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Determination of cobalt in iron-rich materials by X-ray fluorescence spectrometry after solvent and anion-exchange extraction
Chemical Geology, 51 (1985) 3--8
3
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
DETERMINATION OF COBALT IN IRON-RICH MATERIALS BY X-RAY FLUORESCENCE SPECTROMETRY AFTER SOLVENT AND ANION-EXCHANGE EXTRACTION IWAN ROELANDTS Department of Geology, Petrology and Geochemistry, University of Lidge, Liege 1 (Belgium) (Received March 6, 1984; revised and accepted October 12, 1984)
Abstract Roelandts, I., 1985. Determination of cobalt in iron-rich materials by X-ray fluorescence spectrometry after solvent and anion-exchange extraction. Chem. Geol., 51: 3--8. After sample dissolution in a HC1--HNO3 mixture, most of the Fe was removed by solvent extraction, whereafter Co was fixed by anion exchange in 9 M HC1 on a Dowex ® 1X8 resin using batch extraction technique. The dried resin was spread evenly over a disk of self-adhesive foil backed with a cellulose support and then used for X-ray fluorescence analysis. Synthetic standards prepared following the same procedure were employed for calibration. The technique was tested by analyzing a reference sample of magnetite ore, SARM 12. Limits of detection, precision and accuracy of the results are discussed.
1. Introduction In recent years, the determination and characterization o f trace elements in various matrices have received increasing attention, particularly for geochemical, environmental, metallurgical and industrial applications. Co is a technologically important trace metal. It is present in a vast variety o f products, including industrial metal products and wastes. This transition element is also of significance for geochemical investigations related to the fractionation processes and to solving other problems, e.g. the m o d e o f formation of ferromanganese nodules on ocean bottoms which are now considered as practically unfailing resources (Broecker, 1974). When reviewing the literature it appears
0009-2541/85/$03.30
that conventional analytical techniques such as emission spectroscopy, colorimetry, atomic absorption spectrophotometry have been frequently employed for the determination of Co in geologically interesting materials. All these methods are affected by interelement interferences, in particular from Fe which is a main constituent. Neutron activation analysis (NAA) has been often r e c o m m e n d e d as one o f the most precise and accurate methods. Unfortunately, reactor facilities, sophisticated and expensive instruments are the principal obstacles to the use o f this technique, considering the limited financial possibilities o f most research and industrial laboratories. On the other hand, the access o f X-ray fluorescence spectrometry (XRF) is more widely available and this powerful analytical tool has been extensively applied
© 1985 Elsevier Science Publishers B.V.
to a considerable variety of materials mainly for the routine determination o f major components. The accurate analysis of Co traces in Fe-bearing rocks, minerals, ores and concentrates presents major difficulties. Our initial attempts of direct X R F analysis of Co on classical pressed powder pellets w i t h o u t prior isolation from bulk Fe were unsuccessful. Indeed, the determination of small concentrations of Co in a matrix containing Fe at ~ 1000--3000 times the concentration of this element is unreliable because o f the severe spectral overlap b e t w e e n the most sensitive Co-Ks peak and the Fe-K# line, and this unreliability increases as the Co/Fe ratio decreases. Furthermore, absorption and enhancement effects are serious; Co may be present at low concentration in a matrix that is u n k n o w n and variable; well-calibrated Fe ore reference materials are not always easy to obtain. A separation step of Co prior to X R F analysis was therefore to be considered for materials where Fe concentrations are high. In the procedure developed b y Blount et al. (1973), the Fe separation was achieved on columns packed with D o w e x ® 50X8 cation-exchange resin by elution with 1 M HF prior to the batch extraction of the Co, at pH = 9, on a chelating resin, Chelex ® 100, which was then pressed into pellets and measured on the X-ray spectrometer. The pelletizing of Chelex ® 100 resin was, however, f o u n d to present some difficulty. Gulaqar (1974) used sodium diethyldithiocarbamate (NaDDC) to extract the Co into chloroform from a tartrate solution at pH ~> 10.5. The organic extract was then evaporated onto a mixture of cellulose--lithium carbonate (7 : 3) p o w d e r in presence c f acetone and compressed for X R F measurement. In previous papers (Roelandts, 1981, 1983), we have described some applications of the batch anion exchange--X-ray fluorescence m e t h o d to the analysis of geological materials and pointed o u t several attractive features of this technique. Here a combination of solvent and anionexchange extraction was chosen. Most of the
Fe was removed from a 7 M HC1 medium into methyl isobutyl ketone (MIBK) whereafter the Co was collected on an anion-exchange resin from a 9 M HC1 solution using the batch extraction technique. To test the outlined procedure, the analysis o f a South African reference sample of magnetite (SARM 12), whose Co content has already been determined b y independent m e t h o d s was carried out.
2. Experimental section 2.1. Apparatus Batch equilibrations were performed b y means of a Turbula ® system Schatz WAB shaking machine. A stainless steel die and a hydraulic press (Weber ® , Stuttgart, Uhlbach, F.R.G.) were used for pelletizing the support resin. All X R F measurements were carried o u t b y using a CGR ® -alpha 2020 semi-automatic spectrometer (Compagnie Gdndrale de Radiologie, France), equipped with a six-position sample changer. The spectrometer was interfaced to a Hewlett-Packard ® 9815A calculator. The recovery of the ion-exchange procedure was evaluated using the charged particle induced X-ray emission m e t h o d (PIXE) (Johansson et al., 1970) (Experimental Nuclear Physics Institute, University of Liege). The PIXE system was described in detail earlier (Weber et al., 1980).
2.2. Reagents The strongly basic anion-exchange resin D o w e x ® 1X8 (100--200 mesh, chloride form) was supplied by Fluka ® , Switzerland. Cellupowder, Schleicher & Schfill®, No. 123 (Dassel, F.R.G.) was used to prepare the support material for the resin beads. Co stock standard solution (5 mg m1-1 Co) was prepared by dissolving high-purity Co metal (U.C.B., Belgium) in the minimum quantity of a mixture of HNO3--HC1 acids. A 400-pg-m1-1 Y stock standard solution,
prepared b y dissolving Y(NO3)3" 3H20 (Fluka, Switzerland) in distilled water, was used as the internal standard for PIXE analysis. All other chemicals used where o f AR grade purity.
2. 3. Procedure 2.3.1. Cellulose support for the resin. The support for the resin beads used in the X R F analysis consisted to a cellulose disk. A b o u t 2 g o f cellulose p o w d e r were poured into a 27-mm diameter stainless steel die and pelletized under pressure of 104 g cm -2 to give a firm plate 2 mm thick, which proved quite satisfactory. A small round disk o f doublesided self-adhesive Duoplex ® foil (X-film, t y p e DX) was fixed on each cellulose support. 2.3.2. Calibration standards. Known amounts o f Co were pipetted into r o u n d - b o t t o m Pyrex ® centrifuge tubes and carefully evaporated to dryness. The residue was taken up in 25 ml o f 9 M HC1. Portions (200 mg) of dry anionic resin in the chloride form were weighed and added to each solution. The stoppered tubes were shaken mechanically with a Turbula ® mixer for 3 hr. to ensure equilibrium. At the end o f this period, the resin was allowed to settle. The resin was filtered off through a sintered glass crucible o f fine porosity and dried overnight in an oven at 120°C. The resin beads (~ 40 rag) were then spread evenly in a thin layer over the adhesive surface o f the cellulose rigid support. 2.3.3. Determination o f the degree o f adsorption o f cobalt. A k n o w n volume o f the filtrate after the ion-exchange separation was mixed with 100 gg Y in solution (internal standard). Small aliquots (20/~1) of the final mixture was deposited on a cellulosic membrane filter (Metricel ® , GA-6, 0.45 pm, 47 mm, Gelman Sciences, Ann Arbor, Michigan, U.S.A.) which was previously m o u n t e d on a commercial 24 × 36-mm slide frame. After
evaporation o f the solvent, the filter was ready for PIXE analysis using a 20 nA, 2.5 MeV proton beam for irradiation.
2.3.4. Magnetite samples. Approximately 1 g o f magnetite p o w d e r was accurately weighed into a 100-ml Pyrex ® beaker and dissolved in a mixture consisting of 20 ml o f concentrated hydrochloric acid and 5 ml of concentrated nitric acid to ensure complete oxidation o f the Fe. After complete evaporation to dryness on a moderate h o t plate, the attack residue was treated with 10 ml of concentrated hydrochloric acid and again evaporated to dryness. The final evaporation residue was taken up in 25 ml o f 7 M HC1 and transferred to a 250-ml stoppered separatory funnel containing 25 ml o f a MIBK solution pre-equilibrated with 7 M HC1. The mixture was shaken by hand for 2 min. The phases were allowed to separate and the lower oqueous layer (containing Co and small amounts of Fe) was drawn o f f into a second separatory funnel and shaken again with 25 ml of fresh solvent for 2 min. The aqueous phase was drained into a 50-ml beaker and evaporated to dryness. The residue was dissolved in 10 ml o f 9 M HC1 and transferred to the Pyrex ® stoppered t u b e containing a b o u t 200 mg (accurately weighed) of dry anionic resin in the chloride form. Three further 5-ml portions o f 9 M HC1 washing solution were added and the ion-exchange procedure described above was carried out. After equilibration, the resin was recovered b y filtration, dried, and pellets were prepared following the procedure used for the standard samples. 2.3.5. X-ray fluorescence analysis (XRF). The experimental X R F parameters used in this s t u d y are reported in Table I. Background intensity was measured separately at the same 20 Bragg angle of the analyte and under the identical instrument conditions on the " b l a n k " pellet, prepared with Co-free resin. All intensities were corrected for the background so obtained which consistently held
TABLE I Instrumental parameters of cobalt X-ray tube Voltage (kV) Current (mA) Analyzing crystal Collimator (urn) Detector Peak (deg (2e)) Path Counting time (s) Sample spinner
tungsten 50 50 LiF 220 (2d = 2.848 A) 150 scintillation Co-K~, 77.89 vacuum 60 on
at ~ 45 counts per second. In order to compensate for inhomogeneity, two pellets were prepared from each sample and the t w o readings averaged. In the concentration range studied, the reproducibility of the readings was better than 3%. 3. Results and discussion
3.1. Solvent extraction procedure Since the anion-exchange behaviors of Fe(III) and Co are sufficiently similar from relatively concentrated hydrochloric acid solutions (Kraus and Nelson, 1956), a solvent extraction procedure was used in this study for prior Fe removal as outlined by Specker and Doll (1956) and Doll and Specker (1958). The technique is clean and simple: two MIBK extractions with shaking times o f 2 min. remove 99.9% o f the Fe. The amount o f Fe which was f o u n d in the aqueous phase after the second MIBK extraction never exceeded 500 pg for a 1-g magnetite sample, so that the element caused no interference with the subsequent X R F determination o f Co.
3.2. Ion-exchange procedure In a preliminary stage of this work, the effect of agitation time on the adsorption of Co by the D o w e x ® resin was studied. No appreciable difference in the uptake of Co b y the resin was observed after 30 min. All
further data in this work were obtained after a mixing period of 3 hr. The efficiency o f the ion-exchange procedure was evaluated b y PIXE on synthetic solutions. A quantitative adsorption of Co on the anion-exchange resin in the batch procedure described was impossible to achieve even b y using a 24-hr. agitation period. Results obtained showed that the percentage recovery from the solutions remained very constant with a 40 + 2% average value. However, because of the good reproducibility of the recovery tests and the use of calibration standards prepared in exactly the same w a y as the u n k n o w n samples and treated identically, this .disadvantage was of no practical significance. The validity o f this assumption was checked as described in Section 3.5 and Table II.
3.3. Analytical curve Primary synthetic calibration standards of Co were prepared over a large concentration range and used for any t y p e of samples. As can be seen in Fig. 1, the relationship between X-ray intensity (corrected for background) and Co amounts was linear over four orders o f magnitude. The quantity plotted on the ordinate axis was the Co c o n t e n t in the hydrochloric solution (before the ion-exchange procedure).
1o°
/
g, '~,, - 10 5
~
-10 4
Z
-103 10 I
Fig. 1. Linearity test.
102 I
103 J
pg Co/200mg resin
104 I
r'i !
c-
possible in these conditions. The positioning of the spectral background is also complicated. In spectrum B, none of these problems is present: the Co-K s peak is totally resolved from the Fe-K~ peak (in trace amounts after the separation procedure) and stands clearly above the background which is markedly reduced and regularly shaped. Fig. 2 clearly demonstrates the effectiveness of the method described here.
°~
3.5. Limit o f detection, precision and accuracy
I
FeK~
CoK0(
i
X
i
A I 74
I
I 76
I
I 18
I
I HO
2e (goniometer reading)
Fig. 2. Partial X-ray spectra from magnetite sample SARM 12 (South Africa) obtained using the LiF 220 crystal, goniometer speed 2 ° (20) min.-l: (A) untreated sample (Fe-K~ peak overflowed on this scale); and (B) after solvent and anion-exchange extraction.
3. 4. X R F analysis Partial X-ray spectra o f t w o samples of SARM 12 magnetite (South African Committee for certified reference materials) are shown in Fig. 2. Spectrum A was produced b y using untreated sample whereas the upper spectrum B was obtained after the solvent and anion-exchange extraction. Both of the superimposed spectra were traced o u t under identical conditions. It is immediately obvious that in the curve A the peaks are n o t resolved in spite o f the good resolution of the LiF 220 crystal. The Co-Ks peak is obscured b y the overflowed Fe-K~ peak which dominates the X-ray spectrum and makes Co accurate analysis practically im-
Under the above operating conditions, the lower limit of detection (with 95% confidence) has been calculated according to Currie's (1968) convention and found to be equal to 2 ~g Co/200 mg of resin. This is quite sufficient for our purposes. To obtain some idea about the precision and the accuracy of the present procedure, we alyzed a magnetite ore, SARM 12 (bottle /115). " F o u r t e e n independent portions of this reference material were submitted to the entire procedure described above. Duplicate pellets were taken and averaged for analysis. The replicate values are collected in Table II to demonstrate the consistency of the present method; mean values obtained -+ one standard deviation of analytical results are also added. The precision appears to be ~ + 3%. For comparison, the certified value for this sample has also been included in Table II together with the concentration ranges of the available data according to the analytical m e t h o d s employed (Stoch, 1978). It must be noted that X R F results have never been reported previously for this Fe ore. It can be seen that our X R F data obtained b y the m e t h o d described above are in good agreement with the certified value.
~0
Acknowledgement The interest of J. Belli~re is appreciated. J.C. Duchesne is acknowledged for X-ray fluorescence facilities. The author is grate-
TABLE II Results for the determination of cobalt in reference sample of magnetite ore, SARM 12 (in ppm) This work replicate values
mean value .1
234 232 241 240 236 231 245
243 +- 8 (14) *3
243 250 251 249 250 240 254
Certified value .2
233
Concentration range* 2 atomic absorption
emission spectroscopy
neutron activation analysis
110--336 (244)
150--330 (22)
219--235 (12)
• 1 ± 1 standard deviation of analytical results. • 2 F r o m Stoch (1978). • 3 Figures between brackets indicate the number of replicates.
ful to late T.W. Steele (South Africa) for supplying the reference sample of magnetite o r e , S A R M 12 a n a l y z e d in t h i s s t u d y a n d t o G . W e b e r , V. M i o c q u e , G . B o l o g n e a n d G. Delhaze for their assistance. The X-ray equipment used for this study was purchased with funds from the Belgian "Fonds de la Recherche Fondamentale Collective", Collectif Interuniversita~ce de G~ochimie Instrumentale, under Contract No. 2.4521.76.
References Blount, C.W., Leyden, D.E., Thomas, T.L. and Guill, S.M., 1973. Application of chelating ion exchange resins for trace element analysis of geological samples using X-ray fluorescence. Anal. Chem., 45: 1045--1050. Broecker, W.S., 1974. In: Chemical Oceanography. Harcourt Brace Jovanovich, New Yor'.:, N.Y., pp. 89--113. Currie, L.A., 1968. Limits for qualitative detection and quantitative determination -- Application to radiochemistry. Anal. Chem., 40: 586--593. Doll, W. and Specker, H., 1958. Spureanreicherungen in AckerbSden mit selektiver Abtrennung yon Eisen. Fresenius Z. Anal. Chem, 161: 354--362. Gulaqar, O.F., 1974. Dosage de traces de cuivre, de nickel et de cobalt dans les roches par une tech-
nique combin~e extraction--fluorescence-X. Anal. Chim. Acta, 73: 255--264. Johansson, T.B., Akselsson, R. and Johansson, S.A.E., 1970. X-ray analysis: elemental trace analysis at the 10 -~2 g level. Nucl. Instrum. Methods, 84: 141--143. Kraus, K.A. and Nelson, F., 1956. In: Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, Geneva. U.N. Publ., New York, N.Y., 7 : 113--125. Moore, G.E. and Kraus, K.A., 1952. Anion exchange studies, IV. Cobalt and nickel in hydrochloric acid solutions. J. Am. Chem. Soc., 74: 843--844. Roelandts, I., 1981. Determination of light rare earth elements in apatite by X-ray fluorescence spect r o m e t r y after anion exchange extraction. Anal. Chem., 53: 676---680. Roelandts, I., 1983. Determination of thorium in geological materials by X-ray fluorescence spect r o m e t r y after anion exchange extraction. Anal. Chem., 55: 1637--1639. Specker, H. and Doll, W., 1956. Photometrische Eisenbestimmung in Reinmetallen, Legierungen und NE-Erzen nach Abtrennung dutch Verteilen zwischen zwei LSsungsmitteln. Fresenius Z. Anal. Chem., 152: 178--185. Stoch, H., 1978. The preparation and certification of a reference sample of magnetite ore. N.I.M. (Natl. Inst. Metall.), Randburg, Rep. No. 1978, 44 pp. Weber, G., Robaye, G., Delbrouck, J.M., Roelandts, I., Dideberg, O., Bartsch, P. and De Pauw, M.C., 1980. Biomedical application of PIXE in University of Liege. Nucl. Instrum. Methods, 1 6 8 : 5 5 1 ~ 556.
3
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
DETERMINATION OF COBALT IN IRON-RICH MATERIALS BY X-RAY FLUORESCENCE SPECTROMETRY AFTER SOLVENT AND ANION-EXCHANGE EXTRACTION IWAN ROELANDTS Department of Geology, Petrology and Geochemistry, University of Lidge, Liege 1 (Belgium) (Received March 6, 1984; revised and accepted October 12, 1984)
Abstract Roelandts, I., 1985. Determination of cobalt in iron-rich materials by X-ray fluorescence spectrometry after solvent and anion-exchange extraction. Chem. Geol., 51: 3--8. After sample dissolution in a HC1--HNO3 mixture, most of the Fe was removed by solvent extraction, whereafter Co was fixed by anion exchange in 9 M HC1 on a Dowex ® 1X8 resin using batch extraction technique. The dried resin was spread evenly over a disk of self-adhesive foil backed with a cellulose support and then used for X-ray fluorescence analysis. Synthetic standards prepared following the same procedure were employed for calibration. The technique was tested by analyzing a reference sample of magnetite ore, SARM 12. Limits of detection, precision and accuracy of the results are discussed.
1. Introduction In recent years, the determination and characterization o f trace elements in various matrices have received increasing attention, particularly for geochemical, environmental, metallurgical and industrial applications. Co is a technologically important trace metal. It is present in a vast variety o f products, including industrial metal products and wastes. This transition element is also of significance for geochemical investigations related to the fractionation processes and to solving other problems, e.g. the m o d e o f formation of ferromanganese nodules on ocean bottoms which are now considered as practically unfailing resources (Broecker, 1974). When reviewing the literature it appears
0009-2541/85/$03.30
that conventional analytical techniques such as emission spectroscopy, colorimetry, atomic absorption spectrophotometry have been frequently employed for the determination of Co in geologically interesting materials. All these methods are affected by interelement interferences, in particular from Fe which is a main constituent. Neutron activation analysis (NAA) has been often r e c o m m e n d e d as one o f the most precise and accurate methods. Unfortunately, reactor facilities, sophisticated and expensive instruments are the principal obstacles to the use o f this technique, considering the limited financial possibilities o f most research and industrial laboratories. On the other hand, the access o f X-ray fluorescence spectrometry (XRF) is more widely available and this powerful analytical tool has been extensively applied
© 1985 Elsevier Science Publishers B.V.
to a considerable variety of materials mainly for the routine determination o f major components. The accurate analysis of Co traces in Fe-bearing rocks, minerals, ores and concentrates presents major difficulties. Our initial attempts of direct X R F analysis of Co on classical pressed powder pellets w i t h o u t prior isolation from bulk Fe were unsuccessful. Indeed, the determination of small concentrations of Co in a matrix containing Fe at ~ 1000--3000 times the concentration of this element is unreliable because o f the severe spectral overlap b e t w e e n the most sensitive Co-Ks peak and the Fe-K# line, and this unreliability increases as the Co/Fe ratio decreases. Furthermore, absorption and enhancement effects are serious; Co may be present at low concentration in a matrix that is u n k n o w n and variable; well-calibrated Fe ore reference materials are not always easy to obtain. A separation step of Co prior to X R F analysis was therefore to be considered for materials where Fe concentrations are high. In the procedure developed b y Blount et al. (1973), the Fe separation was achieved on columns packed with D o w e x ® 50X8 cation-exchange resin by elution with 1 M HF prior to the batch extraction of the Co, at pH = 9, on a chelating resin, Chelex ® 100, which was then pressed into pellets and measured on the X-ray spectrometer. The pelletizing of Chelex ® 100 resin was, however, f o u n d to present some difficulty. Gulaqar (1974) used sodium diethyldithiocarbamate (NaDDC) to extract the Co into chloroform from a tartrate solution at pH ~> 10.5. The organic extract was then evaporated onto a mixture of cellulose--lithium carbonate (7 : 3) p o w d e r in presence c f acetone and compressed for X R F measurement. In previous papers (Roelandts, 1981, 1983), we have described some applications of the batch anion exchange--X-ray fluorescence m e t h o d to the analysis of geological materials and pointed o u t several attractive features of this technique. Here a combination of solvent and anionexchange extraction was chosen. Most of the
Fe was removed from a 7 M HC1 medium into methyl isobutyl ketone (MIBK) whereafter the Co was collected on an anion-exchange resin from a 9 M HC1 solution using the batch extraction technique. To test the outlined procedure, the analysis o f a South African reference sample of magnetite (SARM 12), whose Co content has already been determined b y independent m e t h o d s was carried out.
2. Experimental section 2.1. Apparatus Batch equilibrations were performed b y means of a Turbula ® system Schatz WAB shaking machine. A stainless steel die and a hydraulic press (Weber ® , Stuttgart, Uhlbach, F.R.G.) were used for pelletizing the support resin. All X R F measurements were carried o u t b y using a CGR ® -alpha 2020 semi-automatic spectrometer (Compagnie Gdndrale de Radiologie, France), equipped with a six-position sample changer. The spectrometer was interfaced to a Hewlett-Packard ® 9815A calculator. The recovery of the ion-exchange procedure was evaluated using the charged particle induced X-ray emission m e t h o d (PIXE) (Johansson et al., 1970) (Experimental Nuclear Physics Institute, University of Liege). The PIXE system was described in detail earlier (Weber et al., 1980).
2.2. Reagents The strongly basic anion-exchange resin D o w e x ® 1X8 (100--200 mesh, chloride form) was supplied by Fluka ® , Switzerland. Cellupowder, Schleicher & Schfill®, No. 123 (Dassel, F.R.G.) was used to prepare the support material for the resin beads. Co stock standard solution (5 mg m1-1 Co) was prepared by dissolving high-purity Co metal (U.C.B., Belgium) in the minimum quantity of a mixture of HNO3--HC1 acids. A 400-pg-m1-1 Y stock standard solution,
prepared b y dissolving Y(NO3)3" 3H20 (Fluka, Switzerland) in distilled water, was used as the internal standard for PIXE analysis. All other chemicals used where o f AR grade purity.
2. 3. Procedure 2.3.1. Cellulose support for the resin. The support for the resin beads used in the X R F analysis consisted to a cellulose disk. A b o u t 2 g o f cellulose p o w d e r were poured into a 27-mm diameter stainless steel die and pelletized under pressure of 104 g cm -2 to give a firm plate 2 mm thick, which proved quite satisfactory. A small round disk o f doublesided self-adhesive Duoplex ® foil (X-film, t y p e DX) was fixed on each cellulose support. 2.3.2. Calibration standards. Known amounts o f Co were pipetted into r o u n d - b o t t o m Pyrex ® centrifuge tubes and carefully evaporated to dryness. The residue was taken up in 25 ml o f 9 M HC1. Portions (200 mg) of dry anionic resin in the chloride form were weighed and added to each solution. The stoppered tubes were shaken mechanically with a Turbula ® mixer for 3 hr. to ensure equilibrium. At the end o f this period, the resin was allowed to settle. The resin was filtered off through a sintered glass crucible o f fine porosity and dried overnight in an oven at 120°C. The resin beads (~ 40 rag) were then spread evenly in a thin layer over the adhesive surface o f the cellulose rigid support. 2.3.3. Determination o f the degree o f adsorption o f cobalt. A k n o w n volume o f the filtrate after the ion-exchange separation was mixed with 100 gg Y in solution (internal standard). Small aliquots (20/~1) of the final mixture was deposited on a cellulosic membrane filter (Metricel ® , GA-6, 0.45 pm, 47 mm, Gelman Sciences, Ann Arbor, Michigan, U.S.A.) which was previously m o u n t e d on a commercial 24 × 36-mm slide frame. After
evaporation o f the solvent, the filter was ready for PIXE analysis using a 20 nA, 2.5 MeV proton beam for irradiation.
2.3.4. Magnetite samples. Approximately 1 g o f magnetite p o w d e r was accurately weighed into a 100-ml Pyrex ® beaker and dissolved in a mixture consisting of 20 ml o f concentrated hydrochloric acid and 5 ml of concentrated nitric acid to ensure complete oxidation o f the Fe. After complete evaporation to dryness on a moderate h o t plate, the attack residue was treated with 10 ml of concentrated hydrochloric acid and again evaporated to dryness. The final evaporation residue was taken up in 25 ml o f 7 M HC1 and transferred to a 250-ml stoppered separatory funnel containing 25 ml o f a MIBK solution pre-equilibrated with 7 M HC1. The mixture was shaken by hand for 2 min. The phases were allowed to separate and the lower oqueous layer (containing Co and small amounts of Fe) was drawn o f f into a second separatory funnel and shaken again with 25 ml of fresh solvent for 2 min. The aqueous phase was drained into a 50-ml beaker and evaporated to dryness. The residue was dissolved in 10 ml o f 9 M HC1 and transferred to the Pyrex ® stoppered t u b e containing a b o u t 200 mg (accurately weighed) of dry anionic resin in the chloride form. Three further 5-ml portions o f 9 M HC1 washing solution were added and the ion-exchange procedure described above was carried out. After equilibration, the resin was recovered b y filtration, dried, and pellets were prepared following the procedure used for the standard samples. 2.3.5. X-ray fluorescence analysis (XRF). The experimental X R F parameters used in this s t u d y are reported in Table I. Background intensity was measured separately at the same 20 Bragg angle of the analyte and under the identical instrument conditions on the " b l a n k " pellet, prepared with Co-free resin. All intensities were corrected for the background so obtained which consistently held
TABLE I Instrumental parameters of cobalt X-ray tube Voltage (kV) Current (mA) Analyzing crystal Collimator (urn) Detector Peak (deg (2e)) Path Counting time (s) Sample spinner
tungsten 50 50 LiF 220 (2d = 2.848 A) 150 scintillation Co-K~, 77.89 vacuum 60 on
at ~ 45 counts per second. In order to compensate for inhomogeneity, two pellets were prepared from each sample and the t w o readings averaged. In the concentration range studied, the reproducibility of the readings was better than 3%. 3. Results and discussion
3.1. Solvent extraction procedure Since the anion-exchange behaviors of Fe(III) and Co are sufficiently similar from relatively concentrated hydrochloric acid solutions (Kraus and Nelson, 1956), a solvent extraction procedure was used in this study for prior Fe removal as outlined by Specker and Doll (1956) and Doll and Specker (1958). The technique is clean and simple: two MIBK extractions with shaking times o f 2 min. remove 99.9% o f the Fe. The amount o f Fe which was f o u n d in the aqueous phase after the second MIBK extraction never exceeded 500 pg for a 1-g magnetite sample, so that the element caused no interference with the subsequent X R F determination o f Co.
3.2. Ion-exchange procedure In a preliminary stage of this work, the effect of agitation time on the adsorption of Co by the D o w e x ® resin was studied. No appreciable difference in the uptake of Co b y the resin was observed after 30 min. All
further data in this work were obtained after a mixing period of 3 hr. The efficiency o f the ion-exchange procedure was evaluated b y PIXE on synthetic solutions. A quantitative adsorption of Co on the anion-exchange resin in the batch procedure described was impossible to achieve even b y using a 24-hr. agitation period. Results obtained showed that the percentage recovery from the solutions remained very constant with a 40 + 2% average value. However, because of the good reproducibility of the recovery tests and the use of calibration standards prepared in exactly the same w a y as the u n k n o w n samples and treated identically, this .disadvantage was of no practical significance. The validity o f this assumption was checked as described in Section 3.5 and Table II.
3.3. Analytical curve Primary synthetic calibration standards of Co were prepared over a large concentration range and used for any t y p e of samples. As can be seen in Fig. 1, the relationship between X-ray intensity (corrected for background) and Co amounts was linear over four orders o f magnitude. The quantity plotted on the ordinate axis was the Co c o n t e n t in the hydrochloric solution (before the ion-exchange procedure).
1o°
/
g, '~,, - 10 5
~
-10 4
Z
-103 10 I
Fig. 1. Linearity test.
102 I
103 J
pg Co/200mg resin
104 I
r'i !
c-
possible in these conditions. The positioning of the spectral background is also complicated. In spectrum B, none of these problems is present: the Co-K s peak is totally resolved from the Fe-K~ peak (in trace amounts after the separation procedure) and stands clearly above the background which is markedly reduced and regularly shaped. Fig. 2 clearly demonstrates the effectiveness of the method described here.
°~
3.5. Limit o f detection, precision and accuracy
I
FeK~
CoK0(
i
X
i
A I 74
I
I 76
I
I 18
I
I HO
2e (goniometer reading)
Fig. 2. Partial X-ray spectra from magnetite sample SARM 12 (South Africa) obtained using the LiF 220 crystal, goniometer speed 2 ° (20) min.-l: (A) untreated sample (Fe-K~ peak overflowed on this scale); and (B) after solvent and anion-exchange extraction.
3. 4. X R F analysis Partial X-ray spectra o f t w o samples of SARM 12 magnetite (South African Committee for certified reference materials) are shown in Fig. 2. Spectrum A was produced b y using untreated sample whereas the upper spectrum B was obtained after the solvent and anion-exchange extraction. Both of the superimposed spectra were traced o u t under identical conditions. It is immediately obvious that in the curve A the peaks are n o t resolved in spite o f the good resolution of the LiF 220 crystal. The Co-Ks peak is obscured b y the overflowed Fe-K~ peak which dominates the X-ray spectrum and makes Co accurate analysis practically im-
Under the above operating conditions, the lower limit of detection (with 95% confidence) has been calculated according to Currie's (1968) convention and found to be equal to 2 ~g Co/200 mg of resin. This is quite sufficient for our purposes. To obtain some idea about the precision and the accuracy of the present procedure, we alyzed a magnetite ore, SARM 12 (bottle /115). " F o u r t e e n independent portions of this reference material were submitted to the entire procedure described above. Duplicate pellets were taken and averaged for analysis. The replicate values are collected in Table II to demonstrate the consistency of the present method; mean values obtained -+ one standard deviation of analytical results are also added. The precision appears to be ~ + 3%. For comparison, the certified value for this sample has also been included in Table II together with the concentration ranges of the available data according to the analytical m e t h o d s employed (Stoch, 1978). It must be noted that X R F results have never been reported previously for this Fe ore. It can be seen that our X R F data obtained b y the m e t h o d described above are in good agreement with the certified value.
~0
Acknowledgement The interest of J. Belli~re is appreciated. J.C. Duchesne is acknowledged for X-ray fluorescence facilities. The author is grate-
TABLE II Results for the determination of cobalt in reference sample of magnetite ore, SARM 12 (in ppm) This work replicate values
mean value .1
234 232 241 240 236 231 245
243 +- 8 (14) *3
243 250 251 249 250 240 254
Certified value .2
233
Concentration range* 2 atomic absorption
emission spectroscopy
neutron activation analysis
110--336 (244)
150--330 (22)
219--235 (12)
• 1 ± 1 standard deviation of analytical results. • 2 F r o m Stoch (1978). • 3 Figures between brackets indicate the number of replicates.
ful to late T.W. Steele (South Africa) for supplying the reference sample of magnetite o r e , S A R M 12 a n a l y z e d in t h i s s t u d y a n d t o G . W e b e r , V. M i o c q u e , G . B o l o g n e a n d G. Delhaze for their assistance. The X-ray equipment used for this study was purchased with funds from the Belgian "Fonds de la Recherche Fondamentale Collective", Collectif Interuniversita~ce de G~ochimie Instrumentale, under Contract No. 2.4521.76.
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