V O L U M E 2 5 , NO, 1 2 , D E C E M B E R 1 9 5 3 Trinitro compounds, such as 1,3,5-trinitrobenzene, are unsuitable because a red color appears merely upon the addition of alkali, in the absence of other reactants. illkaline picrate solutions are less deeply colored and are therefore less objectionable. Alkaline solutions of m-dinitro compounds readily produce colors with many carbonyl compounds, while exhibiting very little color in the presence of base alone. As a rule, ortho-substituted nitro c-ompounds are less reactive than unsubstituted nitro compounds. Multiple ortho substitution serves to inhibit the reactivity of a nitro compound even more strongly-for example, 2,4-dinitromesitylene fails to produce any color in the presence of acetone and sodium hydroxide. For aqueous systems, an alkali-soluble m-dinitro compound, such as 3,5-dinitrobenzoic acid, is a suitable reagent for producing color reactions. In nonaqueous systems, m-dinitrobenzene is probably the most appropriate reagent (12). The reaction leading to the formation of the violet-red creatinine-dinitrobenzoate addition compound is apparently reversible. Color stability is affected by irreversible changes which occur a t the expense of the addition compound. In this process the nitro compound is reduced, largely to azoxy compounds, while creatinine is oxidized to glyoxylate, oxalate, and other products. Another type of irreversible changeis produced concurrently by the introduction of hydroxyl groups into the aromatic ring of the addition product, resulting in destruction of the complex and the produrtion of nitrophenols, which are probably responsible for
1863 the yellow color that appears when a reaction mixture is allowed to stand for some time. ACKNOWLEDGMENT
The author wishes to acknowledge his indebtedness to Isidor Greenwald for advice and guidanre throughout the course of these investigations. LITERATURE CITED
(1) Benedict, S. R., and Behre, J. 4.,J . Bid. Chem., 114, 516 (1936). (2) Bolliger, ii.,J . Proc. Roy. SOC.-V.,S'. Wales, 69, 224 (1936). (3) Carr, J. J., Ph.D. thesis, S e w Pork University, 1952. (4) Carr, J. J., unpublished observations. ( 5 ) Clark, L. C., Jr., and Thompson, H. L., ANAL. CHmr., 21, 1218 (1949). (6) Folin, O., 2. physiot. Chem., 41, 223 (1904). (7) Folin. 0..and Doisy, E. A, J . Bid. Chem., 28, 349 (191i).
(8) International Critical Tables, Vol. I, p. 83, New York, IlcGramHill Book Co., 1926. (9) Jaffe, Il.,Z. physio2. Chem., 10,391 (1886). (IO) Langley, W. D., and Evans, XI., J . Bid. Chem., 115, 333 (1936). (11) Lehnartz, E., 2. physiol. Chem., 271,265 (1941). (12) Zimmermann, W., Ibid., 233,257 (1935).
RECEIVED for review June 13, 1953. Sccepted September 8. 1953. Abstract of a thesis submitted b y t h e author t o the Graduate School of Sew Tork University, June 1952, in partial fulfillment of the requirements for the degree of doctor of philosophy.
Colorimetric Determination of Vanadium(V) and Its Separation from Copper Use of Cupferron HOBART H. WILLARD, ERNEST L. RIARTIN, AND ROBERT FELTHAM' University of New ,Mexico, Albuquerque, N . .M.
This work was undertaken as part of a larger project on the analytical chemistry of vanadium and the use of cupferron with complexing agents. -4 new colorimetric method was developed based on the green color formed by vanadium cupferrate in acetone. Vanadium was separated from copper by precipitation with cupferron at a pH less than 1. This furnishes a good colorimetric method for vanadium in the presence of copper and a convenient separation from copper.
C
UPFERRON has been reported (3, 4 ) as a precipitating agent for vanadium(V) and vanadium(1V) from an acid solution. In most instances the precipitate was filtered, ignited to the oxide, and weighed as such. .4 procedure has been suggested (1) in which the vanadium cupferrate precipitate is dissolved in chloroform and determined by colorimetric means. It was found in this investigation, however, that the color faded on standing. For the determination of relatively large amounts of vanadium this might not be serious, but for the determination of small amounts the error was large. Among other organic solvents tested, acetone was found to give satisfactory results. Iron, titanium, molybdenum, tungsten, tantalum, niobium, palladium, zirconium, antimony(III), bismuth, thallium(III), rare earths, copper, and chromium(V1) interfered. Cupferron was selected as the precipitating agent for vanadium, since it was found to form a relatively stable complex. This complex was not as stable as those formed with iron, titanium, or molybdenum, indicating the possibility that cupferron might 1
Present address, University of Miohigan, Ann Arbor, Mioh.
serve the dual purpose of first removing other elements, under controlled conditions, and then serving as the precipitating agent for vanadium. 4PP4R4TUS 4 Y D Tl4TER14LS
The absorption spectra measurements were made on a Beckman Model B spectrophotometer. A Beckman Model H2 pH meter, the glass electrode of which was calibrated frequently against potassium dihydrogen phosphate buffers, was used for all pH measurements. The colorimetric determinations were carried out n ith the Evelyn photoelectric colorimeter. A solution of ammonium vanadate vias prepared, 1ml of which contained 0.56 mg. of vanadium. From 1 to 25 ml. of this stock solution was used, and the final volume of the acetone solution of vanadium cupferrate was 100 ml. A 6% solution of cupferron was prepared, filtered from dark, insoluble material, and stored in a refrigerator. The acetone and acids were c P. grade. EXPERIV E\TAL
Choice of Organic Solvent. Previous investigators (1) used ether or chloroform to extract the vanadium cupferrate. Most of
ANALYTICAL CHEMISTRY
1864 the solutions in these and other solvents faded on standing. The color produced in ethyl alcohol solution was stable. but the absorption in this solution was not satisfactory. When the vanadium cupferrate is dissolved in acetone at about 20" C. the first color produced is a yellow to brown. Within a few minutes this slowly changes to green. At 30" to 35" C. the change is rapid, but a t 10" C. it may require several days. All measurements reported here have been based on the intensity of the green color. It is believed that the color measured is the result of a reaction between the cupferron and the acetone, and constitutes an indirect method for the determination of vanadium. Therefore, it is absolutely necessary to wash the vanadium c u p ferrate precipitate free of any excess cupferron with 10% sulfuric acid in order to assure accurate results. 50 difficulty has been encountered in this respect. A4fterthe development of the color, the temperature of the solution in the volumetric flask can be adjusted to the temperature selected in the preparation of the standards and the solution can be diluted to volume.
PROCEDURE
The vanadium solution was diluted to 50 ml. and cooled in an ice bath to 0 " to 10" C., and the pH was lowered to less than 1 Filter paper pulp was added along with a calculated excess of freshly prepared 6% solution of cupferron. The pH was again adjusted to less than 1 and the solution allowed to stand for 5 to 10 minutes in the ice bath. The solution was then filtered through a double layer of KO.40 Whatman filter paper and washed with 10% sulfuric acid solution until the washings shon ed no test for cupferron. A 100-ml. volumetric flask was then placed under the funnel and acetone poured over the precipitate on the filter paper. The filter paper was washed clean of the vanadium cupferrate and the final volume diluted to 100 ml. This was allowed to stand for 20 to 30 minutes for the development of the color.
100
90 t
Filtration. When the cupferron precipitate was separated by centrifugation or by use of a filtering crucible, i t was impossible to be certain that the precipitate was free of excess cupferron. If filter paper pulp was added to the solution before precipitation and the solution filtered through a double layer of No. 40 Whatman paper, a clean filtrate was obtained which was free of vanadium. The use of filter paper pulp facilitated the washing of the precipitate. Stability. No variations in the transmittance vere observed over a period of 48 hours. Data for transmittancy-wavelength curves were obtained by measuring the transmittancy in a 1-em. cell, a t room temperature, against an acetone blank, over the range from 350 to 1000 mp (Figure 1). The curves showed a broad band of minimum transmittancy against concentration of vanadium a t 745 mpj and showed good conformity to Beer's 1aJv over the range of concentrations determined (0.0056 mg. to 0 14 mg. per ml.).
80 70
E 60 0
J 50 0
z
2 e 40 5
$ 30 c
20 350
400
500
600
700
800
WAVE LENGTH, my
Figure 1.
Spectral Transmittance Curves for Acetone Solutions of Vanadium Cupferrate
I t does not appear to make any difference when the color is allowed to develop or under what conditions. Studies have been carried out under many different conditions and the results agree regardless of method employed. Solutions have been held at a temperature of 10" C. for 2 days with no tendency to develop the green color, and the volumetric flask was then placed in a water bath at 50" C. and the color allowed to develop. The results check with those in which the color was developed immediately. Sfter the color change begins the reaction is complete in a few minutes. Effect of Concentration of Cupferron. Precipitations carried out with different amounts of cupferron in excess did not affect the results, since the excess was readily washed out of the precipitate with 10% sulfuric acid solution. The wash solutions from the vanadium cupferrate were tested, and washing was continued until the absence of cupferron was assured. pH. Other workers ( 4 ) have stated that complete precipitation is obtained from solutions in which the p H varied from 0 to 3. I n all this work the pH was less than 1 and complete precipitation was obtained. Temperature, Since cupferron solutions are not stable a t elevated temperatures (3), all precipitations were carried out a t 0" to 10' C. The precipitates were filtered immediately to prevent any decomposition of the precipitate or of the excess cupferron. When this procedure was followed, a clean precipitate was obtained. If this precipitate was allowed to remain on the filter paper for over an hour a t 38" to 43" C., there were definite signs of decomposition, but a t temperatures of 21' to 27" C. this was not serious. After the precipitate was dissolved in acetone, temperature had no effect.
The results are shon-n in Table I. The operations should br carried out without undue delay, especially if the room is unusually warm, because cupferrate precipitates tend to decompose on standing. COLORIMETRIC DETERkZINATION OF VAN4DIU31 IN THE PRESENCE OF COPPER
I n addition to precipitating vanadium from an acid solution, cupferron causes precipitation of many metal complexes insoluble in acids. The stability of these complexes varies widely. -4 method has been developed for the separation of copper from vanadium(V), depending upon the fact t h a t when a salt of ethylenediaminetetraacetic acid (Versene) is added to a mixture of copper(I1) and vanadium(V), chelation occurs. The copper coniplex is sufficiently stable so that copper(I1) will not precipitate with cupferron, but vanadium cupferrate will precipitate quanti. tatively. The vanadium can then be determined colorimetrically
Table I. Determination of Vanadium Taken 0.57 0.57 2.85 2.85 2.83 2.85 6.27 6.27
Yanadium(V), X g . Found 0.57 0.59 2.94 2.88 1.96 2 83 8.27 6.13
Error 0.00 +0.02 +0.09 +0.03 +O.ll -0.02 0.00 -0.14
Table 11. Determination of Vanadium(V) with Cupferron in Presence of Copper" Taken
a
l-anadium(V), M g . Found
2.9 2.9 5.7 5.7 8.5 8.5 3.2 mg. of copper were present
3.0 3.1 5.7 5,s 8.5 8.3
in each determination.
Error
+0.1 +0.2 0.0
+O.l
0.0 -0.2
V O L U M E 2 5 , NO. 12, D E C E M B E R 1 9 5 3 by the method already described with an average error of &2.5%. The use of Versene forthe separation of vanadium from chromium, iron, and other elements will be reported later. APPARATUS 4ND MATERIALS
Copper sulfate solution. A solution of copper sulfate was prepared containing 0.63 mg. of copper per ml. Ethylenediaminetetraacetic acid, tetrasodium salt (Versene). A 10% solution prepared from a 34y0 solution was used in these determinations. The Versene was obtained from the Bersworth Chemical Co.
1865
precipitation of the copper was obtained by the addition of 10 ml. of the 6% cupferron solution, even when the pH was less than 0.1. Precipitation of Vanadium in Presence of Copper. In solutions containing varying amounts of vanadium and co per, the precipitation of the vanadium was carried out a t a pH Ess than 1.0, and the vanadium cupferrate precipitate was 6rst washed with a 2% Versene solution, adjusted to a p H of 1 to 1.5, and finally washed with 10% sulfuric acid. The results obtained for the vanadium checked well with the work on solutions which contained vanadium alone (Table 11). The acetone solution of vanadium c u p ferrate was checked for the presence of copper by the dithizone method as described by Sandell ( 2 ) and no copper was found. PROCEDURE
EXPERIMENTAL
Effect of Concentration of Versene Solution on Precipitation of Vanadium. I t has been reported that vanadium precipitates in solutions with a pH of 3 to less than 0 ( I , 3, 4). In the presence of Versene there was no precipitation of the vanadium cupferrate until the pH was below 1.5, and a t a pH of 0.5 precipitation was complete. From 2 to 20 ml. of 10% Versene solution was added to solutions containing 0.0255 mg. of vanadium(V) per ml. The vanadium(l7) precipitated quantitatively with cupferron even in the presence of a large excess of Versene, and the excess could be washed readily from the precipitate. Effect of Versene Solution on Precipitation of Copper with Cupferron. In solutions rontaining 5 ml. of the standard copper solution and from 2 to 10 ml. of the 1 0 7 Versene solution, no
To a solution containing the vanadium and copper salts was added 10 ml. of 10% Versene solution, and the procedure described \vas followed. LITERATURE CITED
(1) Bertrand, D., BzdL
SOC. chim. France, 9, 122 (1942). (2) Sandell, E. R., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 296, Kew York, Interscience Publishers, 1950. (3) Smith, G. F., “Cupferron and Keo-Cupferron,” G. Frederick
Smith Chemical Co., Columbus, Ohio. Drexler, S. J., ,J. Opt.
(4) Strock, L. R., and (1941).
Soe. Amer., 31, 167
R E C E I V Efor D review June 10, 1953. Accepted October 23, 1953.
Automatic Measurement of light Absorption and Fluorescence on Paper Chromatograms J44IES A. BROWN1AND MAX 31. ,MARSH Eli Lilly and Co., Indianapolis, Ind. Previous automatic devices for scanning paper strip chromatograms have either been of the intermittent, nonrecording type or have required the use of an automatic recording spectrophotometer. The attachment described is designed for use with a commercially available monochromator employing a stabilized light source such as the Beckman Model DU spectrophotometer. An innovation in the assembly described is the use of interference filters to
T
HE application of the paper chromatographic technique to
the quantitative as well as qualitative evaluation of complex mixtures continues to receive considerable attention. Recent studies have resulted in the construction of several devices for the scanning of paper strips from the standpoint of light absorption measurements (3-6). The present work represents a continuation of some preliminary observations reported earlier ( 2 )and is an attempt to broaden the utility of the automatic technique, by providing for the measurement of fluorescence on paper strips and by incorporating components which are more accessible to the average laboratory. DESCRIFTIQN OF APPARATUS
The attachment for automatic scanning consists of five major parts-a spacer, a scanning chamber, a strip transporting mechanism, a photomultiplier tube detector with amplifier, and a strip chart recorder. Since paper strip chromatograms can be prepared most conveniently on strips 0.5 inch wide ( 1 ) . the first requirement in designing a scanner was to provide a beam of light a t least that 1
Present addrese, Truesdail Laboratories, Inc., Los Angeles. Calif
permit the measurement of fluorescence on paper strips. Strip chart records, both of absorption us. strip length and of fluorescence us. strip length, have been obtained. The reproducibility of results appears to be better than that obtained with other scanningdevices. Byplottingper cent transmittance rather than absorbancy, greater sensitivity is observed in evaluating low concentrations of absorbing materials on the strips.
wide through which the strip could be transported. Examination of the characteristics of the light beam emitted by the Beckman DU monochromator used in this work showed that in the vicinity of the absorption cells as normally placed in the cell compartment the beam is a vertical rectangle about 1/8 X 3/g inch. K i t h the hydrogen lamp as a source, this beam diverges in the horizontal dimension and converges in the vertical dimension as the distance from the exit slit increases, and a t a distance of about 8 inches it forms a slightly diffuse horizontal rectangle about 3,’~8 X 5 / / ~ inch. With the tungsten lamp as the source, the image a t the %inch distance is a vertical ellipse about )/8 inch wide and 1 inch high but with the brightest portion concentrated X 5 / / ~ inch. Thus, it in a diffuse horizontal rectangle about appeared feasible to transport the strip vertically through the beam a t a distance of 8 inches from the block supporting the filter slide and exit slit. An alternate method would be to place the scanner closer to the block and alter the beam with lenses but this appeared to be an unnecessary complication. The use of the beam at the 8-inch point has a t least two advantages. First, the chamber and scanner are located where there is no interference with the light source housing; secondly, it allows for the transporting of thestripverticallythroughthe beam. Thus, provision is
