DISCUSSION This work expands our formerly graphical methods for the determination of relative pK values without using pH measurements. One objective was the use of the proper function for the fit-the conic section-as suggested in the theory of the observed chemical system. Another problem became apparent in the course of this work: some combinations of series of measurements yield distinctively less reliable results than others. There are several possible reasons for this fact which cannot be investigated in advance. Therefore, we used the criteria of the error analysis section in order to exclude those meaningless results. This second part of the computations required a t least 90% of the total computing time which was about 1 min on a fast computer in our example (7 series of measurements, 113 measurements per series, 30 Monte-Carlo tests per combination).
LITERATURE CITED B. J. Thamer, J. Phys. Chern., 59, 450-453 (1955). G. Heys, H. Kinns, and D. D. Perrin, Analyst (London), 97,52-54 (1972). R. Blume and J. Polster, Z.Naturforsch. 6,29, 734-741 (1974). R. Blume, H. Lachmann, and J. Polster, 2.Naturforsch., 6,30, 263-276 (1975). (5) J. Polster, Z.Phys. Chern. (FrankfurfamMain), 97, 113-126 (1975). (6) H. Mauser, "Formale Kinetik", 1st ed.. Bertelmann Univ.-Verlag, Cusseldorf, 1974, p 328, (7) E. Hardtwig, "Fehler- und Ausgleichsrechnung", 1st ed., Bibliographisches Institut, Mannheim, 1968, p 159. (8) Y. Bard, "Nonlinear Parameter Estimation", Academic Press, New York, 1974, p 46. (9) Anal. Chern., 46, 2258 (1974). (1) (2) (3) (4)
RECEIVEDfor review December 23,1975. Accepted June 3, 1976.
Determination of Trace Amounts of Beryllium in Rocks by Ion Exchange Chromatography and Spectrophotometry F. W. E. Strelow," R. G. Bohmer,' and C. H. S. W. Weinert National Chemical Research Laboratory, P.O. Box 395, Pretoria 000 1, Republic of South Africa
A method is described for the accurate determination of trace amounts of beryllium in rocks. Elution from a column of AG 50W-X8 cation exchanger with 2.0 M nitric acid in 70% methanol separates beryllium from Mg, Ca, Mn(ll), Fe(lll), AI, and many other elements which are retained. In a second cation exchange step, Ti( IV), Cu(ll), the alkalies, and several other elements are eluted with 0.3 M hydrochloric acid in 90% acetone, followed by 0.5 M sulfuric acid and then by 0.5 M nitric acid, all containing 0.015 % hydrogen peroxide. Beryllium is again eluted with 2.0 M nitric acid in 70% methanol. Determination is carried out by measuring the absorbance of the complex of beryllium with chromearurol S in the presence of benryldimethylhexadecylammonium chloride at a wavelength of 610 nm. Additional purification steps are described for samples containing interfering elements and very low beryllium concentrations. The practical sensitivity of the method is about 0.02 fig of beryllium in 1 g of rock, or 0.02 ppm. The relative standard deviation for rocks containing more than 1 ppm beryllium is about 2 % . Relevant elution curves and results for several standard rocks are presented.
Beryllium occurs in rocks in concentrations of a few ppm or less. Published values for this element in international standard rocks show large amounts of scattering (1-3). Often only the estimated sensitivity limit of the method used is given, indicating that the berylliurn concentration should be less (1-3).Since the final accuracy of the determination of a t;ace component in a complex matrix is related to the purity of the material on which the final determination is carried out, it appears that an effective separation of beryllium from the major and minor elements occurring in rocks should be of some use. Many instrumental techniques can be used for the f i n d determination. A spectrophotometric approach will be Department of Inorganic and Analytical Chemistry of the University of Pretoria. 1550
described here because the instrumentation is relatively inexpensive and generally available. It has been shown that the sensitivity of the determination of beryllium by spectrophotometry of its complex with chromeazurol S can be considerably enhanced by the addition of micelle forming agents such as quaternary amines containing one long carbon chain, e.g. zephiramine (benzyldimethyltetradecylammonium chloride) ( 4 ) or benzyldimethylhexadecylammonium chloride (BDHA) ( 5 ) .The second reagent in the presence of 0.1 M ammonium chloride increases the molecular absorption of the complex of beryllium with chromeazurol S to t = 85 000 at 611 nm and to 90 000 at 620 nm, as compared with a value of about 20 000 for emax in the absence of a micelle forming reagent. Sommer and Kuban (6) have used polyvinyl alcohol for micelle formation and obtained a value for emax of about 52 000. Many multivalent elements interfere, but the addition of Ca-EDTA suppresses most interferences, provided the interfering elements are not present in excessive amounts. Zirconium and uranium(V1) interfere nevertheless (6).Furthermore, though the amount of aluminum causing less than f 2 % deviation from the true value increases from 0.2 times to 50 times the amount of beryllium when Ca-EDTA is added, this is not sufficient for rock samples with aluminum-beryllium ratios as large as lo5.The same argument applies to iron(II1) and titanium for which the limiting ratios to beryllium are 32 and 10, respectively, when Ca-EDTA is present (6). A recently developed method (7) using cation exchange chromatography in nitric acid-methanol for the separation of beryllipm seemed to be promising for the purpose. The method separates beryllium from all major rock forming elements with the exception of titanium and the alkalies. In addition some trace elements such as Cu(II),Bi(III), Pb(II),and Hg(I1) also accompany beryllium. Possibilities to include the separation of beryllium from these elements in an extended method were investigated. I t has been shown that titanium can be separated from beryllium by selective elution with 0.5 M sulfuric acid containing hydrogen peroxide (8). Cu(II),
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
’
0,5M H,SO,
+
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0,015% H202
12.5
HN03+0,015% H20@,0M 117 .
HN03in 70%MEOH
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M’ E p A T E Figure 2. Elution curve for Ti(lV)-K-Be-Mg 0
200
j400
800
800
1000
1200
1400
1600
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Conditions similar to Figure 1, but 5 mmol of magnesium instead of 1 mmol
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Figure 1. Elution curve for Ti(lV)-K-Be-Mg Column of 90 ml [20 X 2.5cm] AG BOW-X8 resin, 200 to 400 mesh. Flow rate 3.0 f 0.5 ml/min. 1 mmol of each element.
Bi(III), Pb(II), and Hg(I1) can be eluted selectively with hydrochloric acid-acetone mixtures (9),and the alkalies with 0.5 M nitric acid (IO).From a combination of these methods, a very selective separation of beryllium from rock forming elements has been developed and applied to the accurate determination of beryllium in standard rocks.
EXPERIMENTAL Reagents a n d Apparatus. Beryllium nitrate “extra pure” and titanium(II1) chloride solution (15% in 4%HCl) “for synthesis” from Merck A.G., Darmstadt, Germany, were used as reagents for these two elements. All other chemicals were of A.R. grade purity. Water was distilled and passed through an Elgastat deionizer. The resin was the Bio-Rad AG 50W-X8 sulfonated polystyrene cation exchanger of 200 to 400 mesh particle size, marketed by BioRad Laboratories, Richmond, Calif. Borosilicate glass tubes of 25or 20-mm bore, fitted with a No. 2 porosity glass sinter and a burette tap at the bottom and a B19 joint a t the top were used as columns. A Techtron AA-5 and a Zeiss PMQII instrument were used for atomic absorption and absorption spectrophotometric work, respectively. Elution Curves. One Column. Solutions containing about 1mmol of each Ti(IV), K, Be, and Mg were measured out and mixed. The solution was adjusted to a volume of about 50 ml and to contain about 0.3 M hydrochloric acid and 0.1% hydrogen peroxide. It was passed through a column containing 90 ml(30 g) of AG 50W-X8 cation exchange resin of 200 to 400 mesh particle size. The resin column was 20 cm in length and 2.5 cm in diameter. The resin was in the hydrogen form, and had been equilibrated by passing through 50 ml of 0.1 M nitric acid containing 0.03% hydrogen peroxide. The solution containing the elements was washed onto the column and the beaker rinsed with 0.1 M nitric acid containing 0.03% hydrogen peroxide. Titanium(1V) followed by potassium were eluted with 350 ml of 0.50 M sulfuric acid containing 0.015%hydrogen peroxide followed by 650 ml of 0.50 M nitric acid containing 0.015% hydrogen peroxide. Beryllium followed by magnesium was then eluted with 2.0 M nitric acid containing 70% methanol. A flow rate of 3.0 f 0.5 ml/min was maintained. Fractions of 25-ml volume were taken from the beginning of the first elution step using an automatic fractionator. The excess acid from the fractions containing nitric acid was removed by evaporation, and the amounts of K, Be, and Mg were determined by atomic absorption spectrometry after suitable dilution. Titanium was determined spectrophotometrically as the complex with hydrogen peroxide in 1 M sulfuric acid. The experimental curve is shown in Figure 1. Figure 2 shows a curve using exactly the same experimental conditions, except that the amount of magnesium was increased from 1 to 5 mmol. Two Columns. Amounts of 1g of the granite NIM-G (0.1% MgO) and the pyroxenite NIM-P (25.2% MgO) were dissolved and spiked with 0.2 mmol of beryllium. Columns containing 90 ml of resin as described above were used, and adsorption was carried out from dilute mineral acid (0.1 to 0.3 M hydrochloric plus perchloric acid) containing 0.1% hydrogen peroxide. Elution was carried out with 2.0 M nitric acid in 70% methanol containing 0.03% hydrogen peroxide. The
experimental curves are shown in Figures 3 and 4.The alkali metals, Bi(III), Hg(II), Pb(II), and oxyanion forming elements such as Mo(VI), W(VI), and Nb(V) are eluted together with or ahead of beryllium, though not indicated in the figures. Copper(I1) is eluted about together with titanium(1V) and also is not separated satisfactorily. No Ca, Fe(III), or A1 appeared in the first 1000 ml of eluate. 2nd S t e p . Amounts of standard solutions containing 0.1 mmol of Cu(II), 2 mmol of Ti(IV), 1 mmol of K, and either 0.5 or 1.0 mmol of Be were mixed and diluted to a volume of about 50 ml containing 0.3 M hydrochloric acid and 0.1% hydrogen peroxide. The solution was passed through a column containing 60 ml(20 g) of AG 50W-X8 cation exchange resin of 200 to 400 mesh particle size. The resin column was 20 cm in length and 2.0 cm in diameter, and the resin had been equilibrated by passing through 50 ml of 0.3 M hydrochloric acid containing 0.05% hydrogen peroxide. The solution was washed onto the resin with small portions of the equilibrating solution and copper(I1) was eluted with 250 ml of 0.3 M hydrochloric acid in 90% acetone containing 0.015% hydrogen peroxide. The bulk of the titanium(1V) was then eluted with 200 ml of 0.5 M sulfuric acid containing 0.01% hydrogen peroxide. Residual titanium(1V) followed by potassium and finally by beryllium were eluted with 0.5 M nitric acid containing 0.015% hydrogen peroxide, using a flow rate of 3.0 f 0.5 ml/min throughout. Fractions of 25-ml volume were taken with an automatic fractionator and analyzed as indicated above for Ti(IV), K, and Be, and using atomic absorption spectrometry for the determination of copper(I1). The experimental elution curve is presented in Figure 5 . Bi(III), Hg(II), and Pb(I1) are eluted together with or ahead of copper(I1). Procedure for Separation of Beryllium from Rock Samples. About 1g of finely ground rock material was weighed out accurately and dissolved by heating with a mixture of hydrofluoric, hydrochloric, and perchloric acids in Teflon beakers as described previously (11). Any insoluble residue was separated by filtration and decomposed by heating with a mixture of hydrofluoric, perchloric, and phosphoric acids in q platinum crucible until a viscous mass of polyphosphoric acid remained (12, 13). The fusion was dissolved by heating with 3 portions of 5 ml of 1M hydrochloric acid and diluted to 100 ml, adding 0.05% hydrogen peroxide. The solution then was passed through a column containing 90 ml of AG 50W-X8 cation exchange resin as described above for Figures 3 and 4,followed by the main filtrate in a volume of about 100 ml and containing 0.1 to 0.3 M mineral acid (hydrochloric plus perchloric) and 0.1% hydrogen peroxide. The ions were washed onto the resin and the beakers rinsed with several small portions of about 0.1 M nitric acid containing 0.015% hydrogen peroxide. Beryllium accompanied by part of the titanium, the alkali metals, copper, etc. was eluted with 750 ml of 2.0 M nitric acid in 70% methanol a t a flow rate of 3.5 f 0.5 ml/min. Only 650 ml of eluting agent were used for NIM-P (Figure 4) and 600 ml for NIM-D. The first 100 ml of the eluate was discarded. The other eluate, after the addition of 150 ml of water, was evaporated on the water-bath, and residual salts were dissolved in 5 ml of 3 M hydrochloric acid. Two drops of 30% hydrogen peroxide were added and the solution was diluted to about 50-ml volume. It was passed through a column containing 60 ml of AG 50W-X8 resin and Cu(I1) plus chloride complex forming elements, followed by titanium(IV), and the alkali metals were eluted as described in the previous paragraph for Figure 5. Four hundred and fifty milliliters of 0.5 M nitric acid containing 0.015% peroxide was used to elute the alkali metals. The flow rate was 3.0 & 0.5 ml/min. Finally beryllium was eluted with 400 ml of 2.0 M nitric acid in 70% methanol at a flow rate of 2.5 & 0.5 ml/min. About 150 in1 of water was added to the beryllium containing eluate and the solution
ANALYTICAL CHEMISTRY, VOL. 48, NO. 11, SEPTEMBER 1976
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Figure 3. Elution curve for 1 g of NIM-G spiked with 0.2 mmol of beryllium 100
Eluting agent 2.0 M HNO3 In 70% methanol. Column size and flow rate as for
200
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600
700
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1000
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Figure 1
Figure 5. Elution curve for Cu(ll)-Ti(lV)-K-Be 2120
1
Column of 60 ml 120 X 2.0 cm] AG 50W-X8 resin, 200 to 400 mesh. Flow rate 3.0 f 0.5 mi/mln
zoo
5
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100
z
100
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300
400
600 mi ELUATE
500
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Figure 4. Elution curve for 1 g of NIM-P spiked with 0.2 mmol of beryllium
Eluting agent 2.0 M HN03In 70 methanol. Column size and flow rate as for Figure 1
Table I. Results for the Determination of Beryllium in Rock Samples Sample
Be found, ppm
NIM-G NIM-L NIM-N NIM-S NIM-P NIM-D CAAS-S1 a Ref. 3. b Ref.
7 . 4 7 t 0.05 25.8 f 0.5 0.27 t 0.02 1 . 4 5 I0.02 0.13 * 0 . 0 2
0.03 * 0.02 28.6 I0.4 14.
No. of analyses
3 3 3 3 3 3 4
Range of published values, ppm Be
Not Not Not Not
1-13a 2-31a detected-la detected-20 detecteda detectedfl 21-55b
was evaporated to dryness on the water-bath. The final evaporation was carried out in a small beaker, and organic material was destroyed by fuming to incipient dryness with a sulfuric-nitric acid mixture when necessary. The dry salts were dissolved in 5 ml of 5 M hydrochloric acid and evaporated to dryness on the water-bath. Two and a half milliliters of 0.1 M hydrochloric acid were added and, after dissolution of salts, the solution was made up to volume when required. Determination of Beryllium, Either the whole sample (less than 2.5 pg Be) or a suitable aliquot was taken for analysis into a small beaker. The following solutions were added in the sequence given: 0.1 M hydrochloric acid to make the total 2.5 ml in case an aliquot was taken; 2.5 ml of a solution 0.05 M in Ca-EDTA and 0.01 M in CaC12; 2.5 ml of 0,001 M chromeazurol S; 2.5 ml of 0.01 M BDHA; and 2.5 ml of 1M hexamethylene tetramine. The pH was then adjusted to a value of 6.65 & 0.05 with a pH meter, using 0.2 M aqueous ammonia. The solution was transferred into a 25-ml volumetric flask and made t o volume with deionized water. The absorbance was read against a reagent blank at 610 nm after about 1h and compared with a series of standard containing up to 0.1 ppm beryllium. Blank runs were taken through the whole dissolution and separation procedure and subtracted from the results. Results for several international standard rocks are presented in Table I. 1552
Special Addition to Procedure. (Very low beryllium or much chromium). The eluate from the second ion exchange step after being taken to dryness was treated with 2 ml of perchloric plus a few ml of nitric acid, and taken to fumes of perchloric acid to oxidize chromium. The solution was then diluted to about 100 ml with deionized water and passed through a column containing 60 ml of AG 50W-X8 resin as described above for Figure 5. Chromium(V1)and residual amounts of hydrolyzable elements were eluted with 150 ml of 0.1 M nitric acid. Beryllium was then eluted with 600 ml of 2.0 M nitric acid containing 70% methanol, discarding the first 100 ml. The other eluate was evaporated to dryness and, in a small beaker, fumed with 0.5 ml of perchloric acid and 2 ml of nitric acid, driving off most of the perchloric acid. After dilution to 20 ml, the solution was passed through a column containing 6 mi (2 g) of AG 50W-X8 resin of 200 to 400 mesh particle size. The resin column was about 7.5 cm long and 1.0 cm in diameter. Residual chromium(V1) and impurities were eluted with 25 ml of 0.1 M nitric acid. Beryllium was then eluted with 60 ml of 1.2 M nitric acid. The eluate was evaporated to dryness and beryllium converted to the chloride by repeated evaporation with about 2 ml of concentrated hydrochloric acid. One ml of 0.6 M hydrochloric acid was added followed by 1-ml volumes of the reagents used for the determination as described in the previous paragraph. The solution was made up to 10 rnl volume and the absorption read at 610 nm.
DISCUSSION Figure 1shows that for rocks with low amounts of magnesium (
