Spectrophotometric Determination of Cobalt after Extraction of the Thiocyanate Complex with Acetylacetone W. B. BROWN and J.
F.
STEINBACH
Department o f Chemistry, hiversify of Kentucky, Lexington, Ky. ,Elements that interfere with the determination of cobalt are extracted from an aqueous solution with acetylacetone. Cobalt thiocyanate is then extracted with the same solvent and the absorbance of the solution measured spectrophotometrically. The absorbance curve exhibits a maximum which obeys Beer’s law between 2 X lo-* and 5 X 10” mole of cobalt per liter. The extractability of the blue complex is constant between pH 1 and 6 and is independent of the alkali cation of the thiocyanate salt used in the analysis. The method is suitable for the determination of cobalt in steels.
E
of the blue thiocyanatacobalt(I1) complex with ethers, alcohols, esters, or ketones has been the basis of colorimetric determinations of cobalt. This method is exemplified by the work of Treadwell (8) and summarized by Stengel (7). Cobalt(I1) ion must be isolated from interferences such as iron and copper which also form colored, extractable thiocyanate complexes. Iron has been eliminated by precipitation (6, S), masking (2, 3), and extraction with ether (1). Copper has been masked by thiourea to prevent interference (3). This paper describes a solvent extraction technique using acetylacetone (2,4pentanedione) to remove interferences. The same solvent extracts the cobalt thiocyanate complex after the interferences have been removed. Investigation of the variables involved has shown that the procedure described below is suitable for the determination of cobalt in steels. XTRACTION
APPARATUS A N D REAGENTS
A Beckman Model DU spectrophotometer was used for absorbance measurements which were made a t 625 mp with a slit width of 0.086 mm. The pH of the solutions was measured with a Beckman Model G pH meter. Extractions were performed in 30-ml. glass-stoppered bottles. Commercial acetylacetone was purified by extraction with water followed by distillation. The acetylacetone was then saturated with water. The so-
dium thiocyanate solution was 1.1M in thiocyanate ion and was saturated with acetylacetone. All reagents met ACS specifications of purity. PROCEDURE
To prepare a Beer’s law curve, use aliquot portions of a standard cobalt solution, adjust these aliquota to a p H of 4, and dilute to 50-ml. The concentration of cobalt in these samples should range from 2 X 10-3 to 5 X 10-6 mole per liter. Place 10-ml. portions of these solutions in 30-ml. bottles and add 10 ml. of acetylacetone. Shake the solutions for 1minute, allow the phases to separate, and transfer 5 ml. of the aqueous phase to a 30-ml. bottle. To extract the cobalt, add 5 ml. of the thiocyanate ion solution and 10 ml. of acetylacetone and shake for 1 minute. After the phases separate again, measure the absorbance of the organic phase at 625 mp, the maximum value of the absorbance curve. Table I shows the data used for the plot of absorbance u3. concentration. Dissolve the samples to be analyzed in aqua regia and evaporate the solutions to near dryness. Dilute to a p proximately 50 ml. and adjust the pH to 4 with dilute ammonium hydroxide. Transfer the solutions to 100-ml. volumetric flasks and dilute to volume with distilled water. Then pipet a 10-ml. aliquot of each solution into a 30-ml. bottle. To extract interfering cations, add 10-ml. portions of acetylacetone and discard the organic phase after each extraction. When the last added portion of acetylacetone remains color-
Table 1. Concentration and Absorbance Values of Solutions Used to Prepare Calibration Curves
Concentration of Cobalt X 10’ Mole/Liter 0.232 0.580 1.16 1.52 2.90 3.04 5.80 7.60 8.70 15.2 16.7
Absorbance at 625 Mp 0.023 0.058 0.117 0.152 0.280 0.296 0.572 0.747 0.855 1.50 1.62
leas, pipet 5 ml. of the aqueous phase to a 30-ml. bottle. Add 5 ml. of a 1.1M solution of thiocyanate ion, previously e a t u r h d with acetylacetone, and 10 ml. of acetylacetone saturated water. Shake the resulting mixture for 1 minute and allow the phases to separate. Measure the absorbance of the organic phase a t 625 mp, and from the Beer’s law curve read the concentration of cobalt. Table I1 shows the results of the cobalt determination performed on two National Bureau of Standards alloys (No. 153 and 161). Prepare the reference standard by shaking together 5 ml. of thiocyanate ion solution, 5 ml. of distilled water, and 10 ml. of acetylacetone. The organic phase of this system serves as the reference solvent in the photometric measurements.
Table II. Results of Analysis on National Bureau of Standards Samples
Alloy 153: Mn 0.21970, Cu 0.099%, Ni 0.107%, Cr 4.14%, V 2.04%, Mo 8.38’%, W 1.58%, Co 8.45%. Remainder reported a9 iron-
Sample Taken, G. 0.0182 0.0646 0.1012 0.0935 0.0699 0.0501
Cobalt, Absorbance 0.274 0.903 1.40 1.30 0.995. 0.740 Av.
%
8.60 8.21 8.56 8.26 8.43 8.63 8.45 %error 0.00 Std. dev. 0.16
Alloy 161: Mn 1.2970, Cu 0.04%, Ni 64.3%, Cr 16.90/,,V 0.03%, Mo 0.005%, Co 0.4770,Fe 15.0070. Sample Taken, Cobalt, G. Absorbance % 0.4016 0.327 0.49 0.570 0.7036 0.48 0.821 1.0186 0.48 1.0307 0.892 0.51 0.6459 0.580 0.53 0.6405 0.570 0.48 0.646 0.7704 0.50 0.729 0.8606 0.50 0.699 0.8254 0.50 Av. 0.50 yoerror 6.40 Std. dev. 0.017
VOL 31, NO. 11, NOVEMBER 1959
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Table 111. Effect of Cation Present ond pH on Distribution Coefficient
Source of Thiocyanate Ion, 1.1M Ba( SCN)* KSCN
KH,SCN NaSCN NaSCN
pH of Solution Absorbance 2
1.62
2 -
1 f_. it -
2 2
1 69 1 62 1 62
6
I n this procedure, transfer of a h i ] . portion of the aqueous phase, after removal of interferences, was neceswry because the few remaining drops of acetylacetone on the aqueous phase would alter the volume of the acetylacetone pipetted in for the final extraction. ‘llis step could probably bc elimin:tted if the extraction were carried out in an accurately graduated bulb. Effect of Variables. Ratios of thiocyanate ion t o cobaltous ion above 5 X 10: produce constant absorbances. Belolv this ratio the absorbance is a function of the thiocyanate ion concentration. As seen in Table 111, the distribution coefficient of the complex is the same a t p H 2 and 6. Variation of the alkali cation of the thiocyanate salt has no apparent effect on the distribution coefficirnt of the complex. DISCUSSION
Acctylacrtonr rradily extracts c o y
pcr(II), beryllium(II), manganese(III), iron(III), zirconium(IV), and vanadium(IV) from an aqueous solution at a p H of 4. These elements can be separated from the cobalt before the thiocyanate ion is introduced into the aqueous phase. Chromium(II1) acetylacetonate forms so slou ly that it may be considered unextractable by this procedure (6). Cobalt(I1) and nickel(I1) acetylacetonates are not extracted, because they form dihydratcd acetylacetonates insoluble in acetylacetone. Nickel thiocyanate is insoluble in scetylacetone. The solubility of acetylacetone in water varies only slightly with ionic strength. There is a change of 0.5 gram of acetylacetone per 10 mi. of water over a range of ionic strengths from 0 to 1 at room temperature. Below an ionic strength of 0.04, the solubility of acetylacetone in water is constant. Because the solutions used in this procedure have relatively low ionic strengths, the solubility of acetylacetone may be assumed constant, as evidenced by the straight line obtained for the Beer’s law curve. Solutions of the blue cobalt thiocyanate complex fade in some solvents (4). However, solutions of the complex in acrtylacetone maintain the same absorbance value over a period of 10 days. Decausc a 50-60 mixture of chloroform and acetylacetonc has twcn used as a solvent in a similar systcni for the determination of molj.bdc.num ( 5 ) , this mixture n n s tried in th(3 cvhnlt dctrr-
mination. Unfortunately, the extractability of the cobalt thiocyanate is a function of the chloroform concentration. Because cobalt thiocyanate complex is insoluble in chloroform, the distribution coefficient of the complex is much smaller in a 1 to 1 chloroformacetylacetone mixture than in pure acetylacetone. The molar absorbance index of the blue complex in acetylacetone is 984 f 10 based on moles of cobalt per liter. ACKNOWLEDGMENT
The authors gratefully acknon lvdge the gift of acetylacetone from the Union Carbide Chemicals Co. LITERATURE CITED
(1) Degray, R. J., Rittershausen, E. P., JND. ENG. CHEM.,ANAL. ED. 15, 26 (1943). (2) Foglino, N., Bertalchi, S., Ann. chim. a p p l . 32,206 (1942). (3) Kinnunen, J., Merikanto, B., LVennerstrand, B., Chemist Analyst 43, 21 (1954). (4) Kitson, R. E., ANAL. CHEM.22, 664 (1950). ( 5 ) McKaveney, J. P., Freiser, H., Ibid., 29, 291 (1957). (6) Putsche, H. hl., Malooly, W. F., Ibid., 19, 236 (1947). ( 7 ) Steneel. E., Die Chnnis 56, 47-9 ‘ (1943r ’ (8) Treadwell, F. P., 2. anorg. Chenl. 26, 108 (1901). ’
RECEIVEDfor review April 18, 1958. Arrrptcd .illgust 13, 1959.
Determination of Isomeric Distribution in Mixed 0-and p-Toluenesulfonamides by Ultraviolet Absorption FREDERICK N. STEWART, JAMES E. CALDWELL, and ARTHUR F. UELNER John
F. Queeny Plant, Monsanto Chemical Co.,
,The industrial need for a reliable means of estimating the proportion of eoch isomer in toluenesulfonamides has led to the development of a rapid spectrophotometric technique. The method involves estimation of the para fraction from the ratio of the ultraviolet absorbance at an ortho peak to that at the isoobsorptive point. interferences are removed by extraction. Provision i s also made for an estimation of total toluenesulfonomide content. Standord deviations are within 1% absolute.
T
BE individual 0- and ptoluenesulfonamidc isomers and their mixtures in controlled proportions are used
1806
ANALYTICAL CHEMISTRY
St. louis, Mo.
as intermediates in the manufacture of such diverse products as \iharmaceuticals, condimrnts, plasticizers, and resins. A rapid estimation of the isomeric distribution is frcqucntly necessary. Classically thr isomrric proportions have been rstimntcd from freezing point or self-solution point dctermin:ttions with reference to binary phase diagrams. This technique requires careful control of temperature differentials and equilibrium conditions. Moreover, it is essentially limited to two component systpms, being subject to ( w o r from small amounts of impurities such as sulfones. The ultraviolet absorption spectra of 0- and p-toluenesulfonnmides (1) are significantly different. The difference
is anic,nable to application of the absorhancc rat,io technique (@, involving :tnnlJ.sis of R two-component system from thc ratio of the absorbance a t a w w lrngth where the absorptivitiw arc. iipprccirtbly different, to the absorbmc’c at, the isoahsorptive point (where both components have equal ahsorptivitirs). I n this work, ditolyl sulfone which alisorbs strongly in the ultraviolet, and other 1:ossible neutral or basic organic itnpuritirs were removed by extracting alkaline solutions of the tolucnrsdfonaniitlm \\ ith chloroform. APPARATUS A N D MATERIALS
A C:iry ultraviolet recording spectrc photometer, Model 14M, with I-cm,
