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Anal. Chem. 1905, 57, 1416-1418
Absorptiometric Determination of Tungsten in Rocks after Selective Adsorption on Sephadex Gel Ushio Hase,l Kazuhisa Yoshimura,* and Toshikazu Tarutani Department of Chemistry, Faculty of Science, Kyushu University 33, Hakozaki, Higashiku, Fukuoka 812, J a p a n
Adsorption behavior of tungsten on Sephadex G-25 gel was Investigated for selectlve concentration and determlnatlon of tungsten In rocks. Adsorbablllty of tungsten(V1) on the gel depends on pH value and tungsten concentratlon in equlllbrated solution. At low concentration less than I t jd I iol/dm3, tungsten(V1) Is strongly adsorbed In the pH regil3n c f 2-5.5. The effect of coexlstlng Ions can be ellmlnated by pretreatment of a sample solutlon wlth hydroxylamlne and CyDTA at 80 O C for 60 mln and by passlng the solutlon through a Sephadex gel column at pH 2.5. The tungsten is desorbed from the column with EDTA solution and determlned absorptlometrically wlth Pyrogallol Red as colorlng agent. Amounts of more than 1 mg/kg tungsten in rocks can be determined by the present method. Tungsten contents In standard rocks, JB-I and JG-1, and in shale were presented.
I t is difficult to separate tungsten(V1) from vanadium(V) and molybdenum(V1) due to the similarity of their chemistry. Tungsten occurs a t trace level in geological samples such as rocks and natural waters and its content is generally less than those of vanadium(V) and molybdenum(V1). Tungsten in rocks has been determined by absorptiometry after ion-exchange separation ( 1 , Z), which sometimes contains timeconsuming steps and suffers from difficulty in determination of tungsten in the presence of large amounts of molybdenum. Although such methods as neutron activation analysis (3)and inductive coupled plasma emission spectroscopy ( 4 ) have been frequently used, investigators cannot always have access to the instrumentation. Selective concentration methods for boron, vanadium(V), and molybdenum(V1) have been developed by taking advantage of the reversible adsorbability on Sephadex G-25 gel. These oxo anions show maximal adsorption behavior in the p H region of 9.5-11, 3-7, and 3-4, respectively (5-7). The adsorption may be due to complex formation of the oxo anions with hydroxyl groups of the dextran matrix. On the other hand, chromatographic investigation of polyhydroxyl compounds has been carried out by using tungsten-impregnated cellulose paper. There is strong interaction between polyhydroxy1 compounds and tungsten(V1) in the paper (8). By the molecular sieve effect, chemical species of tungsten(V1) in high concentrations are separated into three fractions by column chromatography with Sephadex G-10 gel (9). A tendency for interaction with the gel is observed to increase with decreasing p H values of the eluent. However, the behavior of tungsten(V1) has not been investigated in low concentrations, in which isopolytungstates are not present. In this study, characteristic adsorbability on Sephadex (2-25 gel has been investigated and applied to the separation and concentration of tungsten(V1) for the purpose of determining Present address: Resources and Environment Protection Research Laboratories,NEC Corp., Miyazaki, Miyamaeku, Kawasaki 213, Japan.
tungsten in rocks. Tungsten(V1) can be determined absorptiometrically with highly sensitive Pyrogallol Red as coloring agent in the presence of cetyltrimethylammonium bromide (10).
EXPERIMENTAL SECTION Reagents. All reagents used were of analytical grade and water deionized-distilled. Pyrogallol Red reagent was prepared by dissolving 0.060 g of mol/dm3 cetyltrimethylPyrogallol Red in 50 cm3 of 1.5 x ammonium bromide and diluting to 500 cm3 with water. CyDTA (trans-l,2-cyclohexanediamine-N,N,N‘,N‘-tetraacetic acid) solution was prepared by dissolving 9.11 g of CyDTA in sodium hydroxide solution and diluting to 100 cm3 with water. The gel used was Sephadex G-25 gel (medium). Sample Preparation. A powdered rock sample of less than 10 g was moistened with water in a Teflon beaker, and 30 cm3 of concentrated nitric acid, 10 cm3 of 10 mol/dm3 sulfuric acid, and 100 cm3of concentrated hydrofluoric acid were added. The mixture was heated on a hot plate and evaporated to fumes of sulfuric acid. After cooling, 50 cm3of concentrated hydrofluoric acid was added and the mixture again evaporated to dryness. Tungsten was extracted from the dry residue with 50 cm3 of 2 mol/dm3 nitric acid solution for 2-3 h at 80 OC on a hot plate. The solution was filtered through a 0.45-hm membrane filter (Millipore) and diluted to 100 cm3 with water. Separation and Concentration of Tungsten. To an aliquot of sample solution containing 0.5-20 fig of tungsten, 15 cm3 of 5 mol/dm3 hydroxylamine hydrochloride solution and 2 cm3 of 0.25 mol/dm3 CyDTA solution were added. The solution was adjusted to pH 2.5 with ammonia solution, diluted t o about 100 cm3with water, and heated a t 80 “C for 60 min. After cooling, the solution was passed through a glassware column (Bio-Rad; 1.5 cm i.d., 10 cm in length) containing 17.5 cm3of Sephadex G-25 gel, which had been previously conditioned with acetic acid solution (pH 2.5). The column was washed with 50 cm3 of acetic acid solution (pH 2.5) and 25 cm3 of acetic acid solution (pH 3.7-3.8). The tungsten(V1) was desorbed with 50 cm3 of 0.01 mol/dm3 EDTA (disodium salt) solution warmed at 30-40 OC and the effluent collected in a glassware beaker and concentrated to less than 5 cm3 by evaporation. Determination ofTungsten(V1). The wall of the beaker was rinsed with a small amount of water. Then 1cm3 of 2 mol/dm3 acetate buffer (pH 4.7) and 2 cm3 of Pyrogallol Red reagent solution were added, and the solution was transferred to a 10-cm3 volumetric flask. After 30 min, the absorbance was measured with a 2-cm cell at 576 nm against reagent blank. A calibration graph was obtained by taking standards throughout the gel adsorption procedure. In the case of the standard addition method, the method blank should be estimated by processing the method procedure above. Distribution Measurements. To investigate the adsorption of tungsten(V1) at tungsten concentrations of 1 x IO”, 1 x and 1 X mol/dm3 on Sephadex G-25 gel, the distribution ratio, D, was measured by the batch technique. For the lowest concentration 250 cm3of solution and 0.2 g of the gel were used, and for other concentrations 100 cm3of solution and 0.5 g of the gel. Each solution was adjusted to a required pH value with sodium hydroxide solution and dilute hydrochloric acid solution, and the ionic strength of the solution was maintained at 0.1 mol/dm3 with sodium chloride. The mixture of the gel and solution was stirred overnight and the tungsten in the equilibrated solution was de-
0003-2700/85/0357-1416$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
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\
40
I
0
2
4
6
I0
PH
Flgure 2. pH dependence of the adsorption of polytungstate(V1) on
PH Figure 1. pH dependence of the adsorption of tungsten(V1) on Sephadex gel in low tungsten concentration: gel, Sephadex G25, 0.20 g; solution, mol/dm3 initial concentration of tungsten, 0.1 mol/dm3 NaCI, 250 cm3; dashed line, mole fraction diagram of each monomer species.
mol/dm3, (e) lo-' Sephadex gel: gel, Sephadex G25, 0.50 g; (A) mol/dm3 initial concentration of tungsten, 0.1 mol/dm3 NaCI, 100 cm3.
termined by the Pyrogallol Red method or atomic absorption spectrometry. If necessary, the tungsten(V1) in equilibrated solution was concentrated by the method described above before determination of tungsten(V1).
RESULTS AND DISCUSSION Effects of the Adsorption of Tungsten(V1). Figure 1 shows the effect of the pH on distribution ratio, D, the extent of tungsten(V1) adsorption on Sephadex G-25 gel from the equilibrated solution a t tungsten concentrations lower than 1 x lo4 mol/dm3. Tungsten(V1) shows maximal adsorption behavior in the pH region of 3.7-3.8. The maximum D value was about 900, which was very large compared with those of boron, vanadium(V), and molybdenum(V1) (5-7), The mole fraction diagram of each species of tungstate, also shown in Figure 1,was calculated by using the acidic dissociation constants of H2W04: pK1 = 3.5 and pK2 = 4.6 (11). The features of the pH dependence of the D value almost agree with the mole fraction diagram of HW04-. If there were no interaction between tungsten(V1) and the dextran matrix, the D value would be in the region of 0 to 1 (12). The high value of D for HWO, indicates that the HW04- species has a suitable configuration for adsorption on Sephadex gel. The interaction is similar to those of polyols and several oxo anions such as borate, vanadate, and molybdate and therefore due to the complex formation between HW04- species and the hydroxyl groups of the gel matrix (13-15). Isopolytungstates exist in the pH region 2 to 5, even if tungsten concentration is mol/dm3. It is reported that tungsten(V1) in high concentrations is adsorbed on Sephadex G-10 gel and on activated carbon from acidic solutions (9,16). D values were also measured in high concentrations and the results are shown in Figure 2. In the systems of 1 X lo4 and 1x mol/dm3, almost all tungsten(V1) exists as isopolytungstates in the pH region where the distribution was measured. The values increase with decreasing pH. The adsorption behavior is similar to those on Sephadex G-10 gel and on activated carbon. In the system of 1 X mol/dm3, however, tungsten(V1) also shows maximum adsorption behavior at pH 5.5. Although at least two isopolytungstates other than HWQ4- are expected to take part in the adsorption on the gel, they show much weaker adsorbability than HW04-. The behavior of HWO, should be important on concentrating tungsten a t trace level in geological samples, When tungsten in a rock sample is determined, it is necessary to separate the tungsten from large amounts of elec-
2ok
0 ' 3 0'4 0'5 Concent rat ion ( mo l i d d )
0'1 012
'
Flgure 3. Effect of electrolytes on the adsorption of tungsten(V1) on NH,OH.HCI (A): gel, Sephadex gel: NaCl (O),NaNO, (0),NaCIO, (A), Sephadex G25,0.20 g; solution, lo-' mol/dm3 initial concentration of tungsten, pH 3.7, 250 cm3.
trolytes. Figure 3 shows the effect of the presence of various salts on the D values. The adsorption of tungsten(V1) was not changed significantly in the presence of these salts up to 0.5 mol/dm3. The adsorbability of oxo anions on the gel can be effectively applied to analysis of samples containing large amounts of salts. Selective Concentration of Tungsten(V1) on Sephadex G-25 Gel Column. Boron and vanadium(V) are reversibly desorbed from the column by changing the pH of the eluent (5, 6). Tungsten(V1) is adsorbed on Sephadex gel more strongly than boron and vanadium(V). It is necessary to desorb tungsten(V1) from the gel column by a chelating agent. When 0.01 mol/dm3 EDTA (disodium salt) solution warmed a t about 30-40 "C was used as eluent, almost all of the tungsten(V1) was always recovered in the first 50 cm3 effluent fraction. The desorption a t low temperature (below 20 "C) results in lowering the recovery of tungsten, When tungsten(V1) was adsorbed at pH 3.7 where tungsten shows maximal adsorption behavior, tungsten(V1) in up to 1500 cm3 solution was completely concentrated with the column (Bio-Rad; 1.0 cm i.d., 20 cm long) containing 4.7 cm3 of the gel. Molybdenum(V1) and vanadium(V) are also adsorbed on a Sephadex gel column in the pH region of 3.7-3.8 and interfere in the absorptiometric determination of tungsten. All of the geological samples contain not only tungsten but also molybdenum and vanadium. It is necessary to remove or mask the elements. Hydroxylamine can serve as masking reagent, whereas the rate of the masking reaction is very slow at room temperature. In Figure 4, the recovery of tungsten(V1) and molybdenum(V1) is plotted against heating time a t 80 "C in
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 ~
Table 11. Determination of Tungsten Content in Rocks (n = 5) sample
I 0
ignesus rocka basalt JB-1 granite JG-1 sedimentary rock shale marine-1 marine-2 marine-3 brackish
t 20
40
60
80
"Certified
100
content, mg/kg 21.1 f 0.2 2.0 i 0.5 2.6 f 0.3 3.7 f 0.5 4.1 i 0.7 1.2 i 0.4
value (ref 13): JB-1, 19 ma/ka; JG-1, 1.3 mg/ka.
Hedtlng t~m(mn)
Figure 4. Dependence of heating time at 80 OC on masking of molybdenum with hydroxylamine and CyDTA (A, concentration of molybdenum, 5 X mol/dm3; 13,concentrationof tungsten, 5 X lo-' mol/dm3): solution, 100 cm3 (pH 2.5)containing 0.75 mol/dm3 in hydroxylamine and 0.005 mol/dm3 in CyDTA. The absorbance was measured after concentration by the column method.
Table I. Effect of Foreign Ions on the Recovery of Tungsten species added
molar ratio" (foreign ion/W)
amt o f W found, wg
re1 error,
Al(II1) Fe(II1) Mn(I1) Mo(V1)
100 000 100000 10 000
2.52 2.48 2.60 2.53 3.25 2.51 2.40 2.42
+0.8
100
Ti(1V)
1000 10000 10 000
V(V)
1000
SiOz
100-cm3 sample solution containing 2.5 wg of
%
-0.8 +4.0 +1.2 +30 +0.4 -4.0 -3.2
W.
the presence of hydroxylamine. The absorbance for molybdenum decreases with time and is equal to zero after 60 min and that for the tungsten decreases slightly. Therefore, if each sample solution is heated a t 80 "C for 60 min in the presence of hydroxylamine, the tungsten can be concentrated without influence from a 100-fold excess of molybdenum(V1) by the present column method. Vanadium(V) can be masked simultaneously by this procedure. Much coexisting iron and aluminum have to be separated before tungsten can be determined in many cases. The interaction of hydroxo complexes of aluminum and iron(II1) with tungsten(V1) results in lowering of the tungsten recovery. The lower the pH of solutions, the smaller their interference becomes. Therefore, tungsten was adsorbed on the column at pH 2.5. The D value is about 100 at this pH and is adequately high for the purpose of separating and concentrating tungsten. CyDTA can form very stable complexes with many metal ions other than molybdenum(V1) and tungsten(V1). CyDTA, therefore, is added for masking foreign metal ions. The recommended procedure was applied to 100 cm3 solutions containing 2.5 Fg of tungsten and various amounts of
foreign ions. As shown in Table I, it is expected that any interferences by coexisting ions can be overcome in analysis of geological samples. Iron and aluminum do not interfere with the recovery of tungsten by 105-foldamounts of tungsten. Determination of Tungsten i n Rocks. The proposed method was applied to determination of tungsten in rocks. The slope of the graph obtained by the standard addition method was identical with that of the calibration graph. It indicates that the recovery of tungsten in sample solution was quantitative. The tungsten contents in shale from Kumamoto Prefecture and in JB-1 and JG-1 are listed in Table 11. JB-1 and JG-1 are reference samples provided from the Geological Survey of Japan. The results obtained by the standard addition method agreed with literature values (17). Registry No. Tungsten, 7440-33-7;Sephadex G-25,9041-35-4; pyrogallol red, 32638-88-3.
LITERATURE CITED (1) Chan, K. M.; Riley, J. P. Anal. Chim. Acta 1987,3 9 , 103-113. (2) Kawabuchi, K.; Kuroda, R. Taianta 1970, 17, 67-73. (3) Hamaguchi, H.; Nakai, T.; Ideno, E. Nippon Kagaku Zasshll961,8 2 , 1493-1498. (4) Wunsch, G.;Czech, N. Fresenius' Z.Anal. Chem. 1984,317, 5-9. (5) Yoshirnura, K.; Karlya, R.; Tarutani, T. Anal. Chim. Acta 1979, 109,
. .- .-..
115-191
(6) Yoshimura, K.; Kaji, H.; Yamaguchl, E.; Tarutani, T. Anal. Chin?. Acta 1981, 130, 345-352. (7) Yoshimura, K.; Hiraoka, S.;Tarutani, T. Anal. Chim. Acta 1982, 142, 101-107. (8) Angus,-H. J. F.; Briggs, J.; Sufi, N. A.; Weigel, H. Carbohyd. Res. 1978,66, 25-32. (9) Ortner, H. M.; Krainer, H.; Daimonego, H. J . Chromatogr. 1973,82, 249-261. (10) Shijo, Y.; Takeuchi, T. BunsekiKagaku 1973,2 2 , 1341-1345. (11) Schwarzenbach, G.; Geier, G.; Littler, J. Helv. Chim. Acta 1982,4 5 , 260 1-26 12. (12) . . Fisher. L. "An Introductlon to Gel ChrornatograDhy"; North-Holland: Amsterdam, 1969. (13) Reimerdes, E. H.; Rothkitt, K.-D.; Schauer, R. Fresenius' Z.Anal. Chem. 1984,318, 285-266. (14) Bourne, E. J.; Hutson, D. H.; Weigel, H. J . Chem. SOC. 1980, 4252-4256 .- - - - - (15) Bourne, E. J.; Hutson, D. H.; Weigei, H. J . Chem. SOC.1981,35-38. (16) Ohashi, K.; Murakami, K.; Yamamoto, K. Bunseki Kagaku 1983,3 2 , E3 13-E319. (17) Ando, A. 3C17, Annual Meeting of the Geochemical Society of Japan, Shimlzu, Shizuoka, 1961.
Received for review November 26,1984. Accepted February 21, 1985.
