V O L U M E 24, N O . 2, F E B R U A R Y 1 9 5 2 0.1 M sulfuric acid, 0.2 M sodium sulfate, and 8.75 x 10-6 M sodium molybdate. The nitrate reduction currents were obtained by measuring the current a t -0.75 volt us. S.C.E. for each nitrate concentration and from this subtracting the current a t that potential for a blank solution. The results are shown in Table 11. In Figure 2 the reduction currents due t o nitrate have been plotted against nitrate concentration. The relationship of reduction current to nitrate concentration is not linear, and thus it is necessary to establish a calibration curve when determining nitrate. The nitrate concentration must be 2 X 10-4 M or greater to obtain measurable current increments. The ratio of current to concentration is comparatively small, as the “diffusion’’ current is apparently governed by the rate of reaction between nitrate and molybdenum(II1) rather than by the rate of diffusion of nitrate. Because the nitrate reduction current is not diffusion-controlled, the nitrate reduction product cannot be ascertained through application of the IlkoviE equation. Determination of Molybdate in Presence of Nitrate. The enhanced reduction current for acidic molybdate solutions containing nitrate may be utilized to measure lower molybdate concentrations than is possible in the absence of nitrate. Polarograms were obtained for varying concentrations of sodium molybdate in 0.1 M sulfuric acid solutions containing 0.2 M sodium sulfate and 0.05 M sodium nitrate. The reduction current was calculated by measuring the current a t -0.70 volt us. S.C.E. for each molybdate concentration and from this subtracting the current a t that potential for a blank solution. The results are shown in Table 111. In Figure 3 the reduction currents have been plotted against molybdate concentrations. For concentrations less than 2 X 10-6 M , the reduction current is proportional to the concentration of molybdate. In more concentrated solutions the reduction
369 current no longer increases linearly with concentration, so that a calibration curve is needed to interpret the results in this range. Molybdate solutions as dilute as 4 X 10-7 M may be estimated by this method, though obviously the accuracy is not so great when measuring diffusion current differences of this magnitude. In solutions more concentrated than those listed in Table 111, the reduction step no longer exhibited a “leveling off” section in the polarogram, so that measurements in that conoentration range are not recommended. By this method, molybdate in the concentration range 4 X lo-‘ to 4 X M may be determined. LITERATURE CITED
Heyrovskg, J., and IlkoviC, D., ColEection CzechosEov. Chem. Cornmum., 7,198 (1935).
Hokhshtein, Ya. P., J . Gen. Chem. U.S.S.R., IO, 1725 (1940). Holtje, R., and Geyer, R., 2. anorg. allgem. Chem., 246, 258 (1941).
IlkoviE, D., Collection Czechoslov. Chem. Communs., 6,498 (1934). Kolthoff, I. M., Harris, W. E., and Matsuyama, G., J . Am. Chem. SOC.,66,1782 (1944).
Kolthcff, I. M., and Lingane, J. J., “Polarography,” p. 228, 9 e w York, Interscience Publishers, 1941. McReynolds, R. C., IND.E m . CHEM.,ANAL. ED., 14, 386 (1942).
Meites, L . , J . -4m. Chern. Soc., 73,4115 (1951). Tokucka, M., and Ruzicka, *J., Collection Czechoslov. Chem. Communs., 6,339 (1934).
Uhl, F. -4., 2.a d . Chem., 110, 102 (1937). Yagoda, H., and Fales, H. A., J . Am. Chem. SOC.,58, 1494 (1936). RECEIVED March 7, 1951. Presented in part before the First Northwest Regional Meeting, AMERICAN CHEMICAL SOCIETY, Seattlo, Wash., October 20. 1945.
Determination of Nitrogen Modified Kjeldahl Procedure Using Thiosalicylic Acid PHILIP McCUTCHAN AND WALTER F. ROTH Union Oil Co. of California, Wilmington, Calif. The determination of nitrogen in nitro-type compounds is a tedious operation unsuited to control operations. A simple modification of the Kjeldahl pmcedure permits the rapid determination of nitrogen in compounds such as nitrobenzene and nitromethane. Thiosalicylic acid is used to assist the conversion of the oxidized nitrogen to ammonia. This modification extends the scope of the method and gives greater assurance of obtaining accurate values on samples of unknown history. It is eminently suited for control operations, as some twenty determinations can be made per man-day.
T
HE macro-Kjeldahl procedures generally used for the
determination of nitrogen in petroleum fractions do not give quantitative results with samples containing nitrogen iri a higher state of oxidation, such as nitro compounds. Among the modifications that have been proposed for reducing the nitrogen to its lower valence are reduction with hydriodic acid and red phosphorus as given by Friedrich (a),and tbe procedure listed by the Association of Agricultural Chemists ( 1 ) rising salicylic acid and sodium thiosulfate. The micro-Kjeldahl reduction method described by Friedrich obtains good results with many compounds, but requires a con-
siderable amount of manipulation Each determination requires more individual attention than with thr macro-Kjpldahl method and it is difficult to assure the requisite digestion teinperature. The AOBC method gives less than quantitative r e s u b with nitrobenzene and 2,4-dinitrophenol. The method gives good results with most compounds but, because of the initial r e a h o n period prior to the addition of sodium thiosulfate and digestiori. takes somewhat longer. If the AO.4C method is modified by preliminary heating n i t h salicylic acid, followed by cooling and addition of sodium thio-
A N A L Y T I C A L CHEMISTRY
370 sulfate before continuance of digestion, remlts are as good as with the thiosalicylic acid method. This additional step of cooling and adding sodium thiosulfate is, however, time consuming and requires a hood, because of evolution of fumes. With this modified AOAC method, a chemist can make about 12 determinations per day.
Table I. Recovery of Nitrogen Nitrogen Recovery, To by Weight AOAC Regular salicylic Thiosa!icylic Theoretical Kjeldahl acid acid Value method 1 method2 procedure Sample Material 0.86 0 97 0.988 0.86 Nitrobenzene blended in 1.00 0.89 gas oil, sample 1 0 95 0.92 0.95 0.92 0.95 0.82
Av. Nitrobenzene blended in g m oil, sample 2
0.268
Av.
Nitroethanea gas oil
blended in
1.04
0.91
0 97
0.15 0.14
0.21 0.21
0 26 0.27 0.26 0 26 0 26
0 14
0 21
0 26
0 99 1 04
0.95
0.95
0.95
0.95
Av.
0 66 0 70
13.83
0.51
Av.
2,4-Dinitrobenzene propyl nuifidea
12.84
2.4-Dinitrophenolb
16.2
Petroleum distillateaC Sample 1
1 02
0.95
0.59
13.01
10.40 10 14 Av. 10.27 12.87 14.90 13.34 14.70 13.41 15.00
Av. 13.86
13 21
13.20
14.87
0.167
0.172
Sample 2
0 033
Sample 3
0 079
0.032 0,032 0.080 0.080
0.170
Purity unknown 6 Eastman chemicals. Results are given on theee petroleum products to show reproducibility as compared t o regular Kjeldahl method on compound8 of nitrogen found in petroleum. a
I n the course of a research invwtigation, two samples were submitted for routine analysis by the Bureau of Mines-Union Oil Co. of California method (3); one sample was 2,4dinitrochlorobenzene, and the other was the reaction product of this compound and an amyl mercaptan. The following results were obtained. \Titlogen Content as oc b i
Ekiglit
Tlieorl
.Inalys~s
2 4-Dinltroclilorobenzene.
13 83
2,4-Dinitrobenzene amyl siilhde CsH:( NOn)l-S-CaHii
11 7'6
0 56 0 70 0 51
('sHaCI(N0o)z
REAGENTS
Thiovalicylic acid. Eastman's T 2805 is suitable. Sulfuric acid, concentrated, reagent grade. Potassium sulfate, reagent grade. Mercury clean metallic. Sodium hydroxide-sodium sulfide solution. This solution is pre ared in quantity with distilled water to contain 40% sodium hygoside and 3% sodium sulfide nonahydrate. Boric acid solution. A 5% solution is prepared by dissolving the appropriate amount of boric acid, reagent grade, in boiling distilled water and decanting when cool from any that may crystallize out. Standard sulfuric acid. Approximately 0.1 and 0.01 N solutions of sulfuric acid in distilled water are standardized to the nearest 0.0001 normality unit against a standardized aqueous sodium hydroxide solution of approximately the same normality, using methyl purple indicator. A 10-ml. buret, graduated in twentieths of a milliliter, should be used for all titrations. Methyl purple indicator, aqueous solution, approximately 0.1% ' active constituent (may be purchased from Fleisner Chemical Co., Benjamin Franklin Station, Washington, D. C,).
13 17 13 14 13 33
12.71 13,Ol 13.23
9.89 13.07 12.66
gated. With nitrobenzene, thio-2-naphthol gave 95% recovery of nitrogen and thiophenol gave 100% recovery, but the latter was inconvenient to use in the laboratory because of noxious fumes. A new procedure using thiosalicylic acid as the reducing agent has been developed. It appears to be suitable for the determination of nitrogen in nitro-type compounds and in all basic and neutral forms of nitrogen compounds found in petroleum or shale oil fractions. The procedure is simple and an operator can make some 20 determinations per day.
11 71 9 85
Theee data indicated that pretreatment of nitro-type compounds waa not always necessary and that the sulfur present In the molecule migh: be the factor affecting more coinplete recovery. A limited study was made of the effect of various organic aulfur compounds on the determination of nitrogen in nitrotype compounds and petroleum fractions In addition t o thioaalicylic acid, both thio-2-naphthol and thiophenol were investi-
PROC EDI'R E
Introduce approximately 1 gram of thiosalicylic acid into the KjeldLLhl flask and add 20 nil. of concentrated sulfuric acid. .4dd a sample of the proper size from a weighing pipet if a volstile liquid, or by weighing in a small porcelain crucible-e.g., Coors OOOOO size-if a solid or nonvolatile liquid. (While a 1-gram sample is desirable because of ease of digestion temperature control, it may vary from 0.1 to 3 grams according to expected magnitude of nitrogen content, 3). After addition of the sample, wash down the neck of the flask with an additional 20-ml. portion of sulfuric acid. Heat until the mixture boils and spattering occurs. This gives a temperature of 274' t o 288' C. Cool t o room temperature and add 20 gmms of potassium sulfate and 1.3 grams of clean metallic mercury, Complete as in the regular Kjeldahl with a digestion temperature of approximately 365' C. Pertinent data obtained by this procedure, compared to those obtained by the regular Kjeldahl and the AOAC methods, are shown in Table I. CONCLUSIONS
The addition of a preheating step with thiosalicylic acid t o the regular Kjeldahl procedure permits the quantitative deterniination of nitrogen in nitro-type compounds. The modified procedure is accurate, requires no interruption of digestion to add sodium thiosulfate for reduction, and enables a chemist to make :LS many as 20 determinations per day ACKNOWLEDGMENT
The authors wish to express appreciation to the Union Oil ('n. of California for permission to publish these data. LITERATURE CITED
(1) dssoo. Offic. Agr. Chemists, "Official and Tentative Methods of
Analysis," 6th ed., Method A.O.A.C., February 27, 1945. ( 2 ) Friedrich, A., Z. physik, Chem., 216, 68 (1933). (3) Lake, G.R., et aE., ANAL.CHEM.,23, 1634 (1951). RECEIVED June 6, 1951.
