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as to prevent the alkali in the distilling flask from being carried over mechanically into the distillate. The Davisson scrubber has been found satisfactory for this purpose. The zinc-iron method invariably gives results too high in mixed fertilizers containing organic matter. The reduced iron method has a tendency to give high re-
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sults, but with proper precautions is much more satisfactory than the zinc-iron method. When nitrates are present the Devarda alloy method has proved more satisfactory than either of the official methods. I n all cases where nitrates are not present the magnesium oxide method should be employed.
The Determination of Dicyanodiamide' By C. D. Garby FIXED NITROGEN RESEARCHLABORATORY. WASHINGTON, D. C.
ICYANODIAMIDE is one of the important deriva- cyanodiamide and sufficiently insoluble. It was noted, tives of cyanamide. Interest in a suitable method for however, that guanylurea often formed precipitates more its determination is occasioned, not only by the in- readily than did dicyanodiamide, and that in some cases the creasing importance of dicyanodiamide for the syntheses of precipitation appeared to be quite complete. various chemicals, but also in connection with its occurrence The insoluble character of nickel guanylurea was observed , ~ a method for the determinain fertilizers containing calcium cyanamide in which it is by Grosman and S ~ h u c kand objectionable. tion of nickel based on this fact was developed by them and I n the work a t this laboratory there has been occasion to their co-workers. The reverse of this procedure-i. e., the determination of guanylurea by predetermine dicvanodiamide. not onlv in crude calcium cyanamide and in cipitation as a nickel salt-was Several methods for the determination of s u g g e s t e d by von Dafert and some of the simpler fertilizer mixdicyanodiamide, particularly in connection iLliklauzs as a method for the d e tures containing it, but also in mixwith the use of calcium cyanamide as a tures that have in some cases cont e r m i n a t i o n of dicyanodiamide. fertilizer, have been suggested, but have Their method, in brief, consisted tained urea, (dicyanobeen found unsuitable. After a careful in evaporating a nitric acid soludiamirjine), guanidine, biguanide, study of the various suggested methods the amidodicyanic acid, and melamine. thus of hydrolyzing tion the sample to the dryness dicyanotwice, nickel guanylurea method was developed. The quantitative determination of The method as described in this paper has diamide to guanylurea, and then dicyanodiamide in the presence Of been in use in the Fixed Nitrogen Research precipitating the latter compound these compounds is rendered particLaboratory for the past two years, and in from a solution of the residue b y 'larly because Of the 'lose all cases has given far better results than means of nickel nitrate in a strongly chemical relationship of many of any of the other methods. alkaline solution. The reactions them to dicyanodiamide. in these two steps of the procedure Several methods have been suggested for the determination of dicyanodiamide, particularly are expressed by the equations: in connection with the use of calcium cyanamide as a fertilizer. NHzC(NH)NHCN HOH HNOs + NH*C(NH)NHCOTheir unsuitability, even for relatively simple mixtures, has NHz.HNOs already been pointed out by Harger2 and others, and the 2C2HsN40.HNOsf Ni(NO& f 4NaOH + Ni(CZHjN10)2.2H20 4NaNOs 2H10 method devised by Harger has likewise been found to have serious limitation^.^ The more recent method for dicyano- These authors stated that the precipitation of nickel guanyldiamide suggested by Johnson' is subject to a very large urea was not affected by the presence of ammonium salts, number of corrections and is limited in its application. After guanidine salts, and urea, even when the latter was present a careful study of the various suggested methods, the de- in quantities ten times greater than that of dicyanodiamide, velopment of a more suitable and a more generally applic- but that cyanamide interfered on account of its conversion able method was undertaken. to guanylurea on hydrolysis. Their method was carefully investigated. Preliminary Experiments The method was first tried out on various sized samples Attempts were first made to devise a simple and direct of pure dicyanodiamide, following closely their procedure, method for the determination of dicyanodiamide, such as except that the precipitate of nickel guanylurea was weighed direct precipitation by a heavy metal. I n this connection directly, after drying, instead of being ignited to the oxide and solutions of salts of various metals-copper, gold, cadmium, then weighed. This modification was made since it was found thallium, antimony, bismuth, chromium, gadolinium, ura- that consistently high and somewhat variable results were nium, iron, cobalt, and nickel-were added to both acid and obtained if the precipitates were ignited to the oxide, owing alkaline solutions of dicyanodiamide and also to solutions probably to partial oxidation of the nickel oxide (NiO). of cyanamide, urea, guanylurea nitrate, guanidine nitrate, Satisfactory results were not obtained in any of the experiand amidodicyanic acid. Mannitol was used to prevent the ments; the precipitates in twenty-five determinations correprecipitation of metal hydroxides in alkaline solutions. No sponded to 91.0-97.2 per cent of the dicyanodiamide acprecipitates were obtained which were both specific for di- tually used. It is probable that by igniting the precipitate to the oxide von Dafert and Miklauz obtained results agree-
D
+
+
1
+
+
Received September 24, 1924.
* THISJOURNAL, ia, 1107 (1920). a A m . FerfiZizcr. S4. 49 (1921): THISTOIJRNAL.. 14.. 143 (1922). . 4 THIS~ o u R N i t ,is, 533 i i g z i ) .
Bcr., 39, 3356 (1806). Z . landw. Versuchsw. Daulschosterr. 22, 1 (1919); J . Sac. Chem. I n d . , 88, 837 (1919); Chcm. Zcntr., 90, 109 (1919). 6
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March, 1926
ing more closely with the theoretical, owing perhaps, as suggested above, to partial oxidation of the oxide. Experiments were then performed in which the procedure was varied as to quantity of acid used, number of times the sample was evaporated to dryness, quantity of nickel reagent, and temperature of precipitation. The results of these experiments also were unsatisfactory and clearly showed that the conditions for carrying out the hydrolysis and precipitation would have to be -re definitely established. It was evident, too, that the following conditions would have to be fulfilled if the method were to be satisfactory as to accuracy: (1) quantitative hydrolysis of dicyanodiamide to guanylurea, avoiding further hydrolysis to guanidine, ammonia, and carbon dioxide, (2) complete precipitation of guanylurea as the nickel salt, and (3) absence of compounds that would be hydrolyzed to guanylurea or precipitated with it. A study of each of these three conditions was therefore made. Hydrolysis of Dicyanodiamide to Guanylurea
The effect of acid concentration on the hydrolysis of dicyanodiamide was first determined. I n each case 0.1-gram samples of pure dicyanodiamide were taken to dryness on a steam bath twice with successive additions of 20 cc. of nitric acid. From 1.5 to 2 hours were required for the two evaporations. The results are shown in Table I. Table I-Effect
of Acid Concentration on Hydrolysis of
Dicyanodiamide Average per cent Concen- dicyanodiamide obtained Concentration as guanylurea tration of HNOa (3 analyses) of HNOs 5 A’ 80.0 N/4 In86.1 N/5 ‘V/2 87.0 N/6 ,v/3 90.6
Average per cent dicyanodiamide obtained as guanylurea (3 analyses: 96.4 96 0 95.9
The guanylurea formed was precipitated in accordance with the procedure given by von Dafert and Miklauz. This procedure does not give satisfactory results, as was later found, but the foregoing hydrolysis data are nevertheless quite comparable and serve to show the necessity of using dilute acid. From these results it appears that an acid concentration of about N/4 is the optimum. Concentrations much greater than this result in a partial hydrolysis of guanylurea, and hence lead to low results. The effect of time of digestion (evaporation) was next studied. I n these experiments 0.1-gram samples of dicyanodiamide were treated with 20 ce. N / 4 acid and evaporated to dryness only once. The rate of evaporation can easily be controlled by adjustment of watch glasses over the containers. Table 11-Effect Duration of digestion Hours 1.5
1.75 2
of Duration of Digestion on Hydrolysis of Dicyanod i a m i d e b y N/4 Nitric Acid Per cent Per cent dicyanodiamide Duration of dicyanodiamide obtained as digestion obtained as guanylurea Hours guanylurea 97.7 2.5 97.8 98.8 3 93.8 99.0 4 91.8
These results are comparable, though actually somewhat high owing to a deficiency of ammonium hydroxide in the solution, and show that under the conditions described the best results are obtained by taking the solution to dryness in from 1.75 to 2.25 hours. A loqger period of digestion results in the decomposition of guanylurea. Precipitation of Guanylurea
The optimum conditions for hydrolysis applicable t o analytical procedure having been quite definitely established, an investigation to determine the conditions for most complete precipitation was then made. With 0.0100-gram
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samples of guanylurea phosphate and following the precipitation procedure described by von Dafert and Miklauz, only about 60 per cent of the theoretical weight of nickel guanylurea was obtained. Increasing the quantity of sodium hydroxide had but little effect except to lengthen the time required for precipitation, and the addition of various salts and organic compounds to the solution also failed materially to reduce the solubility of nickel guanylurea. The expedient of saturating the precipitating medium with the nickel guanylurea was therefore adopted. This procedure is particularly applicable to the dicyanodiamide determination by the guanylurea method, since the residue after hydrolysis can be taken up with a solution saturated with nickel guanylurea. There is no salting-out effect due to urea, ammonium salts, etc., which might be present. It was found that the solubility of nickel guanylurea in the precipitating solution is practically the same as in a 2 per cent ammonia solution-about 10 mg. in 25 cc. a t room temperature. A 2 per cent ammonia solution saturated with nickel guanylurea a t room temperature was therefore used to dissolve the residues after hydrolysis. The concentration of ammonium hydroxide in the solution from which nickel guanylurea is precipitated is very important. Too low a concentration results in a precipitate that contains both nickel guanylurea and nickel hydroxide, even in the presence of mannitol, while too high a concentration gives low results due to the solubility of the precipitate. To avoid a loss of ammonia from the solution during precipitation, closed containers are necessary. The presence of ammonium nitrate in the solution is advantageous and may be present up to about 10 per cent by weight of solution without increasing the solubility of nickel guanylurea. To insure a proper concentration of ammonium nitrate, this salt is added in definite amounts to the nickel reagent. The necessity of saturating the reagents with nickel guanylurea and the accuracy obtainable using the modified conditions of precipitation are seen from Table 111. The conditions of precipitation are described later. Table 111-Completeness of Precipitation of Nickel Guanylurea N o NICKELGUANYLUREA ADDEDTO REAGENTS NICKEL GUANYLUREA ADDED Grams guanylurea phosphate Grams guanylurea phosphate Used Found Used Found 0.0050 0.0012 0.0050 0.0053 0.0078 0.0100 0,0099 0.0100 0.0500 0.0484 0.0500 0.0503 0.0986 0.1000 0.0998 0.1000 0.1974 0 2000 0.2000 0.2002
Influence of Various Compounds on Determination of Dicyanodiamide
A study was made of the effect of the presence of substances such as are frequently associated or obtained with dicyanodiamide on the determination of that compound. Contrary to the statement of von Dafert and Miklauz, it was found that cyanamide does not interfere with the determination. It is quantitatively hydrolyzed to urea during the hydrolysis of dicyanodiamide while the acid solution is still dilute, and is later further hydrolyzed forming ammonia and carbon dioxide. Urea, guanidine salts, melamine, amidodicyanic acid, and sodium nitrate do not interfere. It was found that when biuret was present care must be taken to avoid an excess of sodium hydroxide, since otherwise complete precipitation of nickel guanylurea will not occur. If guanylurea is present, it must be removed before hydrolysis of dicyanodiamide. Biguanide is precipitated as nickel biguanide under the conditions for nickel guanylurea, and hence it must also be eliminated. If the sample is in solution, both guanylurea and biguanide can be eliminated by precipitation as nickel salts as described under ( d ) in “Procedure.” If the sample is dry, it can be extracted
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with absolute acetone in which dicyanodiamide is soluble but in which most salts of guanylurea and biguanide are insoluble.
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ness, and then the oil dissolved out by absolute ether, in which dicyanodiamide is extremely insoluble. The ether solution can be decanted directly from the residue. If the oil were not removed it would be impossible to obtain quantiGuanylurea Method tative hydrolysis of the dicyanodiamide. REAGENTS(c) Cyanamide-Acid Phosphate Mixtures. Mixtures of saturated with (1) Two per cent ammonia solution ("8) Cyanamid and acid phosphate prepared under certain connickel guanylurea (75 cc. concentrated NHpOH per liter). (This solution should be prepared fresh every ditions contain dicyanodiamide, urea, and guanylurea. The procedure for such mixtures is the same as in (b), since urea 6 months.) (2) One hundred grams of mannitol made up t o 1000 cc. does not interfere and guanylurea is eliminated by extracting with Solution 1. (3) Nickel nitrate 40 grams; ammonium nitrate 20 grams; the sample with acetone. The guanylurea salts, sulfate or phosphate, which occur in such mixtures are practically 100 cc. Solution 1 (filter). insoluble in acetone. (100 cc. distilled acetone dissolve (4) Twenty per cent sodium 'hydroxide solution. ( 5 ) Dilute ammonium hydroxide (5 cc. concentrated ",OH 0.0009 gram guanylurea sulfate and 0.0020 gram guanylurea per liter). phosphate at 27" C.) ( d ) Solutions Containing Dicyamdiamide, Guanylurea, PRocEDURE-(a) Technical Dicyanodiamide. For the determination of dicyanodiamide in the technical product, and Biguanide. It is sometimes necessary to determine diwhich usually contains no guanylurea, the procedure is as cyanodiamide in aqueous solutions. Since guanylurea and follows: A 0.1-gram sample is treated with 20 cc. of N / 4 biguanide are the only two interfering compounds that have nitric acid and taken to dryness on a steam bath in from 1.75 been noted, the procedure for the determination of dicyanoto 2.25 hours (until nitric acid fumes are no longer evolved). diamide in their presence will be described. Although these The residue is dissolved in about 40 cc. of Solution 2, about two compounds can be removed from the solution under the 3 cc. of Solution 3, and sufficient sodium hydroxide Solution same conditions as in the regular precipitation of guanylurea, 4 is added drop by drop to produce a greenish yellow color in it is necessary to employ smaller quantities of reagents to avoid obtaining a sirupy mass on evaporation, which would the solution. The solution turns green if dicyanodiamide is absent, and greenish yellow if dicyanodiamide is present in make quantitative hydrolysis impossible. The strong alkaline solution also has a slight action on dicyanodiamide on large amounts. Usually about 2 to 4 cc. of sodium hydroxide long standing. The following procedure must be adhered to solution are required. The solution must be covered to prevent the loss of ammonia which would result in a partial closely to obtain satisfactory results: To remove the guanylurea and biguanide contained in the precipitation of nickel hydroxide, as well as some of the nickel sample 25 cc. of the solution, preferably containing from guanylurea used to saturate the solution. A large, wideform, glass-stoppered weighing bottle is convenient for use 0.1 to 0.4 gram dicyanodiamide and the guanylurea and biguanide salts, are placed in a 50-cc. calibrated flask. Threeas a precipitating vessel. The solution is allowed to stand tenths of a gram of mannitol (3 cc. of Solution 2) and 2.5 to overnight. The greater the excess of sodium hydroxide the 3 cc. of concentrated ammonium hydroxide are added and longer is the time required for complete precipitation. The then 1 to 3 cc. of Solution 3 and sufficient sodium hydroxide solution, which should have a green color, is filtered through a weighed Gooch crucible and washed with about (Solution 4 ) to produce the color change previously described. solution is stoppered and allowed to stand for from 2 to 100 cc. of Solution 5. The Gooch crucible is dried at about 3The hours. It is then made up to volume, fibered, and a 25-cc. 125" C. for about 1hour, thus eliminating the water of crystallization, and the precipitate weighed as Ni(C~NJI~O)n.aliquot is acidified with nitric acid so that the resulting solution is N / 4 . The procedure from this point on is carried out The conversion factor to dicyanodiamide is 0.645. A blank as described under (a). The error introduced by neglecting should be run on the reagents. the volume of precipitate in making the solution up to volume (b) Commercial Calcium Cyanamide. A sample, prefer- is ordinarily quite negligible. In the case of very large preably of such size that the aliquot used will contain 0.05 to cipitates the solution may be filtered, the precipitate washed, 0.2 gram of dicyanodiamide, is extracted with acetone and the combined filtrate and washings made up to a definite (amount of dicyanodiamide soluble in 100 cc. of acetone at volume. An aliquot of the solution is then used for the de26.1" C., 0.7900 gram) in a shaking machine for 2 to 3 hours, the solution filtered, and an aliquot taken to dryness on a termination. The precipitate of nickel biguanide and nickel guanylurea steam bath. The procedure from this point on is the same is contaminated with nickel hydroxide on account of the smalas in (a). If it is necessary to use a water extraction of cal- ler amounts of ammonium hydroxide and mannitol used, but cium cyanamide, the following precautions should be obthis is of no importance because the combined precipitates served: Calcium is always present in water extracts of cal- are filtered out. Under the conditions prescribed the bicium cyanamide as the acid salt and interferes with the deguanide and guanylurea precipitate rapidly enough so that termination of dicyanodiamide. The calcium can be removed no decomposition of dicyanodiamide results from the 2 to by anhydrous oxalic acid. The powdered acid is added until 3 hours' exposure to the strongly alkaline solution. the solution is just acid as shown by an indicator. The calcium oxalate is filtered off and an aliquot made N / 4 with nitric acid, and the dicyanodiamide determination carried out International Annual Tables as usual. Should the ratio of cyanamide to dicyanodiamide Dr. Charles Marie, general secretary of the International be more than 10 to 1, difficulty may be experienced in the Commission charged with the compilation and publication of final stage of the hydrolysis because of the urea present, which "International Annual Tabl6s of Constants and Numerical Data, keeps the materials in the liquid state a t the temperature of Physical, Chemical, and Technological," announces the publication of Volume 5, Part 1, covering the world's literature for the steam bath. It is therefore very difficult to determine the the years 1917-1922, inclusive. Members of the AMERICAN correct time of hydrolysis. The acetone extraction is strongly CHEMICAL SOCIETY are entitled to a special discount on all orders placed before May 1, 1925. Distribution in the United States recommended wherever it is possible to use it. I n case the commercial calcium cyanamide contains oil, is made through the University of Chicago Press, Chicago, Ill., which will furnish full information, subscription rates, etc., as is usually the case in material prepared for fertilizer use, on request. The Annual Tables must not be confused with Inthe sample is extracted with acetone as before, taken to dry- ternational Critical Tables now in preparation under the auspices of the National Research Council.
