Ion Exchange Dissolution Method for Silicate Analysis Kuppusami Govindaraju Centre de Recherches Pe‘trographiques et Gkochimiques, Nancy, France
A simple procedure for dissolving silicate rock samples and eliminating the Si is described. The sample is fused with an alkali borate and the powdered fusion product is stirred for a few minutes in water with a strongly acidic cation exchanger (column or batch technique). The effluent contains only B and Si, whereas all other elements are recovered by elution with diluted HCI. Analytical applications for the determination of major and trace elements in silicates are discussed using atomic absorption, and chemical and spectrochemical methods. Particularly attractive is the analysis of the resin itself (after the adsorption step) using a tape machine for determining trace elements and even major and minor elements.
DECOMPOSITION of silicate rock samples is usually achieved either by an alkaline fusion (NaOH, Naz02,Na2C03, Na2C03 H3B03, LiB02) followed by dissolution of the fused melt in a suitable acid or by direct acid attack (HF, HC104, HCl, HNOI,H~SO~). Schafer ( I ) has described an ion exchange procedure in which finely ground phosphate rock sample is dissociated by shaking overnight in water with a cation exchanger. Additional references to such methods of dissolution of slightly soluble salts (phosphates, sulfates) are given by Samuelson (2, p. 247) and Inczedy (3, p. 136). Although most of the borates are soluble in water, some of them (Ba, Be, Ca, Fe, Mg, Pb, Sc) are insoluble or slightly soluble in water. Application of the ion exchange dissolution method to the field of silicate analysis so far has not been reported. This paper reports a simple method for dissolving silicate rocks and removing silica. The sample is first fused with an alkali borate or with a mixture of alkali carbonate and boric acid. The powdered fusion product is then stirred in water for a few minutes with a strongly acidic cation exchanger. The filtrate and the washings contain only B and Si. An elution with HCI permits the recovery of all other elements. Alkali borate is capable of decomposing even such refractory minerals as corundum, rutile, spinel, zircon, etc. which are not completely attacked by a mixture of H F HC104. Furthermore, the borate fusion-ion exchange dissolution technique described here is faster than the acid attack. However, borates can be dissolved more rapidly in cold “ 0 3 (4,5), HCI (6), citric acid (7), or EDTA (8). In these methods, all elements including Si are brought in solution. Although this is an advantage for the determination of the major constituents of silicate samples, the presence of large quantities of B and Si is troublesome for concentrating such solutions for trace element analysis. In atomic absorption analysis,
+
+
the high salt content soon leads to deposits in burner slots and to uneven flames. These difficulties are overcome by the procedure presented in this paper. EXPERIMENTAL Chemicals. Johnson, Mathey & Co., Ltd., high purity chemicals were employed. Resin and Columns. Convert Dowex 50 W (X8, 100-200 mesh), strongly acidic cation exchanger, to H+ form with 4 M HC1. Wash with deionized distilled water to remove the last traces of acid and keep the resin dry in a hot oven (105’ C). The ion exchange column is made of borosilicate glass and is 30 cm long, 27-mm 0.d. by 24-mm i.d., with glass wool plug or sintered glass disk as resin support. Fusion. Most of the preliminary studies were carried out with granite G R (9) previously enriched with up to 200-ppm amounts of 20 trace elements (Ba, Be, Co, Cr, Cu, Cs, Ga, La, Li, Mn, Mo, Ni, Pb, Rb, Sc, Sn, Sr, V, Yb, Zn). One gram of rock sample is fused with 2 grams of KzC03 and 2 grams of H3B03. Platinum or graphite crucibles are used for the fusion. The former were preferred during the preliminary studies. The fusion is carried out at 1000° C for 20 minutes. The cooled melt is ground to a fine powder with a mechanical vibrating grinder (AUREC) provided with tungsten carbide bowls. Procedure. Normally the ion exchange separation is carried out by a column method: Close the column at the bottom with a rubber stopper. Add 50 ml of deionized distilled water, 5 grams of dry resin, and 1 gram of the ground fusion product, Introduce inside the column a small Plexiglas tube (40 cm long, 6-mm 0.d. by 4-mm i.d.) connected to a supply of compressed air. Regulate the air supply so that the resin particles are in constant flotation and do not form a resin bed on the disk. After 30 minutes of constant agitation, remove the rubber stopper and receive the effluent in a 400-ml beaker. Remove the Plexiglas tube and wash any adhering resin particles into the column. Wash the column with 250 ml of deionized distilled water and combine it with the effluent. Elute with 200 ml of 2M HCl. The effluent and the eluate fractions are qualitatively analyzed by a spectrographic method. About 5 to 10 drops of the solution are evaporated on the flat top of a graphite electrode (7-mm i.d., height 10 mm). The electrode is then burnt by dc arc (26 A) for 10 seconds and the spectrum is recorded on a prism spectrograph (Jobin-Yvon) or on a dual grating spectrograph (Bausch and Lomb). The detection limit is about 1 ppm for most of the elements in solution. A Perkin-Elmer atomic absorption spectrophotometer (Model 303) and an A.R.L. Quantometer are used for quantitative analysis.
RESULTS AND DISCUSSION (1) H. N. S. Schafer, ANAL.CHEM., 35,53 (1963). (2) 0. Samuelson, “Ion Exchange Separations in Analytical Chemistry,” Wiley, New York, 1963. (3) J. Inczedy, “Analytical Applications of Ion Exchangers,”
Engl. ed., Pergamon Press, London, 1966.
(4) N. H. Suhr and C. 0. Ingarnells, ANAL. CHEM., 38,730 (1966). ( 5 ) G. Durnecke and J. Wiegrnann, Chim. Anal., 48,388 (1966). (6) L. Braicovich and M. F. Landi, Spectrochim. Acta, Suppl., 1957, p. 51. (7) K. Govindaraju, Appl. Spectry., 20, 302 (1966). (8) K. Govindaraju, unpublished work, 1965.
24
ANALYTICAL CHEMISTRY
Qualitative spectrographic analyses indicate that the effluent contains only B and Si, whereas these elements are absent in the acid eluate. All the cations adsorbed on the resin are eluted simultaneously. Almost all of them are eluted with the first five fractions of 10 ml. Exceptions are Ba, Be, and Sr which tail down to the tenth fraction of 10 (9) M. Roubault, H. de la Roche, and K. Govindaraju, Sci. Terre, 9, 339 (1964).
ml. However, 90% of these elements are eluted with the first five fractions, B and Si are absent in all these fractions, A total recovery of all elements with a quantitative removal of Si is thus achieved with 200 ml of 2M HCI. This is confirmed by the fact that no further elution of elements occurs, even with 6M HCl. Confirmation is provided by a comparative study of the proposed cation solution technique with another known technique-evaporation of 1 gram of the same fusion product (granite GR) with a mixture of HF HC104 and dissolution of the residue with 200 ml of 2M HCI. A comparison of the spectra indicates that these solutions are identical. A quantitative determination of some major elements (Ca, Mg) and some trace elements (Cs, Li, Rb, Sr, Zn) by atomic absorption spectrophotometry gave almost identical results for these two solutions. The concentration of HCl (0.2M to 1 O M ) has no influence on this elution curve, except that less acid is required with increasing strength of the acid. For example, with 4M HCl, only 100 ml are needed for complete elution. The elution process just described is independent of the diameter of the column (8 mm to 40 mm) and of the flow rate up to 20 ml/minute (column i.d. 24 mm). Thus, the washing and the elution steps can be carried out rapidly by applying pressure to the column. However, an exaggerated flow rate may lead to an incomplete recovery of trace elements. The adsorption of cations on the resin can also be achieved in a beaker containing a magnetic stirrer (batch method). The washing and elution steps are carried out on a sintered gooch crucible. Mo, Sn, and Zr have also been detected in the effluent solution. Apart from these exceptions (see Discussion), it was noted that despite the competition with a large excess of K+, all the cations irrespective of their valency are adsorbed on the resin. A strongly acidic cation exchanger usually has less preference for monovalent ions and a greater affinity for multivalent ions. Also, only 5 grams of resin are needed for the dissolution of 1 gram of fusion product. Brochmann-Hanssen (IO) recommends 10 grams of resin for the dissolution of 100 mg of PbSOa and Schafer ( I ) uses 5 to 10 grams for 50 mg of phosphate rock samples. The explanations for these apparent contradictions with known ion exchange rules or practices should be looked for in the particular nature of the proposed ion exchange reaction and of the experimental conditions: (1) The exchange reaction is always favored to the right by the continuous removal of hydrogen ions present originally in the resin. Thus, silicic and boric acids are formed and are dissolved in water :
+
--
+ K2Si03 2 R.SO,K + H2Si03 R ~ S O I H+ LiB02 + H 2 0 R.S03Li + H3B03 2 R*SO&I + Mg(B02)z + 6 H20 2 R*SOsH
+
(R*SO&Mg
+ H3B03 + 4 HzO
Thus, with a periodical removal of the aqueous solution containing silicic and boric acids and the addition of fresh water, it is even possible to use only 4 grams of resin with 1 gram of fusion product without any leakage of cations. (2) According in Samuelson (2, p. 11l), “The break-through capacity is diminished when the acidity of the solution is raised.” The table on “Break-through Capacities for Dif(IO) E. Brochmann-Hanssen, . I . Am. Pharm. Assoc., 43, 487 (1954).
Table I. Comparison of Results for CaO and MgO CaO, Absorption valuesa
Z Recommended valuesb
MgO, Absorption values”
2.40 2.50 2.40 Granite GR 2.60 2.48 Granite GA 0.88 Granite GH 0.80 0.68 0.0 14.20 13.89 13.40 Basalt BR This paper, average of two analyses. Recommended values taken from reference I2.
z Recommended valuesb 2.40 0.97 0.03 13.21
0
ferent Acidities” by the same author ( 2 , p. 111) shows that the maximum break-through capacity is attained at zero concentration of HCl. In the case of the exchange reaction described in this paper, it takes place in aqueous medium and, therefore, the break-through capacity is at its maximum. This is one of the reasons why only a small quantity of resin is necessary for the reaction. However, in the case of silicate samples particularly rich in trace elements-for example, biotites and ultrabasic rocks-the quantity of resin may have to be increased from 5 to 6 grams or the aqueous solution should be removed periodically and fresh water added. (3) It may be recalled that the aqueous suspension of resin and fusion product is kept in constant flotation. Under these circumstances, the resin particles have almost equal chances of catching the cations. Thus, the cations are adsorbed homogeneously on the resin. This fact explains also the simultaneous elution of all the cations with dilute HCl. (4) Inczedy (3, p. 53) while discussing selectivity theories states that “the selectivity coefficient of a multivalent ion depends on the concentration of the univalent ions to be exchanged in solution. If this concentration is low, the selectivity is considerably increased to the advantage of the multivalent metal ion, but when this concentration is high, the resin becomes more selective for the univalent ion.” This phenomenon of “electroselectivity” may partly explain the fact that monovalent ions are not lost into the effluent. It is not fully understood why Mo, Sn, and Zr are taken up only partially by the resin. However, in the case of Mo, the formation of a stable anionic complex (silicomolybdate) is presumed. As far as Sn and Zr are concerned, an acid medium is necessary for their uptake by cation exchangers. The main features of the borate fusion ion exchange technique for geochemical purposes are as follows : Effluent Solution. The ion exchange method of eliminating Si is faster than by classical methods. There is no need for fume hoods. The effluent solution can be used for the colorimetric determination of Si employing a method similar to that of Ingamells (11). Cation Solution. It can be used for the determination of major and minor elements using chemical or spectrochemical methods. Flame photometric or atomic absorption spectrophotometric methods can serve for the determination of alkali and alkaline earth elements. Preliminary results obtained on some standard rock samples using atomic absorption spectrophotometry are shown in Tables I and 11. These results are in general agreement with known values obtained by different methods and by different laboratories ( 1 2 - 1 4 . (11) C. 0. Ingamells, ANAL.CHEM., 38, 1228 (1966). (12) M. Roubault, H. de la Roche, and K. Govindaraju, Sci. Terre, 11,105 (1966). (13) M. Fleischer, Geochim. Cosmochim. Acra, 29, 1263 (1965). (14) F. Flanagan, Ibid., 31,289 (1967). VOL 40, NO. 1, JANUARY 1968
25
Table 11. Comparison of Results for Rb and Li Rb, ppm Li, PPm AbsorpAbsorpPublished tion Published tion values values" values values" Granite G-l* 216 220 20 24 32, 35, 63 GraniteG-2c 160 168, 203, 513 32 20, 23, 60, GraniteGRd 172 50, 160, 175, 76 GraniteGAd
150
GraniteGHd
370
183, 196, 240 99, 143, 114, 96 220 253, 317, 366, 56 420 2100f 1400
Biotite mica-Fee 2250
84 53, 93, 100, 129 29, 40, 47, 60 1600f
This paper, average of two analyses. Published values taken from reference 13. c Published values taken from reference 14. d Published values taken from reference 12. e Biotite standard recently processed at Nancy. f Unpublished spectrographic results. 0
b
Table 111. Precision of Ion Exchange-Tape Method Twelve analyses on granite Gr Re1 std Concn, Std Published values4 ppm dev dev, 2 . 2 400, 640, 850, 950, 1030, 988 22 Ba Sr
535
13
2.4
cu
295
8
2.7
Cr
99
2.7
2.7
Li V
62 58
3.6 2.9
5.8 5.0
Ni
47
2.5
5.3
Ga
25 8 8 5.1
2.5 1.6 2.4 0.3
10.0 20.0 30.0 5.9
sc
co Be 0
26
1100, 1280, 1400, 1460, 2450 210, 330, 355, 460, 510, 570, 580, 604, 740, 750 270, 325, 340, 370, 380, 440, lo00 15, 64, 100, 105, 110, 120, 126, 400 20, 23.2, 60, 84 30, 55, 61, 70, 84, 90, 100, 145 -10, 25, 35, 45, 60, 68, 69, 80, 100 15. 19.5, 21, 22 6 . 5 , 7, 8, 13, 19 < 2 0 , 4 , 6 , 10, 10, 10, 15, 16 2 . 5 , 4 , 5.4, 5 . 6 , 6 , 8, 8
Published values taken from reference 12.
ANALYTICAL CHEMISTRY
If the separation of alkali and alkaline earth elements into two groups is desired after the adsorption step, the column is treated with 200 ml of 2x sulfosalicyclic acid (pH 7.3) and the eluate solution is received directly in a second column containing an anion exchanger (Dowex 1, citrate form). The first column retains Mg, Ca, Sr, Ba, and part of Al, whereas, alkali elements, Fe, Ti, and the rest of the A1 pass through. The second column fixes Fe, Ti, and A1 and the alkali metals are recovered in the final effluent. The use of sulfosalicyclic acid suggested itself from the work of Maynes (15) who employed this reagent for separating Ca and Mg from Al, Fe, and Ti but under different conditions. Ion Exchange-Tape Technique. The direct analysis of the resin after the adsorption step for trace elements and even for major and minor elements (Al,Ca, Fe, Mg, Mn, Ti) represents the most interesting application. The resin is washed with water and dried at 105' C. It is then fed onto a moving adhesive tape using a tape machine (16,17). Trace elements analysis is carried out with a direct reading spectrometer (ARL Quantometer). High reproducibility characterizes this method, as can be observed from Table I11 which presents the precision data calculated from 12 individual experiments. It can thus be seen that the borate fusion-ion exchange technique is a reproducible process. This leads us to visualize the cations as grafted on the resin grains in a uniform and homogeneous manner. This is important as it has always been difficult to prepare homogeneous powder samples, particularly for trace elements analysis by spectrographic methods. And this has been one of the reasons why solution methods are usually preferred. It may also be mentioned that straight line working curves are obtained up to a lower limit of 5 ppm for most of the elements listed in Table 111. To summarize, the borate fusion-ion exchange technique contributes to the development of a new method for the dissolution of silicate samples, a new method of homogeneous sample preparation for analysis with tape machine and favorable conditions for ion exchange separations. RECEIVED for review May 8, 1967. Accepted October 6, 1967. (15) A. D. Maynes, Anal. Chim. Acta, 32,211 (1965). (16) A. Danielsson, F. Lundgren, and G. Sundkvist, Spectrochim. Acta, 15, 122 (1959). (17) A. Danielsson and G. Sundkvist, Zbid.,15,126 (1959).
