results may be due to reduction of chromium during equilibration. OXIDATION OF CHROMIUM.The reabtion between chromium and diphenylcarbazide, in either aqueous or organic solution, takes place only if the chromium is in the sexivalent state. Chromium is extractled by tri-n-octylphosphine oxide only as the sexivalent ion. As chromium is generally found in the trivalent state, a method for oxidation to sexivalent chromium was needed which is compatible with the determination to follow. Argentic oxide, available from Merck & Co., Inc., was satisfactory for this purpose. Application of Method to Determination of Chromium in Synthetic Solutions of Sodium-Potassium Alloy. Synthetic aqueous solutions of sodium-potassium alloy were prepared t o which a few micrograms of chromium were added. The samples consisted of 5.68 grams of sodium chloride, 3.36 grams of potassium chloride, and 2 to 10 y of trivalent chromium in 80 ml. of solution. This is the equivalent of 4 grams of sodium-potassium dissolved in 80 ml. to yield a solution 1.22M in sodium chloride and 0.56N in potassium chloride, with a chromium concentration of 0.5 to 2.5 p.p.m.
Table IV. Determination of Trace Quantities of Chromium in Synthetic Solutions of Sodium-Potassium 5 ml. of 0.251 TOP0 Aqueous/organic, 14 Chromium, y Taken, Found, Difference, A B B-A 2 2.7 0.7 2.5 0.5 2.5 0.5 4 3.8 -0.2 4.1 0.1 4.0 0.0 8 8.0 0.0 7.9 -0.1 10 9.6 -0.4 9.7 -0.3 Coefficient of variation, 9%.
The chromium in the samples was oxidized with argentic oxide, after which the solutions were made 1M in sulfuric acid and extracted with 5 ml. of 0.2M tri-n-octylphosphine oxide in benzene. After the extracts had dried over silica gel, a 3-ml. aliquot was diluted together with reagent to 10 d. with ethanol (Table IV). The average absorbance for the tests
which involved 2 y of chromium was 0.128 for a 1-cm. cell. I n these solutions the concentration of chromium in the 10-ml. final solution was 0.12 y per ml. If 0.02 is assumed as the minimum significant absorbance that can be measured, the practical sensitivity limit is lowered to about 0.01 y per ml. of final solution or approximately 0.15 y of chromium in the sample solution. LITERATURE CITED
Bose, &I., Anal. Chim. Acta 10, 201 (1954). Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., Interscience, New York, 1950. (3) Urone, P. F., ANAL.CHEW27, 1354 (1955). (4)White, J. C., Ross, W. J Oak Ridge National Laboratory, ““Extraction of Chromium with Tri-n-octylphosphine Oxide,” ORNL-2326 (July 9, 1957). RECEIVED for review September 30, 1957. Accepted January 22, 1958. Division of Analytical Chemistry, 132nd Meeting, ACS, New York, September 1957. Work carried out under contract No. W-7405eng-26 at Oak Ridge National Laboratory, operated by Union Carbide Nuclear Co., a division of Union Carbide Corp., for the Atomic Energy Commission.
Kjeldahl Determination of Nitrogen Extension to Nitro and Nitrogen-Nitrogen Single-Bond Compounds W. E. DICKINSON F. S. Royster Guano Co., I300 Manor Place, S. W., Atlanta IO, Go.
,The refractoriness of nitro nitrogen, nitrogen-nitrogen single-bond nitrogen, and pyrazolone nitrogen to the Kjeldah1 method, challenged analytical chemists for 60 years. Nitro nitrogen and nitrogen-nitrogen single-bond nitrogen are reduced almost quantitatively to amino compounds by zinc dust in solution in such nonoxidizing media as formic, acetic, phosphoric, and hydrochloric acids. In the conversion to the amino derivative, the nitrogen loses its refractoriness. These forms of nitrogen are now amenable to the rapid, accurate, and multiple analytical process of the Kjeldahl method. Only pyrazolone nitrogen still defies this method.
T
men who pioneered the modified Kjeldahl method found salicylic acid unique, in that it was the only compound which, in solution in sulfuric acid, was converted by nitrates to a HE
992
ANALYTICAL CHEMISTRY
nitro compound, which in turn could be reduced to an amino compound by zinc dust or sodium thiosulfate, with only a digestion necessary to deliver the nitrogen quantitatively as ammonia. Many of the compounds tried were converted to nitro compounds, indicating that failure to deliver nitrogen quantitatively was due not to the immunity of these compounds to nitration, but to incomplete reduction of the nitro group to an amino group, or to failure of the digestion. However, as amino compounds are not refractory, the failure must be charged to incomplete reduction of the nitro group. The inference was that not many nitro compounds would be amenable to the Kjeldahl method. REFRACTORINESS OF COMPOUNDS COMPARED
When nitrosalicylic acid is digested
with sulfuric acid and potassium sulfate, the intramolecular reaction a t the breakdown of the compound enables the sulfuric acid to deliver about 80% of If salithe nitrogen present as “4. cylic acid be included with the nitrosalicylic acid, the recovery of nitrogen will be about 90%. With sodium thiosulfate, zinc dust, or iron powder included with the nitrosalicylic acid, the recovery of nitrogen will be 99.9% (2). On the other hand, digesting mnitrochlorobenzene with sulfuric acid and potassium sulfate gives a recovery of only about 25% of nitrogen. Including sodium thiosulfate, zinc dust, or iron powder raises the recovery to about SO%, and 1 gram of 1-naphthol added to the reduction step raises the recovery to about 70%. As none of these reductants can be depended upon to reduce nitro compounds other than nitrosalicylic acid quantitatively
t o amino compounds in sulfuric acid solution, and the addition of outside compounds is ineffective, nitro compounds have been regarded as requiring preliminary reduction (S), not amenable to the Kjeldahl method (4, not convertible quantitatively to ammonia ( I ) , and together with nitrogen-nitrogen single-bond nitro compounds as rendering the Kjeldahl method unreliable (6). The lack of a macro-Kjeldahl procedure for nitro compounds suggested the need for investigating the method with particular reference to their reduction to amino compounds. The problem was that of quantitatively reducing KO2 to NH2, without precluding conversion of the NH2 to NH4 by the Kjeldahl method. The data indicated that this problem might be solved if the refractory nitro compounds could be completely reduced. Subjecting them to the high potential zinc-hydrochloric acid reduction in nonoxidizing acid solutions prior to Kjeldahl digestion held most promise. Exploratory work with solution in glacial acetic, 88% formic, and 85% phosphoric acids and 95% alcohol gave better results on very refractory nitro compounds than reported (1, 4). The results in Table I were arrived a t by an arbitrary technique applied uniformly throughout, and were intended to show only the relative effectiveness of solvents when nitro compounds are reduced by zinc dust. Hydrochloric and phosphoric acids, functioning as both solvent and acidulant, can recover about 97% of the nitrogen in Z,Pdinitrophenol, and 30% formaldehyde acidulated with hydrochloric acid recovers about 90%. Apparently any solvent miscible with hydrochloric or phosphoric acid will give good reduction with a metal dust under conditions of a potential equal to the refractoriness of the compound. Undoubtedly other acidulants may be used. Not every satisfactory solvent may be of equal service in this work. Acetic acid is an excellent solvent, but in the digestion with sulfuric acid that follows the reduction, it carbonizes, prolongs the digestion] and materially depletes the acid. For this reason not more than 10 ml. of acetic acid should be used in reductions; ketones are contraindicated. Formic acid, a strong reducing agent, is the most satisfactory solvent in this work. After it has served its purpose and the sulfuric acid is introduced] the formic acid is decomposed into volatile products and leaves the amino compound in solution in the sulfuric acid, with only a little water. The relative effectiveness of some metal dusts in reducing nitro compounds] under the conditions imposed, is shown in Table 11. The three metals are about equally effective and give results close to theo-
~
Table
I.
~~
Percentage of Nitrogen Recovered from Nitro Compounds a
Glacial 88% 85% Acetic Formic Phosphoric 95% Compound Acid Acid Acid Alcohol Calcd., 70 2,PDinitrophenylhydrazine 27.60 27.88 26.96 27.56 28.29 p-Nitroacetanilide 15.28 15.36 15.32 15.12 15.55 m-Nitrochlorobenzene 8.28 8.48 8.32 7.64 8.89 2,PDinitrophenol 15.08 15.00 12.48 14.96 15.22 a Dissolved in 5 ml. of solvents plus 5 ml. of HCl, and reduced by 2 grams of zinc dust, followed by digestion with HaSO,. Table II.
Effectiveness of Metal Dusts in Reducing Nitro Compounds (In solution in 10 ml. of formic acid and 5 ml. of HC1, followed by digestion in H2S04)
Compound Picric acid p-Nitroacetanilide m-Nitrochlorobenzene 2,PDinitrophenol
Zinc Dust (2 Grams), % 18.12 15.44 8.84 15.24
retical. This precision does not always follow. This is a heterogeneous reaction and results may vary according to technique or lack of technique. If the solution is too acid, the powder will dissolve before all the compound can come in contact lvith the metal surface, where the reaction takes place. If the solution is too low in acid content, long-drawn-out agitation or gentle boiling is needed to keep the nitro conipound in contact with the metal surface. It is not permissible to keep adding dust until the reduction is complete. Maximum reduction must be brought about with a minimum of metal. The ferric and aluminum sulfates formed in the digestion are not soluble in sulfuric acid, and these solids, particularly aluminum sulfate, tend to cause burning of the product. For this reason 1 gram of iron or 0.5 gram of aluminum is about the maximum amount permissible in 25 nil. of sulfuric acid digestion. The data secured in developing the exploratory results reported in Tables I and I1 indicate that formic acid is the preferred solvent in this work, and that with appropriate changes in technique, zinc, iron, and aluminum may be used as reductants. These data have been included in a technique by which the writer has been able to get excellent nitrogen results in nitro compounds. METHOD I
Weigh out 0.35 gram of a nitro compound into a 500- or 650-ml. Kjeldahl flask. Wash down the neck of the flask with a mixture consisting of 5 ml. of 88% formic acid and 2 ml. of hydrochloric acid, and heat on a steam bath until the greater part of the sample is in solution. To the swirling contents of the flask add 2 grams of zinc dust (nitrogen-free) and continue to swirl for 2 minutes. Place on the steam bath for 5 minutes. Add 1 gram of iron
Iron Po-wder (1 Gram), % 18.44 15.64 8.80 15.16
AIuminum Powder ( 0 . 5 Gram), % 18.36 15.48 8.90
15.20
Calcd., 18.35 15.55 8.89 15.22
powder (by hydrogen), and swirl for 2 minutes. Add 2 ml. of hydrochloric acid and 5 ml. of alcohol (for refluxing), and place on the bath for 5 minutes. Repeat addition of hydrochloric acid every 5 minutes until the iron is almost all dissolved. Under a good hood add 25 ml. of sulfuric acid a very little a t a time with swirling of the flask, and swirl in a Bunsen flame until the excessive ebullition has subsided. Add 12 to 14 grams of potassium sulfate or anhydrous sodium sulfate, and digest for 1 hour. Distill into a receiving flask, observing the appropriate Kjeldahl technique. Titration completes the Kjeldahl cycle. This method gives good precision, and a recovery of about 99% of the nitro and nitrogen-nitrogen single-bond nitrogen. The time required for a determination is about 3 hours. Discussion. If a compound does not dissolve in formic acid, but melts under heat, vigorous swirling t o keep the compound in contact with the metal will usually give theoretical results. I n the rare case of a sample that will neither melt nor go into solution in 5 ml. of formic acid and 2 ml. of hydrochloric acid, and a different solvent is required, 5 ml. of acetic acid or 5 ml. of 85% phosphoric acid may be substituted for the 5 ml. of formic acid, to give good precision and a recovery of about 98.5% of the nitro nitrogen. If the iron powder is omitted, reduction will result in a recovery of about 97% of the nitrogen in very refractory compounds. The use of iron powder is indicated rather than additional zinc, not because it is a better reductant, but because it does not agglutinate, or revert to the massive form in acids and lose its effectiveness as a powder, as zinc does. The iron powder is used to scavenge the residue of nitro nitrogen that escapes the zinc reduction. VOL. 30, NO. 5, MAY 1958
993
Table 111.
Nitrogen Determination
Method I 2,4Dinitrophenol p-Nitroacetanilide m-Nitrochlorobenzene
1,4-Dichloro-2-nitrobenzene
+Dinitrobenzene p-Dinitrobenzene o-Nitrobenzoic acid 5-Nitrosalicylic acid 3-Nitrosalicylic acid (anhydr.) Picric acid 2,4-Dinitrophenylhydrazine 2,4Dinitrophenylhydrazone of
dimethyl ketonea
2,4Dinitrophenylhydrazone of
diethyl ketoneo Nitroethane Nitropropane (practical) w-Nitrostyrene Nitrobenzene
Nitrogen Content, Method I1
Calcd.
15.18 15.50 8.86 7.24 16.60 16.56 8.35 7.60 7.62 18.24 28.12
15.12 15.40 8.82 7.20 16.48 16.50 8.28 7.60 7.56 18.18 24.40
15.22 15.55 8.89 7.30 16.68 16.68 8.39 7.65 7.65 18.35 28.29
23.40
. .
23.52
20.88 18.52 15.51 9.36 11.30
9.60 10.96 8.68
21.05 18.66 15.71 9.40 11.38
. .
a Supplied by C. T. Lester, Chairman, Department of Chemistry, Emory University, Emory, Ga.
There are several ways of recovering about 97.5% of the nitrogen in such refractory compounds as the nitrohydrazines and nitrohydrazones. When 1 gram of iron powder is introduced into 5 ml. of phosphoric acid containing 0.35 gram of such compounds in solution, followed by digestion in 25 ml. of sulfuric acid, recovery of nitrogen will be about 97.5%. About the same recovery may be had from 1 gram of zinc dust and 10 ml. of formic acid, on a steam bath 30 minutes, followed by the usual digestion. With compounds containing less than 10% nitrogen, the factor weight had best be raised to 0.7 gram, with appropriate changes in the amount of acid in the receiving flask. The digestion in this method is the original, not the modified Kjeldahl digestion. No reducing agents or catalysts are needed. The experience gained in developing Method I, in which nitro compounds are changed chemically to make them amenable to the Kjeldahl method, led to the conviction that the Kjeldahl method itself could be modified to cope with nitro compounds. The use of zinc dust and iron powder in this effort is contraindicated, because they can be depended upon for complete reduction only in the case of nitrosalicylic acid. The use of a metal higher in the electromotive series was indicated, and aluminum was taken as the metal most likely to be effective. It does not react appreciably with sulfuric acid solutions of nitro compounds a t 0" C., but a t 30" C. the heat of reaction is a little more than is dissipated, and heat buildup (and reaction) is progressive. With excessive aluminum this results in thermal decomposition of the product. I n Method I the nitro compounds in nonoxidizing acids may be reduced a t temperatures up to 100" C.; in the 994
ANALYTICAL CHEMISTRY
second method, the temperature during the reduction in sulfuric acid must not exceed 50" C., because the oxidation potential of sulfuric acid is very greatly increased by heat, and a t this point begins to nullify the reduction. METHOD II
Weigh 0.35 gram of a nitro compound into a 500- or 650-ml. Kjeldahl flask. Wash down the neck of the flask with 20 ml. of sulfuric acid, and heat to solution on a steam bath. Adjust the temperature to 30" C. in a water bath, add 0.4 gram of aluminum powder (Venus aluminum, extra brilliant, U. S. Bronze Powder Works, Closter, N. J.), and shake well. Let stand about 10 minutes or until the contents just begin to foam, or feel tepid to the touch. Swirl the flask against the palm of the hand until the flask begins to feel warm. Alternately swirl the flask and lower the temperature to tepid in the 30" u-ater bath, until there is not enough unreacted metal left to raise the temperature above the merely warm stage. Swirl the flask occasionally for 30 minutes, or until most of the metal is dissolved. Add 12 to 14 grams of potassium sulfate or anhydrous sodium sulfate, digest for 1 hour, then distill. The time required for completing a determination by this method is about 3 hours. Alternative Method. The nullifying effect of high temperatures in the reduction may be conveniently avoided and the nitrogen recovery increased by about 0.20% by adding the aluminum in 0.10-gram portions a t intervals of about an hour. All additions must be swirled for 5 minutes with each rise of temperature. No addition should be made until the small temperature rise has subsided. When a compound contains as much as 15% nitro nitrogen, addition of a single 0.10-gram portion of aluminum will cause a rise in temperature of about
10" C., and the nitrogen found on digestion will be about 10%. Addition of three 0.lO-gram portions of aluminum suffices for compounds containing 20% or less of nitro nitrogen. When the nitro nitrogen exceeds the reduction capacity of the metal, the hydrogen evolved will be absorbed by the nitro compound, and there will be little or no foaming. Addition of 12 to 14 grams of potassium sulfate or anhydrous sodium sulfate follows. The time required for d e termination by the alternative method is about 5 hours. Discussion. Zinc and iron would reduce nitro compounds in solution in sulfuric acid, except that they do not react sufficiently with the acid at temperatures below its oxidizing range. The technique is further complicated by the fact that with increasing temperature and decreasing nitro content, the unreacted hydrogen raises the aluminum particles to the surface of the pool and localizes the reaction and concentration of the heat. Complete reduction of nitro compounds in solution in sulfuric acid is obtained by vigorous agitation to prevent solution of the aluminum in acid in which all the nitro compound has been reduced, while maintaining a temperature below the oxidizing range of the acid. Method I1 is applicable only to aromatic nitro compounds. It gives good precision and a nitrogen recovery of about 99%, but its results are more affected by the technique than are those of Method I. In the writer's hands Method I1 gives a recovery of about 60% of the nitrogen in saturated aliphatic nitro compounds, and about 90% in unsaturated aliphatic nitro compounds, as indicated by the recovery from the unsaturated aliphatic nitro side chain of w-nitrostyrene. Method I is applicable to both aliphatic and aromatic nitro compounds. It is equally effective with ortho, meta, and para derivatives, and with mononitro, dinitro, and trinitro compounds. It is only slightly less effective with nitrogen-nitrogen single bond compounds. Neither method is effective with the pyrazolone compound aminopyrine. Table I11 lists nitrogen results by both methods in some representative nitro compounds. LITERATURE CITED
Bradstreet, R. B.,AXAL.CHEM. 26, 235 (1954).
Dickinson, W. E., I b i d , 26,777 (1954). Kirk, P. L., Ibid., 22,354 (1950). McCutchan, Philip, Roth, W. F., Ibid., 24, 369 (1952).
Parks, T. D., Bastin, E. L., Eligio, J. A., Brooks, F. R., Ibid., 26, 229 (1954).
RECEIVEDfor review May 21, 1957. Accepted January 25, 1958.
