-extract from the sodium fusion with ferroussulfate and then cooling and acidifying with dilutesulfuric acid. The development of a blue coloration ora dark blue precipitat,e is confirmatory for nitrogen.It seems that the development of a blue colorationand/or precipitate in a positive test depends on pHand on temperature. With pH only just below 7, thedevelopment of coloration is inhibited. It is, thus,necessary to ensure the presence of an excess of acid.Similarly, cooling of the solution is advisable becausedevelopment of the blue coloration appears to he inhibitedin hot solutions.Test for HalogenThis is straightforward if sulfur and nitrogen areabsent but even so should cause little difficulty wheneither or both are present in solutions. It must heremembered that the solution should always be acidifiedwith nitric acid and if method (3) has been used, thesolution must he boiled with excess nitric acid to ensurecomplete destruction of sodium carbonate.The chief difficulty arises when cyanide or sulfideions are present and these are removed by boiling inacid solut,ion. The problem is that the use of toovigorous conditions also tends to remove any halideions present. Thus, to avoid loss of halide ions, acidificationshould be with DILUTE nitric acid and thesolution boiled to approximately half its originalvolume. The use of concentrated nitric acid shouldbe avoided.Note. If a good result is obtained for nitrogen, thisindicates a thorough fusion and the presence of a highconcentration of cyanide ions. Thus, if halide ions arealso present it may be expect,ed that a high concentrationof halide ions will he present and with silver nitratea dense precipitate of silver halide will be obtained.If only a faint halide test is obtained, removal of theinterfering ions should he repeated, first under milderconditions (e.g., boiling for a shorter period of time oruse of wealcer acid) and then under stronger conditions(e.g., boiling the solution to a smaller volume or use ofmore concentrated acid). The results should indicatewhether the faint precipitate with silver nitrate resultedfrom cyanide ionr, that had not been destroyedor from halide ions.ConclusionsThis article is based on difficulties experienced bythe authors when using Lassaigne's test. The methodsdescribed above have been tried and are generally foundto work under the circumstances indicated hut anyparticular preference for certain methods is a matterfor each operator to decide for himself.AcknowledgmentThe authors would like to thank Dr. I. L. Finar,Senior Lecturer in Organic Chemistry at NorthernPolytechnic London, for advice and encouragement receivedconcerning this article.Literature Cited(1) LASSAIGNE, J. L., Annalm, 48, 367 (1843).(2) LAXSAIGNE, J. L., Cmpt. Rend. Hebdousadaires des Seancesde L'Acadamie des Science, 16, 387 (1843).(3) JACOBSEN, 0.) Ber., 12, 2318 (1879).(4) MULLIXEN, S. P., AND GABRIEL, G. L., Original eommunication-8thIntern. Congr. of Appl. Chem., 6, 208 (1912);reference obtained from Chem. Abs. 6, 3094 (1912).(5) HAYNES, B., "Qualitative Organic Analysis," MacMillan.London, 1966.(6j MANN, F. G., AND SAUNDERS, B. C., "P~actical OrganicChemistrv." (3rd ed.). Lonemans. Green. London. 1952.- -- ., ...(8) AYRES, D. C., J. CHEM. EDUC., 42,270 (1965).(9) TUCKER, S. H., J. CHEM. EDUC., 22,212 (1043).(10) CHERUNIS, N. D., AND ENTRIKIN, J. B., "Semimicro QualitativeOrganic Analysis," T. Y. Crowell, Co., New York,lo*" .7=..(11) LINSTEAD, R. P., AND WEEDON, B. C. L., "QualitativeOrganic Chemical Analysis," Butterworth, London, 1956.(12) OPENSHAW, N. T., "Laboratory Manual of QualitativeOrganic Analysis," Cambridge University Press, Cambridge,England, 1955.(13) Bann~, W. J., PhD, BSc, LRAM, Private Communication.(14) FIESER, L. F., "Experiments in Organic Chemistry," D. C.Healh and Co., New York, 1935.(15) HUDSON, H. R., PhD, BSc, FRIC, Private communication.Volume 46, Number 9, September 1969 / 607