edited -~ ~-~ bv ~, GEORGEL. GILBERT Denison University Granville, Ohio 43023
The Separation of Cobalt from Nickel by Anion Exchange Chromatography
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Suehnlmo BY
George B. Kauffman and Matthew L. Adams Calllornls Slate Unlverslly. Ffesno Fresno. CA 93740
Thomas R. Tuttle, Jr. Brandals Unlrerslty Wallham. MA 02254 cobalt (atomic number 27) and nickel (atomic number 28) are two transition metals that occupy adjacent positions in the periodic table, have almost identical atomic weights, and are closelv associated in their occurrence in nature. Their separation, therefore, is of more than routine importance. The extensive anion exchanne studies carried out bv Kurt A. Kraus and co-workers during the 1950's a t the 0 i k Ridge National Laboratory provide a method for accomplishing this separation (I),which has been adapted as reproducible experiments using tracer amounts of the ions in 7 M (2)or 8 M ( 3 , 4 )hydrochloric acid. Cohalt is detected radiochemically Poco), and nickel is detected by the spot-test reagent dimethvlelvoxime. We have modified these exneriments. using "m&> amounts of the ions, and have dkterminedthe optimum conditions t o ~ r o d u c ea visuallv observable demonstration that illustrates several fundamental principles of the general chemistry course. Principles Moore and Kraus (la) found that in hydrochloric acid solutions of a wide range of concentrations cohalt(I1) forms an anionic complex, written variously as Co(H20)C18- or CoCla2-, while under the same conditions nickel(I1) forms no anionic complexes:
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CoZ+(pink) 4C1- F? CoCh2- (blue) (1) Conseauentlv. ".cobalt(I1) . . can be adsorbed on anion exchanee resins such as the strongly basic quaternary amine polystyrenedivinylhenzene resin Dowex-1, while nickel(I1) is not adsorbed. I n accordance with Le Chhtelier's principle, lowering the Cl- concentration shifts the equilibrium in eq 1to the left, resulting in desorption (elution) of the cobalt(I1). If the process is carried out on a column, a simple, rapid, and complete separation of cobalt(I1) from nickel(I1) can beeffected.
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Procedure In a preliminary dpmonstration to show the revcrsibility of the reaction shown ineq 1, enough CoClr. 6HjO togive a distinrtly pink solurion isdisbolved by swirling ins400-mL beaker wrth-100mLof
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water. Addition of excess concentrated hydrochloric acid (caution: he careful not to inhale the chokinefumes)withswirline chanees the oink color t o .~ u .m l and e then to c o b t hl& as anionic comolexes are formed. Addition of more water with swirling results in the rrsroration of the original pink rolor. The romplexing and deromplcxing reactions ran he demonstrated repeatedlv hy alternak addrtrma of acid and water. An enimesrhsnge column is prepared by pouring a slurry of-7.5 g of Dowea-1,XB,chloride form 150.100 mesh!, in -6.0 ml. of 9 M HCI inm a 10-mLburet (a buret with a funnel tor, ia wpferred), in which a small plug of glass wool has been inserted above the stopcock to retain the resin. Excess 9 M HCL mav he added to wash the resin into the buret. 9 M HCI may be preparid hy diluting 73 mL of concentrated (12 M) acid to 100mL withdi3tilled water. Usedresin of a smaller particle sire (e.g., 1W200 mesh! will give a hetter separation but requires more time than is availahlein a usuallecture period (50min).The stopcock is opened, and acid is allowed to drain while maintaining the liquid level ahove the top of the resin column witb added acid, resultine in a resin column-8.0 mL in volume. The top of the light yellow resin column should not he higher than the 3.00-mL graduation on the buret. Care should he taken never to allow the liquid level to fall helow the top of the column, which would result in channeling within the resin bed. A 0.5-mL sample of a deep blue stock solution (1.00 g of NiC4. 6Hz0 and 0.30 g of CoCI2. 6H20 in 10 mL of 9 M HCI) is carefully added with a 1.0-mL pipet to the top of the column and allowed to seep into the resin by adjusting the stopcock. (Equimolar mixtures of cobalt and nickel, e.g., 0.3 g each of NiCIz.6Hz0 and CoClz. 6H20 in 10 mL of 9 M HCI mav he used. but since the color of the nickel effluent fraction is pal; extra nickel is preferable.) In order to ensure that none of the stock solution adheres to the buret walls ahove the resin bed and that the sample has completely seeped into the resin, three successive 0.3-mL samples of 9 M HCI are added and drained, while heing careful not to disturb the resin bed. The effluent is discarded. Five milliliters of 9 M HCI is added with a 5.0-mL pipet to the top of the column, heing careful not to disturb the resin bed. In order to save lecture time all of the ahove can be performed before the lecture, if desired. The stopcock is adjusted to produce a flow rate not exceeding 1.5 mLImin (-1.0 mL1min is recommended),and the effluent is collected in a 10-mL graduated cylinder. The cobalt(I1) is adsorbed as a green band -5-6 cm (-2 mL) long at the top of the column, while the nickel(I1) travels down the length of the column and emerges as a visually green solution after about 1-2 mL of effluent have been collected. At the first tinge of green in the effluent the cylinder is replaced with a second 10-mL eraduated cvlinder. and the ode grien nickel(ll) fractiun (-2 m ~ is; collected. The r.ylinder is ;hen replaced with a third cylinder, and any 9 V HCI remaining in the buret is collected. The buret is then filled witb water. As the column is eluted with water, the green cobalt band becomes lighter and begins to travel down the column. When it reaches the bottom of the buret (-4-5 mL of effluent for this fraction), another 5 mL of water is added. The third cylinder is replaced with a fourth cylinder, and a fraction (-1-1.5 mL) of cobalt is collected. The fraction may be blue (com-
Journal of Chemlcal Education
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plexed or partially complexed) or pink (uncomplexed). The cylinder is reolaced with a fifth cvlinder. and a oink.. uncomolexed cobalt fracFion (-1.0-1.5 mLJ is colle&ed. If the first ccibalt fraction (fourth cylinder) is blue, it may be diluted uith a minimum volume (0.5-1 mL, of water, whereupon the color changes tu pink. The entire run requires -15-25 min. To prepare the column for another run any excess water is allowed to drain, another 10-20 mL of water is similarly passed through the column, and -10 mL of 9 M HC1 is passed through the column. Although the colors of the fractions are visible a t close range, they may not be intense enough to be readily visible a t a distance. The fractions may be rendered more readily visible by placing a white background behind the graduated cylinders, by collecting 1.0-mL fractions in white spot-test plates, or by adding a suitable spot-test reagent (5) to the graduated cylinders. Addition of dimethylglyoxime (10 mL of a 1%etbanolic solution), followed by sufficient concentrated aqueous ammonia to render the solution basic (caution: heat is euolved during neutralization) produces a scarlet precipitate with the nickel fraction and a yellowish brown color with the cobalt fraction after some time elanses. If desired. the C-Ni stock rulution may be 'apiked" with "'Cu, aid tlrr c d d t fractim rrray I* detected with a Geiger -Muller counter as well ax visually (caution:
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be careful in working with and discarding solutions containing radioisotopes).
Acknowledgment T h e authors wish t o acknowledge the helpful suggestions of Kurt A. Kraus and Frederick Nelson. both formerlv of the s sp;nrerof Oak Ridge National Laboratory, a n d of ~ a r n e N. Franklin and Marshall College, Lancaster, Pennsylvania. 1. la) Moore, G. E.; Kraua. K. A. J Am. C k m . Soe. 1952.74.843; (b)Krau8.K. A.; Moore, G. E. J. Am. Chem. Soe. 1353.75.1460: Id Kraua, K. A,; Nelson, F.: Smith,G. W. J. Phys.Chem. 1954,58,11;ld)Kraus,K. A.;Nelson,F.J Am. Chem.Soc. 1954,76,9&1: (el Nelaon,F.;Kraua,K.A. J.Am. Ckm.Soc. I954,76,5916:I0 Nslson,F.;Kraus, K. A. J. Am. Chem. Soc. 1955, 77,4508: (g) Kraua, K. A,: Nelson, F, "Anion Exchange Studiesof MetalComplexes" in Tho SfrucfureofElaefrolytic Solutions:Hamer. W .
J.,Ed.; Wiley: NBW York. 1959:pp 3 4 M M .
2. Rudin. N., Ed. Radioisotope Experiments for the Chsmislry Curiculum; NuclearChieagocorp.:Chicago. IL, 163%Expt. G 7. 3. Overman, R.T.; Coffey, D. L.; Muae, L. A. J. Cham. Edue. 1958,35,296. 4. Spencer,J. N. A Rodiorh~mieolStudy of the Ion E~ehongeSeparation of Cobolt and Niekel:Modular Lshorslor~Proeram In Chemistry: Ewt. RADI-028:Grant: Beaton, MA, 1971 (out of print). 5. Feigl, F.: Anger, V . Spot Teata in Inorgonie Anolyaia: Orrpcr, R. E., Transl.; Elacvier: ~
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Amsterdam, 1972; pp 196203,325331.
Volume 66
Number 2
February 1969
167
