V O L U M E 2 5 , NO. 9, S E P T E M B E R 1 9 5 3 Table IV. Insecticide Pyrethrins
1369
Quantitative Recovery of Pyrethrins, Allethrin, and Furethrin from Kraft Paper Added, M g . 5.2
Allethrin
13.9
Furethrin
5.1
Recovered, Mg. 5.4 5.0 5.7
Recovery, c/o 104 96 109
14.7
105 95
13.2
;:;
90 94 99
bv. recovery
amount of insecticide recovered. The acetone method seems to be more efficient because it gives a smaller correction blank for untreated paper and yet extracts the insecticide quantitatively. The applicability of this method as an assay for concentrates of allethrin was tested with three samples of technical allethrin. The purity, or percentage concentration, of these three samples was determined by analysis as described in this paper, the absorbance per milligram being compared with that of a sample of cydl-trans-allethrin (crystalline). Table V shows the percentage of allethrin in the three samples according to four methods of assay. ACKNOWLEDGMEKT
Table V.
Determination of Allethrin in Technical Material
Method Hydrogenolysis (alkali activation) Hydrogenolysis (correcting for anhydride) Ethylenediamine Colorimetric
Sample 1
Sample 2
Sample 3
72.5
94.1
88.9
66.3
91.7 89.8
89.8 85.3 84.1
73.0
66.0
90.0
centrations studied, there is no variation introduced in the analysis of pyrethrins by varying the ratios of pyrethrin I to pyrethrin
11. The quantitative recovery of the three insecticides from kraft paper by the extraction method described herein is given in Table IV. Pyrethrins were present as a commercial coating; allethrin and furethrin were added to the kraft paper in an acetone solution, and the paper was allowed to dry. Extraction of the insecticide from coated paper with ethyl alcohol, according to the method described by Edwards and Cueto (5),gives slightly higher uncorrected results. However, after a correction is made for the untreated paper blank, there are no significant differences between the two methods of extraction in the
The authors are indebted to Hamilton Laudani and S. 4 . Hall for many helpful suggestions, and to ill. S. Konecky and D. J. Glover for hydroqenolysis determinations of allethrin. All are members of the Bureau of Entomology and Plant Quarantine. REFERENCES
(1) Assoc. Offic. Agr. Chemists, “Official Methods of Analysis,” 7th ed., pp. 72-3, 1950.
(2) Chen, Y. L. and Barthel, W. F., J . Am. Chem. SOC.,in press. (3) Edwards, F. I., and Cueto, Cipriano, ANAL.CHEM..24, 1357-9 (1952). (4) Feinstein, Louis, Science, 115, 245-6 (1952). (5) Hagsett, S. N., Kacy, H. W., and Johnson, J. B., unpublished work. (6) Konecky, XI., Schechter, LI. S., Storherr, R. W., Green, K.,and La Forge, F. B., unpublished work. (7) JIatsui, Masanao, LaForge, F. B., Green, X., and Schechter, 31. S., J . Am. Chem. Soc., 74, 2181-2 (1952). (8) Oiwa, T., Inove, Y., Veda, J., and Ohno, LI.,Botyu-Kagaku. 17, 106-22 (1952). (9) Schechter,’M. S., La Forge, F. B., Zimmerli, A., and Thomas, J . hl., J . Am. Chem. SOC.73, 3541 (1951). (10) Thompson, R. B., Chenicek, J. A., and Syman, Ted, I n d . E ~ Q . Chem., 44, 1659-62 (1962). RECEIVED for review March 28, 1953.
Accepted June 25, 1953.
Small Amounts of Uranium in the Presence of Iron Colorimetric Deter rninatio n with 8-Quinolinol LOUIS SILVERMAN, LAV.4D-4 MOUDY, AND DOROTHY W. IIAWLEY Atomic Energy Research Department, North American Aviation, Inc., Downey, Calif, The direct colorimetric determination of uranium, in the presence of moderate or large amounts of iron, i s not possible by other methods. By the 8-quinolinol method, uranium may be determined in the presence of 500 mg. of iron, 50 mg. of copper, 20 mg. of nickel, or 20 mg. of cobalt without prior separations. In conjunction with the hydrolytic separation by which uranium is separated from most of the colored ions but not iron, uranium may he determined colorimetrically, as a routine technique. With an ordinary Beckman spectrophotometer 107 of uranium are easily determined.
M
ANY procedures have been suggested for the colorimetric determination of small amounts of uranium in the presence of foreign elements. Rodden (IS) extensively outlined the peroxide methods showing their applicabilities and shortcomings and noted the effects of interfering elements, such as iron, chromium, and excess reagents. The ferrocyanide method (9,11, IS, 16, 17) also has been investigated. The objection to this method is that uranyl ferrocyanide is colloidal, and such a dispersed precipitate must be kept in a suspended state; furthermore, the ferrocyanide ion reacts with many other ions. The thiocyanate method is currently of interest ( 8 , S, IS). I n the visible spectrum the necessary reagents do not interfere
greatly; but in the ultraviolet range important interferences are noted (2, S,6). Rodden (1s) and Ware (18) offered a list of organic reagents which form colored solutions or suspensions, and from these reports the reagent 8-quinolinol was chosen for the present investigation. Greenspan ( 4 )determined uranium as the colored uranyl 8-quinolinolate in alcoholic solution. Hubbard ( 7 ) separated the insoluble uranyl quinolinolate [U02(C9H60N)2.CpH,0N 1, then coupled the 8-quinolinol to diazotized sulfanilic acid. However, Smales and Wilson (16) obtained the yellow to brown uranium quinolinolate in chloroform solution in order to determine uranium. This last technique is satisfactory for pure uranium
ANALYTICAL CHEMISTRY
1370 solutions, but iron is a major interfering element. Preliminary extraction of iron 8-quinolinolate a t p H 2.8 to 4.3 ( 1 5 ) is not a t all satisfactory, and to date other separations procedures have been necessary. The present investigation was undertaken to obtain a method for the determination of uranium in the presence of iron-a method by which a t least 0.1 mg. of uranium could be detected in the presence of 100 to 500 mg. of iron, and one in which interference by a majority of the elements would be negligible. Iron and 8-Quinolinol. Ferric iron is precipitated by 8quinolinol throughout the pH range of 2.8 to 12 (14). (Uranium is limited to the p H range of 5.7 to 9.8 for complete precipitation.) It is therefore necessary to remove completely ferric ions from the solution before 8-quinolinol is introduced. There are many methods for removing iron from solution. Ferric chloride, in 1 to 1 hydrochloric acid solution, may be extracted with ethyl or isopropyl ether ( 6 ) , but a residual amount of 1 to 2 mg. of iron is left in the aqueous solution. This is sufficient to interfere with the usual 8-quinolinol procedure. Separation of 100 to 200 mg. of iron by cupferron, either by filtration or by extraction, is cumbersome, and frequently decomposition products of the cupferron reagent are left in the aqueous solution. The combination of ether extraction and cupferron extraction offers no advantage because of these same organic residues. Alkaline hydroxide, carbonate, and peroxide separations (IS) are not desirable since some of the uranium will be found in the iron portion while the uranium portion usually retains a little iron. The formation of complex ferric ions, such as fluoride, phosphate, thiocyanate, ferricyanide, or tartrate does not prevent the iron quinolinolate reaction. Ferrocyanide ion, however, does not react with 8-quinolinol in alkaline solution. Formation of Ferrocyanide Ion. The iodide reaction with ferric iron was chosen as the method for conversion of the ferric ions to the ferrous state, since the end product of the reaction, elementary iodine, is easily taken up by the thiosulfate and itself acts as indicator. Sodium cyanide reacts with ferrous halides to form sodium ferrocyanide, but it is not a t all feasible to add the cyanide solution to the ferrous solution, for LL ferrous ferrocyanide precipitate forms. Instead, the ferrous solution is poured into the cyanide solution thus avoiding the formation of a foreign precipitate. At times, if the uranium content is high, the yellow uranyl ferrocyanide may appear and then decompose, without effect on the results. After acidity adjustment, the uranyl 8-quinolinolate may be extracted. Uranyl bQuinolinolate. Examination of the spectrophotometric curves and the concentration-absorbance curves showed that Beer's law was followed a t several wave lengths. A selection of wave lengths was thus available, to permit a choice of curves for the varying concentrations of uranium and to minimize interference by other elements (such as aluminum).
Table I. F e Present, Mg.
Chart for Addition of Reagents
Acidity (to Be Adjusted with HCl), ?&
KI to Be Added, Grams
Thiosulfate, Grams per 100 MI. of Solutlon
NaCN, . Gramsin25 MI. of Solution
Volumetric. Titrate the chloride or the perchlorate with sulfatoceric acid after reduction with lead reductor. PROCEDURE
Removal of Foreign Elements. Foreign elements which ordinarily do not interfere with the uranium reactions may cause difficulties when present in large quantities and should be separated. The following list may contain an appropriate technique: EXTRACTION. Uranium nitrate is extracted from nitrates of the elements (IS) with an ether. Few elements, and these only in smal! amount (except thorium and rare earths), accompany the uranium. The uranium is recovered from the ether by water extraction and determined. Vanadium, iron, molybdenum, phosphates, and sulfates cause difficulties. MERCURY CATHODE.Foreign elements are separated by use of the mercury cathode. Only uranium, boron, aluminum beryllium, vanadium, zirconium, titanium, and certain rare eiements remain in the electrolyte (6). HYDROLYSIS (PYRIDINE).Uranium is precipitated hydrolytically a t pH 6.2 with pyridine (12,IS). This effects a separation from elements such as copper, cadmium, zinc, nickel, and cobalt. HYDROLYSIS (ALKALI METAL CARBONATES). Elements are precipitated hydrolytically in the presence of alkali metal carbonates. Most metals are insoluble under these conditions, but uranium forms a soluble complex. Reprecipitation is usually necessary (IS). CUPFERRON PRECIPITATION. Hexavalent uranium is not precipitated by cupferron in acid solution, and in this manner uranium is separated from many elements ( 6 , I S ) ; results are slightly low. Tetravalent uranium can be precipitated by cupferron.
REAGENTS
CHLOROFORM, reagent grade. ~ Q U I N O L I NINO L CHLOROFORM SOLUTIOX.2.5 Grams per 100 ml. of chloroform. The life of the solution is dependent on the purity of the reagent. Poorer grades are usable for only 3 to 4 days.
POTASSIUM IODIDE, C.P.
SODIUM CYANIDE, C.P. Do not unnecessarily expose to air. PENTAHYDRATE. 5 Grams, 10 grams, SODIUMTHIOSULFATE and 20 grams per 100 ml. of solution. Discard after 30 days. URANIUM STAKDARD SOLUTION.There are two methods of preparation. Dissolve 143 grams of UOt . zHz0 in 110 ml. of perchloric acid (70%), fume, cool, and dilute to 1 liter with water. Approximate concentration of uranium is 119 me. Der ml.: dilute accordinelv for relative standards. Dissolve 253 grams of uranyl nitrate hexahydrate in 10 ml. of nitric acid (sp. gr. 1.42) and 100 ml. of water and dilute to 1 liter with water. Approximate concentration of uranium is 120 mg. per ml. METHODS OF STANDARDIZATION. Gravimetric. Precipitate with ammonium chloride-pyridine at p.H 6.2, ignite a t just 860" C. and weigh as UOz.ess.
-.
V
I
WAVE LENGTH, Ma
Figure 1.
SpectrophotometricCurves for Uranium 8-Quinolinolate in Chloroform
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3
1371
ACID DECOMPOSITION.Fuming the samples with nitric and perchloric acids removes excessive amounts of free mineral acids and also decomposes the interfering or-hydroxycarboxylic acids. Active oxidants should be decomposed by boiling with hydrochloric acid, and those foreign elements which would interfere because of high concentration should be separated by methods noted above. Solutions should be reduced in volume to about 25 ml., and the acidity adjusted according to Table I. Formation of the Ferrous Complex. Stir in solid potassium odide (Table I). The formation of a brown iodine color indicates the presence of elements such as iron and copper. Let stand for several minutes, slowly titrate with sodium thiosulfate solution (Table I) until the brown color disappears, and then add 0.2 to 0.5 ml.of sodium thiosulfate solution in excess. Prepare the required sodium cyanide solution in a 100-ml. beaker (Table I), then slowly and with stirring pour the uraniumiron solution into the cyanide solution. (A thistle tube with drawn-out stem is convenient.) The ferrous iron is converted to ferrocyanide and map form a brown uranyl ferrocyanide coloration or precipitate. Allow the solution to stand for several minutes so that the precipitate may redissolve. At this point the p H of the solution is greater than 9, and is reduced t o 8.7 to 8.9 by careiul addition of concentrated hydrochloric acid.
Preparation of the Standard Curves. The curves plotted in Figure 1 were obtained from 25-ml. solutions of the uranium extracts. A Beckman Model DU spectrophotometer was used, with 10-mm. cuvettes. The curves are plotted from the exact data obtained. No correction was made for the absorbance of the 8-quinolinol unless of poor quality. The correction was then of importance for the lower ranges of uranium and was easily obtained by plotting curves for known contents of 8-quinolinol in chloroform. For the I a r ~ e ramounts of uranium, a higher wave length was chosen a t which the quinolinolate did not absorb the monochromatic light. For this reason, a family of curves was drawn for three wave lengths, 400, 420, and 440 mG (Figure 2). Known amounts of uranyl nitrate and ferric chloride (25, 50, 100, 200, and 500 mg.) were mixed in 100-ml. beakers, and the technique described in the procedure was followed (Table 11).
Table 11. Determination of Uranium in the Presence of Iron Synthetir Samples, I f g . Fe U U added taken founda 1 25 0.668 0.667 50 0.668 0.665 2 25 0.779 0.762 50 0.779 0.780 0 . 7 0 1 0 .727 100 3 4 0 101 0.103 200 0.690 200 5 0.690 0.901 200 0.895 6 0.913 0.920 200 7 1.145 1.157 200 8 0.140 0.148 9 500 0.280 10 500 0 281 0.288 11 0.292 500 0,527 0.531 12 500 0.975 0.970 13 500 Determination a t 440 mp.
Sample
3 440 m
/
t
/
/
a
Ratio of
U/Fe
37/1 74 32 64 143 1980 290 222 219 174 3570 1790 1740 950 513
-
E,
%
2.6 1.3 3.0 1.5 0.70 0.051 0.34 0.45 0.45 0.57 0.028 0.056 0.058 0.11
0.19
I
RESULTS AND DISCUSSION
I l l 1 0
I l l 1
0.5 1.0 CONCENTRATION
I l l 1 1.5
1 1 1 1
1 1 1 1 2.0
I I l I 2.5
I I
3.0
OF U IN MG/25 ML. OF CHCls Figure 2. Absorbance Curves for Uranium 8-Quinolinolate in Chloroform
Extraction of the Uranium. Transfer the yellow-to-brown solution to a separatory funnel (Kimball type, with its small exposure of stopcock grease, is satisfactory) and add 5 ml. of the 8quinolinol-chloroform reagent. Mix for 30 seconds, let stand for 2 minutes or until the layers separate, and draw the lower (chloroform) layer into a second separatory funnel. Repeat the extractions until the chloroform-oxine layer is colorless. The volume of extract is dependent upon the amount of uranium present. About 0.05 mg. or less of uranium should be contained in 10 ml. of chloroform; 2.5 mg. of uranium require four 5nil. extractions and can be contained in a 25-ml. volumetric flask; proportionately larger volumes of chloroform-quinolinolate are required for higher amounts of uranium. Measurement of the Uranium. Slowly drain the combined c>xtracts into the volumetric flask, retaining any water in the funnel. Dilute the chloroform extracts to volume with pure chloroform and miu well. Maintain constant temperature, if desired. Obtain the absorbance a t several preselected wave lengths with a spectrophotometer; with the data obtained from the standard curves, calculate uranium concentration.
The investigation was directly concerned with the determination of uranium in the presence of iron. Other elements which may interfere may be separated by any of many procedures now applicable. However, so many analyses concern uranium with only iron as the important foreign element that the investigation leading to a satisfactory method for the determination of small amounts of uranium in the presence of iron was justifiable. Formation of the Iron Complex. The ferric iron-iodide reaction is infrequently used (19). At a p H of 1 to 2 (low enough to prevent hydrolysis of iron), the iodide and ferric ions react to form ferrous iodide and free iodine. The use of thiosulfate to remove the free iodine from solution permits the ferric-iodide ion reaction to go to completion. The conversion of the ferrous iron to the complex ferrocyanide was a mommtary problem. If the iron content of the test solution was low (5 to 10 mg.), the conversion was satisfactory; but for higher iron contents, a blue precipitate formed. This coloration and precipitation could be prevented only by pouring the test ferrous solution (pH 4) into the cyanide solution. Obviously, the ferrocyanide was reacting with the excess iron present to form the blue ferrocyanide. With the successful formation of the ferrocyanide, a yellow to brown color or cloud formed. This may be attributed to the formation of uranyl ferrocyanide, which later redissolved because of the high alkalinity (pH 9). Elimination of Interference by Iron. Test runs were made with analyzed uranium perchlorate samples to which known amounts of iron were added. In this investigation, an upper limit of 500 mg. of iron per sample was chosen. Table I1 shows that 0.1 to 1.1 mg. of uranium may be determined in the presence of 0 to 500 mg. of iron without interference. Extraction of the Uranium. Foreign Elements. The pH of the solution was adjusted to just 8.9 and was ready for extraction. .4t this p H aluminum, bismuth, cadmium, cobalt, copper, iron, lead, nickel, uranium, zinc, and others were extractable into
ANALYTICAL CHEMISTRY
1372 chloroform with 8-quinolinol. Many of these ions could be removed in a preliminary step. Thus, uranium in a solution which had been treated with pyridine a t pH 6.2 would separate from cadmium, cobalt, copper, nickel, and zinc; aluminum and iron would precipitate along with the uranium. After dissolving the precipitate in acid, and following the procedure for extraction of the uranium, it was found that aluminum accompanied the uranium, as quinolinolate. The point of maximum absorption for aluminum quinolinolate is 395 mp (IO) which would have interfered with the uranium quinolinolate readings. This interference was only partly circumvented by using alternative wave lengths (440 mp) for the readings for uranium. For this reason, instead of plotting a single standardization curve for the uranyl 8-quinolinolate, a series of standard curves a t selected wave lengths was plotted, and the results for uranium were then checked a t several wave lengths. Values for uranyl 8quinolinolate extract, which are nearly identical a t 440 and 420 mp but higher a t 400 mp, would clearly indicate the presence of an interfering quinolinolate. If aluminum was the interfering element and if its content was much less than that of uranium, it could be compensated for by comparing with a mixed uraniumaluminum 8-quinolinolate curve whose readings are obtained a t 400,420, and 440 mp. Tolerances for foreign elements are 500 mg. of iron, 50 mg. of copper, 20 mg. of nickel, 20 mg. of cobalt, and 20 mg. of chromate (iron absent); aluminum content must be less than half that of uranium and corrections may be made. Thorium and zirconium should not be present.
Applications for the Procedure. The procedure was applied t o mixtures of graphite with uranium compounds. The known inipurities in the graphite include silicon, iron, aluminum, calcium, and magnesium, all in small percentages. Boron, titanium, and manganese may have been present in trace quantities. The particular samples tested contained 0.1% or more of chromium. For test runs, a weighed amount of graphite (1.5 grams) was ashed, the residue weighed, then dissolved in perchloric acid. The perchloric acid solution was diluted and duplicate aliquots were taken for colorimetric analysis. Table I11 illustrates the results of these analyses. The second column shows the precision which may be expected for duplicate aliquots by two analysts. When these aliquot figures are multiplied by appropriate factors, the values of milligrams of uranium per gram of graphite are found for the third column. The fourth column is the original ash weight calculated as impure uranium. Colorimetric methods have another advantage over the gravimetric procedure for determination of uranium as the oxide. According to Blitz and Muller ( I , 8), the uranium-oxide ratio changes with temperature, which means that the gravimetric factor for uranium is dependent on temperature control. Table IT'
Table IV. Effect of Variable Furnace Temperature on Gravimetric Determination of Uranium hlilliernms ~....~~-~~~. U by ashing U, ash dissolved in and weighing HClO4 and U de- Difference % ' of Sample as oxide termined by colora Color Vilues Furnace Left at hlarimum Temperature Setting 1 21.5 22.3 +4% 22.4 ~
Table 111. Colorimetric Determination of Uranium in Graphite (Ashed) Milligrams U per Gram Impure U calculated of graphitea from graphite ashb 3.88 4.16 3.88
1
U per aliquot 0.690 0.690
2
0.913 0.920
3.99 4.03
4.38
3
0.635 0.640
4.14 4.18
4.51
4
0.714 0.720
4.57 4.60
5.05
Sample
2
22.0
23.0 23.0
+4.4%
3
20.6
21.5 21.6
+4%
4
25.5
26.6 26.7
+4%
5
26.1
26.9 26.9
+3qo
28.8
28.8 28.8
+I%
1
5
1.270 3 38 5.07 1.290 3.43 Determinations were made at 440 mp. b Each ashed sample contained identical amounts of silica, iron, aluminum, calcium, and magnesium. Amounts of chromium varied.
Temperature Manually Controlled at 850' C. 11.2 11.7 +5% 11.8
Q
2
11.4
11.3 11.4
0
3
13.1
12.9
-2%
12.9
Uranium Curves. The family of curves presented in Figure 1 was obtained from measurements of uranyl quinolinolate [UOZ( C ~ H B O N ) ~ . ( C ~ H , OinN )25-ml. ] volumes with chloroform as solvent. These curves do not exhibit wave lengths of maximum absorption, but do show working ranges for the uranium concentrations. For example, a t 420 mp the absorbance spread is from 0.3 (absorbance) for 0.1 mg. of uranium to 1.3 (absorbance) for 1.5 mg. of uranium. Figure 2 shows that the 440 mp curve should be used for the higher amounts of uranium and that the 400 mp (or 390) curve should be used for the smaller amounts of uranium. If the amount of uranium present is much less than 0.1 mg., then only two 5-ml. portions of 8-quinolinol reagent are required. In this case, an absorbance corresponding to a concentration of 0.05 mg. of uranium in 25 ml. of solution will actually be 0.02 mg. of uranium per 10 ml. of solution. This appears to be the lower operating limit for uranium, The upper operating limit is set by the volume of extractant necessary for the uranium. It is suggested that 0.5 to 2.5 mg. of uranium per 25 ml. of extractant be used. Test solutions which contain more than 3 mg. of uranium should be divided into aliquot port,ions.
5
4
13.1
5
13.2
6
15.5
-2% +2%
- 1%
Determination a t 440 mp.
Table V.
Comparison of Results by Colorimetric Method and Alpha-Counting Technique Sample 1
a
12.9 12.9 13.4 13.5 15.3 15.3
U, Mg. U per M1. Solution 8-Quinolinol Alpha counting0.045 0.050 0,047
2
0.52 0.52
0.51
3
0.55 0.56
0.61 0.63
4
0.62 0.63
0.62
5
0 92 0.93
0.92
Results by C. T. Young, this laboratory.
V O L U M E 25, NO, 9, S E P T E M B E R 1 9 5 3 contains a comparison of colorimetric uranium values obtained by burning mixtures of uranium and graphite in a small furnace. These samples are divided into tn.0 sets, I n the first set, the furnace was set a t maximum and no subsequent adjustment ninde, while in the second set, the temperature was manually controlled a t approximately 850’ C. -1 comparison of results by the 8-quinolinol colorimetric method and the alpha-counting technique mas made. Table V contains these values. Effect of Impurities in 8-Quinolinol. Some samples of 8quinolinol gave colored solutions in chloroform, but part of the original color remained in the aqueous extract. Better grades of the rengeiit werr colorleqs in chloroform solution. Effect of Illumination. The chloroform-quinolinol solution is greatly affected by illumination from fluorescent lighting, and poor grades of 8-quinolinol solution must be discarded after 4 days. On the other hand, uranyl quinolinolate solutions extracted into chloroform from colorless chloroform-8-quinolinol stable if mnintaincd a t 25’ C. for 16 solutions weie found to hours. LITER.4TURE CITED
(1)
Blitz, W.,and Muller, IT., Z . anorg.
16.
allgem. Cheni., 163, 296
(1 927).
( 2 ) Currah. J. E.. and Beamish. F. E.. AXAL.CHEM..19. 609 (1947). ( 3 ) Gerhold, M., ’and Hecht, F., Microchemie veT. Mikrochim. Acta, 36, 1100-5 (1951). (4) Greenspan, J., Schuler, 31. J., Goldenberg, H., Taub, D., and
1373 Carlson, A. S., L7.S. Department of Commerce, Office of Technical Services, Washington, D. C., R e p t . D-12 (April 1 , 1946). (5) Henicksman, A. L., I b i d . , R e p t . LA-1394 (March 15, 1952). (6) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” iiew York, John Wiley & Sons, 1929. (7) Hubbard, B., E. S. Department of Commerce, OfficeofTechnical Services, Washington, D. C., R e p t . CC-2134 (Sept. 15,19441,; (8) Kats, J. J., and Rabinowitz, E., “The Chemistry of Uranium, Part 1 , 1st ed., Kew York. RlcGraw-Hill Book Co., 1961. (9) Lafferty, R. H., McCormick, T . J . , and Myers, H., U.S.DeparG ment of Commerce, Office of Technical Services, Washington, D. C., R e p t . XAC-54-1348 (April 1 , 1946). (10) Rloeller, Therald, IHD.ENG.CHEM..ASAL. ED., 15, 346 (1943). (11) hIuntz, J. A, V. S. Department of Commerce, Office of Technical Services, Washington, D. C., Rept. MUC- JIW-599. (12) Ostroumow, E. -4., 2. a n a l . Chem., 106, 244-8 (1936). (13) Rodden, C. J., “Bnalytical Chemistry of the Manhattan Project,” Chap. 1, New York, AIcGraw-Hill Book Co., 1950. (14) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., Kew York, Interscience Publishers, 1950. (15) Smales A. A, U. S. Department of Commerce, Office of Technical Services, Washington, D. C., Rept. BR-r12 (April 17, 1944). (16) Smales, A. A., and Wilson, H. X . , I b i d . , Rept. BR-150 (Feb. 22, 1943). (17) Tissier, Marguerite, and Benard, Henri, C‘ompt. Tend. SOC. biol., 99, 1144 (1928). (18) . , Ware. E., V. S. Department of Commerce, Office of Technical Services, TTTashington, D. C’., R e p t . MDDC-1432 (August 1945, declassified Nov. 7, 1947). (19) White, L. J., Coke a n d Gas, 14, 285-8 (1942). RECEIVED for review April 16, 1953. Accepted June 22, 1953. Based on studies conducted for the Atomic Energy Commission under C o n t r a c t A T - l l 1-Gen-8. Presented at -4nalytical Chemistry Information Meeting, Oak Ridge, Tenn., M a y 19 to 21, 1953.
Determination of Diacetyl DUANE T. ENGLIS, E,MILY J. FISC“, AND SHIRLEY L. B.4SH University of Illinois, Urbana, I l l . Dimethylglyoxime shows an intense absorption peak near 226 mp in the ultraviolet region of the spectrum. The absorption by the compound follows Beer’s law and serves as a basis for a direct determination of diacetyl in starter distillates or distillates from other food products after its conversion to dimethylglyoxime, but without the supplementary formation of a metal complex. The results obtained with this spectrophotometric procedure compare favorably with those obtained with the gravimetric method in which the nickel compound is formed. The spectrophotometric procedure is of high sensitivity. Concentrations of 0 to 10 p.p.m. may be easily estimated. The method should be advantageous for laboratories equipped with instruments for work in the ultraviolet range.
D
ISCETYL has long been recognized as a constituent occurring in small amounts in vinegar, fermented cane juice, cultured milk products, and a number of other food materials. Because the amount normally present is small, relatively large samples are usually necessary to furnish enough of the compound to make possible a quantitative determination. The widely used method of Barnicoat (2) involves a steam distillation of the diacetyl, its conversion to dimethylglyoxime, precipitation with a nickel salt, and final gravimetric estimation as the nickel compound. Colorimetric methods have also been employed. As an alternative procedure, Barnicoat has dissolved the nickel precipitate in chloroform and estimated the material by a standard series comparison with similarly prepared solutions of known concentration. Stotz and Raborg (8) suggested a sensitive method based upon an assumed tetravalent nickel dimethylglyoxime compound. Okac and Polster (5) confirmed the sensitivity of the method, but differed as to the nature of the colored compound. Mohler and 1
Present address, University of California Loa .4ngeles. Calif
dlmasy (4)detected diacetyl by the specific absorption of the ferric complex. Prill and Hammer (6) employed the rose-red ferrous complex for the quantitative microdetermination of diacetyl. They reported that it was possible to detect as little as 0.001 mg. of diacetyl per 5 ml. of solution and to measure conveniently 0.01 to 0.5 mg. per 10 ml. (1 to 50 p.p.m.) of solution. More recentlv, Speck ( 7 ) has developed a method for diacetyl based on the formation of a purple dye with chromotropic acid in the presence of concentrated sulfuric acid. This method may be used for quantities from 0.03 to 0.10 mg. per ml. (30 to 100 p.p.m.), which is not quite as sensitive as that of Prill and Hammer. A considerable number of substances interfere or similarly respond to the reagents. Diacetyl has only a moderate absorption in the ultraviolet region of the spectrum ( 3 ) . However, when converted to the dimethylglyoxime it has intense absorption. This property appeared to offer promise as a means of determination of diacetyl in various food products without the necessity of conversion to a metallic compound.
