958
Anal. Chem. 1985, 57, 958-959
Microdetermination of Nitrogen in Organic Compounds by the Sodium Fusion-Spectrophotometric Method Ernest J. Breda E. I. du Pont de Nemours & Company, Inc., Beaumont Works, P.O. Box 3269, Beaumont, Texas 77704 In the absence of the preferred Dumas nitrogen apparatus or the more sophisticated nitrogen analyzers, a micro-Parr bomb can serve to determine microquantities of nitrogen in organic compounds. The sample or compound, either solid or nonaqueous liquid, is decomposed by fusing with metallic sodium in a sealed nickel bomb (1). The nitrogen is converted to sodium cyanide. The excess sodium is decomposed with absolute ethanol. The solution is adjusted to pH 7.1-7.2 with dilute hydrochloric acid and analyzed for cyanide by the Chloramine-T and mixed pyridine/pyrazolone reagent method (2,3). The absorbance of the blue color formed is measured with a spectrophotometer a t 615 nm. The amount of cyanide found is converted to the equivalent nitrogen in the compound. Sodium fusion is used in classical chemistry for the qualitative identification of nitrogen in organic compounds. Of course some stable compounds are incompletely decomposed by this technique. With a micro-Parr bomb on hand and in the interest of expedience, the sodium fusion technique was tried for quantitative nitrogen estimation in some research compounds. Surprisingly, some fairly decent results were obtained on some known compounds by this means. The method is not as rapid as desired but it is handy, simple, and economical. As with any micro or semimicro method, this procedure is sensitive to technique. Compounds must contain carbon and be essentially free of moisture. E X P E R I M E N T A L SECTION Apparatus. The sodium fusion bomb was the same as described by Lohr, Bonstein, and Frauenfelder ( I ) , except that the collar was made of Type 304 stainless steel. Other details were the same. The bomb was tested and utilized as described in the reference. It was held in a cast iron clamp behind a safety shield. The bomb was heated with a Tirrell gas burner. A semimicro or five-place balance was used to weigh approximately 100 mg of sample containing small amounts of nitrogen or a microbalance to weigh 4 to 6 mg of pure compounds. A Beckman Model DU spectrophotometerwith 1-cmand 10-cm matched cells was used for absorbance measurements. However, any equivalent instrument will suffice. A pH meter equipped with a thin combination glass and calomel electrode was used to measure pH. Reagents. All reagent solutions were prepared from reagent grade chemicals and distilled water and stored in glass or polyethylene bottles. Other materials were as follows: sodium spheres, to '/4 in, diameters (Matheson, Coleman and Bell No. CB 1035), stored under kerosene; ethyl alcohol, absolute; hydrochloric acid/water, 1/3 by volume; hydrochloric acid, 0.1 N; sodium hydroxide, 0.1 N; Chloramine-T solution, 1% aqueous; pyridine, l-phenyl-3methyl-5-pyrazolonesolution, prepared as in ref 2; bis(pyrazolone), Eastman No. 6969; mixed pyridine/pyrazolone solution, prepared as in ref 2; stock potassium cyanide solution, prepared as in ref 2 , 1 mL = 1mg of CN- (approximately), analyzed by the Liebig titration method against silver nitrate ( 4 ) ,solution loses strength gradually and must be rechecked every week; standard potassium cyanide solution, dilute 10 mL of the stock KCN solution to 1 L with water, 1 mL = 10 pg of CN- (approximately); working standard KCN solution, dilute 10 mL of standard KCN solution to 100 mL with water, 1.00 mL = 1.0 pg of CN- or equivalent t o 0.54 pg of N (approximately),prepare daily; standard silver nitrate solution, 0.0192 N, prepared and standardized as in ref 2. Calibration. From a pipet 1- to 6-mL amounts of working KCN standard solution were transferred to separate 25-mL volumetric flasks and diluted to 15 mL with deionized water. Then
0.2 mL of Chloramine-T solution was added to each flask and stoppered. The mixture was swirled several times and allowed to stand 1 to 2 min. From a pipet, 5 mL of mixed pyridine/ pyrazalone solution was added and diluted to 25 mL with water, stoppered, and mixed by inverting several times. The solution color was allowed t o develop for 25-30 min and the absorbance measured at 615 nm in 1-cm cells. The absorbance was plotted on the ordinate vs. micrograms of nitrogen in 25 mL on the abscissa of natural coordinate paper. Lower concentrations of cyanide were measured in longer path cells. Preparation of Sample. Weigh by difference on a semimicro balance approximately 0.1 g of sample in a long stemmed glass weighing tube or weighing boat and place the sample into the bomb cylinder. Dry a pellet of sodium between pieces of filter paper and place the pellet on top of the sample in the bomb and seal the bomb. Tighten the plug with a wrench while holding the bomb in a vise. Place the sealed bomb in the cast iron clamp and hold it at an angle of approximately 45O from the horizontal. Heat the bomb over a Tirrill burner for 15 min. During 10 min of the heating time, the lower 1-to 2-cm portion of the bomb cylinder should be at a cherry red heat. CAUTION The heating of the bomb should be conducted behind a safety shield. Conduct the sodium elimination and pH adjustment in a hood. After heating, allow the bomb to cool under a jet of air. Remove the bomb from the clamp and wash with distilled water. Dry off with a filter paper and discard the paper and washings. Open the bomb carefully and remove the gasket. Any material adhering to the gasket is washed with distilled water into a 100-mL beaker containing 5 mL of absolute ethyl alcohol. Add absolute ethyl alcohol dropwise to the bomb cylinder to destroy the excess sodium as indicated by the evolution of gas. When no further evolution of gas is visible, carefully pour the contents of the bomb into the beaker containing the ethanol and the washings from the gasket and wash off the edges of the bomb cylinder with dropwise addition of ethanol. Again, add alcohol to the bomb cylinder to destroy any residue of sodium. Repeat the operation if necessary until all sodium is destroyed. Finally, wash out the bomb and the sides of the bomb with a fine spray of water from a wash bottle. The volume of the solution in the beaker should be approximately 20-25 mL. Place the beaker in a dish containing ice water. Carefully neutralize the solution in the beaker with 1/3 hydrochloric acid/water to near a pH between 7.1 and 7.2 using a pH meter. Approach the pH 7.1-7.2 region from the high side using 0.1 N hydrochloric acid to make small adjustments in pH. Should the pH drop below 7, readjust with 0.1 N sodium hydroxide. Use a glass rod for stirring the solution during pH adjustment. Transfer the solution including any unburned carbon into a 50or 100-mL volumetric flask depending on the level of nitrogen content present. Dilute the solution to the mark with water. Allow the carbon to settle in the flask before sampling. Determination of Nitrogen as Cyanide. Transfer a suitable aliquot (usually 5 or 10 mL) of the sample solution to a 25-mL volumetric flask and treat as in the preparation of the calibration curve. From the calibration curve read the nitrogen content in 25 mL corresponding to the observed absorbance of the solution at 615 nm in the appropriate cell. Calculate the parts per million of nitrogen content of the sample. Analysis of Pure Nitrogen Compounds. In a glass weighing tube, accurately weigh 4 to 6 mg of sample by difference. Place the sample into the bomb cylinder and treat as above under Preparation of Sample, After adjustment of pH to 7.1-7.2, transfer the solution to a 1-L volumetric flask. Dilute to 1L with distilled water. Analyze a 5- or 10-mL aliquot of the solution the same as in the Determination of Nitrogen as Cyanide. Calculate the percent nitrogen. pg of N from graph X aliquot factor 70 N = 10 x mg of sample
0003-2700/85/0357-0958$0 1.50/0 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
Table I. Analyses of Pure Nitrogen Compounds compound
70 nitrogen calcd found deviation
acetanilide," C8HgN0 10.36
10.33 10.22 10.48 10.45 10.20 10.12 p-carbomethoxybenz. 7.82b 7.80 amide, C9H9N03 7.70 7.69 tris(hydroxymethy1)- 11.44d 9.84 methylamine: C4HllN03 9.88 19.88d 20.12 p-nitroaniline," CI3HGNZ02 20.08 24.13 23.82 dimethylglyoxime,' C4H4N202
disodium ethylenediaminetetraacetate dihvdrate! Na2C10H14N208'
-0.03 -0.14 +0.12 +0.09 -0.16 -0.24 -0.02
type of N
linkage amide
amide
-0.12 -0.13 -1.72
amine
-1.68 +0.24
amine and nitro
+0.20 -0.31
oxime
7.45d
23.92 7.50
-0.21 +0.05
amine
8.60d
7.35 7.30 8.65
-0.10 -0.15 +0.05
nitrile
7.65d
8.73 8.45 7.52
+0.13 -0.15 -0.13
nitro
7.40 0.05
-0.25
2Hz0
methyl p-cyanobenzoate,' C9H7N0Z
methyl p-nitrobenzoatd potassium nitrate:
13.86
potassium salt
KNO, 0.08
KNOB+ benzoic acid'
13.86
1.37
sodium nitrate," NaN03
16.48
1.50 0.05
NaN03 + benzoic
16.48
0.05 2.65
sodium salt
acid'
3.00 Reference standard, Eastman Kodak. Theoretical. Eastman Kodak, reagent grade. Calculated from purity. 'Eastman Kodak, ACS reagent. f Aldrich Chemical, reagent grade. Fisher Certified standard. Fisher Certified ACS reagent.
RESULTS AND DISCUSSION Table I shows data on the recoveries of nitrogen as cyanide when essentially pure compounds were analyzed. The essential requirement for this analysis is that the sample must contain carbon to form the cyanide. In the analysis of noncarbon compounds, a substance such as benzoic acid or other easily decomposable non-nitrogen carbon compound must be added to make the analysis work. Since a truly non-carbon organic type compound was not available' or found, the chemistry was not studied as to what happens in the absence of carbon. One can only speculate on a possibility being the formation of free nitrogen which we know will not analyze like cyanide. Several organic nitrogen compounds with different N linkages were analyzed on a high spot basis. With one exception all the tested compounds gave good recoveries of nitrogen.
959
Tris(hydroxymethyl)methylamine,C14H1103N7, gave rather poor recovery (see data). The reason for this is not known a t present without more detailed study. I t is possible that this compound did not decompose completely a t the conditions of the test. I t may also be that some species other than cyanide formed or something that affected the color formation in the Chloramine-T/pyridine/pyrazolone test. A remote possibility may be that the compound was not as pure as represented. No separate purity analysis was made on any of the compounds tested since they were represented to be essentially of reagent grade quality. The melting points were determined and found to be a t or approximately those reported for the pure compounds. Purities were accepted as represented and nitrogen contents calculated on the purity basis. The known compunds were all dried before analysis. Those with melting points above 100 "C were dried a t 100-102 "C in an oven for 1 h and then kept in a Pz05desiccator. Compounds with melting points below 100 "C like methyl p-nitrobenzoate, mp 94-96 "C, were dried over P205in a vacuum desiccator a t 35 "C for 24 h. Two inorganic compounds, potassium nitrate and sodium nitrate, were fused and analyzed. As expected, no cyanide formed. The brucine method for nitrates ( 5 )showed nearly all the nitrate present in either case. Fusing the same nitrate compounds with benzoic acid present showed about 10-16% of the nitrate converted to cyanide. The rest of the nitrogen remained intact as the nitrate. This behavior raised the question of what happens if an organic nitrate compound is fused with sodium, but this was not tested. In one case-acetanilide-the repeatability of the analysis was tested. The variation of the results was approximately that of the pyridine/pyrazalone method for cyanide or about 2 parts in 100 parts. A note of caution should be observed. Both sodium and cyanide are extremely hazardous substances. The analyst must wear appropriate safety equipment. The hands should be covered with surgical or thin PVC gloves in handling the cyanide solutions. Exercise care when working with metallic sodium, it should not come in contact with an aqueous wet sample nor should excess bits and pieces of sodium be discarded indiscriminately. Pieces of sodium must be returned to the kerosene layered storage bottle. Alternately destroy unusable pieces of sodium carefully by placing them in absolute ethanol, 1-propanol, or 1-butanol until all evolution of gas ceases, then flush away with water. Registry No. Na, 7440-23-5; acetanilide, 103-84-4; p-carbomethoxybenzamide, 6757-31-9;tris(hydroxymethyl)methylamine, 77-86-1; p-nitroaniline, 100-01-6; dimethylglyoxime, 95-45-4; disodium ethylenediaminetetraacetate dihydrate, 6381-92-6;methyl p-cyanobenzoate, 1129-35-7; methyl p-nitrobenzoate, 619-50-1; nitrogen, 7727-37-9; chloramine T, 127-65-1;pyridine, 110-86-1; pyrazolone, 39455-90-8.
LITERATURE CITED (1) Lohr, L. J.; Bonstein, T. E.; Frauenfelder, L. J. Anal. Chem. 1953, 25, 1115-1 117.
(2) "Standard Methods for the Examination of Water and Wastewate", 13th ed ; American Public Health Association: New York, 1971; pp 402-403, 404-406. (3) Epstein, J. I n d . Eng. Chem., Anal. Ed. 1947, 19, 272-274. (4) Kolthoff, I. M.; Sandell, E. B. "Textbook of Quantitative Inorganic Analysls", revised ed.; MacMillan: New York, 1943. (5) "Standard Methods for the Examination of Water and Waste Water", 13th ed.; American Public Health Association: New York, 1971; pp 461-464.
RECEIVED for review November 16,1984. Accepted January 10, 1585.
