I N D USTRIAL A N D ENGINEBRING CHEMISTRY
848
Ash.3 Readings in degrees Fahrenheit taken every 5 minutes were as follows: 1482,1608,1716, 1806, 1878, 1950,'2022,2094, 2146, 2198, 2250, 2300, 2354, 2410, 2458, 2510, 2562, 2615, 2666, 2725, 2770, 2828, 2880, 2935, 2990,3015,3045
The temperature measurements were made with a Leeds & L~~~ No. 4391 ; Telescope No. Northrup 63,359; Ammeter No. 66,693; it was standardized by the U. S. Bureau of Standards, Washington, D. C. (B. S. Test No. Ttp-33,767), on April 4, 1922. It was rechecked on 8
Proc. Am. SOC.Testing Materials, 20, 796 (1920).
Vol. 16, No. 8
April 15,1924, by checking the fusing points of samples of coal ash, the fusing points of which were determined by the s' Bureau Of Mines author a t the laboratories Of the through the courtesy of A. C. Fieldner, superintendent of the Pittsburgh Station. The same batch of cones was used and the results checked within 20' F. a t 2900" F. The rise in temperature is surprisingly uniform and a reducing temperature can be maintained through the entire run. The most pleasing feature of this experiment is the durability of the crucible. Seventy-five runs have already been made with it, and it shows no signs of wear. It promises to be good for another fifty runs.
Preparation of Guanidinium Salts from Calcium Cyanamide' By J. S. Blair and J. M.Braham FIXEDNITROGEN RESSARCHLABORATORY, WASHINGTON, D. C.
HILE guanidinIt is believed, however, Guanidinium salts can be advantageously produced only from ium salts havenot that the method described cyanamide or its derivatives. The methods heretofore deoeloped for as yet become inin the present paper, involvits production have entailed either the ammonation, in nonaqueous dustrially important, there ing as it does the utilization solution, of cyanamide itself, or the ammonation or hydrolysis of appear to be a number of of neutralized aqueous exdicyanodiamide. The use of cyanamide in its most readily and uses to which they may be tracts of calcium cyanamcheaply available form-i. e., in the aqueous solution in which it may devoted when they become ide, offers important adbe obtained by extraition of crude calcium cyanamide with water-has cheaply available in large vantages over any methods apparently not hitherto been considered practicable. The presquantity. Nitroguanidine, using d i c y a n o d i a m i d e, ent paper describes the conditions under which about 80 per cent of for example, seems to give since, as will later be shown, the cyanamide contained in such solutions may be convetted to considerable promise as an it gives a better yield, a guanidinium salts by heating under pressure with ammonium salts. explosives i n g r e d i e n t . purer product, and constiGuanidinium hydroxide is tutes a more direct process. an unusually strong organic base,2 and guanidinium carbonEXPERIMENTAL PROCEDURE ate, which can be prepared in a very pure state, has been Solutions of free cyanamide, HsCNZ, were' obtained as suggested as a standard in alkalimetry.* Crude calcium cyanamide was gradually added to follows: In the early methods for the preparation of guanidinium salts, by the fusion of ammonium thiocyanate,4 cyanamide is about five times its weight of cold water, and after vigorous thought to be formed as an intermediate. These methods are mechanical agitation for 2 hours the insoluble material much inferior to the processes that have been devised since was filtered off and washed; the filtrate was immediately the development of the cyanamide process of nitrogen fixa- neutralized accurately with sulfuric acid, the calcium sulfate tion, which gives a more direct and much cheaper source of filtrated off and washed, and the neutral solution concentrated the cyanamide. The methods described in the literature for to about 375 grams cyanamide per liter at a temperature not synthesis of guanidinium salts from cyanamide itself6 in- exceeding 80" C. This was accomplished by heating in a volve the isolation of solid cyanamide ina relatively pure state, large dish on a steam bath and passing a blast of air over the a step which it would be advantageous to avoid. The use of surface. The precise cyanamide content of this solution was cyanamide in aqueous solutions such as are readily obtained then determined by analysis, and measured quantities of from calcium cyanamide has not heretofore been considered ammonium salt and water were added to give the solution the practicable, apparently because of an insufficient understand- composition desired for heating. The solution was then anaing of the conditions governing the stability of cyanamide lyzed for total nitrogen, cyanamide, ammonia, dicyanodiamide, solutions. For this reason, the more recently developed and urea. The two last-named compounds were always presmethods have involved the utilization of dicyanodiamide, ent in very small amounts. I n all but a few preliminary experiments 100 cc. of solufrom which guanidine can be produced either by hydrolysis6 tion were used, the heating being performed in small steel or by ammonation.? autoclaves with enameled interior surfaces, heated by means of Received March 18, 1924. oil baths. The temperatures and pressures inside the autoOstwald, J . p r a k t . Chem., 121 88, 367 (1888). claves were measured in some cases. d Dodd, J . SOC. Chem. I n d . , 40, 89T (1921). In most of the experiments ammonium nitrate was used, 4 Delitsch, J . prakl. Chem., [21 9, 1 (1874). 5 Erlenmeyer, AWL, 146, 259 (1868). Bannow, Bey., 4, 161 (1871); because guanidinium nitrate is only moderately soluble in Volhard, J . prakl. Chem., [2] 9 15 (1871); Ewan and Young J . Soc Chcm. water and is hence more easily recovered from solution in a Ind., 40,189 (1921). (Volhard did not use aqueous solutions as stated by pure state than are the other common guanidinium salts, Ewan and Young but fused together solid cyanamide andsn ammonium salt.) which are extremely soluble. The method is otherwise not Ulpiani, D. R. Patent 209,431 (October 16, 1907). Stolle and Rrauch, B e . , 46, 2337 (1913); Levene and Senior, J . B i d . Chem., 25, 623 (1916); restricted to the use of any particular ammonium salt, Davis, J . A m . Chem. Soc., 48, 669 (1921). however. 7 Spandau Stickstoffwerke, D. R . Patent 222,552 (October 30, 1908); The contents of the autoclaves after beating consisted of Werner and Bell, J . Chem. SOC.( L o n d o n ) , 4 8 ,669 (1921); Ewan and Young, yellow solutions of disagreeable odor. Small quantities of bc. dl.; Davis, J . A m . Chem. SOC.,48, 2234 (1921).
W
0
August, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
yellowish brown water-insoluble material were also present. When ammonium nitrate was used, considerable quantities of guanidinium nitrate had crystallized, since the solutions were saturated with respect to that salt.
849
ide was estimated gravimetrically as the nickel salt, and cyanamide by precipitation with ammoniacal silver nitrate and subsequent determination of nitrogen by the Kjeldahl method. Total ammonia was determined by the aeration method de9
flywe 2 Ef&d of Temperature o f Heating.
Temperature ofHea+iq“c
scribed by Matthews.10 The usual distillation method is not applicable, because all the compounds present in these solutions are extensively hydrolyzed in hot alkaline solution to carbon dioxide and ammonia. The alkalinity of the solution was taken as a measure of the concentration of free ammonia (ammonium carbonate), since the guanidinium and biguanidinium salts present are not hydrolyzed and hence give neutral solutions. Combined ammonia was then estimated COMPOUNDS PRESENT AND METHODSUSED FOR THEIR by difference between total and free ammonia. Ammelide DETERMINATION was precipitated from the solution by acidifying with acetic The compounds always found in these autoclave solutions acid, and determined by direct weighing. The precipitate are guanidinium salt, urea, ammonium salt, free ammonia, evidently was not pure ammelide, however, since one sample and carbon dioxide, as well as the water-insoluble material. was found to contain 44.5 per cent nitrogen, while the theoI n a few cases in which special conditions prevailed, cyanam- retical value is 43.77 per cent. The granular! yellowish brown water-insoluble material, ide, dicyanodiamide, biguanidinium salt, or ammelide were previously mentioned, has not been identified. It is insoluble also present. Satisfactory methods for the determination of all these com- in hot as well as in cold water, and in cold alkalies and mineral pounds have not been developed, but in general about 96 per acids. Six different samples, obtained in as many different cent of the cyanamide used in these experiments was accounted experiments, were analyzed for nitrogen, giving as the two for. Gusnidinium salt was determined gravimetrically as the extreme values 48.22 and 48.88 per cent. Analysis of one picrate by precipitation in ammoniacal ammonium picrate sample by the Devarda modification showed the absence of solution as described by Vozarik.8 It was found to be es- nitrate nitrogen. Hot concentrated potassium hydroxide sential to use care in adjusting the relative concentrations of converted the material to a substance having the solubility precipitant and of guanidinium salt, but a special experiment characteristics of ammeline or ammelide. No further work showed that under proper conditions the results were accurate was done on the identification of this material, which is reto within l’per cent. Urea was determined by the urease ferred to as “insolubles” in this paper. m e t h ~ d which , ~ gives very accurate results. Dicyanodiamide RESULTSOF PRELIMINARY EXPERIMENTS was determined by conversion to guanylurea and determinaSeveral. preliminary experiments were made to determine tion of the latter as the nickel salt, by a method which will be described in a later paper from this laboratory. Biguan- approximately the conditions as to concentrationof cyanamide, relative proportions of cyanamide and ammonium salt, tem8 Z. angew. Chcm., 16, 670 (1902).
The entire contents were filtered through a tared Gooch crucible and washed with warm water until only the yellowish brown insoluble material remained on the filter, the guanidinium nitrate being brought completely into solution. The filtrate and washings were made up to a definite volume for analysis, while the weight of the dried insoluble material was determined.
* Fox and Geldard, Tars JOURNAL,
16, 743 (1923).
10
J . Agr. Sn., 10, 72 (1920).
INDUSTRIAL AND ENGINEERING CHEMISTRY
850
perature, and duration of heating which favor the formation The conditions maintained in the final experiments were based on the information thus obtained. These preliminary experiments will not be described in detail, since the final experiments give more conclu-
of guanidine in good yield.
fihwe .5 A
sive results with regard to guanidine formation. The following observations were made, however, which do not enter into the final experiments, and are worth recording. It may be mentioned that, while numerical results are not given, these experiments were quantitative and just as dependable as the later ones.
'
1-Cyanamide does not react with ammonium nitrate in aqueous solution a t 100" C. to form guanidinium salt. Some conversion to dicyanodiamide and to urea does take place. This is to be contrasted with the quantitative conversionof cyanamide to guanidinium halide, when heated with ammonium halides in alcohol solution a t the same temperature." SchmidP states that cyanamide will react with ammonium hydroxide in aqueous solution a t 100" C. to form guanidinium hydroxide, but the correctness of his interpretation is rendered doubtful by the probability of guanidinium salt formation by hydrolysis of dicyanodiamide during the course of analysis, by his method. 2-When cyanamide is heated in aqueous solution a t 180" C., a conversion to guanidine (guanidinium carbonate) does take place even in the entire absence of ammonium salts in the original solution. This is probably due to reaction with the ammonium carbonate formed in large quantities by hydrolysis under such conditions. Ammelide, urea, and dicyanodiamide are also formed. The addition of ammonium nitrate to the original solution has the effect of tremendously increasing the extent of guanidinium salt formation, and of reducing the formation of ammelide and ammonium carbonate. A large excess of ammonium salt results in complete absence of ammelide, and also of dicyanodiamide in the autoclaved solution. 3-Guanidinium nitrate, when heated alone in aqueous solution except for a small amount of ammonium hydroxide such as is normally formed in the autoclave experiments, suffered a considerable hydrolysis to urea and ammonium nitrate a t 165" C. When heated a t 185O C. the guanidinium nitrate was hydrolyzed to a much greater extent, large quantities of ammonium carbonate being formed in addition. Dicyanodiamide, biguanidinium salt, ammelide, or "insolubles" were not formed. This shows t h a t in the other experiments these substances were formed byreactions in which the cyanamide takes part. 11
1s
Erlenmeyer, loc. 6% Arch. Pharm., 264, 626 (1918).
Vol. 16, No. 8
FINAL EXPERIMENTS Expt. 1-Effect of Ammonium Salt-Cyanamide Ratio Four solutions having identical concentrations with respect to cyanamide (168.8 grams per liter) but varying as regards concentration of ammonium nitrate were heated a t 165' C. for 3 hours. The results are shown in Fig. 1. Expt. %-Effect of Temperature Five 100-cc. portions of a solution containing 156.2 grams cyanamide per liter and 2.27 mols of ammonium nitrate per mol of cyanamide were heated for 3 hours a t varying temperatures. Results are shown in Fig. 2 . Expt. 3-Effect of Duration of Heating Five 100-cc. portions of a solution having the same composition as that used in Expt. 2 were heated a t 166" C. for varying periods of time. It was known that the changes occurring in the first few minutes of heating, during which the conditions of heating are of necessity not constant, are very significant. Therefore, a thermometer and pressure gage were inserted in the autoclaves used in the shorter experiments, and the conditions existing inside the autoclave, as registered by these instruments, are shown in Fig. 3-A. The analytical results are shown in Fig. 3-B. Both must be taken into account in interpreting the results of the experiment. Expt. ,$-Effect of Rapidity of Heating Two portions of a solution containing 162.03grams cyanamide per liter and 1.88 mols ammonium nitrate per mol of cyanamide, were each heated a t 156' C. for 3 hours. In Expt. 4-A, however, the solution was heated slowly, whereas in 4-B the autoclave was plunged into the hot oil bath, as in all the preceding experiments. The conditions of heating, as well as the thermal behavior of the solutions in the two cases, can be best seen by reference to Fig. 4. The analytical results are given in Table I. TABLEI CYANAMIDE CONVERTED TOAmmonGuaniium Dicyanodinium Carbon- InsolSum of ExDt. CONDITIONSdiamide Urea Salt ate ubles Percentaees 4-A , Heated slowly 1.7 6.6 77.7 1.9 4.5 92.4 4-B Heated rapidly 0.0 73.8 2.5 5.4 13.3 95.0 -PERCENTAGE
e 60
$
54
b
Conc. HzCN2 156 ZgperLi?er E a f h Zmp. I65'C Molal Ratio ",NQ f o &Cnl,- 2.2 7 ~
b P
I
I
ffor&,on of h'csfmg. M,irufeS
Expt. &-Effect of Concentration of Cyanamide Four solutions, each containing ammonium nitrate and cyanamide in the same molecular ratio (1.88) but varying in concentration, were slowly heated up to 155°C. andmaintained a t that temperature for 3 hours. The results are shown in Fig. 5.
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1924
Expt. 6-Eflect of Adding Urea to the Autoclave Charge The desirability of using a large excess of ammonium salt would obviously involve, when operating on a large scale, the recovery of unconverted ammonium salt for further use. Such recovered ammonium salt would, from solubility considerations, be contaminated with urea. Expt. 6 was performed, therefore, to determine the effect of added urea on the formation of guanidine.
85 1
termined decrease in total ammonia with the ammonia shown by the guanidine determination to have been used u p to form guanidine. Its estimation from the alkalinity of the solution, as in the case of the other salts, was obviously impossible. One minor observation is worthy of notice. Although the ‘(insolubles” obtained in the four experiments varied greatly in appearance, the nitrogen content was quite uniform, showing that the character of the insoluble material is not affected by the anion present.
Expt. 8-Reaction of Dicyanodiamide with Ammonium Nitrate A solution containing 165 grams dicyanodiamide per liter and twice the theoretical quantity of ammonium nitrate was slowly heated up to 155O C . and maintained a t that temperature for 3 hours. The experiment was performed in duplicate as a check on reliability of results, the two autoclaves being simultaneously heated in the same oil bath. One of the autoclaves was fitted with a thermometer and pressure gage, the readings of which showed that the reaction was only slightly exothermic as compared with the reaction between ammonium nitrate and cyanamide.
Expt. 8-A 8-B
Three portions of a solution containing 164.7 grams cyanamide per liter and ammonium nitrate in the ratio of 1.95 mols per mol of cyanamide were heated slowly up to 155’ C. and maintained a t that temperature for 3 hours. To one portion no urea was added, to the second and third, amounts of urea were added which corresponded to 10 and 20 per cent, respectively, of the cyanamide present. The cyanamide used, however, contained 1.95 per cent urea, so that the actual proportion of the urea to cyanamide is expressed in the first column of Table 11. TABLEI1 Urea as Per -PERCENTAGE CYANAMIDE CONVERTED TO-cent of Guanidinium Ammonium Cyanamide Salt Urea Carbonate Insoluble 1.95 79.6 8.6 3.6 6.0
83.8 83.1
24.9 17.1
54.4 60.0
3.4 2.9
3.2 3.1
1.53 1.63
3.7 3.6
91.1 88.0
Time in Hinufes
Tme in Minutes
11.95 21.95
TABLE IV PERCENTAGE DICYANODIAMIDE CONVERTED TO DicyanoAmmondiamide Biguaniium (Uncon- Guanidin- dinium Carbon- Insol- Sum of Perubles centages Salt verted) ium Salt Urea ate
2.2
5.2 6.4
-0.66
7.4
97.8 98.6
I n this experiment .ammonium nitrate was compared with the three other commonly available ammonium salts-namely, the chloride, sulfate, and phosphate. The concentration of cyanamide was 104.43 grams per liter, the molal ratio of ammonium d t t o cyanamide was 2:l and the autoclaves were heated gradually to 155’ C . and maintained a t that temperature for 3 hours. The results are shown in Table 111. TABLE I11 ---PERCEXTAGE CYANAMIDE CONVERTED TODicyanoGuanidin- Ammonium Insol- Sum of Perdiamide Urea ium Salt Carbonate ubles centages
84.1 59.7 78.6 31.4
3.2 15.3 5.5 56.5
5.1 8.7 6.8
2.4
I
figure 5 Effect o f Concsnfrafion Bo
Expt.7-Effeot of the Particular Ammonium Salt Used
,
90
I
I
I
Sum of Percentages
8.8 97.8 a Some of the urea originally present was converted to other forms.
Expt. Chloride 1.4 3.2 Sulfate 0.0 9.0 Nitrate 0.0 6.4 Phosphatea 0.0 0.2 a Diammonium phosphate.
The results are shown in Table IV. No explanation is offered for the discrepancy in the results of the two experiments, as the autoclave charges were portions of the same solution, and the conditions as to heating were in the main identical. That the discrepancy exceeds any possible error of analysis is shown by the fact that guanidine, dicyanodiamide, and ammonia analyses agree in showing that the reaction is more complete in one case than in the other.
96.0 92.7 97.3 90.5
The free ammonia formed by hydrolysis in the case of the phosphate was estimated by comparison of the directly de-
30.
B
3,
F
9
Durafion o f Hesfmg 3 Hours TemDersfure o f Heafinu /55“C
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
852
DISCUSSION OF RESULTS The three important primary reactions that take place on heating aqueous solutions of cyanamide and an ammonium salt are: (1) Ammonation of cyanamide to guanidinium salt HzNCN N H Z = HzNC(:NH)NHsX (2) Hydration of cyanamide t o urea HzNCN HzO = HzNCONHz (3) Polymerization of cyanamide to dicyanodiamide 2HzNCN = HzNC( :NH)NHCN
+ +
These three-reactions may be thought of as competing for the cyanamide in such systems. Reactions 1 and 3 are highly exothermic and account for the great evolution of heat as shown in Fig. 3. Reaction 1 is probably independent of the titer of the solution, but Reaction 2 is greatly catalyzed by either hydrogen or hydroxyl ions. Reaction 3 is also catalyzed, by alkalinity only, however. Important secondary reactions are: (4) Hydrolysis of urea t o ammonium carbonate HzNCONHz 2Hz0 = (NH4)zCOs ( 5 ) Ammonation of dicyanodiamide, first to biguanidinium salt HzNC( :NH)NHCN N H Z = HzNC( :NH)NHC(:NH)NHPX (6) Which may be then further ammonated H2NC( :NH)NHC(:NH)NHsX 4-N H Z = 2HzNC( :NH)NHsX
+
+
Other reactions of less importance represent the formation of ammelide, which, however, takes place only in dilute solutions or solutions containing little or no ammonium salt. It may perhaps be written, in the absence of further information :
+
(7) 3CHzNz 2Hz0 = CaH40zN4 f 2"s (8) And the reaction which results in the formation of the insolubles. The latter reaction, of course, cannot be written in detail on the basis of the present information as to the nature of this material.
SUMMARY OF ESSENTIAL FEATURES OF METHOD
1-A neutral cyanamide solution is first prepared and then concentrated to about 165grams per liter. As shown by Expt. 5, an increased yield of guanidinium salt is obtained by concentrating the cyanamide solution, contrary to the expectation that polymerization would be increased. 2-A 100 per cent excess of ammonium salt was shown to be necessary, This increases tremendously the proportionate extent of guanidinium salt formation a t the expense of dicyanodiamide. Unfortunately, however, urea formation is also promoted. 3-The solutions should be slowly heated up to 155' C. A higher temperature favors hydrolysis of all the compounds present to ammonium carbonate, as shown by the results of Expt. 2, A lower temperature, however, seems to favor the formation of dicyanodiamide, thus reducing the yield of guanidinium salt. The preliminary experiments had shown that a t temperatures as low as 100' C. no guanidinium salt is formed, while polymerization of cyanamide takes place to a considerable extent. The same effect, in less degree, is also shown by the results of Expt. 2. The undesirable effects of too high temperatures are also encountered if the autoclave is heated too rapidly a t least in the absence oi an extremely efficient cooling system capable of carrying off the large quantities of heat suddenly liberated by the ammonation and polymerization of cyanamide. 4-Under the foregoing conditions the proper duration of heating appears to be 3 hours, in spite of the fact that 90 per cent of the guanidinium salt is formed within the first few minutes by direct ammonation of cyanamide. Although plant practice may make a shorter period advisable, the 3-
Vol. 16, No. 8
hour period seemed desirable, not only because some of the remaining 10 per cent of the cyanamide has in the same time gone to form dicyanodiamide and biguanidinium salt, requiring the remaining time for their transformation to guanidinium salt, but, as is pointed out below, the absence of dicyanodiamide greatly increases the ease of isolating the pure guanidinium salt. Under these conditions the conversion of the small quantities of dicyanodiamide to guanidinium salt seems complete; if however, larger quantities of dicyanodiamide are formed, as in the case of a deficiency of ammonium salt, or when dicyanodiamide rather than cyanamide is used as the starting material (Expt. 8), the conversion to guanidinium salt is incomplete. &The recovery of the guanidinium salt in the case of the nitrate is quite simple. The autoclaved solution is filtered hot, concentrated, and cooled. An experiment in this laboratory, using a typical solution of guanidinium nitrate as obtained by the present processl showed that in the absence of dicyanodiamide 80 per cent of the guanidinium nitrate could be obtained with a purity, as determined by the picrate method, of more than 99 per cent by a single crystallization. This is impossible when dicyanodiamide is present, but, as has been repeatedly stated, its complete absence may be assured by adherence to the foregoing conditions. SOLUBILITY OF GUANIDINIUM NITRATE I n connection with the recovery of guanidinium nitrate, a study was made of its solubility in water, alcohol, and acetone at 0', 25 ', and 50' C. Equilibrium between salt and solution was in each case attained by agitation a t these temperatures for 24 hours. Weighed amounts of the saturated solutions were evaporated to dryness (constant weight) in an electric oven a t 100" C. The results of the experiments are given in Table V. These results are in good agreement with a single determination made by Thiele,I3who found that 100 cc. water dissolved 10.79 grams guanidine nitrate at 16' C. The absolute ethyl alcohol (scientific) used was stated by the manufacturers to contain not more than 0.1 per cent water. The acetone was a chemically pure product. It was found that evaporation of 22.4076 grams of this acetone left 0.0047 gram of oily residue. A correction was applied on this basis to the solubility determinations in this solvent. Temgerature
C. 0 25 50
18
TABLE V GUANIDINIUM NITRATE GRAMS-7 $er 100 Grams Per 100 Grams' Per 100 Grams Water Solution Alcohol Solution Acetone Solution 4.43 14.07 29.20
0.85 1.62 3.28
0.677 0.671
..
Ann., 270, 76 (1892).
Calendar of Meetings British Association for the Advancement of Science-Toronto, Canada, August 6 to 13, 1924. American Chemical Society-68th Meeting, Ithaca, N. Y., September 8 to 13, 1924. The Franklin Institute-Centenary Celebration, Philadelphia, Pa., September 17 to 19, 1924. American Mining Congress-Sacramento, Calif ., September 29 t o October 4, 1924. American Electrochemical Society-Detroit, Mich., October 2 to 4, 1924. American Association for Advancement of Science-Washington, D. C., December 29, 1924, t o January 3, 1925. National Chemical Equipment Exposition and American Institute of Chemical Engineers-Providence, R. I., June 22 to 27, 1925.
