WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. 863 LXX1V.-The Constitution of Carbumides. Part IV. The Mechanism of the Interactioiz o f Urea and Nitrous Acid. By EMIL ALPHONSE WERNER. THE decomposition of urea by nitrous acid is generally considered to be properly expressed by the following simple equation: CON,H + 2HN0 = CO + 2N + 3H,o, and this reaction is frequently cited in text-books as additional evidence iu support of the " carbamids " structure of urea. Theoretically this reaction should be available for the estimation of urea as is commonly suggested in the literature; it is never used for this purpose and i t is doubtful whether it ever has been since experiment has proved i t to be quite valueless. On the1 other hand, i t constitutes a well-known metho'd f o r the estimation 'of nitrous acid with a very fair degree of accuracy on the supposition that ther above equation is true.No doubt for this reason and on account of the employment of other methods for the estimation of urea this reaction has been considered a11 along as a normal change, scarcely deserving of any further inve'stigation. I n continuation of the author's work on the constitution and pro-perties of urea its behaviour towards nitrous acid has been sub-mitted to a careful quantitative study. The following are some of the more important facts which have been observed and whilst the 864 WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. show what an erroneous ooaception has been generally entertained regarding the nature of this reaction they fully justify the neces-sity f o r a proper investigation into the true mechanism of the change.1. Usea and pure nitrous acid in aqueous solution did not inter-act. (Expta. IX X.) 2. The presence of a strong acid (hydrochloric or nitric) quickly promoted a brisk interaction even in dilute solutions and the reac-tion was coImp_letad in a relative’ly short time. 3. The presence of a weak acid such as acetic acid did not pro-mote an interaction unless the concentration was abnormally high, and qven then the velocity of the reaction was extremely slow. 4. The1 volume of nitrogen evolved was not a direct measure of the amount of urea decomposed calculabd on the basis -of the above equation ; the quantity decomposed was much greater * than that indicated by the evolved nitrogen.5. Only when urea was present in considerable excess was the volume of nitrogen evolved an approximately true estimate of the amount of nitrous acid decomposed. 6. The volume ratio of carbon dioxide t o nitrogen (1 2) required by the equation has never been obtained; thel proportion of carbon dioxide was always much higher; moreover the composition of the gas was liable to much variation with small changes in concen-tration. It is obvious that so far as the usual explanation of this reaction is concerneld all thesel facts stand out as anomalies f o r which the ordinary equation offelrs no explanation. Now anomalies in such a reaction can have no reality; their apparent existeace is the natural consequence of an erroneous con-ception of the change and when the true constitution of urea is considelred they appear as normal phenomena which reveal the true mechanism of the interaction.Mechanism of the Interaction of Urea and A’itrous Acid. I n the course of a recent investigation on the properties of pure nitrous acid RBy Dey and Ghoah (this vol. p. 414) noticed that a solution of the acid ( N / 32-HNO,) was practically without action on urea “ no matter how much urea was added.” They found that the addition of gulphuric acid was necessary to promohe and complete a reaction. This anomaly they remark “was without any appa-rent rea-son,” a just comment when urea is believed to be carb-amide. Pure nitrous acid in aqueous solution does not react with * See remarks on page 87 WERNER THE CONSTITUTION OF CARBAMIDES.PART IV. 865 urea until an amino-group is presented for attack a condition brought about by the production of a salt of urea on the addition of a sufficiently strong acid thus: NH,,HX HN:C<xH3+HX = HN:C< OH The first stage of the reaction then takes place in accordance (a) HX:C<Z29HX + HNO = N + HNCO + 2H,O + HX. the equation * : with The cyanic acid is decomposed in two ways as fast as i t is generated. It is hydrolysed,? thus: and directly attacked by nitrous acid according to the equation : ( b ) HNCO + H20 = NH3 + Cog, ( c ) HNCO + HNO = CO + N + H,O (see Expts. V I I and VIII). Both these decompositiom proceeld simultaneonsly with the primary reaction (a) but the relative proportions in which they take place can be varied a t will within certain limits by adopting suitable conditions which will be presently shown.The production of cyanic acid has been easily demonstrated by its isolation in the form. of the silver salt; thus when urea was attacked by nitrous acid in the presence of silver nitrate and a small excess of nitric acid a yield of pure silver cyanate was obtained equal to 42 per cent. of the theoretical calculated on the equation : N + AgOCN + 2HN0 + 2H,O. Considering the favourable oonditions for hydrolysis and the very sensible solubility of silver cyarrate in dilute nitric acid such a result was even more successful than could re’asonably have been ex-pected. It will be seen nolw that when urea (in the form of an salt) and nitrous acid interact a certain proportion of nitrogen from the urea is always fixed as an ammonium salt and herein liw the fallacy of the reaction so far as the estimation of urea is concerned.The variations observed in the ratios of carbon dioxide to nitrogen are thus easily explained since the volume of nitrogen evolved is lowered in proportion to the amount of cyanic acid hydrolysed. The latter change can be only partly suppressed even under the * No doubt this decomposition originates through the medium of diazotisation. t Cyanic acid in water alone is hydrolysed to urea 2HNCO+H20= CON2H4+C0 (Normand and Cumming T. 1912 101 1859); in the presence of mine ral acid of course the change is aa above 866 WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. most f avourable conditions (that is high concentration and nitrous acid in excelss) with the result that the ratio of carbon dioxide to nitrogen evolved is never that which has been erroneously assumed.Now acoording to the above explanation the interaction of urea and nitrous acid is theuretically clearly divisible into two stages, during the first of which one molecule of urea is completely decom-posed by one molecule of nitrous acid instead of by two molecules, as has been commonly but falsely supposeld. This has been elasily proved experimentally by adopting the exact conditions which the theory rigorously demands namely (1) the presence of urea in excess a t the outset (2) a low concentration of nitrous acid (3) the presence of mineral acid in excess of that required to neutralise ammoiiia generated from the hydrolysis of cyanic acid and so to maintain the proper configuration of the urea molecule.Under these conditions the decomposition of cyanic acid by nitrous acid can be almost completely suppreased in favour of its decomposition by hydrolysis. A knowledge of the amount of cyanic acid hydrolysed compared with the volume of nitrogeln evolved is an all-important factor by means of which a very clear insight into the mechanism of the reaction has been obtained. The following results illustrate the degrele of success which has been realised in experimentally proving the problem which is indi-cated by the theory of the change now put forward. TABLE I. I. 11. 111. CON$34+HN0,. CON,H,+HNO,. CON,H,+HNO,, Molecular ratios 1 l 1.5 1 2 1 Nitrogen evolved, calculated on the theoretical ......92.5 per cent. 95.73 per cent. 99.34 per cent. HNCO hydrolysed 57.0 , 96.0 , 99.5 ,) posed by HNO 13.0 , 4.0 Y 9 0.5 ,? HNCO decom-Proportion of urea actually de-composed by one molecule of HNO ... .... .. . .. 79.5 , 91.73 , 98.84 ,) CO,=43*1 , CO2=43-2 , co,=44.2 ,, N =54*5 , N =54.3 ), NO = 3.2 , NO = 2.2 , NO = 1.3 ,, Ratio CO t o N.. . 1 1.24 1 1-26 1 1-22 It will be seen on viewing the results of the above experiments (the full details of which are given under Expts. V. VII. an WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. 867 VIII.) that the amount of urea decomposed by one molecular proportion of nitrous acid according t o the equation N2 + HNCO + 2H,O + HX was less than that indicated by the volume of nitrogen evolved.The difference was most marked when the exact proportions (equal molecules) of urea and nitrous acid required by the equation were used since the conditions were less favourable for a quantitative realisation of the second change namely, HNCO + H20 + HX=NH,X + CO,, than when a considerable excess of urea was present. In the latter case the desired object was almost fully attained (111) and the true nature of the primary stage of the reaction thereby established. As regards the composition of the evolved gases the ratio of carbon dioxide t o nitrogen was in each case approximately 1 1.20; this of course was not the true value since a very sensible amount of carbon dioxide was held in solution in the residual liquid; when corrected in the case of result 111 for example the true ratio was CO,= 1 N= 1.02 or 1.1 as required by the combination of the two equations. It' may be well t o direct attention here t o the constant presence of a small amount of nitric oxide in the evolved gas; whilst this was no doubt due to the decomposition of a corresponding pro-portion of nitrous acid thus 3HN0,=HN03+ H,O + 2N0 i t was not found possible to eliminate it completely even when urea was in excess and the concentration of nitrous acid a t t'he outset of the reaction was as low as N / 2 0 . Under such conditions as are commonly adopted in the estimation of nitrous acid by the aid of urea the proportions of nitric oxide may easily amount to between 6 and 8 per cent. of the evolved gases according to the particular concentration of the solution used.This fact appears to have been generally overlooked. It is obvious when the ratios HN02:N and 3HN02:2N0 are compared that the presence of nitric oxide must lead to a result in excess of the true value; for example in the case of result 111, table I if the nitric oxide found was included as nitrogen. the yield of the latter would appear as 101.8 per cent. of the theoretical. RBy and his co-workers (Zoc. cit.) using a solution of nitrous acid of concentration N/32 obtained a result which they found was in excess of the true value in nearly the same proportion as above and no doubt for the same reason. This decomposition o 868 WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. nitrous acid readily explains why a solution of comparatively high concentration say HNO = N / 6 can slowly attack urea ; the generation of nitric acid gradually brings about the required condition.The results under table I were obtained by adding the theoretical proportion of nitrous acid slowly and a t intervals to the acid solution of urea. A very low concentration of nitrous acid was thus ensured throughout the progress of the reaction. When the molecular proportion of nityous acid was added all at once the concentration a t the outset being HNO,=N/6 the results as was to be expected were very different as shown below (Expts. IV. and V.). TABLE 11. I. 11. 111. CON,Hd + HNO,. CON,H + HNO CON,H + HNO, Molecular ratios 1 l 1.5 1 2 l Nitrogen evolved 91-46 per cent.94.40 per cent. 96.48 per cent. HNCO hydro-HNCO decom-lysed.. ............. 7 1.5 .. 74.5 , 76-0 ,, posed by HNO 28.5 ) 25.5 )) 24.0 ), Urea actually de-composed by HNO ............ 62.96 .. 68-90 ,) 72.48 ,) Whilst the volume of nitrogen evolved was only slightly below that previously observed the amount of urea decomposed was in each case much less than before This was the natural result of the much greater facility offered for the decomposition of cyanic acid by nitrous acid a t the higher concentration. The latter was also responsible for the slight increase in the proportions of nitric oxide. The constancy* to be observed in the proportions of carbon dioxide and nitrogen in the evolved gases as shown in both tables in spite of the fairly wide differences in the propor-tions of urea decomposed is easily explained when the ratios of cyanic hydrolysed to cyanic decomposed by nitrous acid are con-sidered.As regards the very slow reaction which was noticed to take place between urea and nitrous acid in the presence of acetic acid (when HNO,=N/4) this was entirely due to the gradual decom-* Within the limits of experimental error the rate of mixing for instance, which affects the velocity of the reaction has a deoided-influence on the above ratios WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. 869 position of the former acid. Urea acetate has been described by Matignon (Compt. rend. 1891 112 1369) as a compound which was completely dissociated in aqueous solution hence acetic acid could not establish the essential condition required t o promote the desired interaction (Expts.IX. and X.). Decomposition of UreG in the Presence of Two Molecular Proportions of Nitrous Acid. The results recorded in table I have conclusively proved that urea can be completely decomposed by one molecular proportion of nitrous acid; that this is not accomplished under the conditions which are commonly employed is solely due to the disturbing effect of the secondary reaction ( c ) . Now according t o the usual interpretation of the change two molecular proportions of nitrous acid should be required t o decompose one of urea. The effect after treating urea directly with nitrous acid in these proportions and at different concentrations,. in the presence of hydrochloric acid to promote the change are given below (for details me Expts.I. II. and 111.). TABLE 111. Nitrogen evolved. Composition of evolved gases. Per cent. Per cent. 72-02 HNO N/3 I. Urea N/6 (2 C.C. N-HC1) 71.99 C0,=32.1 N2=68.2 NO=9.G 69.19 C 0 2 ~ 32.6 N2-57*3 NO = 10.0 HNO N/6 11. Urea N/12 HNO N / 8 111. Urea N/16 HNO N/10 IV. Urea N/20 (3 C.C. N-HC1) (2 C.C. N-HC1) 72.07 CO,=31.1 N2=58*1 NO=10.4 1 I (2 C.C. N-HC1 I n each case it was readily proved that all the urea had been decomposed whilst an excess of nitrous acid remained yet in round numbers only about' 70 per cent. of the theoretical proportion of nitrogen was evolved. The remainder of the nitrogen was of course present as ammonium chloride* in the residual solution * It is interesting t o note that Clam (Ber.1872 4 140) long ago noticed the formation of ammonia when nitrous acid reacts with urea ; thus iie gave the following equation for the reaction in the cold; BCON,H,+N,O, 870 WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. It will be noticed also that even without allowing for carbon dioxide held in solution the ratio of carbon dioxide t o nitrogen was still well below that of 1 2 as required. An equally marked divergence from the theoretical results was obtained by carrying out the reaction in two separate stages. It will be obvious that according to the usual but false explanation of the change such a procedure should give similar results for each stage. Whilst the first stage was repre-sented by the values given under I table 11 the volume of nitrogen evolved in the second stage was equal to only 26-38 per cent.of the original amount of urea present hence 62.96 + 26*38= 89-34 per cent.; nitrogen from the remainder of the urea (10.66 per cent.) was fixed as ammonium chloride in the second stage (see Expt. IV.). The composition of the evolved gases namely Co2=22.4 per cent. N2=50*7 per cent. NO=26*8 per cent. was very different from that of the gases set free in the first stage. A rather remarkable paradox makes its appearance when the results are compared an the basis of the false and of the truer equations; thus according to t,he usual interpretation of the reac-tion the amount of urea decomposed was roughly 30 per cent. greater (table 111) than indicated by the volume of nitrogen set free whereas in reality the amount of urea decomposed was much less than required by the volume of nitrogen evolved.The para-dox of course is but a phantom; its existence is just as unreal as the usual explanation of the change is incorrect. A contemplation of the results just recorded and so easily demonstrated makes i t almost impossible to believe that the behaviour of urea towards nitrous acid has ever been seriously studied with the object of obtaining evidence to support the sup-posed (' carbamide " formula." Whilst the present study of the reaction has supplied further proof of the cl'yclic formula it has also brought to light yet another This was not so. (NH4),C0 + 2N2 + C 0 2 . It was assumed however that urea was hydrolysed to ammonium carbonate during the process apparently in-dependent of the reaction with nitrous acid since the proportion of carbon dioxide to nitrogen evolved is shown to be the same as in the usual equation ; probably for this reason the observation is never mentioned in the text-books.* Emmerling (A. 1886 50 747; the original paper in Landw. Ver-suchs-Stat. 1886 440 was not available) studied the decomposition of urea by nitrous acid in the presence of nitric acid and acetic acid respectively both in cold and in hot solutions. The volume of nitrogen evolved was found never to be equal to the theoretical but no apparent attempt was made to offer any explanation of the results WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. 871 of the many fallacies which abound throughout the chemistry of urea.The origin of these is not far to seek. Instead of a careful study of the properties and reactions of urea being made the groundwork for solving the problem of its constitution an almost infallible belief in the truth of the carb-amide formula has all along been the predominant factor in deter-mining what these properties and reactions should be. Secondary changes seemingly unimportant by-products apparent abnormalities in certain reactions and so forth have been pushed aside as of little consequence so long as the end result could be made to fit in with the “carbamide” structure. E X P E R I M E N T A L. With one exception all the experiments were made with the aid of a Lunge nitrometer. The specimen of sodium nitrite used for the generation of nitrous acid contained 97.18 per cent.of NaNO,; * a proportionate weight (71 69) corresponding with the theoretical required was used in each case. Action of Nitrous Acid o n Urea in Molecular Proportions of Two t o One. Expt. 1.-0.03 Gram of pure urea and 0.071 gram of sodium nitrite dissolved in 1 C.C. of water were introduced into the nitro-meter over mercury and 2 C.C. of N-hydrochloric acid directly added. Concentration a t outset HNO =N/3 CON,H,=N/6. The reaction was apparently coiripleted within thirty minutes whilst more than four-fifths of the gas had been evolved after five minutes. I n this and all other experiments not less than one hour was allowed to elapse before the gas was measured and analysed.Gas evolved=31.4 C.C. a t 18O and 766.5 mm. CO,=11 c.c., NO = 3 c.c. N,= 17.4 C.C. Volume of nitrogen a t N.T.P.=16.134 c.c. =72*02 per cent. of the theoretical. (Theory = 22.4 C.C. a t N.T.P.) Expt. 11.-As above but HNO = N / 6 CON,H = N / 12. Gas evolved=29*7 C.C. at’ 16O and 763.5 mm. C0,=9*55 c.c., NO=2*85 c.c. N,= 17.3 C.C. * Estimated by the thiourea method (T. 1912,101 2190 and Coade and Werner T. 1913 103 1221) 872 WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. Volume of nitrogen a t N.T.P.=16.126 c.c. =71.99 per cent. of the theoretical. Expt. irZI.-The last experiment was repeated to prove the cause f o r the deficiency of the evolved nitrogen. After the evolution of gas had ceased the residual liquid was well washed out of the nitrometer.It required for neutralisation, using methyl-orange as indicator 6.1 C.C. of N/lO-sodium hydr-oxide. Since 1 C.C. of N-hydrochloric acid was directly neutralised in liberating nitrous acid there should have remained according to the usual equation free acid equivalent to 10 C.C. of N / l O -sodium hydroxide. Hence 10 - 6-1 =3*9 C.C. of N/lO-hydrochloric acid were neutralised by ammonia from the hydrolysis of cyanic acid. Now according to equations ( a ) and (b) (p. 865) the maxi-mum amount of acid that could be thereby neutralised would be 5 C.C. of N/10-hydrochloric acid therefore 78 per cent. of the theoretical proportion of cyanic acid was hydrolysed or so much of its nitrogen was fixed as ammonium chloride. The remainder of its nitrogen namely 22 per cent.was set free (equation c ) , together with that from urea in accordance with equation (a). Since all the urea was decomposed with the liberation of half of its nitrogen we have 50+22=72 per cent. of the total nitrogen set free which was in complete agreement with the result obtained from Expt.. 11. As the results with urea and nitrous acid (1 2) a t lower con-centrations (table 111) were obtained in a manner similar t o the above further details are unnecessary. Action of Nitrows Acid on Urea (2 1) in Two Stages. Expt. ZV. First Stage.-O*O6 Gram of urea and 0.071 gram of sodium nitrite were dissolved in 4 C.C. of water and 2 C.C. of N-hydrochloric acid directly added. Concentration CON,H = N / 6 HNO,= N / 6. Gas evolved=37.5 C.C. a t 1 8 O and 763 mm.CO,=13*7 c.c., Volume of nitrogen a t N.T.P.=20.49 c.c. =91*46 per cent. of the theoretical. Second Stage.-The gas having been expelled from the nitro-meter (from a repeated experiment) 0.071 gram of sodium nitrite dissolved in 1 C.C. of water was added and then 1 C.C. of N-hydro-chloric acid. The evolution of gas was very much slower than in the first stage and five hours were allowed for the completion of the reaction. NO = 1.6 c.c. N2=22*2 C.C WERNER THE CONSTITUTION OF CARBAMIDES. PART IV. 873 Gas evolved=25*3 C.C. a t 1 8 O and 769.3 mm. C0,=10*25 c.c., NO=2.35 c.c. N2=12*7 C.C. Volume of nitrogen a t N.T.P. = 11.82 C.C. =52*76 per cent. of the theoretical. An analysis of the residual solution after the first stage showed t h a t 7.15 C.C.of 2\7/10-hydrochloric acid had been neutralised, equivalent t o 71.5 per cent. of cyanic acid hydrolysed therefore 28.5 per cent. of the evolved nitrogen was t h e result of the reaction between nitrous acid and cyanic acid (equation c ) ; hence 91.46 - 28.5 = 62.96 per cent. of the urea present was decomposed in this stage. Therefore only 37.04 per cent. of urea remained t o be attacked by nitrous acid i n the second stage and since 52.76 per cent. of nitrogen was evolved it follows t h a t 52-76 - 37.04 = 15-72 per cent. of the nitrogen set free in this stage waL due to the above reaction (equation c ) . A comparison of the results from the two stages is not without interest. First stage. Second stage. Per cent. Per cent. Urea decomposed ..................= 62.96 37-04 HNCO hydrolysed .................. ~ 7 1 . 5 0 57.60 HNCO decomposed by HNO ... = 28.50 42-40 CO =36.5 40.5 50.1 9.3 Composition of evolved gas.. . . Ratio CO N 1 1.62 1 1.23 Since nitrous acid was in considerable excess i n the second stage (which should not be the case according t o the usual equation), the proportion of cyanic acid attacked by it to cyanic acid hydro-lysed was much greater than in the first stage. Decomposition of Urea by Oibe Molecdar Proportion of Nitrous Acid. In order to illustrate how the re'sults given under table I were obtained it will be sufficient to state1 the details of the most succw-f ul e.xperiment. Expt. 1.'.-0*12 Gram of urea was dissolved in 3 C.C. of N-hydro-chloric acid and the solution introduce'd into the nitrometer; 0,071 gram of sodium nitrite dissolved in 2 C.C.of water was placed in the cup (previously rinsed) of the nitrometer and added gradually in four separate portions to the urea solution. The1 reaction which was hastened by shaking t o ensure rapid mixing was allowed to completle itself before each addition of the sodium nitrite. VOL. (3x1. M 874 WERNER THE CONSTITUTION O F CARBAMIDES. PART IV. Gas evolved (after one hour) =44*2 C.C. a t 1 8 O and 766.4 mm. ; Volume of nitrogen a t N.T.P.=22*252 c.c.=99-34 per cent. of the theoretical. The residual solution from a similar experiment required 10.05 C.C. 'of N / 10-sodium hydroxide for neutralisation. Hence 20 (2 C.C. of 1";-hydrochloric acid originally free) - 10.05 = 9.95 C.C.of N/lO-liydrochloric acid were neutraliseld as the result of cyaiiic acid hydrolysis which was therefore alniost complete. There-fore the amount of urea actually decomposed was 99*34-0.5= 98.84 per cent. 'of the theoretical only 0.5 per cent. of the evolved nitrogen being derived from cyanic acid. Therefore one molecule of urea was decomposed by one molecule1 of nitrous acid. CO,=19*6 C.C. ; NO =0*6 C.C. ; N,=24*0 C.C. Isolation of Cynnic Acid as the Silver Salt from the Interaction of Urea and Nitrous Acid. Ex@. VI.-0*6 Gram of urea and 0.71 gram of sodium nitrite were dissolved in 40 C.C. of ice-cold water and t o the solution 1.7 grams of silver nitrate previously dissolved in 5 C.C. of water and 5 C.C. of iY-nitric acid welre added. As t h e pale cre'am-coloured pre-cipitate 'of silver nitrite which was immediately formed gradually disappeared it was replaced by a snow-white precipitate of silver cyana te.During the progress of the reaction further 5 C.C. of N-nitric acid were added. After an hour the precipitate was collected washed, and dried. It gavel none of the reactions for nitrous acid and contained Ag = 71-84 per cent. (AgOCN requires Ag = 72 per cent.) ; on adding a few drops of sulphuric acid to the dry salt the character-istic pungent odour of cyanic acid was evolved. The w-eight of silver cyanate obtained was 0.63 gram which was equal t o 42 per cent. of the theoretical for equation (d). The Interaction uf Cyanic Acid and Nitrous Acid. As this change does not appear to have been hitherto examined, the following experiments were made in order to prove the validity of equation ( c ) already given.Expt. VII.-0*081 Gram of pure potassium cyanate and 0.071 gram of sodium nitrite werel dissolved in 2 C.C. of water and intro-duced into the nitro8meter; 3 C.C. of AT-hydrocliloric acid were added, t h a t is 1 C.C. of acid in excess t o counteract the neutralising effects of hydrolysis. Concentration a t outset LINO and HNCO = A ' / 5 WERNER THE CONSTITTJTION OP CARBAMIDES. PART IV. 875 The evolution of gas was very rapid and the reaction was practi-Gas evolved after ‘one hour=34’3 C.C. a t 16O and 757.8 mm.; C02=19.1 c.c.; NO-3.6 c.c.; N2=11*6 C.C. Volume of nitrogen a t N.T.P.=10*72 c.c.=47*8 per cent. of the theoretical. Therefore 52.2 per cent.of cyanic acid had been hydro-lysecl. The residual solution required for neutralisation 8.5 C.C. of N / 10-sodium hydr-oxide inste’ad of 4.78 C.C. as required by the gasometric analysis. The apparent discrepancy was easily elxplained when the above results were considered. Tho volume of nitric oxide evolved (3.33 C.C. a t N.T.P.) represents a decomposition of 22.3 per cent. of nitrous acid with the generation of nitric acid equivalent to 0.74 C.C. of N / 10-sodium hydroxide whilst the proportion of cyanic hydrolysed showed that free nitrous acid remained equivalent t o 3 C.C. of NI10-sodium hydroxide. Hence 4.78 + 0.74 + 3.0 = 8.52 C.C. of AT/ 10-sodium hydroxide were required which is in complete agree-ment with the value actually found. The presence of unchanged nitrous acid in the residual liquid was easily proved.Therefore the reaction between cyanic acid and nitrous acid takes place theoretically between equal molecular proportions but a t a concentration of N / 5 the velocity of hydrolysis of cyanic acid is slightly higher than that of the primary change. cally completed witshin five minutes E,rpt. Vlll.-The above experiment was repeated. The Behaviour of Urea towards Pure Nitrous Acid alone 04 in the Preseitce of Acetic Acid. Expt. ZX.-O*O3 Gram of urea and 0.071 gram of sodium nitrite were dissolved in 2 C.C. of water and 2 C.C. of N-acetic acid were added. There was no preceptible evolution of gas until after a consider-able time; thus after twenty-four hours 7.4 C.C. had been evolved and a t the end of ninety-six hours when the experiment was stopped the volume of the evolved gas was=12*6 C.C.a t 1 3 O and 758 mm. The volume of nitrogeln at N.T.P. was=6.56 c.c. and the original gas contained 9.5 per cent. of nitric oxide. The slow action was primarily brought about as a result of the gradual decomposition of nitrous acid whereby urea nitrate was slowly generafed. RSy and his co-workers (Zoc. cit.) have shown that even a t Oo the most concentrated solution of nitrous acid, stable for only a short time was approximately N/5.5. Expt. X.-The same proportions of urea and sodium nitrite as before were dissolved in 29 C.C. of water and 1 c.c of N-acetic acid was added. Concentration of HNO = N / 4. Concentration of HNO =A’/ 30. &I M 876 ROBINSON A THEORY OF THE MECHANISM OF THE After remaining for three days iii the nitrometer the voluine of gas evolved was 0.8 C.C.Yet when 2 C.C. of N-hydrochloric acid were added a fairly brisk reaction was quickly promoted and even a t this low concentration was almost completed at the end of half an hour. &urn mary . (1) Urea is not attacked by pure nitrous acid alone or even when a second very weak acid is present. (2) When a salt of urea is produced by the presence of a sufficiently strong acid i t is immediately attacked by nitrous acid, because an amino-group is thereby presented for such attack. (3) One molecule of urea (as a salt) requires but one molecule of nitrous acid for its deconipositioii into nitrogen cyanic acid, and water since only one amino-group is present. (4) Cyanic acid and nitrous acid react in equal molecular pro-portions with the production of nitrogen carbon dioxide and water. (5) The usual interpretation of the reaction between urea and nitrous acid which has been hitherto accepted is incorrect; first, because it is in contradiction to the experimental facts and, secondly because it is based on an erroneous conception of the constit'ution of urea. UNIVERSITY CHEMICAL LABORATORY, TRINITY COLLEQE DUBLIN. (Received July Zlst 1917.