Ultraviolet Spectrophotometric Determination of Chromium as the Peroxychromic Acid-2,Z-Bipyridine Complex Gordon A. Parker1 and D. F. Boltz Department of Chemistry, Wayne State University, Detroit, Mich. 48202
of the peroxy complexes, the nature of the complex formed by peroxychromic acid and 2,2'-bipyridine was investigated. As a result of this investigation, a new ultraviolet spectrophotometric method for the determination of trace amounts of chromium is proposed. Lassner and Puschel have reported the preparation of complexes between the peroxy complexes of titanium, niobium, and tantalum with nine different metallochromic indicators (1). Two previous studies utilized the infrared spectrum of the peroxychromic acid-2,2'-bipyridine complex as an aid in elucidating the structure of peroxychromic acid (2,3). Of the following ligands, pyridine, piperidine, 4-(2-pyridyl azo)-resorcinol (PAR), pyrocatechol violet, xylenol orange, bromothymol blue, phenylarsonic acid, anthranilic acid, 1 $10-phenanthroline, and 2,2'-bipyridine which were tried as the auxiliary ligand with peroxychromic acid, only 1 ,IO-phenanthroline and 2,2'-bipyridine exhibited a significant change in the ultraviolet spectrum of the complex as compared to that of the ligand used. The peroxychromic acid-2,2'-bipyridine complex was selected for further study because its absorbance maximum occurs at a longer wavelength and is more removed from the cut-off wavelength of the ethyl acetate, the most suitable extractant for the complex. IN A SPECTROPHOTOMETRIC study
EXPERIMENTAL
Apparatus. Absorbance measurements were made with a Cary Model 14 spectrophotometer in 1.000-cm silica cells. Solutions. Standard Chromium(V1) Solution. Dissolve 0.0934 gram of reagent grade potassium chromate, which has been dried at 110" C for 2 hours, in distilled water and dilute to 1 liter. This solution contains 25.0 ppm chromium (VI). 2,2'-Bipyridine Solution. Dissolve 50 mg of reagent grade 2,2'-bipyridine in distilled water and dilute to 500 ml. This is a 0.01 solution of 2,2'-bipyridine. Sulfuric Acid Solution. Dilute 13.9 ml of commercial reagent grade sulfuric acid to 250 ml of solution with distilled water. This solution is 1.OM in sulfuric acid. Diverse Ion Solutions. Weigh a reagent grade sample of each of the various materials tested, dissolve in distilled water, and dilute to a specific volume. Prepare stock solutions containing 1000 ppm of each ion (or less for those species in which solubility prevents this amount from dissolving) and use an appropriate aliquot for each interference test. Hydrogen Peroxide Solution. Commercially available reagent grade 3 % hydrogen peroxide was used. Ethyl Acetate. Reagent grade. Present address, Department of Chemistry, University of Toledo, Toledo, Ohio 43606. (1) E. Lassner and R. Puschel, Mikrochim. Ichnoanal. Acta, 753 (1 ,- 9M). - - .,. (2) J. E. Ferguson, C. J. Wilkins, and D. F. Young, J. Chem. SOC. 1962, 2136. (3) W:P. Griffith, Ibid.,p 3948.
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ANALYTICAL CHEMISTRY
280
320 340 WAVELENGTH, r n N
300
360
Figure 1. Absorption spectrum of peroxychromic acid-2,2 '-bipyridine complex 1. 9 ppmof Cr 2. 6 ppmof Cr 3. 3 ppmof Cr
Recommended General Procedure. Treat the sample by appropriate means so that a solution of chromium(V1) results. Transfer an aliquot of this solution (choose the aliquot so that its volume is less than 20 ml and the final chromium concentration per 25-ml sample will be within the optimum concentration range of from 2 to 5 ppm chromium) to a 125-ml. separatory funnel and acidify with 1 ml of 1.OM sulfuric acid. Add sufficient distilled water to bring the total volume to 20 ml and then add about 20 ml of ethyl acetate. Cool the funnel and its contents at IO" C for 0.5 hour. After cooling, add 1 ml of a 3 % solution of hydrogen peroxide which has also been cooled at 10" C and immediately extract for 30 seconds by vigorously shaking the separatory funnel. After the layers have separated, discard the aqueous layer. Add to the ethyl acetate layer 10 ml of a 0.01 aqueous solution of 2,2'-bipyridine which has also been cooled at 10" C and immediately extract for 30 seconds by vigorously shaking. After the layers have separated, discard the aqueous layer and transfer the ethyl acetate layer to a 25-ml volumetric flask. Dilute to the mark with additional ethyl acetate. Shake and allow any water droplets in the flask to settle. Transfer a sample from the upper portion of the flask to a spectrophotometer cell and measure the absorbance at 308 mp. Use a reagent blank prepared in the same manner as the sample but containing no chromium as the reference solution.
RESULTS
Effect of Variables. CHROMIUM CONCENTRATION. The ultraviolet absorption spectrum of the peroxychromic acid2,2'-bipyridine complex is shown in Figure 1. The peroxychromic acid-2,2'-bipyridine complex shows conformity to Beer's law for the concentration range 0-10 ppm chromium. A Ringbom plot ( 4 5 ) for peroxychromic acid-2,2'-bipyridine shows the optimum concentration range for chromium by this method to be from 2-5 ppm chromium. The molar absorptivity of the peroxychromic acid-2,2 '-bipyridine complex is 7.89 X lo3 liter per centimeter per mole. 2,2'-BIPYRIDINE CONCENTRATION. The effect of varying the 2,2 '-bipyridine concentration upon a given concentration of chromium was investigated. It was found that incomplete reaction occurs until the chromium, as peroxychromic acid, and 2,2'-bipyridine are present in a 1 to 1 molar ratio. An excess of 2,2'-bipyridine does not affect the absorbance at 308 mp. Time and Temperature. The necessity of cooling solutions to 10' C or below before formation of peroxychromic acid has been established. Even with this precaution, aqueous solutions of peroxychromic acid begin immediate decomposition while ethyl acetate solutions of peroxychromic acid are fairly stable for about 30 minutes. This complex shows only negligible changes (a change of & 3 S X or less is considered negligible) in the absorbance value at 308 mp for at least 24 hours. Once the complex has formed, it may be kept at room temperature without danger of decomposition. Other Variables. Brookshire and Freund (6) list the following optimum conditions for peroxychromic acid extraction : hydrogen peroxide concentration, 0.02 molar; pH of aqueous solution, 1.7 ZIZ 0.2; and number of extractions, 3. Using the recommended general procedure results in a hydrogen peroxide concentration of 0.04 molar, a pH of 1.0, and 1 extraction. Although these conditions differ slightly from those recommended, they are conveniently achieved without undue precautions and the results obtained are satisfactory and reproducible. Diverse Ions. The effect of the presence of other species was studied using a solution containing 3 ppm of chromium (VI). The diverse ion solution was added to the chromium(V1) solution, the resulting solution treated to form the peroxychromic acid-2,2'-bipyridine complex, and the complex extracted. A relative error of i3.5 or less in the observed absorbance was considered negligible. The following ions present to the extent of 1000 ppm resulted in a negligible error: acetate, aluminum, ammonium, arsenic(V), bromate, bromide, cadmium, calcium, cerium(III), chlorate, chloride, citrate, iodate, iron(III), lithium, magnesium, manganese(II), nitrate, perchlorate, phosphate, potassium, sodium, strontium, sulfate, thallium(I), and thorium(1V). Table I lists the interfering ions. DISCUSSION
Under the conditions specified, the method is applicable to the determination of 2-5 ppm of chromium but has been shown to obey Beer's law over the range from 0-10 ppm of chromium. Once the peroxychromic acid-2,2'-bipyridine complex has been formed, it is stable. The recommended conditions for the formation of peroxychromic acid differ only slightly from those recommended by Brookshire and Freund. (4) A. Ringbom, 2. Anal. Chern., 115, 332 (1939). (5) G . H. Ayres, ANAL.CHEM., 21,652 (1949). (6) R. K. Brookshire and H. Freund, Ibid., 23, 1110 (1951).
Table I. Interfering Diverse Ions Amount added,
Relative error,
PPm 20 40 220 20 40 100 20 lo00 40 20 800 40 240 20 20 40 10 1200 10 20 24 lo00 640 lo00
- 87 -5
- 10 -4 -7 -5 -7.5 -13 - 13 -5 0 - 10 -5 +4.5 -4.5 - 25 - 14 -8 -5.5
600
a
20 40 20 20 20 40 20 20 20 20 20 Ethylenediaminetetraacetate.
-5 -4 -7.5 - 24 - 14 -4 - 100 -100
Permissible amount, PPm 8 4 20 8 4 20 4 100 4 4 800 4 24 2 8 4 4
200 4 4 12 500 16 500 20 0 0 0 0 0 0 0
-60
- 100 -100
- 100 - 16 - 100 - 21 - 100 - 100
0 0 0
0
A pH of 1 for the aqueous solution prior to the addition of hydrogen peroxide was found suitable in this procedure. Also only one extraction is recommended instead of three extractions for the complete removal of the chromium from the aqueous layer. Glasner and Steinberg also recommended one extraction in their method for the determination of chromium as peroxychromic acid (7). There are three major effects exhibited by those ions found to interfere with this proposed method. First, certain ions such as barium and lead(I1) form precipitates with the sulfuric acid. The possibility of circumventing this interference by using perchloric acid or hydrochloric acid was not investigated. Another interference arises with those ions which react with either chromium(V1) or the hydrogen peroxide. Iron(II), arsenic(III), and manganese(VI1) are typical examples. A third type of interference is caused by ions which also form peroxy complexes exhibiting ultraviolet absorptivity-cg., molybdenum(V1) and titanium(1V). Vanadium(V), although forming a peroxy complex, does not interfere. This proposed ultraviolet spectrophotometric method based on the peroxychromic acid-2,2'-bipyridine complex is more sensitive than methods based on the formation of peroxychromic acid. This method compares favorably in sensitivity with the direct spectrophotometric determination of chro(7) A. Glasner and M. Steinberg, Ibid., 27, 2008 (1955). VOL 40, NO. 2, FEBRUARY 1968
e
421
mium(V1) as chromate ion and, in addition, does not suffer from as many interferences. It is more sensitive and less subject to interferences than the spectrophotometric determination of chromium(V1) as dichromate (8). The peroxychromic acid-2,2'-bipyridine method is not as sensitive or as simple as the s-diphenylcarbazide method for chromium (9). All three
methods are subject to some interferences from excess amounts of certain metal ions, but in this method fairly large amounts of vanadium do not interfere as in the s-diphenylcarbazide method. An indication of the precision of this method for the determination of chromium was found by determining 16 samples containing 3.0 ppm of chromium with a standard deviation of 0.016 or a relative standard deviation of 3.5%.
(8) G . Charlot, "Colorimetric Determination of Elements," Elsevier Publishing Co., Amsterdam, 1964, pp 226-30. (9) E. B. Sandell, "Colorimetric Determination of Traces of Metals," 3rd Ed., Interscience, New York, 1959, pp 390-8.
RECIEVED for review August 22, 1967. Accepted November 13, 1967. Presented at the Anachem Conference, Detroit, Mich., October 3-5, 1967.
Oxidation of Some Arylamines by Copper(l1) in Acetonitrile Byron Kratochvill and David A. Zatko2 Department of Chemistry, University of Wisconsin, Madison, Wis. 53706
DIPHENYLAMINE was suggested as an oxidation-reduction indicator for dichromate titrations by Knop in 1929 (1). The more convenient water-soluble sulfonic acid derivative was introduced in 1931 by Sarver and Kolthoff (2). The mechanism of the oxidation of diphenylamine by dichromate had been studied earlier by Kolthoff and Sarver (3), who established that the reaction takes place in two steps, first formation of diphenylbenzidine, then diphenylbenzidine violet.
H
Diphenylbenzidine
0fW-Q / \
/ \
H H Diphenylbenzidine Violet
In the second step, an insoluble, unstable green intermediate was obtained which was postulated to be a 1:l diphenylbenzidine-diphenylbenzidine violet adduct. This green intermediate has been suggested as an indicator because of its small blank correction, but it must be used as a suspension in water ( 4 ) . The exact nature of the intermediates formed in the homogeneous oxidation of diphenylamine to diphenylbenzidine violet has not been completely clarified, although some data have been obtained by chronopotentiometry (5, 6). Additional information can be gotten by using aprotic nonaqueous solvents to reduce solubility difficulties and permit assessment of hydrogen ion effects. Oxidation by copper (11) in acetonitrile has been investigated by 1 Present address, Department of Chemistry, University of Alberta, Edmonton, Alberta. * Present address, Department of Chemistry, University of Alabama, University, Ala., 35486.
(1) J. Knop, J . Am. Chem. SOC.,46,263 (1924). (2) L. A. Sarver and I. M. Kolthoff, Ibid., 53, 2902 (1931). (3) I. M. Kolthoff and L. A. Sarver, Ibid., 52, 4179 (1930). (4) H. H. Willard and P. Young, ANAL.CHEM., 5, 154, 158 (1933). ( 5 ) R . N. Adams, J. H. McClure, and J. B. Morris, Ibid.,30,471 (1958). (6) H. B. Mark, Jr., and F. C. Anson, Ibid., 35,722(1963).
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ANALYTICAL CHEMISTRY
potentiometric titrations and electron spin resonance spectrometry in order to evaluate solvent effects on the reactions of diphenylbenzidine and related compounds and to determine the analytical applicability of copper(II), thus extending the range of this reagent over that reported previously (7). EXPERIMENTAL
Reagents. Acetonitrile (Matheson, Coleman, and Bell, Norwood, Ohio) was purified by the method described previously (7). The water content, determined by Karl Fischer titration, was less than 5 X 10-4M. Anhydrous copper(I1) perchlorate solutions were prepared by dissolving tetrakis(acetonitri1e) copper(I1) perchlorate, prepared from nitrosyl perchlorate and copper metal, in anhydrous acetonitrile (7). The solutions were dispensed from automatic burets and were protected from the atmosphere by drying tubes of magnesium perchlorate. Where exclusion of water was not required, solutions were prepared by dissolving hexaquo copper(I1) perchlorate in commercial acetonitrile, filtering to remove a small amount of white precipitate that formed, and storing the solution in a glass stoppered bottle. Tetramethylbenzidine (TMB) (Eastman Organic Chemicals, Rochester, N. Y.)was recrystallized from acetonitrile before use (m.p. 195-6"; reported 198"). Diphenylbenzidine (DPB) (G. Frederick Smith Co., Columbus, Ohio) was recrystallized from acetonitrile (m.p. 248" ; reported 245 "). Tetramethylbenzidine Orange (TMB Oranges 2), the twoelectron oxidation product of TMB, was prepared by potentiometric titration of a TMB suspension in acetonitrile with copper(I1) to the second equivalence point, filtration of the orange-red needles that formed, and recrystallization from warm acetonitrile. Tests for trace copper in the product were negative. Diphenylamine (DPA) (Eastman Organic Chemicals) was used as received (m.p. 53.0-53.2"; reported values range from 51-55 "). Titrations of diphenylamine recrystallized from an ethanol-water solution (m.p. 53.0-53.2"), and of recrystallized material which was vacuum sublimed (m.p. 53.1-53.4") gave essentially the same results. Apparatus. A 75-ml weighing bottle with a Teflon lid was used as a titration cell. A platinum wire indicating electrode and a silver-0.0100M silver nitrate in acetonitrile reference electrode were used in conjunction with a k e d s & Northrup Model 7401 pH meter, and all potentials are reported relative to this reference couple. The reference electrode was isolated by two ultra fine glass frits and a 0.1M (7) B. Kratochvil, D. A. Zatko, and R. Markuszewski, ANAL. CHEM., 38, 770 (1966).
