Spectrophotometric Determination of Cobalt with 1,2,3-Cyclohexanetrione Trioxime W. JOE FRIERSON, NANCY PATTERSON, HARRIET HARRILL, and NINA MARABLE Department of Chemistry, Agnes Scoft College, Decatur, Ga.
b The purpose of this investigation was to study the reactivity of 1,2,3cyclohexanetrione trioxime with inorganic ions, in particular the cobalt(l1) ion which reacts with it to give a stable yellow complex. This complex does not give an absorption maximum, but shows increased absorbance from 575 to 375 mp. The wave length selected for this study was 400 mp, since the reagent shows essentially no absorbance a t this value. The reaction is carried out in a solution with the p H adjusted between 3 and 4. The optimum concentration range for cobalt in the colorimetric procedure developed is 1 to 4 p.p.m. O f the diverse ions checked, nickel, iron, and copper interfere. A preliminary study has been made of this reagent for the determination of these three elements.
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HE FOLLOWING COMPOWNDS have been reported as uscful for the spectrophotometric drtermination of cobalt: N',N'-bis(3-dimethylaminopropy1)dithio oxamide @), nitroso-R salt (7), 1-nitroso-2-naphthol (8), 2nitroso-1-naphthol (9), oxamidoxime (6), potassium thiocyanate (I), 1(%pyridylazo)-2-naphthol ( 2 ) . In this paper the application of a new Eastman reagent, 1,2,3-ryclohexanetrione trioxime, to the detwmination of cobalt will be discussed.
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EXPERIMENTAL
Apparatus. Absorbance measurements in connection with thc study of the effects of pH and wave length were made on the Bausch and Lomb Spectronic 20 spectrophotometer. All other absorbance measurements were made with a Bcckman Model DU spectrophotometer with matched 1.00cm. Corex cells. The DU was operated a t varying sensitivity using a slit width of approximately 0.5 mm. All pH measurements were made with a Beckman Zeromatic pH meter, with glass electrodes. A syringe microburet, Model No. SB2, was used for measuring volumes of solution. Reagents. The standard solution of cobalt was prepared by drying Baker Analyzed reagent grade cobalt chloride hexahydrate (CoC1&HzO) of 99.3% purity a t 140" C. for a period of several hours. A t this tcmpcrature the salt goes to the blue anhydrous
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ANALYTICAL CHEMISTRY
form. Dissolve 55.40 mg. of this dried salt in 250 ml. of redistilled water to give a 100-p.p.m. solution of cobalt. Dissolve 147.3 mg. of Eastman Organic Chemical No. 7660 (white label), 1,2,3 - cyclohexanetrione trioxime, in 100 ml. of 95% ethyl alcohol. This gives a colorless solution of 8.6 X 10-3211 concentration. Solutions of several ions tested quantitatively for interference were prepared a t concentrations of 100 p.p.m. from reagent grade chemicals or from the metals. The following were used: aluminum metal, antimony trichloride, ber llium oxide, bismuth nitrate pentahy%ate, chromic nitrate nonahydrate, chromous sulfate heptahydrate, copper sulfate, germanium dioside, irov metal, magnesium sulfate, mangaqese sulfate monohydrate, mercurous nitrate monohydrate, mercuric nitrate monohydrate, nickel nitrate hexahydrate, and zinc oxide. Procedure. Solutions of known concentration were prepared so that 4-ml. aliquots eventually diluted to 10 ml. would contain a concentration of cobalt within the range of 1 to 4 p.p.m. These aliquots were pipetted into 50-ml. beakers, 0.8 ml. of reagent was addcd, and the pH was adjusted to the desired range. The solutions were transferred to 10-ml. volumetric flasks and diluted to the mark. After 30 minutes, absorbance readings were taken a t 400 mp against a reagent blank
From the average of the absorbance readings from four aliquots for each known concentration of cobalt, the Beer's law graph was prepared. The color reaction shows adherence to Beer% law from 0.025 to 5.0 p.p.m.; however, when readings are made between 20 and 80% absorbance, the concentration range a t 400 mp is 1 to 4 p.p.m. RESULTS
Color Stability. Cobalt reacts with 1,2,3-cyclohexanetrione trioxime to give a yellow complex which ie stable t o light. No change was detectable in the absorbance readings taken on 10 different solutions before and after they were exposed to light for 18 hours. The reaction also is unaffected when tlie solution is heated to 80" C. prior to taking absorbance readings. Rate of Reaction. Absorbance readings taken a t 5-minute intervals reach a maximum approximately 15 minutes after the reagent is added t o the cobalt. I n order to ensure complete color development, each solution should be allowed to stand for 30 minutes after addition of the reagent before the absorbance is measured. Wave Length. A study of the absorption spectrum of the cobalt complex from 285 to 575 ma revealed no absorption maximum. The working portion of the curve is shown in Figure 1. The absorbance of the complex read against a reagent blank decreases steadily as the wave length increases. The reagent shows almost no absorbance from 400 to 550 mp, but the absorbance increases sharply below 375 ma, approaching the absorbance of the complex a t 293 ma. As a compromise between the maximum absorbance for the complex and the minimum absorbance for the reagent, 400 mfl was selected as the wave lcngth for making all subsequent readings. Effect of pH. The effect of hydrogen ion concentration was investigated over a p H range of 0.08 to 11.0 (Figure 2). Solutions containing 0.15 ml. of a 100-p.p.m. solution of cobalt, '-4- 4 . \ .- - ___ __ * -*--e _-_ 1 mi. of reagent, and enough water to 375 425 75 575 WAiC L E I 6 permit immersion of the electrodes were adjusted to the desired pH with Figure 1. Absorbance curve of cobalt either dilute hydrochloric acid or dilute complex and reagent sodium hydroxide. These solutions 0 Complex w r e then transferred to 10-ml. volA Reagent 4
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umetric flasks and diluted t o the mark. Absorbance measurements were then made against a reagent blank at 400 mp, The complcx exhibits maximum absorbance betwcen the p H values of 3 and 4. Solutions containing the reagent alone were also adjusted to various pI1 values and absorbance readings were made against a water blank. Above pH 7 the reagent has a yellow color similar to that of the complrx and the absorbance is quite high. In the p H range from 2 to 6 the reagent is colorless and shows essentially no absorbance. Amount of Reagent. When the 1,2,3-~yclohexanetrionetrioxime concentration is 8.6 X 10-JM, 0.25 ml. of the solution is necessary to complex 15 pg. of cobalt. This value was found by adding varying volumes of the reagent to a constant volume of the cobalt solution until maximum absorbance was attained. In order t o ensure the necessary excess of reagent when the cobalt concentration is 4 p.p.m. or greater, 0.8 ml. of the reagent was added to each subsequent sample. Nature of Color Reaction. The combining ratio of cobalt and the reagent was investigated by two methods: the mole-ratio method of Yoe and Jones (10) and the continuous variation method of Job as modified by Vosburgh and Cooper (6) (Figures 3 and 4). These results show that cobalt reacts with 1,2,3-cyclohexanetrione trioxime in a 1 to 3 ratio. Unknowns. I n order to test the accuracy of the method, nine solutions of concentrations unknown to the analyst were prepared. Three to four aliquots were taken from each and treated according to the procedure described previously. The average
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Figure 2, complex
Effect of pH on cobalt
A
Complex Reagent
Table I. Determination of Cobalt in Solutions of Unknown Concentrations Cobalt, pg.
Applied 28.0 40.0 22.0
Found 28.0 39.8 21.7
38.4 18.4
38.3 18.4
25.6
25.7
30.4 17.6 36.0
30.0 16.8 35.4
standard deviation was 0.72% and the maximum deviation was 1,46%. Results are recorded in Table I. DISCUSSION
The spectrophotometric determination of cobalt with 1,2,3-cyclohexanetrione trioxime offers several advantages. The method is simple and
Figure 3. Mole-ratio method applied to cobalt complex
accurate as is evident from thc stcps in the proccdure and the deviations recorded for the unknowns. Solutions of both the reagent and the complex are stable, which makes it possible to kecp a stock reagent solution on hand and to make absorbance readings a t any convenient time after thc reagent nnd the cobalt have been mixed and nllowd to stand 30 minutes. The pH range is broad enough to minimize tiny error due to this factor. Although the optimum concentration rang0 for making absorbance rradings at 400 mp is not very broad, this rangc may be extrnded by making the readings a t a higher wave length. Since the reagent is colorless in solution and shows very little absorbance a t the recommended wave Icngth, the nrcessaq excess of reagent introduces only minimum error in absorbance rcadings. If a large amount of cobalt is prescnt, the complex tends to precipitate; however, it is unlikcly that the method would be used under these conditions. This precipitation may be avoided by the addition of some protective colloid
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t
Figure 4. Continuous variotion method applied to cobalt complex
such as gelatin. I n connection with the color reaction, when the reagent is first added to the cobalt, a brownishpink color forms; then the color changes rapidly t o yellow. Since the formation of octahedral complexes with cobalt(I1) involves the promotion of an electron from a third to a fourth orbital, this makes the cobalt(I1) easily oxidizable to cobalt(II1) (4). In fact, the oxidation oceurs with such ease that it is feasible to suggest that the cobalt in the complex will be oxidized in air, thus accounting for the intermediate color. A number of diverse ions have been investigated quantitatively for the possibility of interference. Those which showed no interference a t concentrations as high as 4.00 p.p.m. were aluminum, antimony, beryllium, bismuth, chromium(I1 and 111), germanium, magnesium, manganese, mercury(1 and II), and zinc. Copper, nickel, and iron(II1) interfere a t concentrations down to 0.5 p.p.m. With the reagent, copper forms a brown complcx and nickel forms a purple complex. Iron forms a complex which varies from pink through purple to yellow, depending on the pH. The copper complex shows decreasing absorbance between 385 and 665 mp. The nickel complex shows maximum absorbance a t 420 and 560 rnl. The iron complex shows maximum absorbance a t 520 mp. Tests were made on spot piates with 1,2,3-cyclohexanetrione trioxime and the following 22 inorganic ions: Ag+, As+a, Ba+?, Ca+2, Cd+l, Cs+, Gd+*, La+*, Li+, Nd +?, Rb+, Reo4-, Rh+* S C + ~Sm+a, , Sn+‘, Sr+l, Ta04-8, Th+‘, T1+, WO4-2, Y+*. No positive results were observed in any of these tests which were made by adding one drop of the reagent t o one drop of the inorganic ion solution containing 0.1 mg. of the metal per mi. of solution. VOL. 33, NO. 8, JULY 1961
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ACKNOWLEDGMENT
Financial support from the Rescarch Corp. and the National Science Fomdation i R gratefully acknowledged. LITERATURE CITED
(1) Brown, W.B.,Steinbach, J. F., ANAL, CHEW31, 1805-6 (1959).
(2) Coldstein, G., Manning, D. R., Menis, O., Ibid., 31, 192-5 (1959).
(7) Welcher F. J., "Organic Analytical
Reagents," Vol. 3,p, 332,Van Nostrand, Princeton, N. J. 8 ) Ibid., [9) Yoe, iof'Barton, C. J., IND.ENQ. CHEM.,ANAL.ED. 12, 405-9 (1940). (10) Yoe, J. H.,Jonee, A. L., Ibid., 16, 111 (1944).
(3) Jacobs, W.D.,Yoe, J. H., Anal. Chim. A c h 20, 43543 (1959). (4) Paulin Linu? "The Nature of the Chemic&?Bond, p. 149, Cornell University Press, Ithaca, N. Y., 1980. (5) Pearse, G. A., Pflaum, R. R., ANAL. CHEM.32,213-16 (1960). (6) Vosburgh, W. C., Coo er, R. U., J. An.Chem. Soc. 63,437 rl941).
fI'
RECEIVED for review September 12, 1900. Accepted March 10, 1961.
Stability of the Curcumin Complex in Boron Determination D. EMERTON WILLIAMS and JAMES VLAMIS Deparimenf o f Soils and Plant Nutrition, Universiiy o f California, Berkeley, Calif.
b The determination o f , boron using curcumin-oxalic acid reagent i s somewhat limited b y a rather rapid hydrolysis of the complex at room temperature, necessitating reading of samples within a 2-hour period. The stability of the dry complex or of alcoholic solutions of the complex when stored at room temperature and at 0 ' C. was investigated. Samples stored in akohol were stable for 5 days at 0" C. but for only a few hours at room temperature. Dry samples stored up to 3 days at. room temperature and at 0' C. gave the same readings after 2 hours in alcohol as freshly dried samples. Samples stored dry at room temperature for 1 to 4 weeks had a 2-hour lag period before maximum color was regained. Because of this lag, dry storage samples should not be read sooner than 2 hours following alcohol addition.
ethyl alcohol, the stabilit was compared a t room temperature a n i a t 0" C. The sam les mere removed from storage, reaxon a Nett-Summerson colorimeter (540 mcc) at 2, 24, 48, 72, and 96 hours following alcohol addition, and returned to storage between readings. All solutions stored at 0" C. were allowed to reach room temperature just prior to reading on the colorimeter. A second group of samples was also separated into two sets, which were stored dry at room temperature and at 0" C. Complex samples containing boron concentrations of 1.0 and 2.0 p.p.m. were removed from storage at varying periods of time, dissolved in alcohol, and read on the colorimeter at 0, 1, 2, and 3 days. Following each determination the samples were returned to storage to be reread the following day. The initial reading for each sample was made 2 hours after alcohol addition. .4 third group of samples was stored dry a t room temperature. Samples were removed from storage a t 0 and 24
alcohol prior to reading on a colorimeter. After addition of alcohol, the complex must be read within 2 hours to prevent loss of color from a slow hydrolysis of the complex to curcumin and boron compounds. A larger number of boron samples could be prepared in 1 day if the boroncurcumin color were more stable and allowed for more flexibility in the scheduling of analyses. An examination of the stability of the boron-curcumin complex was undertaken t o determine how long samples could be kept after formation of the complex. EXPERIMENTAL
Boron-curcumin samples were pre%red using 0.2-, 1.0-, and 2.0-p.p.m. oron concentrations. Following development of the rcd complex, on a 55" C . water bath, the solutions were examined for color stability in a variety of ways. After the residues were dissolved in
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HE determination of boron, using curcumin dye t o develop a red boron-curcumin complex, was described by Dible, Truog, and Berger (1). The complcx is taken up in 95% ethyl
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Figure 1. Decrease in color values with time at room temperature ANALYTICAL CHEMISTRY
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Figure 2. Changes in boron-curcumin readings as a function of time of storage in alcohol solution
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