ACKNOWLEDGMENT
The authors gratefullv acknowledge the help given by R. S. Bederka, L. G. Jordan, and E. J. Weber in performing the many analyses and thank A. J. LaVine for drawing their attention to the work of Warf, Cline, and Tevebaugh. LITERATURE CITED
(1) Armstrong, W. D., IND. ENG.CHEM., ANAL.ED. 8, 384 (1936). (2) Armstrong, W. D., J. Am. Chem. Soc. 55, 1741 (1933). (3) Boruff, C. S., Abbott, G. B., IND. ENG. CHEM..ANAL. ED. 5. 236 (1933). (4) Clark, H. S., ANAL. CHEX.23, 659 (1951).
(5) Daniel, 2. anorg. Chem. 38, 290 (1904). (6) Ebere, W.F., Lamb, F. C., Lachele, C. E., IND.ENG. CHEM.,ANAL. ED. 10, 259 (1938). (7) Fedoruk, J. C., General Chemical Division, Morrietown, N. J., pri-
Aluminum Co. of America, private communication. Reynolds, D. S., J. Assoc. Ofic.Agr. Chemists 17, 323 (1934).
Ibad., 18, 108 (1935). Reynolds, D. S., Hill, T
vate communication.
(8) Hoskins, W. M., Ferris, C. A., IND. Ibid., 3, 366, 371 ENG. CHEM., ANAL. ED. 8, 6 (1936). (9) Kimball, R. H., Tufts, L. E., ANAL. ists 11; 225 (1928).” CHEM.19, 150 (1947). (20) Rowley, R. J., Churchill, H. V., (10) McClendon, J. F., Foster, W. C., IND.ENG. CHEM.,ANAL. ED. 9, IND.ENG.CHEM.,ANAL.ED. 13, 551 (1937). (21) Warf, J. C., Cline, W. D., Tevebaugh, 280 (1941). R. D., ANAL.CHEW26,342 (1954). (11) McClure, F. J., Ibid., 11, 171 (1939). (22) Willard, H. H., Winter, 0. R., JND. (12) Matuseak, M. P., Brown, D. R., ENG. CHEM., ANAL. ED. 5, 7 Ibid., 17, 100 (1945). (1933). (13) Milton, R. F., Liddell, H. F., Chivers, J. E., Analyst 72, 43 (1947). RECEIVEDfor review January 3, 1957. (14) Moss, hi. L., Research Laboratory Accepted January 16, 1958.
Extraction of Chromium with Trioctylphosphine Oxide from Acidic Solutions of Alkali Metal Salts Determination in Situ as Chrom um-D phenylcarbazide Complex C. K. MANN and J. C. WHITE Oak Ridge National laboratory, Oak Ridge, Tenn. For determination of microgram amounts of chromium in concentrated solutions of alkali metals, chromium is extracted in the sexivalent state with a 0.2M solution of tri-n-octylphosphine oxide in benzene from either a chloride or sulfate solution of the alkali metals. The chromium-diphenylcarbazide complex is then formed directly in the benzene solution b y addition of an alcoholic solution of diphenylcarbazide. As chromium is extracted b y tri-noctylphosphine oxide only in the sexivalent state, an oxidant is added to ensure complete oxidation of chromium. Argentic oxide was a satisfactory oxidizing agent in sulfuric acid solutions. When hydrochloric acid is present, bromine water is the preferred oxidant. The coefficient of variation of the method is 9% for 2 to 10 y of chromium extracted from approximately 10 grams of sodium and potassium chlorides in 80 ml. of solution. O n a weight basis this range is equivalent to 0.2 to 1 pap.m.
A
for the spectrophotometric determination of chromium is based on the formation of the characteristic red-violet color with diphenylcarbazide, after extraction of chromium from aqueous solution into an immiscible nonaqueous solution of tri-nMETHOD
octylphosphine oxide (TOPO). White and Ross (4) found that dichromate can be extracted from acidified aqueous solutions into hydrocarbon solutions It of tri-n-octylphosphine oxide. seemed possible, therefore, that chromium might be separated from solutions having such a high ionic strength as to vitiate the usual diphenylcarbazide method and concentrated into a smaller volume so as to increase the sensitivity of the method. REAGENTS
Standard solution of sexivalent chromium, 1.000 mg. per ml. in approximately 0.05M sulfuric acid. Dissolve 2.828 grams of dried potassium dichromate from the National Bureau of Standards in about 800 ml. of water to which 3 ml. of concentrated sulfuric acid has been added. Dilute to 1 liter with water. Standard solution of trivalent chromium, 1.000 mg. per ml. Transfer an aliquot of the standard solution of sexivalent chromium to an Erlenmeyer flask. Pass a stream of sulfur dioxide through the solution for 5 minutes; then boil the solution gently for 30 minutes to remove the excess sulfur dioxide. Use this solution directly or dilute it with water as required. Tri-n-octylphosphine oxide (TOPO), 0.2M. Dissolve 7.72 grams of tri-noctylphosphine oxide (Eastman Organic
Chemicals No. 7440) in 100 ml. of benzene. Protect from light by storing it in an opaque container. Diphenylcarbazide, 0.25 (w./v.)%. Dissolve 125 mg. of diphenylcarbazide in 50 ml. of absolute ethanol immediately before use. Argentic oxide, Ago, reagent grade, hIer ck . PROCEDURE
Transfer an aliquot, no larger than 80 ml., which contains preferably 3 to 10 y of chromium and not less than 1 y in approximately 5 volume sulfuric acid, to a 150-ml. beaker. Add 3 ml. of concentrated sulfuric acid and approximately 100 mg. of argentic oxide. Stir the solution well; then heat to boiling and boil vigorously for 5 minutes. Cool the solution to room temperature. Dilute the cooled solution to approsimately 50 ml., if necessary, with water; then transfer the solution to a 125-ml. separatory funnel and add sufficient 5M sulfuric acid to make the solution about 1M. Complete the following procedure for color development without interruption. Add 5 ml. of 0.2M tri-n-octylphosphine oxide in benzene; then shake the sample for 2 minutes. Allow the phases t o separate and collect the organic phase in a test tube that contains silica gel. (Do not use indicator silica gel, as the color is extracted by the organic solution.) Transfer 2 ml. of diphenylcarbazide solution to a 10VOL. 30, NO. 5, M A Y 1958
989
nil. volumetric flask. Add a 3-nil. aliquot of the extract to the flask; then dilute the sample to volume with ethanol. Measure, within 1 hour, the absorbance of the solution a t 550 mp against a reference solution that contains only the reagents and that has been taken through the entire procedure. Calculate the chromium content by comparing the absorbance with that of standard solutions of chromium that have been carried through the same procedure. Factors Affecting Chromium and Diphenylcarbazide Reaction in Nonaqueous Medium. T o realize the niasimum advantage from the concentration of chromium into a small volume by extraction into tri-n-octylphosphine oxide, the color reaction was conducted in situ in the organic phase of the extraction. This procedure avoids the necessity of stripping the chromium from the organic liquor and the attendant steps in the procedure of preparing the solution for analysis. The sexivalent chromium-diphenylcarbazide complex formed in ethanolic solution is similar to that obtained in aqueous solution. The absorption spectra are essentially identical, except that the absorbance peak occurs a t 550 nip in ethanol, rather than a t 540 mp in the aqueous medium. The molar absorbance index for chromium in ethanol is identical with that in water, approximately 32,000. In both cases, Beer's law is followed up to 0.8 p.p.m. of chromium. Benzene was used primarily as the solvent for the phosphine oxide, although cyclohexane was equally satisfactory. Other similar hydrocarbons would probably be suitable. Several nonpolar solvents were used as diluents for the chromium-diphenylcarbazide reaction medium. The absorbance of the complex was markedly affected by the choice of diluent. For example, 1 y of chromium, allowed to react with diphenylcarbazide in different solvents, showed the following absorbances a t 550 mp: chloroform 0.323, carbon tetrachloride 0.335, benzene 0.308, acetone 0.118, and ethanol 0.450. For this reason, all experimental work described involved extraction with benzene solutions of tri-n-octylphosphine oxide, transfer of an aliquot of the organic extract to a volumetric flask together with the ethanolic diphenylcarbazide, and dilution to volume with absolute ethanol. ORDEROF ADDITIONOF REAGENTS. Bose (1) pointed out that the dichromate solution should be added to the diphenylcarbazide, rather than vice versa, because the reaction involves an oxidation of diphenylcarbazide to diphenylcarbazone with simultaneous reduction of chromium chromate to the chromous state. If dichromate is 990
ANALYTICAL CHEMISTRY
in excess, there is danger of oxidation of diphenylcarbazone and of divalent chromium. This effect was not noted in this work, possibly because a large excess of diphenylcarbazide was used. RATE OF REACTIONAKD STABILITY OF COMPLEX.In the conventional chromium - diphenylcarbazide method, ( 2 ) maximum color intensity is achieved within a few moments, after which the color fades fairly rapidly. When the color is developed in nonaqueous media, its behavior depends upon the conditions of the extraction. If the aqueous solution from which chromium has been extracted is acidified with hydrochloric acid, the complex color shows a fairly rapid increase in intensity for about 1hour, followed by a very gradual increase for a t least a week. During the first hour the increase amounts to about 5% of the initial value; during the second hour it is less than 1%. Satisfactory reproducibility can therefore be obtained if absorbance is measured a t least 1 hour after the solutions are prepared. If chromium is extracted from solutions that contain sulfuric acid, maximum color intensity develops almost immediateIy on addition of diphenylcarbazide. The absorbance remains essentially constant for at least an hour before the color starts to fade. A decrease in intensity amounting to 4% of the initial value was noted after a 4hour delay; the color fades to a pale pink on standing overnight. Extraction from sulfuric acid, therefore, allows immediate measurement of absorbance. A delay in the completion of the procedure after extraction, but before the complex is formed, decreases the ultimate color intensity. A delay of 1 hour caused a decrease of 25% in absorbance. Similarly, a delay after the reagents are mixed, but before dilution to final volume with ethanol, results in an appreciable decrease in color intensity. A delay of an hour was accompanied by a decrease of about 9% in absorbance. TEMPERATURE OF REACTION.The determination of chromium with diphenylcarbazide in aqueous solution is said to be sensitive to changes in temperature. In this study, experiments were carried out a t temperatures varying from 20" to 45" C. to ascertain the effect on either rate of formation or ultimate intensity of color. No significant differences were noted. CONCENTRATION AND CONDITION OF DIPHENYLCARBAZIDE SOLUTIONS. The optimum amount of reagent for the procedure recommended here is 5 mg. per determination. Because of the limited solubility of diphenylcarbazide in ethanol, it is conveniently added as 2 ml. of a 2.5 mg. per ml. solution. This represents a t least a 30-fold molar excess of reagent over chromium.
Large amounts of reagent have no undesirable effect. Solutions of diphenylcarbazide in ethanol vary in stability, some showing a discoloration shortly after preparation and others almost no discoloration even after 24 hours. Solutions that showed no color change had a slightly acid pH and gave more reproducible results than those showing an alkaline reaction. The cause of this variation is not known. An attempt to produce stable reagent solutions by adding acid was unsuccessful; the result is decreased color intensity, similar to that observed if the tri-n-octylphosphine oxide solution contains droplets of the aqueous phase. For very precise determinations, Urone (3) recommends that the reagent solution be prepared just before use and that the time lag between dissolution of the reagent and its use be standardized. On the basis of experience with ethanolic solutions, the latter seems impractical because the rate of discoloration is not reproducible. The data shown in Table I were obtained by extracting 100 y of sexivalent chromium from 10 ml. of 1K sulfuric acid into 5 ml. of 0.1M tri-n-octylphosphine oxide. One series of determinations was performed a t one time; one batch of diphenylcarbazide solution was used to develop the colors. The other series of results was obtained over a period of 2 weeks; a different batch of diphenylcarbazide was generally used for each determination. In all cases, the reagent solution was prepared on the date it was used. The coefficient of variation in the former case is 1%: while that of the latter is 3%. This mdicates the loss of precision to be expected due to the lack of stability of the reagent. ACIDITYAND TYPEOF ACID. Quantitative extractions of chromium can be performed from solutions of several common acids in concentrations of approximately 1 to 8 or 10M (4). When a colorimetric determination follows the extraction, however, the conditions must be more closely restricted (Table II), Extractions on solutions of sexivalent chromium in either hydrochloric or sulfuric acid give satisfactory results. If hydrochloric acid is used, its concentration must be between 1 and 2 X . Chromium is not extracted quantitatively from solutions that contain acid concentrations appreciably less than lM, while full color development is not obtained if the concentration of hydrochloric acid is greater than 2M. If other conditions remain constant, the absorbance is approximately 85% of the maximum on 411.1 solutions and 58% when acid concentration is 6 M . In the case of sulfuric acid, maximum color intensity is obtained when extractions are performed on solutions
with an acid concentration of 1M. Use of either lower or higher concentrations results in decreased color intensity. The absorbance, obtained after extraction from 0.5M solutions, is about 80% of that found after extraction from 1M solutions; from 2M sulfuric the value is about 90% of the maximum. As an indication that the phenomena observed involve extent of complex formation rather than completeness of extraction, if, after solutions of chromium that contain 2.5M sulfuric acid are extracted, the organic phase is equilibrated with 1M acid, a subsequent determination of chromium gives results in approximate agreement with those obtained after extraction from 1M sulfuric acid. If the organic phase is equilibrated with 0.5M sulfuric acid or with water, the color intensity is leas. This would seem to indicate that a definite and very small amount of acid is extracted into benzene from aqueous acid solutions and that the acid concentration obtained by equilibration with 1M sulfuric or 1M hydrochloric is the optimum value for development of the diphenylcarbazide color in nonaqueous media. To illustrate the fact that the amount of acid needed for development of the color of this system is very small, the addition of 1 drop of 0.05M sulfuric acid to the reagent stock solution, which would amount to roughly 1 X 10-4M acid in the final solution, caused a decrease in color intensity averaging 20%. By contrast, the determination of chromium in aqueous solution required 0.1M sulfuric acid; maximum color intensity is not obtained with lower concentrations, while rapid fading occurs with higher concentrations. If the extraction is performed on solutions acidified with nitric or perchloric acid, the results are unsatisfactory. When 1M nitric acid is used, absorbance values are about 40% of those obtained with hydrochloric or sulfuric acid. Increasing concentrations of nitric acid cause a drastic decrease in color intensity, that with 6M acid being nearly zero. White and Ross (4) found that extraction in nitric acid medium is vitiated by reduction of sexivalent chromium, possibly by oxides of nitrogen. This could explain the low results described here. I n extraction from perchloric acid solutions, much the same result is obtained, except that the value for 1M perchloric acid is about 78% of that obtained with sulfuric acid. If after extraction from solutions 4M in perchloric acid, the extract is equilibrated with 1M sulfuric acid, no change in ultimate color intensity is noted. Apparently the effect is caused by something other than extraction of acid into the organic phase.
Table 1.
Effect of Instability of Diphenylcarbazide Solutions on Precision of Determination
Diphenylcarbazide Soh. All detns. with same soln. Different soh. for each detn.
No. of Detns.
Absorbance
Dev.
SM.
Coeff. of Variation, %
7 17
0.495 0.557
0.005 0.018
1.0 3.0
COLOR REACTION.Increase in the ionic strength of the aqueous solution due to presence of other salts has little effect on the color reaction. Extractions on solutions 2M in sodium chloride and 1M in sulfuric acid showed no significant variation from that expected for 1M sulfuric acid alone. When the concentration of sodium chloride was raised to 3 M , the absorbance showed an 18% decrease. In the case of sulfate salts, making the solution 0.4M in ammonium sulfate or 0.9M in sodium sulfate had no effect on the determination. Nitrate salts, as expected, strongly interfere with the determination. The absorbance obtained after extraction of chromium from aqueous solutions 1.8M in sodium nitrate was only 13% of that expected in the absence of nitrate. The tolerance of the method for nitrate was not determined. EFFECTOF VARIABLESON CONCENTRATION OF SEXIVALENT CHROMIUM BY EXTRACTION. The extraction of chromium into tri-n-octylphosphine oxide offers the possibility of materially increasing sensitivity by utilizing extraction for concentrating the chromium. Among the factors that must be considered when the ratio of aqueous volume to organic volume is made larger than unity are concentration of tri-n-octylphosphine oxide in the organic phase, the acid used to acidify the aqueous phase, salts present in the aqueous phase, and the length of time Table 111.
Molarity 1 1 1 1 1
1 H ~ O I
KC1
Table It. Effect of Acidity on Extraction and Determination of Chromium as Diphenylcarbazide Complex in Tri-noctylphosphine Oxide-Benzene Solution
hcid Hydrochloric
Molarity
Chromium Recovered, y6 100
1 2
100 85 58 80 100 90
4 6
Sulfuric
0.5 1 2 1
Nitric
40
6 1
Perchloric
0
58
of equilibration. The results of tests conducted to evaluate these points are shown in Table 111. Although chromium was extracted quantitatively from sulfuric acid solutions, the presence of additional sulfate ion depressed the ultimate recovery of chromium; additional chloride ion had no such effect. It was reported that equilibrium between organic and aqueous phases is achieved within 1 minute (4). In this work, experiments involving equilibration times of 2 and of 10 minutes indicated no improvement with longer shaking. Recoveries after longer periods mere actually about 90% as large as after the shorter period. These
Extraction of Chromium by Tri-n-octylphosphine Oxide under Various Conditions 100 y of sexivalent chromium 5 ml. of tri-n-octylphosphineoxide in benzene
Acid HC1
H2SO4 NaCl
Av.
1 1 1 1
.
A.1 0.04
}
Concn. of
Phase Volume Ratio, A/O
0.05 0.05 0.1 0.1 0.2
10 10
TOPO, M
6
20 10
0.2 0.05 0.1 0.2 0.2
20
0.2
No. of
Av .
Detns.
Recovery, 7'
2 2
98 83 100 92
4
2
2
104
6
101 9R lo 1 87 84
20
8
103
20
2
73
1 2 -
6
20
1 2
-2 2
--
EFFECT OF SALTCONCENTRATION ON VOL. 30, NO. 5, MAY 1958
991
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
