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C. E, REDEMANN, F. C. RIESENFELD, and F. S. LA VIOLA
Research Division, The Fluor Corp., Ltd., Whittier, Calif.
Formation of Biuret from Urea
WHEN
urea is heated to temperatures near, or above, its melting point, ammonia is slowly evolved and several different substances are gradually formed. These so-called side-reaction products are generally produced in very small amounts. If the temperature range is limited to slightly above the normal melting point of pure urea (132’ C.) the principal side-reaction product is biuret. This is not a new compound for Wiedemanri (18) first prepared it by heating urea nitrate to 150’ to 170’ C. The only interest in biuret for many years was in the chelates which it forms in alkaline solution with both copper and nickel ions. Because of their strong colors these compounds are considered for possible analytical reagents. The formation of the colored copper chelate of biuret is the basis of the “biuret test” for proteins. Interest in biuret was renewed when Sanford and coworkers (75) described the injurious effects produced when a foliar spray prepared from a commercial prilled urea was applied to young pineapple plants. These workers conclusively showed that the injury to the pineapple plants was caused by biuret present in the urea as an impurity. Jones ( 8 ) found that when urea containing a n impurity of biuret was applied
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as a foliar spray on citrus trees an apical chlorosis developed. This chlorosis was named “yellow tip” from the characteristic yellow tip produced in the citrus leaf. Later studies by Jones and coworkers ( 9 ) indicated that maximum amount of biuret permissible in urea is 0.25% for foliar application on citrus trees; for application to soil around citrus trees, u p to 2.5% biuret in the urea does not appear to produce any significant amount of damage. Permissible limits for biuret in urea have not been established for other species of plants; however, some species of plants are relatively insensitive to biuret damage. Biuret also has some potentially valuable applications such as
1. A preplanting fertilizer. When first applied it kills weed seeds, but after about 6 to 8 weeks it loses it phytotoxicity, permitting the planting of the intended crop i n a weed-free environment (3) 2. A constituent in plastics of the urea-formaldehyde type (70, 77) 3. Use in lubricating oil additives (7) 4. A blowing agent in making foamed rubber and plastics (73, 76) 5, Raw material for melamine manufacture (2, 74) 6, The basis of certain hypnotics and sedatives (7)
This study is an excellent contribution in a field in which there is little published information.
The presence of biuret in
commercial ureas has delayed the use of urea foliage sprays on crop plants sensitive to biuret for several years.
The
information contained in this article should be useful in the ~
manufacture ’of urea of low biuret content.
I
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H,N-C-NH,0
H No~ rC - N H ~
Urea lke t o - form)
(enol- form)
Ureo
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t
-“3
t Biuret
Tri ure t
Biuret
OIH ZC..
“
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HO/C‘~$C‘~H
C y o n u r i c Acid
Biuret and several additional products may be produced from the decomposition of urea as shown by the above scheme. Although these various individual reactions take place separately it has not been established conclusively that this is the mechanism operating in urea melts. Several additional products are possible either through further interaction of cyanic acid with the indicated substances, or by elimination of water in place of ammonia. These substances include tetrauret, melamine, ammeline, ammelide, and various unidentified polymeric materials. The actual reaction conditions employed determine which of these side reactions may take place. This complexity of competing and consecutive reactions in the urea system makes a true kinetic interpretation of reaction rate data for this system unprofitable. The quantitative data on the rate of the biuret formation from urea by Nukada (77) are far from adequate for understanding the conditions governing biuret formation in urea under plant operating conditions. Some conditions VOL. 50, NO. 4
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-0
4.0
P c 0
Y
3.0
3 r
0
.", c
2.0
0 .e 0
5
1.0
c Q) L
a .-
a
Time, Min.
Figure 1. Biuret formation in d r y urea a t 140" (no agitation)
favorable to forming biuret from urea, given by Olin (72), Harmon ( 6 ) , and Carbo (5), are:
1. Removal of the ammonia as rapidly as it is formed 2. An operating temperature range not too far above the melting point of urea T h e application of vacuo or the addition of some inert solvent with the boiling point in the appropriate operating range may be used to strip ammonia from the melt as rapidly as it is produced. However, no actual rate data are given. Several score samples of urea, both pure and fertilizer grade, domestic and imported, have been assayed for their biuret content. I n general, the fertilizer grades contain much higher biuret content for two reasons. First, purifi-
130
170
150 Temp.
OC.
Figure 2. Biuret formation in dry urea vs. temperature (per hour)
634
C. and
atmospheric pressure
cation by crystallization from a solvent is usually omitted on this grade of urea. Second, fertilizer grade urea is generally offered in a shotted or prilled form, produced by rapidly cooling small spherical droplets of a very concentrated urea solution approaching molten urea in composition. The higher temperatures needed to prepare this urea solution for the prilling operation, result in an increase in biuret content. The biuret content of prilled fertilizer-grade urea samples ranges from 0.96 to 8.147,. One fertilizer grade of urea, offered as a conditioned crystalline material, had a n exceptionally low biuret content of 0.0917,> which is well below that found in all samples of reagent grade urea. Most commercial crystalline ureas had biuret contents of 0.2 to 0.9% by weight. However, several samples had lower biuret contents, and two samples contained 1.3% or more biuret. These figures indicate that low biuret content can be obtained by crystallizing urea. According to agriculturalists, crystalline urea is undesirable for it is more costly to produce and the crystals tend to agglomerate into solid cakes upon storage, Because of this the farmer cannot apply this material with conventional fertilizer distributing equipment. T h e query is frequently made-Where is the biuret formed? The correct answer is-all through the process of manufacture. The greatest amount of biuret formed is a t the point where excess ammonia and water are removed to give essentially a urea melt. Because of the low ammonia and water content and the high temperature, conditions are most favorable for biuret formation a t this point. Experimental
Materials. The urea used was of analytical grade (obtained from J. T.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Baker or Mallinckrodt) and contained from 0.098 to 0.175% biuret. All results were corrected for the amount of biuret initially present in the sample of urea used rather than to prepare a biuret-free urea test control. The biuret used as a n analytical standard in the colorimetric method for assaying the urea samples was a commercially pure product (obtained from Mathieson Chemical Co.) which \$as twice recrystallized from boiling methanol and then dried at 110' C. before use. Determination of its water content by the Karl Fischer method showed that the biuret was anhydrous. Its nitrogen content was determined by the Kjeldahl method. All chemicals used were reagent purity. Analytical Methods. The biuret content of all samples was determined by a modified method of Ellis and Formaini ( 4 ) . This modification used a higher total copper concentration which allowed the absorption of light by the copper-biuret complex to be linear over a lvider concentration range. Also, a single alkaline copper tartrate solution was used in place of the two solution procedure. This modified method has advantages over the original procedure. Preparation of Biuret Reagent. Twenty-five grams of copper sulfate pentahydrate and 45 grams of tartaric acid were dissolved in 300 ml. of water and cooled in an ice-water bath. A second solution was prepared by dissolving 120 grams of sodium hydroxide in 500 ml. of water and then cooled. These two solutions were mixed slowly in an ice-water bath and stirred vigorously. The resulting deep blue solution was again cooled to room temperature and diluted to 1 liter. This solution keeps well; but on prolonged standing a fine sediment sometimes separates and the reagent should be discarded as scattering of light by these fine particles ma)- give high results. The absorbance of this reagent (one volume of reagent to five volumes of final solution) was measured on a spectrophotometer a t 550-mw wave length against a reagent blank. Procedure, Fiveto IO-gram samples of urea were weighed into 150 X 15 mm. borosilicate glass test tubes. These samples were placed in a preheated thermostatically controlled oil bath regulated to the desired operating temperature. In most runs the molten urea was kept in a quiescent state as any stirring changed the surface area in contact with air and thus altered the rate of ammonia removal. Stirred melts lead to erratic and nonreproducible results. Results from static melts were reproducible as the amount of liquid surface was kept constant in contact with the air. Time was counted
BIURET F O R M A T I O N F R O M UREA from the moment the sample was introduced into the bath until it was removed. The sample was then cooled in a cold water bath (20' C.). The time required to melt the sample was assumed to be approximately equal to the time necessary to cool the sample and thus no correction was made for either of these periods. The cooled sample was dissolved in water and diluted to a known volume, usually 100 ml. Aliquot portions of this solution were analyzed for biuret content. From these data the weight of biuret in the total sample was computed and expressed as per cent weight of the original urea sample.
1
2
4
Discussion of Results Figure 1 shows the amount of biuret produced by heating an unstirred melt of pure urea in air a t 140' C. for increasing periods of time. An unstirred melt is specified as the actual rate of biuret formation is influenced by the removal of ammonia resulting from the stirring operation. The curve in Figure 1 is almost linear over this short time intervai and its mean slope is 0.055% biuret per minute. The effect of temperature on rate of biuret formation in an unstirred melt of pure urea, maintained a t the indicated temperature for 60 minutes, is shown in Figure 2. The mean temperature coefficient over the range of 140' to 150' C. is 1.95; for the temperature range of 160' to 170' C., it dropped to 1.43. This change in temperature coefficient is probably due to the large portion of urea converted to biuret a t the higher temperature interval in the 1-hour period. A shorter time interval is probably more desirable. However, these data indicate the importance of temperature as a factor in biuret formation. Figure 3 shows the effect of adding a small amount (570 by wt.) of water to urea melts a t 140" C. Low percentages of water have little effect on the initial rate of biuret formation; after several hours the two curves separate, and after 7 hours only two thirds as much biuret was formed in the presence of water as was produced in the control run. This phenomenon is probably related to the side reactions produced in the two cases. A comparison of the rate of biuret formation in urea melts a t 140' C, when in contact with air and/or in a n ammonia atmosphere of 1 atm. is shown in Figure 4. Under these conditions the biuret formation is only about 60% as great in the presence of ammonia as in air. The effect of changing the partial pressure of ammonia above a melt of pure urea a t 140' C. is shown in Figure
t
6
Ti me, hou rs Figure 3.
A
Biuret formation at.140' C.
Pure dry urea a t 140' C. Pure urea plus 5% water a t
140' C.
-
0 4l W
c 0 V
0
z
6
a
c
e o
.m
Time, Hours
Figure 4. Effect of ammonia on biuret formation in dry urea at 140" C. and atmospheric pressure (ammonia bubbled through melt)
A 0
20
Urea' in air (unstirred) Urea in ammonia atmosphere
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0
: 12 -
3 r
0
z W
E A 13-85 mi. Nb/Mln.
LL L
2
.-m
01
I
I
0.5
1.0
30 NH3/Min.
1.5
NH3 Portio1 Pressure Atmosphsrrs
Figure 5. Effect of partial pressure a t 140" C.
of ammonia on biuret formation in dry urea
One-hour reaction time (ammonia and nitrogen bubbled through melt). at 85 ml. ammonia per minute indicate equilibrium maintained
A.
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Figure 6. Biuret formation under simulated crystallizer conditions
06
n m
A.
100' C.; pressure
E.
100' C.;
2 6 6 mm. at Hg.
266 mm. at Hg. pressure; IC1 urea (contains 1 .O biuret initially) C. 80' C.; 136 mm. at Hg. pressure D. 50' C.; 61 mm. at Hg. pressure Reagent grade urea, containing 0.2% biuret initially, in all runs except E . All points corrected far initial biuret content of urea
04 LL 0 c
E
2 m
8 02
0 Time, hours
from pure urea a t 80" C., curve C, is only 0.04c/,per hour, and a t 50" C., curve D , only O.OOS%l, per hour. Figure 7 shows the results of maintaining two 82% urea solutions-one containing initially O.3yGfree ammonia and the other 1.0% free ammonia, a t 80" C. for long periods of time. Under these conditions the biuret formed is 0.11%8in 1000 hours for the 0.370 initial ammonia content, and 0.082% for the 1.O% initial ammonia content solution. The hydrolysis of urea was much more significant than was the formation of biuret under these conditions. During the 1000 hours, 41.5 and 40% of the urea hydrolyzed in the 0.3y0 and 1.0% initial ammonia content solutions. The rapid initial rate of biuret formation in these two solutions has not been satisfactorily explained. Acknowledgment
5. Partial pressures of ammonia were
ous solutions under much lower temperatures (Figure 6). In each case the solution was saturated with urea at the temperature indicated. Water and ammonia were slowly removed by distillation a t the equilibrium partial pressure during the run. Only the minimum amount of water was removed. At 100" C., curve A , the mean rate of biuret formation is 0.115% per hour when using reagent grade urea. However, a commercial urea (obtained from Imperial Chemical Industries) submitted to the same test conditions gave a lower biuret formation, curve Bnamely, 0.10070 per hour. This decrease is due to impurities present in the commercial urea. Biuret formation
varied by mixing gas streams of ammonia and nitrogen in suitable ratios. When pure nitrogen was used the ammonia partial pressure was zero and consequently the amount of biuret formed was a function of the rate of gas flow, as nitrogen separated the ammonia from the melt. At a nitrogen rate of 30 ml. per minute 10.8% biuret was produced in 1 hour; a t 60 ml. per minute, about 16%. These results are in agreement with those in Figure 4. The results shown up to now have all indicated the amount of biuret produced in urea melts at 140" C., slightly above the melting point, 132" C., of pure urea. However, biuret can be formed in aque-
The authors wish to thank Gene Alley and Robert Evans for assisting with the numerous analytical determinations necessary in this study. The authors also thank The Fluor Corp., Ltd., for permission to p u b k h this material. Literature Cited (1) Adelson, D. E., Larson, R. G. (to Shell Development Co.), U. S . Patents 2,599,736 and 2,599,737 (June 10, 1952). (2) American Cyanamid Co., Brit. Patent 598,175 (Feb. 12, 1948). (3) DeFrance, J. A., Bell, R. S., Odlund, T. E., J . Am. Soc. Agron. 39, 530 (1947). (4) Ellis, G. C., Formaini, R. L., J . Agr. Food Chem. 3, 615 (1955). (5) Garbo, P. W. (not assigned), U. S. Patent 2,524,049 (Oct. 3, 1950). (6) Harmon, J. (to E. I. du Pont de Nemours & Co.), Ibid., 2,145,392 (Jan. 31,1939). (7) Hill, A. J., Degnan, W. M. (to American Cyanamid Co.), Ibid., 2,379,486 (July 3, 1945). (8) Jones, M'. W.! Science 120, 499 (1954). 19'1 Jones. W. W..Embleton. T. W.. Goidall, G. ' E . , Citrus 'Leaoes 35 (11 1 , 1 2 (1 (10) McLeod,-E. \
,
&
co.
(July ,, ,,. ., J . Pharm. Chem., Japan (11) Nukada 23.39 (12) Olin; J. F. (to' Sharples Chemicals, Inc.), U. S. Patent 2,370,065 (Feb. 20, 1945). (13) O'Neal, G. M. (to Sherwin-Williams Co.), Ibid., 2,668,152 (Feb. 2,
Time, hours
Figure 7. Biuret formation at 80" C. in of ammonia
82y0 urea solutions with different amounts
Urea hydrolysis
A. E.
In 0.3% NHBsolution In 0.1 % NHI solution
Biuret formation
C.
D.
636
In 0.3% NH3 solution In 1 .O% NH3 solution
INDUSTRIAL AND ENGINEERING CHEMISTRY
1954). (14) Padsen, J. H., klackay, J. S. (to American Cyanamid Co.), Ibid., 2,658,892 (Nov. 10, 1953). (15) Sanford, W. G., Cowing, D. P., Young, H. Y . , Leeper, R. W., Science 120,349 (1954). (16) Schwarz, H. F.; India Rubber World 114, 211 (1946). (17) Simons, J. K., Weaver, W. I. (to Libbey-Owens-Ford Glass C o . ) , U . S. Patent 2,378,110 (June 12, . . 1945). (18) Wiedemann, G., Ann. Chem. 68, 323 (1848).
RECEIVED for review June 12>1957 ACCEPTEDAugust 13: 1957
