Accurate Determination of Ten Major and Minor Elements in Silicate Rocks Based on Separation by Cation Exchange Chromatography on a Single Column F. W. E. Strelow, C. J. Liebenberg,' and A. H. Victor National Chemical Research Laboratory, Pretoria, Republic of South Africa
A method is developed for the quantitative separation and accurate determination of AI, total Fe, Ti, Ca, Mg, Mn, Na, K, V, and Zr in synthetic mixtures and applied to silicate rock samples. Only a single column of AG 50W-X8 sulfonated polystyrene cation exchange resin is used and the elements are eluted by stepwise changing eluting agents. Ti(IV) and Zr appear in the same eluent fraction, but can be determined spectrophotometrically without interference. All other elements are completely separated from each other and determined very accurately by selected known analytical procedures. Recoveries for major amounts are 100.0 f 0.1% or better for AI, Fe, Ca, Mg, Mn, and V and 100.0 f 0.2% or better for TI, Na, and K. Recoveries for Zr are only about 9 8 % because Hf is not eluted together with Zr. Relevant distribution coefficients, elution curves, results for two synthetic mixtures and six standard rock samples, and a method for the separation of trace amounts of V in rocks from accompanying impurities are presented. Several methods have been published which describe t h e separation of t h e major elements in silicate rocks (1-4). Only t h e method of Strelow et al. (4) apparently seems t o be capable t o give accurate results for t h e major rock forming elements Al, Fe, Ca, Mg, N a , K, a n d Ti, a n d also includes t h e minor or trace elements M n , Zr, a n d V. T h e method uses eluting conditions with large separation factors, which is a n advantage because it allows quite large samples to be separated on relatively small columns; b u t it has t h e disadvantage t h a t three columns have t o be used. I t would be a n attractive improvement if t h e separation could be carried out on a single column. T h e method of Oki et a1 (2) seems to be t h e only one which uses a single column for t h e separation of t h e major rock-forming elements, b u t iron has t o be removed hy extraction prior t o t h e ion exchange separation. T h i s will have a n adverse effect o n t h e accuracy of t h e results produced, because small losses always t e n d t o occur. T h e r e seems to be a tendency t o low recoveries, a n d deviations between results of repeated analyses sometimes a m o u n t t o more t h a n 1%of t h e result. Systematic information on distribution coefficients a n d cation exchange behavior in HC1-acetone solutions ( 5 ) makes it possible t o omit t h e solvent extraction of iron(II1) a n d include t h e separation of iron(II1) in t h e ion exchange procedure. It also provides conditions for a very good separation between manganese(I1) a n d magnesium ( 6 ) , which are eluted toDepartment of Inorganic and Analytical Chemistry, University
of' Pretoria.
Yosimura and H. Vaki, Fresenius 2. Anal. Chem., 161, 393 (1958). Y. Oki, S. Oki, and S. Hidekata, Bull. Chem. SOC.Japan, 35, 273 (1962). A . D. Maines. Ami. Chim. Acta. 32, 211 (1965). F. W. E. Strelow, C. J. Liebenberg, and F. von S. Toerien, Anal. Chim. Acta, 47, 25 1 (1969). F. W. E. Strelow, A . H . Victor, C. R. van Zyl. and Cynthia Eloff, Anal. Chem, 43,870 (19711. F. W. E. Strelow and Cynthia Baxter. J. South Afr. Chem. lnst., 22, 29 (1969). J.
gether by Oki et al. I n addition, " 0 3 provides larger separation factors a n d a better separation of potassium from t h e dismagnesium (7) a n d titanium(1V) because in "03 tribution coefficients of t h e alkalies are lower while those of Mg a n d Ti(1V) are higher t h a n in HC1; a n d finally HC104 a n d "03, respectively, are better eluting agents t h a n HCl for t h e separations of Mg a n d Ca from Al. T h e increases in t h e separation factors a r e not very large, b u t become significant when t h e conditions for t h e separations are rather critical. T h e merits of a separation procedure for silicate rock elements based on these facts, using only one cation exchange column, a n d including t h e elements V a n d Zr have been investigated in detail.
EXPERIMENTAL Apparatus. Borosilicate glass tubes of 25- or 20-mm i.d. with fused-in glass sinters of No. 2 porosity and a buret tap at the bottom and a B 19 ground glass joint at the top were used as columns. A Zeiss PMQ I1 and a Perkin-Elmer 303 instrument were used for spectrophotometric and atomic absorption measurements, respectively. Reagents. Analytical reagent grade chemicals were used throughout. The resin used was the AG 50W-X8 sulfonated polystyrene cation exchanger supplied by Bio-Rad Laboratories, Richmond, Calif. Resin of 200- to 400-mesh particle size was used for column work. Phosphoric acid to be used for fusions was diluted with 4 volumes of H20 and passed through a cation exchange column. 'The cleaned acid was stored in plastic bottles. Three solutions were prepared for column work. Solution A: 11.2 mg of V(V); 22.3 mg of Na; 41.5 mg of K; 70.0 mg of Fe(II1); 15.5 mg of Mn(I1); 60.2 mg of Mg: 71.5 mg of Ca; 79.5 mg of AI; 14.8 mg of Zr and 12.0 mg of Ti(IV) per 50 ml of 1M HC104 containing 0.15%H202. Solution B: 2.2 mg of V(V); 22.3 mg of Na; 41.5 mg of K; 35.0 mg of Fe(II1); 15.5 mg of Mn(I1); 30.1 mg of Mg; 71.5 mg of Ca; 26.9 mg of Al; 1.2 mg of Ti(1V) and 200 Hg of Zr per 50 ml of 1M HC104 containing 0.15% Hz02. Solution C: Similar to A but containing 3.7 mg of Zr instead of 14.8 mg. Furthermore, standard solutions of the 10 elements, each containing one element as chloride or nitrate in dilute HC1 or " 0 3 in amounts per 10 ml as indicated in Tables I and 11. Distribution Coefficients and Elution Sequence. From a critical study of available information on cation exchange distribu(9),H2S04 ( 9 ) , HC104 ( I O ) , and tion coefficients in HC1 (81,"03 "&acetone mixtures ( 5 ) , the elution scheme presented by the distribution coefficients in Table I11 was finally selected for the separation. Coefficients for 0.01, 0.2, 0.75, and 1.25M acids were obtained by plotting the published coefficients against acid concentration and reading the required values from the curves. The resulting elution sequence is presented in Table IY, Elution Curves. The separation factors for some neighboring pairs in the scheme are rather small and the separations are marginal. These separations were investigated in more detail. Na-K-Ti(1V). Fifty milliliters of solution A were measured out, evaporated to fumes of HC104, and fumed until only about 1 nil of HC104 remained. After cooling, about 100 ml of deionized water (7) F. W. E. Strelow, J. H. J. Coetzee, and C. R. van Zyl, Anal. Chem, 40, 196 (1968). (8) F. W. E. Strelow, Anal. Chem., 32, 1185 (1960). (9) F. W. E. Strelow, Ruthild Rethemeyer, and C . J. C. Bothma, Anal. Chem., 37, 106 (1965). (10) F. W. E. Strelow and H. Sondorp, Talanta, 19, 11 13 (1972).
A N A L Y T I C A L CHEMISTRY. VOL. 46, NO. 11, SEPTEMBER 1974
1409
Table I. Quantitative Results for Synthetic Mixture No. 1. Element
Taken, mg
Na
22.23 40.15 59.80 13.37 68.94 79.33 70.61 12.14 3.62 10.76
K Mg Mn(I1) Ca A1 Fe(II1) Ti(1V) Zr
v (V)
Found, mg
22.25 40.14 59.80 13.37 68.94 79.33 70.61 12.15 3.56 10.76
Table IV. Elution Sequence Element
Coefficient of variation, %
i 0.024 i 0.026 i 0.018 i 0.009
v (VI
0.11 0.064 0.031 0.067 0.026 0.028 0.072 0.19 0.59 0.042
+ 0.018 i 0.022 i. 0 . 0 5 1 i. 0 . 0 2 3 i 0.021
i 0.005
Na
K Ti(IV)
Fe(II1) Mn(I1) Mg Ca A1
Table 11. Quantitative Results for Synthetic Mixture No. 2 Element
Taken, mg
Na
22.23 40.12 59.80 13.37 68,96 79.33 70.62 1.21 0.210 0.0563
K Mg Mn(I1) Ca A1 Fe(II1) Ti(1V) Zr
v (V)
Found, mg
22.25 i 0 . 0 2 8 40.14 i. 0 . 0 7 0 59.81 + 0.028 1 3 . 3 6 i 0.011 6 8 . 9 5 + 0.017 79.33 i 0.024 70.63 i 0.022 1 . 2 1 i 0.006 0.210 i. 0.0022 0.0562 i 0.00027
+ Zr
Eluting agent
300 m l of 0 . 0 1 M "03 containing 0 . 1 5 % He02 500 m l of 0 . 5 0 M HNO, containing 0 , 0 5 % H?O, containing Another 450 ml of 0 , 5 0 M " 0 3 0 . 0 5 % Hz01 300 ml of 0 . 5 0 M HzS04 containing 0 . 0 5 % HzO? 350 ml of 0 . 2 0 M H C l in 85% acetone 300 ml of 0 . 7 5 M H C l in 90% acetone 400 ml of 1 . 2 5 M HC104 450 ml of 1 . 2 5 M "OB 250 ml of 3 . OM H C l
and 1 drop of 30% H202 were added and the salts dissolved by stirring. The solution was then passed through a column of 90 ml (30 grams) of resin. The column was 19 cm in length and 2.5 cm in diameter and had been equilibrated by passing through 50 ml of 0.1M HC104 containing 0.15% of HzOz. The beaker was rinsed and the elements were washed onto the column with 3 portions of 10 containing 0.15% of HzOz, and V(V) was eluted ml of 0.01M "03 with 300 ml of the same reagent. The eluate was discarded. Then containing 0.05% Na, K, and Ti(1V) were eluted with 0.50M "03 H202. Fractions of 25 ml volume were taken with an automatic fractionator and the amounts of Na, K, and Ti(1V) in the fractions were determined by a suitable analytical procedure. The experiment was repeated with 50 ml of solution B. A flow rate of 3.0 f 0.3 ml per minute was used throughout. The two elution curves are shown in Figure 1.
Coefficient of variation, 70
0.13 0.17 0,046 0.082 0.025 0.030 0.031 0.52 1.06 0.48
Table 111. Relevant Cation Exchange Distribution Coefficients for Rock Analysis Separation Scheme Eluting agent
0.01M
V(V)
Na
K
1.5
>lo2
>lo2
Ti(1V)
Zr
Mg
Fe(II1)
Mn(I1)
>io4
>io4
>io3
>io3
> i o 4
362
89
71
58
59
41.5
Ca
"03-
H202
0.50M
>io3
> i o 4
>io4
"03-
H2 0 2 0.50M HzS04HzOz 0.20M H C l in 85% acetone 0.75M HC1 in 90% acetone 1.25M HClO4 1.25M "01 3.OM HC1
...
12.7
26.2
...
...
...
...
...
...
...
...
1.8
920
... ...
...
...
...
...
...
3.7
...
...
...
...
,..
... ...
,..
...
... ...
... ...
...
...
... ...
... ...
...
47.3 9.0
4.6
>io3
225 16
... ...
113 -150 >io3
>io3
36 23
...
392 126 >io3
666 69 45 4.7
Ti(IV)-Zr-Mg. Experimental conditions were similar to those described above, but solutions A and C were used. V(V), Na, and K were eluted according to Table IV and the eluates discarded before the elution of Ti(1V) and Zr(1V) with 0.50M HzS04 containing 0.05% _ _ was started. The exoerimental elution curves are D-M A (COLUMN 16%) shown in Figure 2. Manganese(I1)'is retained more strongly and (COLUMN LOAD 8 , ~ ~ ) eluted later than Mg. Mg-Ca. An elution curve was prepared using solution A and the conditions described above. The eluting agents for Na, K, Ti(IV), and Zr, Fe(III), and Mn(I1) were passed through the column and discarded and Mg followed by Ca then eluted with 1.50M HC104. I The experiment was repeated with 1.25M HC104 as eluent. Both I ! elution curves are shown in Figure 3. Ca-AI. Elution curves were prepared with solution A as described using a column containing 90 ml of resin, and all the d e ments except Ca and AI were eluted using the elution sequence summed up in Table IV. Ca was then eluted with 1.50 and in an1 100 200 300 4 1200 other experiment with 1.25M "03. The elution curves are shown mi ELUATE in Figure 4. Complete Elution Curve for All Elements. Fifty milliliters of Figure Elution for Na-K-Ti(lV) with 0 , 5 0 M HN03-0,05% solution C were treated and passed through a column containing H202at various column loads. 90 ml resin as described for Na-K-Ti(1V). The elements were then 90 ml [19 X 2.5 c m ] AG 50W-X8 (H+) resin, 200 to 400 mesh. Flow rate 3.0 eluted using the elution sequence summed up in Table IV. The f 0.3 ml/rnin Ti(IV) + Zr elution was completed at the end of the first day. Fifty I?, I
l1,2
t
,
I
,.
1410
ANALYTICAL CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
4fi
:1 Mg
1
I
I I
5
A ( 1 . 5 0 M HC104)
I
I
B !1,25M HCIO4)
*-x-):
I
-
E
I
n w
I I
a
I I I
10 w
1
0 _1
I cc 0
i!
I '
( \
2
I
II
I
I
ml E L U A T E ml E L U A T E
Figure 2. Elution curves for Ti(lV)-Zr-Mg with 0.50M H 2 S 0 4 - 0 . 0 5% H 2 0 Pat various amounts of Zr 90 ml [ 19 X 2.5 cm] AG 50W-X8 (H+)
Figure 3. Elution curves for Mg-Ca with 1.50 and 1.25MHC104
resin: 200 to 400 m e s h . Flow rate 3.0
f 0.3 ml/min
milliliters of 0.01M HCI was passed through the column to fix the elements in their positions and the elution was continued at the next morning. On the second day, the elution of Ca was completed and the column left running for the night when necessary. The .41 was eluted on the third morning. Fractions of 25-ml volume were taken with an automatic fractionator and the amounts of the elements in the fractions were determined by appropriate analytical procedures. The experimental elution curve is shown in Figure 5. Quantitative Separation of Synthetic Mixtures. Ten-milliliter amounts of standard solutions of the 10 elements in dilute HCI or HNOs were measured out and mixed in sixfold. Three additional 10-ml amounts of each standard solution were measured out and kept separately for comparison. About 5 ml of 60°h HC104 were added to the mixed solutions, and these were then taken to fumes of HC104. The fuming was continued until about 1 ml of HC104 remained, and 100 ml of deionized water were added and the salts dissolved by stirring. Immediately before the sorption step, 1 ml of 30/0 H 2 0 ~was added. The elements were then adsorbed on a column of 90 ml of resin as described for the Na-K-Ti(IV) separation, but 0.1M HC104 instead of 0.OliM HNO:r was used to wash the elements on to the resin. Elution was carried out as indicated in Table IV using a flow rate of 3.0 f 0.3 ml per minute. The vanadium fraction was retained from the beginning of the adsorption step. During the first day VI\'), Na, K, Ti(IVj, and Zr were eluted and the column was left over night under 0.01M HCI as described above. On the second day Fe(II1j. Mn(II), Mg, and Ca were eluted and finally AI on the third day. Before the elution of Fe(III), 50 ml of 0.1M HCI in 50% acetone, which had been boiled for a few minutes, were passed through the column, and after the elution of Mn(II), the acetone was washed from the column by passing through 100 ml of aqueous 0.1M HCI. Both these fractions were discarded. All the other fractions were then evaporated to dryness. About 100 ml of deionized water was added to the Fe(II1)fraction before evaporation. Organic matter was destroyed in the residues when necessary and the amounts of the elements were determined by selected known analytical procedures carrying out the determinations for separated fractions and standards at the same time. Two blank column runs were also carried out and the results were corrected for reagent blank values. The quantitative results are presented in Tables I and I1 while the analytical procedures used are summed up in Table V. Separation of Traces of Zr from Excess HzSOi. Appropriat,e aliquots of the eluate were diluted to 0.25h.f H&04 containing 0.05% HsO:! and passed through columns containing 17 ml (7.5 gram) AG L X 8 resin, which had been equilibrated by passing through 50 ml of 0.25M H2SO4 containing 0.05% H202. The col-
90 ml [ 19 X 2.5 cm] AG 50W-X8 (H+) resin; 200 to 400
+ 0.3 ml/min
MA
x-x-x
mesh. Flow rate 3.0
!l,50M HN03)
B !1,25M " 0 3 )
15 -
E
LLI n
a
Figure 4. Elution curves for Ca-AI with 1.50 and 1.25MHN03 90 ml [ 19 X 2.5 f 0.3 ml/min
cm] AG 50W-XB (H+) resin: 200 to 400 mesh. Flow rate 3.0
umns were 10 cm in length and 1.5 cm in diameter. The Ti(1V) was eluted by passing through 50 ml of the same reagent followed by 50 ml of 0.05M HzS04 to reduce the amount H&O4 left on the column. Zr was then eluted with 200 ml of 4M HC1. Excess of HC1 and residual HzS04 were removed by evaporation on the hotplate and the residue was dissolved in the desired amount of HzS04. Analyses of Standard Rock Samples. About 0.5 gram ofsample material was weighed out accurately and dissolved by heating with a mixture of HF, HCI, and HC104 in a 100-ml Teflon beaker using a temperature controlled air-bath which enclosed the beakers to E of their height. Care was taken to remove all the H F and the solutions were finally fumed until only about 1 ml of HC104 remained. After dilution, any remaining insoluble residues were separated by filtration and the filtrate was kept for the column separation. The residue was dissolved by heating with a HF-"2104HxP04 mixture in a platinum crucible until a viscous transparent mass of polyphosphoric acid had been formed. The melt was dissolved in 10 ml of 0.1M HC104 containing 0.03% HzOz, kept apart,
ANALYTICAL CHEMISTRY, VOL. 46, NO, 11, SEPTEMBER 1974
1411
Figure 5. Complete elution curve for V(V)-Na-K-Ti-Zr-Fe(ll1)-Mn(ll)-Mg-Ca-AI 90 ml [19 X 2.51 AG 50W-X8 (H+)resin: 200 to 400 mesh. Flow rate 3.0 f 0.3 ml/min
Table VI. Standard Rocks Analyzed Table V. Selected Known Analytical Procedures Used for Determination of Elements Element
N a and K
Mg
Mn(I1)
Ca
A1
Fe(II1)
Ti(IV) Zr
v (V)
Procedure
Atomic absorption spectrometry using a propane-butane flame, a flat water-cooled burner and the 589-nm and 766.5-nm lines for N a and K, respectively. Titration with E D T A a t pH 10 a n d 40 "C, eriochrome black T indicator Small amounts ( < 2 mg) by atomic absorption spectrometry, using a n air-acetylene flame and t h e 285 n m line. Titration with E D T A in t h e presence of excess ammonia, hydroxylamine hydrochloride a n d triethanolamine, methylthymol blue indicator. Small amounts ( < 5 mg) by atomic absorption spectrometry, using a n air-acetylene flame a n d t h e 279-nm line. Titration with E D T A in the presence of excess ammonia, methylthymol blue indicator. Small amounts ( < 4 mg) by atomic absorption spectrometry, using a n air-acetylene flame and t h e 422.7-nm line. Addition of excess C D T A and backtitration at pH 5.5 with zinc sulfate, xylenol orange indicator. Small amounts ( < 2 mg) spectrophotometrically with alizarin red S. Addition of excess C D T A a n d backtitration a t pH 5.5 with zinc sulfate, xylenol orange indicator. Small amounts ( < 6 mg) by atomic absorption spectrometry using a n air-acetylene flame a n d t h e 248.3-nm line. Differential spectrophotometry as t h e complex with HzO?in 1 . 5 M H?SOa. Spectrophotometrically as t h e complex with xylenol orange in 0.25M H?SO:. Titration with ferrous iron in perchloric a n d phosphoric acid, barium diphenylamine sulfonate indicator. Small amounts (
