Characterization of lsopolytungstates by Molecular Sieve Chromatography H. M. Ortner Metallwerk Plansee AG. & Co. KG, A-6600 Reutte, Austria
The column chromatographic behavior of tungstate solutions on tightly cross-linked dextran gels is governed by two effects: by chelate complex formation with the hydroxyl groups of the dextran matrix which increases with decreasing pH and by a more or less distinct molecular sieve effect according to the molecular size of various isopolytungstate species. This has already been found in a previous study ( 1 ) . Similar behavior was observed for molybdates and vanadates (2, 3 ) . The following discussion shows that individual isopolytungstates can be characterized with regard to their degree of condensation as Wg, Wlz, and W24 aggregates on the basis of their elution behavior on Sephadex G-10 columns.
EXPERIMENTAL Apparatus. Column chromatographic experiments were carried out on home-made Plexiglas columns. Sample application and elution were carried out by means of a peristaltic pump 4912A from LKB, Bromma, Sweden. Eluate fractions were collected using an LKB 7000 Ultro Rac fraction collector with drop counter. A Philips sequence spectrometer P W 1220/C was used for the quantitative X-ray fluorescence spectrometric determination of tungsten in the eluate fractions. Those measurements were carried out under following conditions: gold target tube, 50 kV, 40 mA; PVC cells with cover and bottom consisting of 6 pm Mylar foil; LiF(ZOO)-crystal; first order; collimator: fine (lamellar distance 160 pm); counting time 20 or 40 sec; measurements were carried out a t the W La-line with proportional and scintillation counter in series; automatic pulse height selection by means of a sine theta potentiometer and fixed window. Reagents. Sephadex G-10 fine was purchased from Pharmacia, Uppsala, Sweden. The tungstate solutions were made up from 99.99% H2W04 supplied by Tungsram, Budapest, Hungary. All other reagents were of AR grade from Merck, Darmstadt, Germany. Procedure. Preparation of the tungstate solutions: The starting material for all column tests was HzW04. This was dissolved in 5 ml 3M NaOH (15 mmoles) per gram of HzW04 (4.0 mmoles). The HzW04 was suspended in about 80 ml of water and dissolved by NaOH addition under slight heating. The solution was then made up to 100 ml. The p H adjustment was carried out by slow dropwise addition of 0.1M HCl with intensive stirring. The p H value of older tungstate solutions was checked every three days and readjusted when necessary, also in every instance before application to the column. Column Chromatography. Parameters for the “small” column: bed dimensions: 10 X 3.0 cm (70.7 ml); flow rate: 14 ml/hr cm2 (100 ml/hr); sample size: 3.0 ml; volume of collected fractions: 3.0 ml; interstitial volume of column, u = 26.7 ml; inner gel volume, L: = 17.0 ml; room temperature (20-23 “C). Parameters for the “large” column: bed dimensions: 57 X 2.0 cm (179 ml); flow rate: 31.7 ml/ hr cm2 (100 ml/hr); sample size 3.0 ml; volume of collected fractions: 3.0 ml; interstitial volume of column, u = 67.7 ml; inner gel volume, ui = 43.2 ml; room temperature (20-23 O C ) . Deionized water was the eluant used for all column chromatographic experiments.
RESULTS AND DISCUSSION According to earlier results (I ), an aging study on tungsten solutions in the neutral range appeared most promising. The coexistence of three tungstate species with different degrees of condensation had been established in this p H region, even on a comparatively small Sephadex G10 column. However, the resolution of that column was not 162
sufficient to separate all three elution peaks and to measure the peak elution volumes precisely. Hence, the behavior of a 0.1M tungstate solution at pH 7.0 is now investigated as a function of its age on a column of suitable size. Owing to pronounced sorption of certain tungstate species on the large bed volume of this column, there is the risk that such species would be completely adsorbed and consequently would not appear in the elution profile. Therefore, a comparative study on a large and a small column seemed advisable. The results of thii study are summarized in Table I and Figure 1. The somebyhat unusual dimensions of the small column permit working with low flow rates (14 ml/hr cm2) despite the relatively high drop rate (100 ml/hr) and thus a t “near equilibrium conditions” ( 4 ). Under these conditions, both fairly flow-rate independent molecular sieve effects as well as flow-rate sensitive sorption effects caused by chelate complex formation ( 1 , 2 )should be clearly apparent. The considerably greater flow rate used with the large column was intended to favor any molecular sieve effects over sorption effects. Although there are diverse opinions as to the number of species of isopolytungstates, their molecular size, stability, degree of protonation, and transformation schemes (1, 5, 6),the transformations shown in Table I1 can be assumed to apply to 0.1M tungstate solutions under the test conditions. All column experiments tabulated in Table I can be interpreted by means of the transformation scheme of Table I1 in combination with the aforementioned chelate complex formation of tungstates with the gel matrix. Degree of Condensation. First, it seemed important to determine whether the three peaks in fact correspond to and Wg, postuthe three degrees of condensation W24, W12, lated by Glemser et al. (5 1: if the Kd averages for the large column from Table I are plotted against the logarithms of the degree of condensation, a straight line results (Figure 2), as is normally the case in molecular size determinations by means of molecular sieve chromatography. This may be taken as indirect proof for the existence of these degrees of condensation under the chosen experimental conditions. Taking into account the greater uncertainty of the Kd determination with the small column, a Kd range of 0 to 0.30 for W24, around 0.6 for W12, and around 1 for w 6 can be assumed for all corresponding tests, which is in agreement with the results in Table I. An actual size determination of the three isopolytungstate species from the obtained Kd values is not yet possible since information on the relationship between Kd or K values and the hydrated ionic radii of proper reference substances has so far been reported only for the Sephadex gels G-15 and G-25 ( 7 , 8 ) . Total Chromatographic Recoveries. Figure 3 shows the variation of the total recoveries with increasing aging of the tungstate solution. The trend is practically identical for the large and small column. The absolute amounts are also comparable. This confirms the assumption that molecular sieve effects on the large column are favored over sorption effects by the greater flow rate. Of interest also is a comparison of the yields obtained in an earlier column test se-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
%
I
I ,
1
j: 1 jj 1
column 100 x 30mm
- fresh
---
26 h
50 h
ml
10
Figure 1. Aging experiments for 0.1M tungstate solutions at pH 7.0. Comparison of elution profiles for Sephadex G-10 columns of different dimensions
ries for 0.01M tungstate solutions a t pH 3.0 ( 1 ). Apart from the expected lower rate of conversion for more dilute solutions, the trend of the total recovery in this test series is comparable with that in the experiments described here,
as is also evident from Figure 3. Because of the lack of a measurement for the 0.01M solution between 2 and 7 days, the assumed trend of the curve between these two points is shown as a dotted line.
Table I . Comparative Aging Experiments with 0.1MTungstate Solutions at p H 7.0 on a Small and a Large Sephadex G-10 Column T o t a l recoverf hO.0
Small column 1 fresh 2 26 hr 3 50 hr 4 4d 5 8d Large column la fresh 2a 24 h r 3a 48 hr 4a 4d 5a 8d 6a 14 d
Peaks, Kd valuesd
up to m l
lst Peak
2nd Peak
60 78 61 79 82
140 140 140 140 140
0.31
not resolved not resolved not resolved 0.49 not resolved
54 71 61 80 84 55
190 190 190 190 190 190
A . of s.*
0.29 0.29 0.13 0.06 0.15 0.14 0.21 0.10
0.14 0.10
... ... strong tailing 0.56 0.60 0.58
3rd Peak
1.2 1.2 1.2 1.0
not resolved
1.3 1.3 1.2 1.05 1.07 1.03
Mean: 0.14 0. 5a 1.05 (No. la-6a) (No. 4a-6a) (No. 4a-6a) a Number of column test. Age of solution. Total chromatographic recovery in 70of the quantity applied t o a column, measured u p to an eluate volume of ml. Kd values of all peaks occurring in an elution profile are tabulated. ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
163
-
-
-
Table 11. Transformations of 0.1MTungstate Solutions at pH 7.0 a n d Room Temperature (5) Tungstate Monotungstate Paratungstate A W P a r a t u n g s t a t e B Species X Metatungstate species d - Metatungst ate / Degree of Species X: W,2condensation WI *w6 $1-Meta: Wz4 Rate of quick, min., min. to hours, Para E- Species X: Species X e transformation for 0 . 1 M equilibrium ! slow Meta: equilibsolutions also Para B 2 Meta rium,i-Metacomplete very slow at pH 7 Meta: very slow at pH 7: weeks to months Formula wo42[w6020 [ W ~ Z O ~ ~ ( O H ) ~ Species ~ I ~ ~ - X: formula [Wl,0,,(OH),]6not yet established; probably identical with Para-B. L-Meta: [w240,,(OH),,Ii2-
--
-
d
Discussion of the Elution Profiles. On both the large and small column, an initial increase and then a decrease of the peak with a Kd value of 0.14 is clearly recognizable. Since only $-metatungstate exhibits a W24-aggregation, this peak must be due to the latter. Similar behavior was observed with 0.01M solutions a t pH 3.0 ( 1 ). Furthermore, it is noticeable that with the small column there are always three peaks (unresolved for young solutions), whereas with the larger column, these are present only when older solutions are chromatographed. Obviously the middle species is practically completely adsorbed from young solutions by the much larger volume of the large column. Similar total recoveries for the large and small column are not contradictory to this total loss of the W12 fraction on the large column since the recovery of the middle species on the small column certainly is incomplete, too. Furthermore, the W12 portion of the total elution profile for the small column and for young solutions is rather insignificant. However, the loss of the total W12 fraction on the large column causes the total recovery to be slightly lower than for the small column. This trend is reversed for older solutions where the Wlz sorption is less pronounced. The K d values of the third peak are also higher for fresh solutions than for old ones. It seems probable that these variations are caused by tungstate species of the same degree of condensation but of different degrees of hydration or protonation. 164
On both columns, the eluted W12 fraction increases with increasing age of the solution. This can be explained by a gradual conversion of $-metatungstate into metatungstate. Of all possible Wlz condensation products, the metatungstate has the lowest content of hydroxyl groups (cf. Table 11). However, these hydroxyl groups are essential for the chelate complex formation with the gel hydroxyl groups according to the reaction scheme proposed in (2). Thus, the sorption tendency for different W12 species may decrease with decreasing hydroxyl content. This would then account for the increasing eluted W12 portion with increasing age of the solution. Hence, this is due to elution of increasing portions of metatungstate. The portion eluted at Kd = 1.2 passes through a maximum during the first 50 hours. This may be due to monotungstate or paratungstate A. According to Figure 2, both species have complete access to the gel interior, so that the probability of sorption by complex formation is very pronounced. This is also proved by the strong tailing of these peaks and the shift of the Kd value from 1.0 to 1.2. Monotungstate, which is assumed to be largely hydrated in aqueous solution, should exhibit a still stronger sorption tendency than paratungstate A. This makes the elution maximum a t 24 hours understandable: the sum of eluted monotungstate and paratungstate A then attains a maximum. At this moment, the sample solution probably does
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
not contain appreciable amounts of monotungstate. However, the partial paratungstate A sorption with its concomitant pH increase during the chelate formation ( 1 ) again causes formation of monotungstate. The elution a t Kd = 1.05 with clearly less tailing which is observed from the fourth day on, points to the presence of paratungstate A mainly. This again is due to the rising elution of W ~ aggreZ gates, the sorption of which also causes an increase in pH and thus a stepwise decondensation. Decondensation processes of tungsten(V1) are slow so that paratungstate A rather than monotungstate is formed during elution. The dependence of the sorption caused by chelate formation on the degree of condensation can thus be summarized as follows: +-Metatungstate ( W Z ~naturally ) exhibits the least sorption tendency. Because of molecular sieving (Kd = 0.14),it contacts only a small part of the total gel and the contact time is short. Chelate formation tendency of the WI2 condensation products increases with increasing hydroxyl group content of the isopolytungstate molecule. This explains the aforementioned absence of the W12 peak for the large column in the case of one- or two-day-old solutions as well as the trend of the total yield (Figure 3). Paratungstate B and species X are considerably more adsorbed (and also more easily decondensated by a pH increase) than the very slowly formed metatungstate. Because of the large bed volume of the large column, paratungstate B does not appear a t all in the elution profile of one- and two-dayold solutions. After two days, a maximum of paratungstate B (and species X) in the test solution leads to a yield mini-
mum. The decrease in the total yield of two-week-old solutions can be accounted for by a decrease in the +-metatungstate concentration in favor of metatungstate formation. Paratungstate A and monotungstate are eluted at Kd = 1.0 to 1.2, with W1 being more strongly adsorbed than We. Thus, the elution profiles obtained can be interpreted on the basis of modern concepts of degrees of condensation and conversion reactions of individual isopolytungstates as well as the chelate complex formation of tungstates on dextran gels. It is hoped that the method of molecular sieve chromatography, especially on tightly cross-linked dextran gels, in combination with conventional methods such as dialysis, electrophoresis, sedimentation, diffusion, etc., may provide valuable new insights into the highly complex chemistry of iso- and heteropoly acids.
LITERATURE CITED (1) H.
M. Ortner.
H. Krainer, and H. Dalmonego, J. Chromatogr., 82, 249
(1973). (2)S.Karajannis, H. M. Ortner, and H. Spitzy, Manta, 19, 903 (1972p (3)H. M.Ortner and H. Dalmonego, J. Chromatogr., 89, 287 (1974). (4)J. C. Giddings, "Dynamics of Chromatography," Part I, Marcel Dekker, New York. N.Y.. 1965. (5)0. Glemser, W. Holznagel. W. Hoitje, and E. Schwarzmann, Z. Naturforsch., 206, 725 (1965). (6)P. Souchay, "Ions Mineraux Condenses," Masson, Paris, 1969. (7)Y. Ueno, N. Yoza, and S.Ohashi, J. Chromatogr., 52, 469 (1970). (8)Y. Ueno, N. Yoza, and S.Ohashi, J. Chromatogr., 52, 481 (1970).
RECEIVEDfor review February 4, 1974. Accepted September 17,1974.
Rapid Simultaneous Determination of Picogram Quantities of Aluminum and Chromium from Water by Gas Phase Chromatography Thomas A. Gosink Institute of Marine Science, University of Alaska, Fairbanks, Alaska 9970 7
The simultaneous analysis of aluminum and chromium has been reported by Genty and coworkers ( I ) , but the procedure is lengthy, requires repeated handling, and, as a consequence the accuracy and precision, though good, suffers some deterioration. In our efforts to adapt fluorinated metal chelate gas chromatography to natural water samples, we have developed the improved procedure described in detail in this report. There are at least two advantages in the use of gas phase chromatography and of chelates for the analysis of aluminum and chromium. The first is that by taking advantage of the electron capture detectors sensitivity to perfluoroalkyl groups, several orders of magnitude greater sensitivity over most standard methods is achieved, and the second is that it appears that the chelation technique will enable investigators to differentiate between dissolved, particulate, and adsorbed forms of the element from the analysis differences for raw, filtered, and acid digested samples which can be prepared, and thus stabilized, at the collection site rather than at a later time after the sample has changed. This aspect will be discussed in an article to be submitted elsewhere.
EXPERIMENTAL Apparatus. A Varian Model 1520B gas chromatograph, equipped with a tritium source electron capture detector and thick walled (0.25-inch o.d., 0.12-inch i d . ) Teflon columns and crossover, was employed. The flow rate of the nitrogen carrier was 70 ml/min. The temperatures for the injector, column, and detector were, respectively: 150, 145 and 185 O C . Column 1. The principal column used was 60 inches in length, and filled with 60-80 mesh silanized glass beads lightly coated with D.C.-710. T h e coating was accomplished by boiling ethyl acetate in a beaker containing one-half the volume of glass beads as solvent, and 2% of the weight of the beads of D.C.-710. The beads were isolated from the hot solvent by suction filtration on a Buchner funnel. Columns 2 and 3. A 46-inch column prepared as above, but using only 0.3% D.C.-710, was similar in retention time to a 32-inch column packed with 5% by weight of SE-30 on silanized chromosorb P; both operated a t 120 O C . Glassware. All glassware was initially cleaned with NOCHROMIX and then silanized with a 25% solution of hexamethyldisilazane in benzene. T h e 15-ml screw cap culture tubes and the Teflon-lined caps thereafter were cleaned by soaking them for several hours in 3M HC1, followed by a thorough rinsing with deionizeddistilled water. In order to ensure uniform low blanks, the screw
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