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James L. Anderson' James G r o~~~. hs ~
a n d Darryl Frick North Dakota State University Fargo, N~ 58105
Mini-Column lon-Exchange Separation and Atomic Absorption Quantitation of Nickel, Cobalt, and Iron An undergraduate quantitative analysis experiment
T h e facile ion-exchange chromatographic separation of numerous first-row transition metal ions based on differential stability of their chloro complexes has been demonstrated by Kraus and Moore ( I ). A number of undermaduate laboratorv experiments have been developed, hasedbn their procedurd. These experiments have been commonly restricted to the separation of pairs of metal ions (2-4)such a s Ni(I1) and Co(I1) (3,4), h u t occasionally they are extended t o the separation of Ni(II), Co(II), and Fe(II1) (5), or more ( I ) metal ions. T h e sources cited adequately cover the theoretical basis of the seoaration. T h e most common ouantitation schemes utilize EDTA titrations (2,4,5). ~ped;ophotometric ( I , 3 ) and radiotracer ( I ) methods have a h o been applied. In separations involving Fe(III), Fe(II1) quantitation is typically omitted (5) or performed via radiotracer methods ( I ). since extensive l.'eilll) precipitation interferes with analssjs under pH conditions fawrahle for EDTA titrations. We have d e v e l o ~ e da simple atomic-absorption analysis scheme which circumients the Fe(II1) . precipitation problem and enables convenient . separation and quantitati& of a mixture containing Ni(II), Co(II), and Fe(II1). One of the objectives of our quantitative analysis courses is to expose students t o a s wide a variety of techniques as possible within t h e constraints of a 10-week quarter. This objective is particularly important t o many of the nonchemistrv maior students. who will not have the oonortunitv .. t o take a course in instrumental analysis. Several of our other experiments use soectroohotometric methods. We alreadv prbvide a n EDTA titraiion experiment (determination df water hardness). In addition, the EDTA titration procedure for Ni(I1) and Co(I1) is moderately time-consuming a n d r e quires considerable care by students (particularly pH control) t o assure acceptable tesults. T h e atomic absorption quantitation scheme described here is substantially faster, less complex, and very sensitive. T h e higher sensitivity of atomic absorption enables use of small sample quantities, illustrating semi-micro techniaues. T h e analvsis can convenientlv b e performed by elutihg microliter sample quantities through a column nreaared . . from a dis~osahlelahoratorv"nioette. .. This approach allows introduction of a n additional analytical techniaue into the limited auarter time frame. while actuallv shortening the duration of i h e laboratory.
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Experimental Procedure All needed glassware and solutions for the ion exchange experiment should be prepared in advance, since the elution Process is relatively rapid. he folluwing laboratory apparatus and glassware items are required: 1 disposable Pasteur pipet ( a 5 mm internal diameter, -10 cm body length) to fit snugly em length of smsll.diamete;flexihle plastic over pipet tip kg., Teflon*) Small glass wool plug Pinch clamp 3-250 ml volumetric flasks Ring stand and clamp
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Author to whom correspondence should be addressed at Department of Chemistry, University of Georgia, Athens, GA 30602.
Also needed are 2 ml dry strong anion exchange resin (Amberlite CG-400,100-200 mesh (Mallinckrodt), or equivalent will suffice), -50 ml each of 9 M HCI, 4 M HCI, 0.5 M HCl. Students are provided with several milliliters of unknown solutions, containing mixtures of hydrated nitrate salts of Ni(II), CdII), and Fe(III), prepared from reagent grade material without drying. To test the procedure, and to afford the class an opportunity for statistical data evaluation, all students were given identical mixtures prepared by mixing equal volumes of 0.500 M stock solutions of all three salts in 9 M HCI. The class is also provided with 0.00500M stock solutions of eaeh salt, prepared by 100:l dilution of the master stock solutions. These stock soiutions hill be used to prepare calibration standards for the atomic absorption spectrophotometer. A quantity of 250 ml volumetric flasks is required for preparation of standards far the atomic absorption measurements. The ion exchange resin can be cleaned by batchwise washing with successive 10 ml portions of hot distilled water, 1 M HCI, distilled water, 0.5 M NaOH, distilled water, ethanol, and distilled water. (Conditioning is not needed far fresh resin but should he used for recycled resin.) A small glass wool plug is tamped lightly into the bottom of the Pasteur pipet to retain the ion exchange resin bed. The flexible plastic tubing is slipped snugly aver the pipet tip. The pipet is held by a clamp on a rinestand. Two milliliters of clean resin are susoended in0.5M " HCI and poured into the Pasteur pipet to form a column -8 cm deep. Addition of 0.5 M HCL is continued until the resin has settled. The column is drained until the liquid leveljust covers the top of the resin bed, and the tubing at the pipet outlet is closed off with apinch clamp. The liquid leuel must not drop below the top of the resin bed. A 10 ml portion of 9 M HCI is passed through the column to equilibrate the column and assure complete charging with chloride ion. Flow is halted when the liquid level just covers the column. A 200.~1 . aliouot . of unknown solution is earefullv..olaced on the too of t h column, ~ hy means of a mirruliter pipet (the Eppendorf type of pipet is particularly nmvenient if available)and allowed todrain into theresin. The N i W iseluted with = 5 mi of 9 M HCI incoa lnhelled 250 ml volumetric flask.The elution band is amewhnt drftirult t o r e , due 1,) the faint rvlur of the Ui(lll sample. The Nitll) remains in ratiunir form under thepeeonditims while Collljand Ferllll are retained on thr rolumnnrCoCL'-and FeCId-, respectively. Thesolution is drained to the top of the resinbed,flow is stopped, and the volumetric flask is set aside. The Co(I1)is eluted into the second labelled volumetric flask with =5 ml of 4 M HCI, until the hand (first blue, and then pink) passes through the column, and the effluent is completely colorless. The solution is drained to the top of the resin bed, flow is stopped, and the volumetric flask is set aside. The Fe(II1) ion is eluted into the third labelledvolumetric flask with a 5 ml 0.5 M HCI, using several milliliters excess after the yellow ~ ~ ( 1 1band 1 ) has passed through the column,and the effluent is eel~ ~ l e sCare s . should be exercised at eaeh stare " to assure eomolete recovery of each ion from the column. All three volumetric flasks are diluted to the mark with distilled water. A series of 4 or 5 standards is prepared for each element from the 0.005W M stock solutions. For the sample concentrations used here, standards for each element can be conveniently prepared with a 10 ml buret, using aliquots of 2.50,5.00,7.50, and 10.00 ml of stock solution, diluted to volume in 250 ml volumetric flasks. To conserve glassware and time, it is convenient for eaeh set of standards to he prepared and shared by groups of 5-10 students. It is vital that these standards he carefully prepared, with all critical concentration data properly recorded. At this point in the experiment, it is a convenient time to take a break, if time constraints require one. Volume 57, Number 7, July 1980 / 521
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The atomic absomtion oortion of the exoeriment is conducted in groupsufbl0. The basic theoretical backsound ofaaumicshwrption is discussed, and the uperalion of the instrument is demonstrated. Hasic operating parameters inuludingehoiceoipas m i x t u r ~ a n dflame conditions are set up by the laboratory instructor. Each student performs an optimization of wavelength, burner height and alignment using a standard; aspirates his or her unknowns as well as a complete set of standards into the flame; and takes absorption readings, under close suoervision of the lahoratorv instructor. The Perkin-Elmer Model i0.1 instrument used here &ot,idra a readout in perCQntab. sorption r100'? minus pcrrent transmittance), which the studen& convrrt to absorbance wing tables. Each student prepare* csl~bratmn curves of absorbance versus known concentration, for each element, and determines the unknown concentrations hy reference t o the calibration curves. Students report concentrations of the constituents in the original unknown sample in parts per million by weight. The table illustrates the results of one group of I students. I t should be pointed out to students that atomic absorption measurements are subject t o interferences which may vary with sample eomposition-e.g., "matrix effects." Consequently, efforts should he made t o assure that final sample compositions are as nearly identical as possible t o the standards. At the dilutions used herefinal sample pH and pCl ranged from 0.1-2 for the 9 M and 0.5 M HCI eluents, respectively, while standards were made up a t pH and pCI near 1.HCI media are not particularly noted for matrix effects. The experimental data do not reveal anv sienificant influence of the samole matrix on ,. the rcsults. Only the negative devia~iunbetween experimentnl and "true" concentration8 fur Co2* is statistically significant according toStudent's t-tcst. and could readily be accounted for by assuming slight excess moisture in the hydrated salts used t o prepare ssmPI~S. As seen in the table, both the accuracy and the precision of the atomic absorption method are goad for spectrophotometric procedures. The atomic absorption procedure is quite rapid, and several groups of students can successfully complete the atomic ahsorption procedure during a standard 4-hr lahoratory period. This experiment has heen in use for a year and has been enthusiastically received by students. The use of the mini-column greatly speeds the Liquid chromatography procedure relative to larger-diameter, longer-column procedures, while the atomic absorption technique greatly simplifies and speeds the quantitation procedure relative t o other techniques such as EDTA titrations. A very efficient student could complete the entire experiment in a single 4-hr lahoratorv oeriod. The rhromntoeraohic seoaration of colored hands is visuallv in~
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sample, followed by a high degree of dilution, illustrates the sensitivity of atomic absorption as a technique far trace analysis, although the actual concentrations used here are still fairly high, and by no means tax the capabilities of the atomic absorption spectrometer. Atomic absorption spectroscopy is relatively immune to the specific chemical form of a metal ion. barring matrix effects. Thus. atomic aburption is su~tahleprimarily to rnea.urr the toml or' all furma uf agivenmetal ion. Sinceeach metal used in th~sexperimentis initially present m unly
