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Hydrothermal extraction of potassium, sodium, rubidium and cesium from rocks by lithium hydroxide and determination at very low natural levels
Analytica Chimica Acta, 169 (1986) 179-193 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
HYDROTHERMAL EXTRACTION OF POTASSIUM, SODIUM, RUBIDIUM AND CESIUM FROM ROCKS BY LITHIUM HYDROXIDE AND DETERMINATION AT VERY LOW NATURAL LEVELS
REINER GOGUEL Chemistry
Division,
D.S.I.R.,
Lower Hutt (New Zealand)
(Received 10th September 1984)
SUMMARY The selective extraction of Na, K, Rb and Cs from rocks is described. The method is particularly designed for low levels of rubidium and cesium in basic and ultrabasic rocks. The rocks are decomposed with lithium hydroxide solution at 180°C. Only part of the aluminium and chromium accompany the alkali metals into solution; all other rock constituents are left behind as insoluble lithium silicate, hydroxides of divalent metals, etc. Concentrations of rubidium and cesium too low to be determined directly by flame emission spectrometry are preconcentrated up to 25-fold by liquid-liquid extraction. Quantitative recovery (>99.5%) of the two metals is achieved by coprecipitation with potassium tetraphenylboron within the organic phase (di-isobutyl ketone) for subsequent backextraction and dissolution in an acidic aqueous phase. Detection limits are 1 mg kg-’ Na or K, 0.1 mg kg-’ Rb and 0.05 mg kg-’ Cs in the rock for the direct determination and 0.003 mg kg-’ Rb and 0.001 mg kg’ Cs after preconcentration. Methods are described for the purification of lithium hydroxide and the potassium nitrate used as carrier. Results are presented for the Na,O, K,O, Rb and Cs contents and the K/Rb values for 23 geochemical reference samples (basic and ultrabasic rocks, and iron formation samples).
The alkali metals (other than lithium) cannot be accommodated in many ferromagnesian silicate minerals. Thus most ultrabasic rocks have very low concentrations of these alkali metals: Na and K at mg kg-’ levels and Rb and Cs at very much lower levels. Lithium can replace aluminium and magnesium in silicate rocks and its concentration does not fall below mg kg-i levels even in ultrabasic rocks. For use with conventional flame spectrometric techniques, a method is needed that will determine low levels of Na, K, Rb and Cs with a minimum of matrix interferences and contamination from the laboratory environment. The dissolution of rocks in nitric acid after hydrofluoric/perchloric acid digestion provides solutions in which Li, Na, K, Rb and Cs can be quantified by direct aspiration into the flame. Atomic absorption or, for greater sensitivity, flame emission signals are measured. Cesium being the least concentrated and least sensitive element gives the poorest signals. In an earlier paper [ 11, matrix interferences in the determination of cesium in acid rock solutions were studied; emission in the air-acetylene flame was used and a detec0003-2670/85/$03.30
o 1985 Elsevier Science Publishers B.V.
180
tion limit of 0.05 mg kg-’ Cs in the rock was established. The method is thus suitable for the majority of rock types but not for ultrabasic rocks and rocks that are very high in iron. The strong spectral background in the presence of high concentrations of iron enhances the baseline noise and increases the detection limit to 0.3 mg kg-’ cesium in iron formation samples. In order to improve detection limits for cesium, three different approaches were tried. In one, iron was removed by liquid-liquid extraction from 6 M hydrochloric acid. This required an additional evaporation step for removal of the acid after the separation. The chloride concentration in the solution has to be minimized because of the high stability of CsCl in the flame, resulting in poor atomization efficiency. In the second approach, cesium was collected on a zirconium phosphate column as suggested by Osterried [2]. Difficulties were encountered with reproducible separation of cesium from the zirconium phosphate. In the third method, cesium was collected on ammonium molybdophosphate (AMP) as described by Feldman and Rains [3] or on ammonium tungstophosphate (ATP) by coprecipitation [4]. Again sorption of cesium from the acidic solution was not very reproducible and hence often not quantitative. Cesium-134 tracer carried through the coprecipitation procedures developed by Sixta et al. [4] gave recovery of only 74% with an in-house basalt standard. In contrast to the separation methods applied to acidic solutions, where the presence of polyvalent cations (iron( III), aluminium, titanium, zirconium, etc.) causes variable recovery of cesium, liquid-liquid extraction from alkaline solution with tetraphenylboron [5] proved to be extremely reliable. It was therefore important to devise a procedure that would replace acidic dissolution of the rock by alkaline decomposition. Lithium hydroxide was chosen for the decomposition of the rock for the following reasons: (1) the solution can be used for the determination of Na, K, Rb and Cs; (2) reagent grade lithium hydroxide monohydrate is less contaminated with Rb and Cs than sodium hydroxide and can, in contrast to sodium hydroxide, be easily purified by recrystallization; (3) silica forms insoluble lithium silicate and is thus also separated from Na, K, Rb and Cs. Development of the method Initially, the rock (a marine basalt used as in-house standard containing 2% Na*O, 0.55% KzO, 10 mg kg-’ Rb and 0.086 mg kg-’ Cs) was fused with 2-3 times its weight of lithium hydroxide monohydrate in a gold crucible at 850°C for 30 min. Leaching of the cake with water at room temperature gave insufficient recoveries for the alkali metals but disintegration of the cake with water (at least 5 times the weight of the lithium salt) in a pressure vessel at 150°C brought all Na, K, Rb and Cs into solution. Recovery for cesium was also checked by 134Cs radiotracer. Under alkaline conditions there was no retention of cesium by the solid. To minimize the chances of contamination, the possibility of omitting the fusion step was investigated. The rock powder (<0.062 mm) was stirred with
181
12-15 times its weight of 4.5 M lithium hydroxide at 180°C. Although it took about 70 h until x-ray diffraction ceased to show the presence of any of the original minerals, all alkali metals were extracted after 16 h. X-ray diffraction spectrometry showed that the new solid phase consisted of lithium metasilicate, Ca(OH)2, Mg(OH)?, Fez03 and unidentified Li-Al-S&O-H compounds. The minerals containing alkali metals are probably broken down first. It was found that several minerals that are slowly attacked by hydrofluoric acid are also slowly attacked by lithium hydroxide. Despite continuous stirring, approximately 4 h is needed for each 10” M layer of kyanite or tourmaline to be broken down. The attack on chromites and spinels is even slower. These minerals are not broken down in 70 h but there is no indication that their dense structure can accommodate any Na, K, Rb or Cs. Fusion of the Chinese chromites DZ-Cr-1 and DZ-Cr-2 broke down the mineral but did not change the results for the alkali metals. Residual interferences The species dissolved in the lithium hydroxide solutions after reaction with the rock consists of all Na, K, Rb and Cs accompanied by only aluminium corresponding to approximately 1% A1203 in the rock and varying amounts of chromium as chromium(V1). No interference from these concomitants could be detected in the flame emission signals for the alkali metals or in the following extraction of rubidium and cesium with tetraphenylboron. On fusion at 85O”C, more aluminium (corresponding to 4% A1203) was made soluble. Fusion also dissolved large concentrations of chromium as chromium(V1) when chromites were analysed. In this case, there was additional spectral background affecting the direct determination of Rb and Cs in the lithium hydroxide solution but chromium was left behind and did not interfere with the tetraphenylboron extraction. Although lithium is only slightly ionized in the acetylene-air flame, it strongly reduces intra-alkali interferences (Table 1). As a result, matching of standards and samples does not need to be very close for the determination of sodium and potassium in a fuel-rich acetylene-air flame. The fuel-rich hydrogen-air flame (Table 1) shows virtually complete freedom from intraalkali interferences even for rubidium and cesium. Because of the more intense emission giving better detection limits for Rb and Cs in the acetylene air flame (e.g., 0.0002 mg Cs kg-’ in the acetylene-air flame vs. 0.0006 mg Cs kg-’ in the hydrogen-air flame) the air-hydrogen flame. For the determination of rubidium and especially cesium in the acetylene-air flame, the potas sium content of samples and standards has to be very closely matched or the method of standard additions has to be used for calibration. Correction factors can also be used [1] . The Cs and Rb results obtained in the present work did not need correction; with the TPB extraction, the potassium concentration of the final solution and the standards is maintained at 1000 + 100 mg kg-‘. Anions, especially chloride, from the rock will also be found in the lithium
182 TABLE 1 Intra-alkali enhancement effect in acetylene-air and hydrogen-air flames, presented as ratios of emission signals (5 x lo* M analyte) in the presence and absence of a second alkali (0.1 M KNO, except when potassium is the analyte and 0.1 M NaNO, is used) Solution
Metal
Enhancement
effect
Acetylene (1 min-’ )*
0.5 M LiOH
1.6 M HNO,
Na K Rb cs Na K Rb cs
Hydrogen (1 min-’ )a
1.16b
1.48c
5.6d
8.2.=
1.02 1.14 1.48 1.99 1.06 1.49 2.29 4.2
1.00 1.10 1.29 1.66 1.02 1.26 1.85 2.74
1.00 1.00 1.04 1.19 1.00 1.00 1.09 1.23
1.00 1.00 1.02 1.02 1.00 1.00 1.00 1.00
‘8.5 1 min-’ air for all flames. -Flame e 1660 K.
temperature:
b2500
K; c2390
K; d1930
K;
hydroxide solution. At the low levels contained in the samples under investigation, the interferences are negligible. The highest concentration found was 0.6% Cl in sample DZ-1 corresponding to less than 0.01 M chloride in the lithium hydroxide solution. At this low level, there was no effect on the atomization of alkali metals in the acetylene-air flame even for cesium which forms the most stable chloride complex [ 11. Variables affecting the extraction of rubidium and cesium as tetraphenylboron complexes Choice of solvent. Feldman and Rains [3] extracted cesium from 0.2 M sodium hydroxide, suggesting a mixture of 4-methyl-2-pentanone (MlBK) and cyclohexane. With a distribution ratio of only 50, their extraction procedure did not allow concentration of cesium. It served the purpose of separating cesium from the molybdenum of the carrier compound that otherwise produced very strong spectral background of the flame emission signal. 2,6Dimethylheptan4-one (di-isobutyl ketone; DIBK) provides sufficient solubility of sodium-TPB (0.1 M) and gives a distribution ratio of 250 (Fig. 1). This solvent also has the advantages of lower mutual solubility with water and lower vapour pressure. However, the solubility of potassium-TPB is low in the organic phase as it is in the aqueous phase (Fig. 1) and the formation of precipitates reduces the concentration of dissolved rubidium and cesium in the organic phase. A study of the extraction behaviour of K-, Rb- and Cs-TPB showed that the formation of the precipitate can be used to advantage if the K, Rb and Cs suspended or dissolved in the organic phase is stripped into a suitable aliquot of dilute nitric acid. When the alkaline aqueous solution is shaken
183
Fig. 1. Distribution ratios of dissolved K, Rb, and Cs-TPB between di-isobutyl ketone and 0.1-5 M alkaline solutions: ( l) LiOH; (0) NaOH; (0) Na,CO,. Molarities are expressed in terms of the alkali metal ion.
with a 3% solution of Na-TPB in DIBK, the K-TPB precipitate forms in the organic phase and remains there. After the aqueous phase has been removed, at least 99.5% of all rubidium and cesium (Fig. 2) remain in the separator-y funnel, provided that sufficient potassium is available for coprecipitation. Otherwise, when no precipitate is formed, 95% of the cesium and only 60%
0
A.. 01
05
1
Ifi
oL.1. 01
05
I
IF1
Fig. 2. Percentage recoveries of (A) Rb (0.6 and 2.6 mg kg-‘) and (B) Cs (0.2 and 1 mg kg-‘), in the presence of several concentrations of K (0, 80, 160 mg kgl) vs. the molarity of alkali metal solution: (0) LiOH; (n)NaOH or Na,CO,. The alkaline solution is extracted with l/5 of its volume of di-isobutyl ketone. Molarities are expressed in terms of the alkali metal ion.
184
of the rubidium are extracted from the aqueous 2 M lithium hydroxide phase (Fig. 2). Dissolved and precipitated K, Rb and Cs are carried into the aqueous phase after shaking with a suitable aliquot of nitric acid. Recovery is incomplete because of the substantial solubility of the alkali metals in the organic phase after irreversible decomposition of TPB in contact with acid. For a single extraction, it varies between 95 and 99% for the volume ratios (organic/aqueous) 5 and 1.5, respectively. This results from the distribution ratios in favour of the aqueous phase; D = 130 for Rb and D = 110 for Cs. Therefore, the final solution is obtained by combining two extractions with equal amounts of acid. This results in virtually complete recovery; 4 ml of final solution obtained by extraction with two 2-ml portions of acid from 15 ml of organic phase gave 99.6% recovery for Cs and 99.7% recovery for Rb. This is the smallest volume applied here. Choice of potassium concentration Potassium has two functions in this method. It acts as a carrier for complete extraction of Rb and Cs from the aqueous phase by TPB/DIBK and as a buffer stabilizing the ionization of rubidium and especially cesium in the acetylene-air flame [l] . Its concentration must therefore be adjusted to give optimum performance on both counts. A concentration of 1000 mg K kg-’ of solution keeps the ionization of Rb and Cs close to its minimum in the air-acetylene flame without measurably increasing the noise level from increased continuum background in the vicinity of the wavelengths used. The amount of potassium to be added to the lithium hydroxide solution before liquid-liquid extraction, in order to achieve the desired concentration in the final solution, depends on several features: (1) the potassium content of the rock; (2) the amount of final solution required for Rb and Cs determination; and (3) the concentration of potassium in the lithium hydroxide solution after extraction. The third of these depends on the lithium hydroxide concentration remaining after the reaction of the solution, which varies with the silica and alumina content of the rock (see below) and affects the solubility of K-TPB (e.g. 1.3 X lo4 M in 0.7 M LiOH or 6 X lo4 M in 3.2 M LiOH). It also depends on the total potassium content of the lithium hydroxide solution which determines the degree of supersaturation of the solution with respect to K-TPB. These variables are taken into account in Fig. 3, which gives values for the amount of potassium to be added. These additions will ensure that the potassium concentration, in the 7 5 g of lithium hydroxide solution obtained from 3 g of rock, rises above the minimum potassium concentration of 80 mg kg-’ needed to achieve quantitative extraction of rubidium and cesium. Solutions which exceed this value without addition of potassium would originate from rocks that contain more than 2000 mg K kg-’ (0.24% K,O), which is a typical value for marine basalts. The maximum concentration of potassium that can be applied is governed by the capacity of the organic extract to suspend solid K-TPB without detrimental effect on the separation of aqueous and
185
Fig. 3. Approximate additions of potassium before liquid-liquid extraction of 90% of the original 75 g of LiOH solution containing 3 g of rock (O-0.6% K,O). The final solution (3, 4, 5 and 10 ml) obtained by acid stripping will then contain about 1 mg K ml-‘. Aluminosilicates (e.g., anorthosite) or silicates (e.g., dunite) require smaller potassium additions than oxides (dashed lines).
organic phase. If the organic phase contains more than 0.03 M K-TPB in suspension, droplets cease to merge. Without addition, this concentration corresponds to 0.72-0.85% KzO in the rock sample and 18 g of final solution. At this or higher levels of potassium oxide in the rock, the rubidium and cesium concentrations are high enough to make TPB extraction unnecessary. Lithium hydroxide concentration For 2.5-3 g of rock, 35 g of 4.5 M lithium hydroxide is applied. The solution loses lithium to the solid phase where it is tied up in silicates and aluminates. For most silicate rocks, the amount of lithium lost from the solution is about l/3 to l/2. The final solution diluted to 75 g therefore contains l1.4 M lithium hydroxide. The sodium hydroxide content of this solution is normally below 0.15 M (albite). Although this sodium content increases the pH of the solution, it has no measurable effect on the recovery of rubidium and cesium during TPB extraction whereas high sodium hydroxide concentrations do affect recoveries (Fig. 2). The TPB extraction recoveries for rubidium and cesium decrease with increasing lithium hydroxide concentrations (Figs. 1 and 2).
186 EXPERIMENTAL Reagents Lithium hydroxide monohydrate is normally not pure enough for direct usage in the determination of alkali metals in ultrabasic rocks. One batch from Merck and several batches from Sigma were available; they gave similar levels of contamination: 5-15 mg Na kg-’ (Sigma), 25 mg Na kg-’ (Merck), 1.6-5 mg K kg-’ (Sigma), 10 mg K kg-l (Merck). Purification by recrystallization from redistilled solvents (ethanol and isopropanol) is straightforward but requires care to avoid recontamination. First, 430 g of lithium hydroxide monohydrate is dissolved in 1900 g of water (distilled in silica or FEP) in a 2-1 polyethylene or FEP bottle at 70°C. After cooling, 1 1 of the solution is transferred to each of two 2-l bottles by decantation and 150 ml of ethanol is mixed into each portion. Then 850 ml of isopropanol is added to each bottle. After the precipitate has settled, the clear supernatant solution is decanted and 250 ml of isopropanol is mixed with the precipitate in each bottle. Decantation and addition of 250 ml of isopropanol are repeated three times. After the last decant&ion, the slush is dried inside the open bottles at 70°C in a vacuum oven (55-60s yield). Compared with the original chemical, the purification reduced the concentration of Na, K, Rb and Cs by factors of ca. 50. The concentrations in the final preparation were typically 0.18 Na, 0.1 K, 0.0007 Rb and 0.0002 Cs (mg kg-‘) corresponding to 0.5 Na, 0.3 K, 0.002 Rb and 0.0005 Cs (mg kg-‘) in the rock. Sodium tetraphenylboron (cat. no. T-4125, from Sigma or 6669 from Merck) typically contains rubidium and cesium levels of 8-10 ng g-’ ; 10 ng g-l produces a blank corresponding to 1.8 pg kg-’ in the rock. 2,6-Dimethylhep tane4-one was from Hopkins and William or from J.T. Baker. Potassium nitrate (analytical-reagent grade) contains little cesium (e.g., 0.08 mg kg-‘) but rubidium concentrations are quite high (e.g., 1.4 mg kg-‘). The Rb and Cs contents can be brought below the detection limit (30 ng Cs or 60 ng Rb per g KNOB) by passing a nearly saturated solution of potassium nitrate (220 mg g-l) very slowly (0.25 ml min-‘) through a column (80-mm high, 12-mm diameter) of potassium hexacyanocobalt( II)-ferrate( II) available as KCF-1 (50-100 mesh; Bio-Rad Laboratories). Apparatus Aluminium pressure vessels with PTFE liners (Perkin-Elmer, Autoklav-2) were provided with 30-mm long PTFE-encapsulated magnetic followers and gold liners to fit the vessels. The gold liners are essential when fusion pre cedes hydrothermal interaction. Other equipment included alkali-free separatory funnels (125-ml nominal capacity) made of Nalgene, FEP, or silica, hot plate magnetic stirrers, and a large laboratory centrifuge with 75-ml polypropylene centrifuge tubes (1OOOg). For flame emission spectrometry, the burner was a 75-mm single-slot
187
burner, water-cooled on the premix chamber with flow spoiler and pneumatic nebulizer. The flame was acetylene-air or hydrogen-air (see Table 1). The monochromator was a Perkin-Elmer 305B instrument (400 mm FL, F7 aperture, with a grating having 1440 lines mm-‘, 600 nm blaze. The wavelengths used were: for cesium, 852.1 nm (signal + background) and 851.6 nm (background only); for rubidium, 794.8 nm (signal + background) and 794.2 run (background only) ; for potassium, 769.9 nm (no background correction); for sodium, 589.6 or 589.0 nm (no background correction). The slit-width settings (bandpasses) were: 2 (0.14 nm) for cesium or rubidium; 4 (1.4 mn) for potassium concentrations above 50 mg kg-’ in rock, or 2 for concentrations below that level; 4 for sodium concentrations above 40 mg kg-’ in rock slit, or 2 for concentrations above that level; slit 2 isolates the less sensitive sodium line at 589.6 nm. The small aperture is used in combination with slit 4 for the higher sodium and potassium concentrations. A diaphragm (5-mm diameter) is placed at 80 mm from the centre of the flame. For the determination of rubidium and cesium, suppression of background emission from high concentrations of lithium hydroxide or potassium ionization buffer is improved by the use of filters inserted by a filter wheel installed before the entrance to the monochromator. The filter used for cesium measurements was a Schott RG830 (3-mm thick; 90% transmission at 852 nm, 0.5% transmission at 795 nm). The filter used for rubidium measurements was a Schott RG780 (3-mm thick; 85% transmission at 794.8 nm, 44% at 780.0 nm, 16% at 769.9 run, 0.5% at 735 nm. The main benefit of this filter is the drastic decrease of the slope of the potassium background near the rubidium line to allow background correction from measurement at a single wavelength. The photomultiplier was a Hamamatsu R955 (0.2~nA dark current) ; voltages applied varied between 300 and 830 V (Na 300 V; K 400 V; Rb 600 Vi Cs 830 V). Calibm tion A set of eight standards containing 0.45 M lithium hydroxide was used. The standards also contained (in mg kgd) 0.1 Cs and 0.4 Rb and: (1) 0.1 Na, 0.1 K; (2) 0.3 Na, 0.4 K; (3) 0.5 Na, 0.125 K;(4) 1.5 Na, 0.375 K;(5) 5 Na, 1.25 K; (6) 15 Na, 3.75 K; (7) 50 Na, 12.5 K;(8) 150 Na, 3.75 K. The first two standards were used for linear work with sodium and potassium; for this purpose, the burner was placed perpendicular to the optical path, and slit 2 without diaphragm was used. The other standards were used for higher concentrations when slit 4 was used in combination with the diaphragm (see above) is used and the burner was parallel to the optical path. Calibration graphs can be drawn but computer evaluation of the data by means of algorithms developed by Patterson [ 71 was used here. All measurements of cesium and rubidium were evaluated by rectilinear calibration.
188
Procedure Powdered sample (<0.06 mm or 230 mesh; 2.5-3 g) was transferred to the weighed pressure vessel containing 35 g of 4.5 M lithium hydroxide (d = l.l), 4 ml 1.6 M nitric acid, and a magnetic follower. After assembly, the vessel was placed on a hot plate controlled at 180-185°C for 60-70 h, with slow stirring at about 150 rpm. The suspension from the cooled vessel was transferred to a go-ml centrifuge tube, water being used to rinse the vessel. After centrifugation for 10 min, decantation of the supernatant solution into a separatory funnel left behind a cake of solids wetted by a little solution (1 - fl). The recovery (fl) of the solution varied between 0.90 and 0.98 depending on the surface properties of the solid particles. The loss (1 - fl) was measured as follows: the centrifuge tube was filled to the original volume with water and shaken for 1 min to bring the solids back into suspension; centrifugation then produced a second solution with a sodium concentration CL,; the ratio of the sodium concentrations C~,/Cn, gave a value for (1 - fl). Between l/60 and l/20 of the solution in the separatory funnel was withdrawn, diluted five-fold and used for the direct determination of alkali metals by flame spectrometry. The corresponding fi values are 0.983 and 0.950. If the remaining solution (75 X fl X fi g) contained sufficient potassium (>80 mg K kg-‘), it was ready for liquid-liquid extraction. A freshly prepared solution of 0.45 g of Na-TPB in 15 ml of DIBK was added to the contents of the separation funnel and shaken for 3 min. Potassium additions for samples with low potassium content were taken from Fig. 3. For example, anorthosite AN-G contains 0.135% KzO. If 4 ml of final solution was wanted for the determination of Rb and Cs, the required potassium addition was 2.7 mg. Measurement of the potassium content of the aqueous phase before and after extraction (5.8 and 1.7 mg of potassium) determined the amount of potassium released to the organic phase (4.1 mg of potassium). Potassium, Rb and Cs were stripped from the organic phase by repeated extraction with equal amounts of 1.6 M nitric acid; 0.5 g was used for each mg of potassium. Extraction was done as follows: after shaking for 2.5 min and a reaction time of 75 min, shaking was repeated for 1.5 min; the second extraction only required 1.5-min shaking. In the case of AN-G, 2 g of acid was used twice. The combined 4 g of solution contained 4.1 mg K (1022 mg kg-‘), 2330 ng Rb (0.583 mg kg-‘), and 128 ng Cs (0.032 mg kg-‘). The content of rubidium and cesium in the rock (in mg kg-‘) was calculated from { [ weighta,,,& - TPB blankai,,, (ng)]/[f& - LiGH blanka,,ti (ng)]) divided by the weight of the rock sample (mg), i.e., for AN-G [(2330 - 4)/(0.934 [(128 - 4)/(0.934
X 0.985) - 4]/3000 X 0.985) - 1]/3000
= 0.842 mg Rb kg-’ = 0.045 mg Cs kg-‘.
Simplified procedure (for the determination of sodium and/or potassium only) Rock powder (<230 mesh; 0.05-0.2 g) was digested with 8 ml of 3 M lithium hydroxide in a 25-ml Pt or Au crucible placed inside the pressure
189
vessel. The suspension was stirred at 180 + 5°C for 60 h. Alternatively, the sample was fused at 850°C with 1 g of lithium hydroxide monohydrate in a gold crucible. The cake was then leached with stirring for at least 6 h in the pressure vessel. The sample suspension was transferred to a preweighed polyethylene bottle and diluted with several water rinses until about 50 g of solution was obtained. The concentrations of sodium and/or potassium were measured by emission or atomic absorption in the hydrogen-air flame. RESULTS
AND DISCUSSION
The certification of the sodium and potassium contents of the older rock standards is based on a larger number of results from many laboratories. Although certification is normally based on statistical evaluation, one compilation applies criteria of selection [8]. The latest comprehensive compilation [9] of elemental concentrations in the older U.S.G.S. rock standards provides the largest set of data available for the content of alkali metals in ultrabasic rocks. However, neither statistics nor selected data obtained with the most suitable methods revealed most probable or consensus values from data more or less evenly spread over three orders of magnitude. The choice of values from selected laboratories allowed Abbey [El] to give an approximate value for sodium in PCC-1 (0.01% NazO) but not for potassium in PCC-1 and DTS-1. One reason for a general bias in the results for sodium and potassium at low levels is sample contamination when the samples are converted to solutions, stored, diluted and chemically buffered, e.g., for flame spectrometry. Potassium at very high levels is used extensively as an ionization suppressor and sodium contamination is also difficult to avoid in the laboratory. As the present work was designed to avoid contamination with alkali metals as far as possible in a standard analytical laboratory and to remove the sample matrix, the low values obtained (Tables 2 and 3) can be regarded as a closer approximation to the true values than the higher literature values. All values represent the average of two to three separate analyses. There were no batches in the lithium hydroxide extraction as only one reaction vessel was available. Values above 0.7% N&O and above 0.1% KzO show good agreement with values that have established common ground: usable values [8], recommended values [lo], consensus values [ 111 and certified values [ 121. In contrast to sodium and potassium, the rarer alkali metals rubidium and cesium have been determined by only a few analysts. Acceptable values cannot be derived statistically but single measurements obtained by isotopedilution mass spectrometry [14, 231 may be regarded as the most reliable results. In the case of cesium, radiochemical [24] or epithermal neutron activation also provides sufficiently strong and selective analytical signals. At higher levels of cesium, concurrent studies [l, 211 of the flame emission signals of cesium in the presence of the entire rock matrix except silica
Origin
A.N.R.T. A.N.R.T. B.A.S. B.A.S. B.A.S. U.S.G.S. S.A.B.S. A.N.R.T. U.S.G.S. XIGMR XIGMR XIGMR XIGMR CCRMP CCRMP CCRMP U.S.G.S. U.S.G.S. A.N.R.T. CCRMP S.A.B.S. S.A.B.S. U.S.G.S.
Symbol
AI-1 AN-G BCS 267 BCS 302 BCS 375 BHVO-1 NIM-D DT-N DTS-2 DZ-Cr-1 DZ-Cr-2 DZ-1 DZ-2 FeR-1 FeR-3 FeR-4 G-2 GSP- 1 IF-G MRG-1 NIM-N NIM-P PCC-1
Albite Anorthosite Silica brick Iron ore Soda feldspar Basalt Dunite Kyanite Dunite Chromite Chromite Ultrabasic Ultrabasic iron ore Iron ore Iron ore Granite Granodiorite Iron ore Gabbro Norite Pyroxenite Peridotite
Rock type
0.017 0.004 0.002 0.019 0.007 0.003 0.026 4.00 2.65 0.028
0.051
11.15 10.89 1.62 0.051 0.088 10.58 2.23 0.0072 0.027 0.004 0.0173 0.0046 0.0023 0.0190 0.0043 0.0056 0.027 4.02 2.71 0.025 0.711 2.45 0.344 0.0067 0.005-0.4 0.002-0.02 0.01-0.13 2-4.87 2.3-5.8 0.003-0.24 0.63-1.33 2.20-2.72 0.27-0.51 0.002-0.05
10.4 [ 151 2.21 [ll]
10.24-10.48 2.10-2.35 0.01-0.08 0.01-0.15 DTS-1: 0.04 [22] 0.007 [lo] 0.025 [12] 0.009 [12] 0.008 [12] 0.028 [12] 0.03 [25] 0.03 [25] 0.05 [25] 4.07 [9] 2.80 [ 91 0.032 [ 271 0.71 [S] 2.46 [8] 0.37 [8 ] 0.01 [8]
10.6 [27] 1.63 [13] 0.06 [15]
9.5-11.8 1.1-2.2 0.05-0.07
0.116 0.004 0.005 0.005 0.011 0.017 0.247 4.44 5.50 0.007
0.641
0.132
HF 0.133 0.135 0.124 0.633 0.763 0.507 0.0048 0.125 0.0009 0.116 0.0057 0.0046 0.0073 0.0101 0.0154 0.248 4.53 5.44 0.0058 0.179 0.242 0.084 0.0012
LiOH
Range
HF Common
This work
This work LiOH
K,G (%) Literature
Na,O (%)
Results obtained for alkali metals in standard rocks
TABLE 2
0.0006-0.06 0.004-0.06 0.17-0.34 2.2-7.8 4.2-7.1 0.001-0.18 0.16-0.22 0.19-0.31 0.06-0.12 0.0003-0.1
0.72-0.85 0.45-0.68 0.00-0.03 0.05-0.21 DTS-1:
0.06-0.20 0.04-0.24 0.12-0.18
Range
Literature
0.12 [22] 0.0012 [lo] 0.11 [12] 0.010 [12] 0.010 [12] 0.009 [12] 0.02 [25 J 0.03 [25] 0.29 125 J 4.49 [ 91 5.49 [ 91 0.012 [27] 0.18 [8] 0.25 [8] 0.09 [8] -
0.79 [15] 0.54 [8 3
0.14 [27] 0.13 [13] 0.14 [15]
Common
A.N.R.T. A.N.R.T. B.A.S. B.A.S. B.A.S. U.S.G.S. S.A.B.S. A.N.R.T. U.S.G.S. XIGMR XIGMR XIGMR XIGMR CCRMP CCRMP CCRMP U.S.G.S. U.S.G.S. A.N.R.T. CCRMP S.A.B.S. S.A.B.S. U.S.G.S.
Al-I
AN-G BCS267 BCS302 RCS375 BHVG-1 NIM-D DT-N DTS-2 DZCr1 DZCr-2 DZ-1 DZ-2 FeR-1 FeR-3 FeR-4 G-2 GSP-1 IF-G MRG NIM-N NIM-P PCC-1
Origin
Symbol
25
1.1
6.6
5.5 [271 6.05 0.84 0.85 ~141 3.65 23.3 48 ~151 46.1 a [161,8.7 ~171.10.9 cl81 8.61 0.218 0.2 c201 4.45 5 c221 0.04-0.06 (DTS-1 1231) 0.041 5.76 0.168 0.172 0.359 0.443 0.491 13-27 1251 16.1 171 1141.168 C231 169 254 [Sl 254 0.308 0.4 [271 7.17 1261 7.10 2.4-7 [20] 3.46 2-a 1201 2.49 0.062 1231 0.061 0.61 0.23 0.14
1.03 <0.05 <0.05 0.0s co.3 0.32 0.70 1.35 0.98
0.08 0.13
1.07
0.37 0.07
0.34 0.046 0.527 1.12 1.01 0.082 0.083 0.114 0.0028 1.05 0.019 0.039 0.068 0.117 0.159 0.668 1.33 0.97 0.056 0.594 0.242 0.140 0.0051
LiOH
HF
Lit..
I-IF LiOH
Ce (mgkg+)
Rb (mgkg-')
6.2 0.21 0.43 Ultrabasic 0.42 Ultrabesic 0.14 Iron ore 0.50 Ironore 16.1 Ironore Granite 165 Grenodiorite 248 Ironore 7.41 Gebbro Norite Pyroxeaite Peridotite
Albite Anorthosite Silicabrick Iron o*e SodaFeldspar Basalt DUIlite Kyenite DUllita Chromite
RocktyPa
Results obtained for alkali metals in standard rocks
TABLE 3
0.5-3 [251 1.33 191 0.95 c91 0.06 1271 0.55 p.211 0.24 L1.211 0.12 Cl.211 0.0053--0.0055 C241
0.08 [161.o.oa clsi,o.o9 cl91 0.05 [l,211 0.21 [l.211 0.005 (DTS-1 C241)
0.34 1271 0.043 1141
Lit.
480 2920 620 490 300 1040 400 490 400 370 620 490 370 410 510 320 486 389 340 450 1170 610 360
rati0
R/Rb atomic
192
produced a useful set of data. In the case of rubidium, x-ray fluorescence signals are sufficiently strong to allow accurate determination at sufficiently high levels, e.g., 8.7 mg kg-’ for BHVO-1 [17]. There is good agreement between the rubidium and cesium data obtained by reaction with lithium hydroxide and those literature values obtained by other methods that are especially suitable for rubidium or cesium determinations in rocks at low levels. For eight of the reference samples analysed for these two metals, no literature values were available for comparison. A set of K/Rb ratios was calculated from the lithium hydroxide data, as K/Rb ratios are used routinely for the study of rock differentiation. The highest ratios are found in rocks with high anorthite content, e.g., basalts and anorthosites. Ultrabasic rocks that are low in feldspar show a relatively low K/Rb ratio. Precision Values for KzO and NazO above 0.1% (1000 mg kg-‘) obtained from different runs for the same sample did not vary by more than 3% (range). The variations are attributed partly to the combined errors of the factors fl, f2, and partly to flame emission fluctuations. Relative standard deviations (RSD) based on six runs of the in-house basalt containing 2% Na,O, 0.55% KzO, 10 mg kg-’ Rb and 0.09 mg kg-’ Cs were 1.2,1.4, 2.5 and 3.8%, respectively. Near the detection limits (0.0005% NazO, 0.003 mg kg-’ Rb and 0.001 mg kg-’ Cs), which are only approached in the case of cesium in DTS-2 and PCC-1, the noise in the flame emission signal of 0.0003 mg kg-’ cesium determines the possible error. For greater precision and accuracy of potassium or sodium alone, as needed in K/Ar age determination, a simple extraction method requiring less sample gave excellent reproducibility of the results (0.4% RSD). These small variations (e.g., 0.511 +- 0.002% KzO in BHVO-1) are of the same magnitude as the noise of the flame emission signal. By atomic absorption measurements in the hydrogen-air flame, precision was further improved (e.g., 0.513 +-0.001% K,O in BHVO-1). Conclusions Lack of a satisfactory method for the quantitative and selective extraction and preconcentration of rubidium and cesium from rocks led to the development of the proposed technique. Data at very low natural levels can be obtained by flame emission spectrometry. Such data could hitherto be obtained only by using isotope-dilution mass spectrometry or neutron activation analysis. The selective extraction of alkali metals from rocks by lithium hydroxide provides not only a very good solution for the preconcentration of rubidium and cesium but also for the direct determination of the more concentrated alkali metals, potassium and sodium. Other cations and anions are present in too low concentrations to produce any interferences.
193
I am grateful to Mr. R. G. Ditchbum, Institute of Nuclear Sciences, for his help with 134Cstracer work. REFERENCES 1 2 3 4 5 6
R. Goguel, Geostandards New&, 5 (1981) 95. 0. Osterried, Fresenius Z. Anal. Chem., 199 (1964) 260. C. Feldman and T. C. Rains, Anal. Chem., 36 (1964) 405. V. Sixta, M. Miksovsky and I. Rubeska, Sbornik Geol. Ved., 19 (1978) 199. K. B. Brown, USAEC Rept. CF-60-5-114 (1960), CF-61-3-141 (1961). T. R. Foisom, N. Hansen, G. J. Parks Jr. and W. E. Weitz Jr., Appl. Spectroscopy, 28 (1974) 345. 7 J. E. Patterson, Anal. Chim. Acta, 107 (1979) 201. . 8 S. Abbey, Geol. Survey Canada, Paper 83-15 (1983). 9 E. S. Gladney, C. E. Burns and I. Roelandts, Geostandards New&, 7 (1983) 3. 10 F. J. Flanagan, U.S. Geol. Surv. Prof. Pap., 840 (1976) 131. 11 E. S. Gladney and W. E. Goode, Geostandards Newsl., 5 (1981) 31. 12 Xian Institute of Geology and Mineral Resources, Chinese Academy of Sciences, Explanatory Note on the Reference Materials of the Chromite and the Ultrabasic Rocks. 13 K. Govindaraju, Geostandards Newsl., 4 (1980) 49. 14 S. P. Verma, Geostandards Newsl., 5 (1981) 129. 15 Bureau of Analysed Samples, England, Supplementary Information, 1981. 16 I. W. Croudace, J. L. Joron, H. Jaffrezic, G. Meyer and M. Treuil, Geostandards Newsl., 6 (1982) 236. 17 B. P. Fabbi and L. F. Espos, U.S. Geol. Surv. Prof. Pap., 840 (1976) 89. 18 P. A. Baedeker, J. J. Rowe and E. Steinnes, J. Radioanai. Chem., 40 (1977) 115. 19 F. J. Flanagan, T. L. Wright, S. R. Taylor, C. S. Anne1 and J. I. Dinnin, U.S. Geol. Surv. Prof. Pap., 840 (1976) 33. 20 T. W. Steele, A. Wilson, R. Goudvis, P. J. Ellis and A. J. Radford, Council for Mineral Technology S.A., Report No. 1945 (1978). 21 S. Terashima and N. Mita, Geostandards Newsl., 5 (1981) 71. 22 K. Govindaraju, Geostandards Newsl., 6 (1982) 91. 23 J. R. De Laeter and K. J. R. Rosman, Geostandards Newsl., 1 (1977) 35. 24 T. Smet and I. Roelandts, Geostandards Newsl., 2 (1978) 70. 25 S. Abbey, C. R. McLeod and Wan Liang-Guo, Geol. Surv. Canada, Pap. 83-19 (1983). 26 S. Abbey, A. H. Gillieson and Guy Perrault, Geol. Surv. Can., CANMET Rep. 79-35 (1979). 27 K. Govindaraju, Geostandards Newsl., 8 (1984) 63.
HYDROTHERMAL EXTRACTION OF POTASSIUM, SODIUM, RUBIDIUM AND CESIUM FROM ROCKS BY LITHIUM HYDROXIDE AND DETERMINATION AT VERY LOW NATURAL LEVELS
REINER GOGUEL Chemistry
Division,
D.S.I.R.,
Lower Hutt (New Zealand)
(Received 10th September 1984)
SUMMARY The selective extraction of Na, K, Rb and Cs from rocks is described. The method is particularly designed for low levels of rubidium and cesium in basic and ultrabasic rocks. The rocks are decomposed with lithium hydroxide solution at 180°C. Only part of the aluminium and chromium accompany the alkali metals into solution; all other rock constituents are left behind as insoluble lithium silicate, hydroxides of divalent metals, etc. Concentrations of rubidium and cesium too low to be determined directly by flame emission spectrometry are preconcentrated up to 25-fold by liquid-liquid extraction. Quantitative recovery (>99.5%) of the two metals is achieved by coprecipitation with potassium tetraphenylboron within the organic phase (di-isobutyl ketone) for subsequent backextraction and dissolution in an acidic aqueous phase. Detection limits are 1 mg kg-’ Na or K, 0.1 mg kg-’ Rb and 0.05 mg kg-’ Cs in the rock for the direct determination and 0.003 mg kg-’ Rb and 0.001 mg kg’ Cs after preconcentration. Methods are described for the purification of lithium hydroxide and the potassium nitrate used as carrier. Results are presented for the Na,O, K,O, Rb and Cs contents and the K/Rb values for 23 geochemical reference samples (basic and ultrabasic rocks, and iron formation samples).
The alkali metals (other than lithium) cannot be accommodated in many ferromagnesian silicate minerals. Thus most ultrabasic rocks have very low concentrations of these alkali metals: Na and K at mg kg-’ levels and Rb and Cs at very much lower levels. Lithium can replace aluminium and magnesium in silicate rocks and its concentration does not fall below mg kg-i levels even in ultrabasic rocks. For use with conventional flame spectrometric techniques, a method is needed that will determine low levels of Na, K, Rb and Cs with a minimum of matrix interferences and contamination from the laboratory environment. The dissolution of rocks in nitric acid after hydrofluoric/perchloric acid digestion provides solutions in which Li, Na, K, Rb and Cs can be quantified by direct aspiration into the flame. Atomic absorption or, for greater sensitivity, flame emission signals are measured. Cesium being the least concentrated and least sensitive element gives the poorest signals. In an earlier paper [ 11, matrix interferences in the determination of cesium in acid rock solutions were studied; emission in the air-acetylene flame was used and a detec0003-2670/85/$03.30
o 1985 Elsevier Science Publishers B.V.
180
tion limit of 0.05 mg kg-’ Cs in the rock was established. The method is thus suitable for the majority of rock types but not for ultrabasic rocks and rocks that are very high in iron. The strong spectral background in the presence of high concentrations of iron enhances the baseline noise and increases the detection limit to 0.3 mg kg-’ cesium in iron formation samples. In order to improve detection limits for cesium, three different approaches were tried. In one, iron was removed by liquid-liquid extraction from 6 M hydrochloric acid. This required an additional evaporation step for removal of the acid after the separation. The chloride concentration in the solution has to be minimized because of the high stability of CsCl in the flame, resulting in poor atomization efficiency. In the second approach, cesium was collected on a zirconium phosphate column as suggested by Osterried [2]. Difficulties were encountered with reproducible separation of cesium from the zirconium phosphate. In the third method, cesium was collected on ammonium molybdophosphate (AMP) as described by Feldman and Rains [3] or on ammonium tungstophosphate (ATP) by coprecipitation [4]. Again sorption of cesium from the acidic solution was not very reproducible and hence often not quantitative. Cesium-134 tracer carried through the coprecipitation procedures developed by Sixta et al. [4] gave recovery of only 74% with an in-house basalt standard. In contrast to the separation methods applied to acidic solutions, where the presence of polyvalent cations (iron( III), aluminium, titanium, zirconium, etc.) causes variable recovery of cesium, liquid-liquid extraction from alkaline solution with tetraphenylboron [5] proved to be extremely reliable. It was therefore important to devise a procedure that would replace acidic dissolution of the rock by alkaline decomposition. Lithium hydroxide was chosen for the decomposition of the rock for the following reasons: (1) the solution can be used for the determination of Na, K, Rb and Cs; (2) reagent grade lithium hydroxide monohydrate is less contaminated with Rb and Cs than sodium hydroxide and can, in contrast to sodium hydroxide, be easily purified by recrystallization; (3) silica forms insoluble lithium silicate and is thus also separated from Na, K, Rb and Cs. Development of the method Initially, the rock (a marine basalt used as in-house standard containing 2% Na*O, 0.55% KzO, 10 mg kg-’ Rb and 0.086 mg kg-’ Cs) was fused with 2-3 times its weight of lithium hydroxide monohydrate in a gold crucible at 850°C for 30 min. Leaching of the cake with water at room temperature gave insufficient recoveries for the alkali metals but disintegration of the cake with water (at least 5 times the weight of the lithium salt) in a pressure vessel at 150°C brought all Na, K, Rb and Cs into solution. Recovery for cesium was also checked by 134Cs radiotracer. Under alkaline conditions there was no retention of cesium by the solid. To minimize the chances of contamination, the possibility of omitting the fusion step was investigated. The rock powder (<0.062 mm) was stirred with
181
12-15 times its weight of 4.5 M lithium hydroxide at 180°C. Although it took about 70 h until x-ray diffraction ceased to show the presence of any of the original minerals, all alkali metals were extracted after 16 h. X-ray diffraction spectrometry showed that the new solid phase consisted of lithium metasilicate, Ca(OH)2, Mg(OH)?, Fez03 and unidentified Li-Al-S&O-H compounds. The minerals containing alkali metals are probably broken down first. It was found that several minerals that are slowly attacked by hydrofluoric acid are also slowly attacked by lithium hydroxide. Despite continuous stirring, approximately 4 h is needed for each 10” M layer of kyanite or tourmaline to be broken down. The attack on chromites and spinels is even slower. These minerals are not broken down in 70 h but there is no indication that their dense structure can accommodate any Na, K, Rb or Cs. Fusion of the Chinese chromites DZ-Cr-1 and DZ-Cr-2 broke down the mineral but did not change the results for the alkali metals. Residual interferences The species dissolved in the lithium hydroxide solutions after reaction with the rock consists of all Na, K, Rb and Cs accompanied by only aluminium corresponding to approximately 1% A1203 in the rock and varying amounts of chromium as chromium(V1). No interference from these concomitants could be detected in the flame emission signals for the alkali metals or in the following extraction of rubidium and cesium with tetraphenylboron. On fusion at 85O”C, more aluminium (corresponding to 4% A1203) was made soluble. Fusion also dissolved large concentrations of chromium as chromium(V1) when chromites were analysed. In this case, there was additional spectral background affecting the direct determination of Rb and Cs in the lithium hydroxide solution but chromium was left behind and did not interfere with the tetraphenylboron extraction. Although lithium is only slightly ionized in the acetylene-air flame, it strongly reduces intra-alkali interferences (Table 1). As a result, matching of standards and samples does not need to be very close for the determination of sodium and potassium in a fuel-rich acetylene-air flame. The fuel-rich hydrogen-air flame (Table 1) shows virtually complete freedom from intraalkali interferences even for rubidium and cesium. Because of the more intense emission giving better detection limits for Rb and Cs in the acetylene air flame (e.g., 0.0002 mg Cs kg-’ in the acetylene-air flame vs. 0.0006 mg Cs kg-’ in the hydrogen-air flame) the air-hydrogen flame. For the determination of rubidium and especially cesium in the acetylene-air flame, the potas sium content of samples and standards has to be very closely matched or the method of standard additions has to be used for calibration. Correction factors can also be used [1] . The Cs and Rb results obtained in the present work did not need correction; with the TPB extraction, the potassium concentration of the final solution and the standards is maintained at 1000 + 100 mg kg-‘. Anions, especially chloride, from the rock will also be found in the lithium
182 TABLE 1 Intra-alkali enhancement effect in acetylene-air and hydrogen-air flames, presented as ratios of emission signals (5 x lo* M analyte) in the presence and absence of a second alkali (0.1 M KNO, except when potassium is the analyte and 0.1 M NaNO, is used) Solution
Metal
Enhancement
effect
Acetylene (1 min-’ )*
0.5 M LiOH
1.6 M HNO,
Na K Rb cs Na K Rb cs
Hydrogen (1 min-’ )a
1.16b
1.48c
5.6d
8.2.=
1.02 1.14 1.48 1.99 1.06 1.49 2.29 4.2
1.00 1.10 1.29 1.66 1.02 1.26 1.85 2.74
1.00 1.00 1.04 1.19 1.00 1.00 1.09 1.23
1.00 1.00 1.02 1.02 1.00 1.00 1.00 1.00
‘8.5 1 min-’ air for all flames. -Flame e 1660 K.
temperature:
b2500
K; c2390
K; d1930
K;
hydroxide solution. At the low levels contained in the samples under investigation, the interferences are negligible. The highest concentration found was 0.6% Cl in sample DZ-1 corresponding to less than 0.01 M chloride in the lithium hydroxide solution. At this low level, there was no effect on the atomization of alkali metals in the acetylene-air flame even for cesium which forms the most stable chloride complex [ 11. Variables affecting the extraction of rubidium and cesium as tetraphenylboron complexes Choice of solvent. Feldman and Rains [3] extracted cesium from 0.2 M sodium hydroxide, suggesting a mixture of 4-methyl-2-pentanone (MlBK) and cyclohexane. With a distribution ratio of only 50, their extraction procedure did not allow concentration of cesium. It served the purpose of separating cesium from the molybdenum of the carrier compound that otherwise produced very strong spectral background of the flame emission signal. 2,6Dimethylheptan4-one (di-isobutyl ketone; DIBK) provides sufficient solubility of sodium-TPB (0.1 M) and gives a distribution ratio of 250 (Fig. 1). This solvent also has the advantages of lower mutual solubility with water and lower vapour pressure. However, the solubility of potassium-TPB is low in the organic phase as it is in the aqueous phase (Fig. 1) and the formation of precipitates reduces the concentration of dissolved rubidium and cesium in the organic phase. A study of the extraction behaviour of K-, Rb- and Cs-TPB showed that the formation of the precipitate can be used to advantage if the K, Rb and Cs suspended or dissolved in the organic phase is stripped into a suitable aliquot of dilute nitric acid. When the alkaline aqueous solution is shaken
183
Fig. 1. Distribution ratios of dissolved K, Rb, and Cs-TPB between di-isobutyl ketone and 0.1-5 M alkaline solutions: ( l) LiOH; (0) NaOH; (0) Na,CO,. Molarities are expressed in terms of the alkali metal ion.
with a 3% solution of Na-TPB in DIBK, the K-TPB precipitate forms in the organic phase and remains there. After the aqueous phase has been removed, at least 99.5% of all rubidium and cesium (Fig. 2) remain in the separator-y funnel, provided that sufficient potassium is available for coprecipitation. Otherwise, when no precipitate is formed, 95% of the cesium and only 60%
0
A.. 01
05
1
Ifi
oL.1. 01
05
I
IF1
Fig. 2. Percentage recoveries of (A) Rb (0.6 and 2.6 mg kg-‘) and (B) Cs (0.2 and 1 mg kg-‘), in the presence of several concentrations of K (0, 80, 160 mg kgl) vs. the molarity of alkali metal solution: (0) LiOH; (n)NaOH or Na,CO,. The alkaline solution is extracted with l/5 of its volume of di-isobutyl ketone. Molarities are expressed in terms of the alkali metal ion.
184
of the rubidium are extracted from the aqueous 2 M lithium hydroxide phase (Fig. 2). Dissolved and precipitated K, Rb and Cs are carried into the aqueous phase after shaking with a suitable aliquot of nitric acid. Recovery is incomplete because of the substantial solubility of the alkali metals in the organic phase after irreversible decomposition of TPB in contact with acid. For a single extraction, it varies between 95 and 99% for the volume ratios (organic/aqueous) 5 and 1.5, respectively. This results from the distribution ratios in favour of the aqueous phase; D = 130 for Rb and D = 110 for Cs. Therefore, the final solution is obtained by combining two extractions with equal amounts of acid. This results in virtually complete recovery; 4 ml of final solution obtained by extraction with two 2-ml portions of acid from 15 ml of organic phase gave 99.6% recovery for Cs and 99.7% recovery for Rb. This is the smallest volume applied here. Choice of potassium concentration Potassium has two functions in this method. It acts as a carrier for complete extraction of Rb and Cs from the aqueous phase by TPB/DIBK and as a buffer stabilizing the ionization of rubidium and especially cesium in the acetylene-air flame [l] . Its concentration must therefore be adjusted to give optimum performance on both counts. A concentration of 1000 mg K kg-’ of solution keeps the ionization of Rb and Cs close to its minimum in the air-acetylene flame without measurably increasing the noise level from increased continuum background in the vicinity of the wavelengths used. The amount of potassium to be added to the lithium hydroxide solution before liquid-liquid extraction, in order to achieve the desired concentration in the final solution, depends on several features: (1) the potassium content of the rock; (2) the amount of final solution required for Rb and Cs determination; and (3) the concentration of potassium in the lithium hydroxide solution after extraction. The third of these depends on the lithium hydroxide concentration remaining after the reaction of the solution, which varies with the silica and alumina content of the rock (see below) and affects the solubility of K-TPB (e.g. 1.3 X lo4 M in 0.7 M LiOH or 6 X lo4 M in 3.2 M LiOH). It also depends on the total potassium content of the lithium hydroxide solution which determines the degree of supersaturation of the solution with respect to K-TPB. These variables are taken into account in Fig. 3, which gives values for the amount of potassium to be added. These additions will ensure that the potassium concentration, in the 7 5 g of lithium hydroxide solution obtained from 3 g of rock, rises above the minimum potassium concentration of 80 mg kg-’ needed to achieve quantitative extraction of rubidium and cesium. Solutions which exceed this value without addition of potassium would originate from rocks that contain more than 2000 mg K kg-’ (0.24% K,O), which is a typical value for marine basalts. The maximum concentration of potassium that can be applied is governed by the capacity of the organic extract to suspend solid K-TPB without detrimental effect on the separation of aqueous and
185
Fig. 3. Approximate additions of potassium before liquid-liquid extraction of 90% of the original 75 g of LiOH solution containing 3 g of rock (O-0.6% K,O). The final solution (3, 4, 5 and 10 ml) obtained by acid stripping will then contain about 1 mg K ml-‘. Aluminosilicates (e.g., anorthosite) or silicates (e.g., dunite) require smaller potassium additions than oxides (dashed lines).
organic phase. If the organic phase contains more than 0.03 M K-TPB in suspension, droplets cease to merge. Without addition, this concentration corresponds to 0.72-0.85% KzO in the rock sample and 18 g of final solution. At this or higher levels of potassium oxide in the rock, the rubidium and cesium concentrations are high enough to make TPB extraction unnecessary. Lithium hydroxide concentration For 2.5-3 g of rock, 35 g of 4.5 M lithium hydroxide is applied. The solution loses lithium to the solid phase where it is tied up in silicates and aluminates. For most silicate rocks, the amount of lithium lost from the solution is about l/3 to l/2. The final solution diluted to 75 g therefore contains l1.4 M lithium hydroxide. The sodium hydroxide content of this solution is normally below 0.15 M (albite). Although this sodium content increases the pH of the solution, it has no measurable effect on the recovery of rubidium and cesium during TPB extraction whereas high sodium hydroxide concentrations do affect recoveries (Fig. 2). The TPB extraction recoveries for rubidium and cesium decrease with increasing lithium hydroxide concentrations (Figs. 1 and 2).
186 EXPERIMENTAL Reagents Lithium hydroxide monohydrate is normally not pure enough for direct usage in the determination of alkali metals in ultrabasic rocks. One batch from Merck and several batches from Sigma were available; they gave similar levels of contamination: 5-15 mg Na kg-’ (Sigma), 25 mg Na kg-’ (Merck), 1.6-5 mg K kg-’ (Sigma), 10 mg K kg-l (Merck). Purification by recrystallization from redistilled solvents (ethanol and isopropanol) is straightforward but requires care to avoid recontamination. First, 430 g of lithium hydroxide monohydrate is dissolved in 1900 g of water (distilled in silica or FEP) in a 2-1 polyethylene or FEP bottle at 70°C. After cooling, 1 1 of the solution is transferred to each of two 2-l bottles by decantation and 150 ml of ethanol is mixed into each portion. Then 850 ml of isopropanol is added to each bottle. After the precipitate has settled, the clear supernatant solution is decanted and 250 ml of isopropanol is mixed with the precipitate in each bottle. Decantation and addition of 250 ml of isopropanol are repeated three times. After the last decant&ion, the slush is dried inside the open bottles at 70°C in a vacuum oven (55-60s yield). Compared with the original chemical, the purification reduced the concentration of Na, K, Rb and Cs by factors of ca. 50. The concentrations in the final preparation were typically 0.18 Na, 0.1 K, 0.0007 Rb and 0.0002 Cs (mg kg-‘) corresponding to 0.5 Na, 0.3 K, 0.002 Rb and 0.0005 Cs (mg kg-‘) in the rock. Sodium tetraphenylboron (cat. no. T-4125, from Sigma or 6669 from Merck) typically contains rubidium and cesium levels of 8-10 ng g-’ ; 10 ng g-l produces a blank corresponding to 1.8 pg kg-’ in the rock. 2,6-Dimethylhep tane4-one was from Hopkins and William or from J.T. Baker. Potassium nitrate (analytical-reagent grade) contains little cesium (e.g., 0.08 mg kg-‘) but rubidium concentrations are quite high (e.g., 1.4 mg kg-‘). The Rb and Cs contents can be brought below the detection limit (30 ng Cs or 60 ng Rb per g KNOB) by passing a nearly saturated solution of potassium nitrate (220 mg g-l) very slowly (0.25 ml min-‘) through a column (80-mm high, 12-mm diameter) of potassium hexacyanocobalt( II)-ferrate( II) available as KCF-1 (50-100 mesh; Bio-Rad Laboratories). Apparatus Aluminium pressure vessels with PTFE liners (Perkin-Elmer, Autoklav-2) were provided with 30-mm long PTFE-encapsulated magnetic followers and gold liners to fit the vessels. The gold liners are essential when fusion pre cedes hydrothermal interaction. Other equipment included alkali-free separatory funnels (125-ml nominal capacity) made of Nalgene, FEP, or silica, hot plate magnetic stirrers, and a large laboratory centrifuge with 75-ml polypropylene centrifuge tubes (1OOOg). For flame emission spectrometry, the burner was a 75-mm single-slot
187
burner, water-cooled on the premix chamber with flow spoiler and pneumatic nebulizer. The flame was acetylene-air or hydrogen-air (see Table 1). The monochromator was a Perkin-Elmer 305B instrument (400 mm FL, F7 aperture, with a grating having 1440 lines mm-‘, 600 nm blaze. The wavelengths used were: for cesium, 852.1 nm (signal + background) and 851.6 nm (background only); for rubidium, 794.8 nm (signal + background) and 794.2 run (background only) ; for potassium, 769.9 nm (no background correction); for sodium, 589.6 or 589.0 nm (no background correction). The slit-width settings (bandpasses) were: 2 (0.14 nm) for cesium or rubidium; 4 (1.4 mn) for potassium concentrations above 50 mg kg-’ in rock, or 2 for concentrations below that level; 4 for sodium concentrations above 40 mg kg-’ in rock slit, or 2 for concentrations above that level; slit 2 isolates the less sensitive sodium line at 589.6 nm. The small aperture is used in combination with slit 4 for the higher sodium and potassium concentrations. A diaphragm (5-mm diameter) is placed at 80 mm from the centre of the flame. For the determination of rubidium and cesium, suppression of background emission from high concentrations of lithium hydroxide or potassium ionization buffer is improved by the use of filters inserted by a filter wheel installed before the entrance to the monochromator. The filter used for cesium measurements was a Schott RG830 (3-mm thick; 90% transmission at 852 nm, 0.5% transmission at 795 nm). The filter used for rubidium measurements was a Schott RG780 (3-mm thick; 85% transmission at 794.8 nm, 44% at 780.0 nm, 16% at 769.9 run, 0.5% at 735 nm. The main benefit of this filter is the drastic decrease of the slope of the potassium background near the rubidium line to allow background correction from measurement at a single wavelength. The photomultiplier was a Hamamatsu R955 (0.2~nA dark current) ; voltages applied varied between 300 and 830 V (Na 300 V; K 400 V; Rb 600 Vi Cs 830 V). Calibm tion A set of eight standards containing 0.45 M lithium hydroxide was used. The standards also contained (in mg kgd) 0.1 Cs and 0.4 Rb and: (1) 0.1 Na, 0.1 K; (2) 0.3 Na, 0.4 K; (3) 0.5 Na, 0.125 K;(4) 1.5 Na, 0.375 K;(5) 5 Na, 1.25 K; (6) 15 Na, 3.75 K; (7) 50 Na, 12.5 K;(8) 150 Na, 3.75 K. The first two standards were used for linear work with sodium and potassium; for this purpose, the burner was placed perpendicular to the optical path, and slit 2 without diaphragm was used. The other standards were used for higher concentrations when slit 4 was used in combination with the diaphragm (see above) is used and the burner was parallel to the optical path. Calibration graphs can be drawn but computer evaluation of the data by means of algorithms developed by Patterson [ 71 was used here. All measurements of cesium and rubidium were evaluated by rectilinear calibration.
188
Procedure Powdered sample (<0.06 mm or 230 mesh; 2.5-3 g) was transferred to the weighed pressure vessel containing 35 g of 4.5 M lithium hydroxide (d = l.l), 4 ml 1.6 M nitric acid, and a magnetic follower. After assembly, the vessel was placed on a hot plate controlled at 180-185°C for 60-70 h, with slow stirring at about 150 rpm. The suspension from the cooled vessel was transferred to a go-ml centrifuge tube, water being used to rinse the vessel. After centrifugation for 10 min, decantation of the supernatant solution into a separatory funnel left behind a cake of solids wetted by a little solution (1 - fl). The recovery (fl) of the solution varied between 0.90 and 0.98 depending on the surface properties of the solid particles. The loss (1 - fl) was measured as follows: the centrifuge tube was filled to the original volume with water and shaken for 1 min to bring the solids back into suspension; centrifugation then produced a second solution with a sodium concentration CL,; the ratio of the sodium concentrations C~,/Cn, gave a value for (1 - fl). Between l/60 and l/20 of the solution in the separatory funnel was withdrawn, diluted five-fold and used for the direct determination of alkali metals by flame spectrometry. The corresponding fi values are 0.983 and 0.950. If the remaining solution (75 X fl X fi g) contained sufficient potassium (>80 mg K kg-‘), it was ready for liquid-liquid extraction. A freshly prepared solution of 0.45 g of Na-TPB in 15 ml of DIBK was added to the contents of the separation funnel and shaken for 3 min. Potassium additions for samples with low potassium content were taken from Fig. 3. For example, anorthosite AN-G contains 0.135% KzO. If 4 ml of final solution was wanted for the determination of Rb and Cs, the required potassium addition was 2.7 mg. Measurement of the potassium content of the aqueous phase before and after extraction (5.8 and 1.7 mg of potassium) determined the amount of potassium released to the organic phase (4.1 mg of potassium). Potassium, Rb and Cs were stripped from the organic phase by repeated extraction with equal amounts of 1.6 M nitric acid; 0.5 g was used for each mg of potassium. Extraction was done as follows: after shaking for 2.5 min and a reaction time of 75 min, shaking was repeated for 1.5 min; the second extraction only required 1.5-min shaking. In the case of AN-G, 2 g of acid was used twice. The combined 4 g of solution contained 4.1 mg K (1022 mg kg-‘), 2330 ng Rb (0.583 mg kg-‘), and 128 ng Cs (0.032 mg kg-‘). The content of rubidium and cesium in the rock (in mg kg-‘) was calculated from { [ weighta,,,& - TPB blankai,,, (ng)]/[f& - LiGH blanka,,ti (ng)]) divided by the weight of the rock sample (mg), i.e., for AN-G [(2330 - 4)/(0.934 [(128 - 4)/(0.934
X 0.985) - 4]/3000 X 0.985) - 1]/3000
= 0.842 mg Rb kg-’ = 0.045 mg Cs kg-‘.
Simplified procedure (for the determination of sodium and/or potassium only) Rock powder (<230 mesh; 0.05-0.2 g) was digested with 8 ml of 3 M lithium hydroxide in a 25-ml Pt or Au crucible placed inside the pressure
189
vessel. The suspension was stirred at 180 + 5°C for 60 h. Alternatively, the sample was fused at 850°C with 1 g of lithium hydroxide monohydrate in a gold crucible. The cake was then leached with stirring for at least 6 h in the pressure vessel. The sample suspension was transferred to a preweighed polyethylene bottle and diluted with several water rinses until about 50 g of solution was obtained. The concentrations of sodium and/or potassium were measured by emission or atomic absorption in the hydrogen-air flame. RESULTS
AND DISCUSSION
The certification of the sodium and potassium contents of the older rock standards is based on a larger number of results from many laboratories. Although certification is normally based on statistical evaluation, one compilation applies criteria of selection [8]. The latest comprehensive compilation [9] of elemental concentrations in the older U.S.G.S. rock standards provides the largest set of data available for the content of alkali metals in ultrabasic rocks. However, neither statistics nor selected data obtained with the most suitable methods revealed most probable or consensus values from data more or less evenly spread over three orders of magnitude. The choice of values from selected laboratories allowed Abbey [El] to give an approximate value for sodium in PCC-1 (0.01% NazO) but not for potassium in PCC-1 and DTS-1. One reason for a general bias in the results for sodium and potassium at low levels is sample contamination when the samples are converted to solutions, stored, diluted and chemically buffered, e.g., for flame spectrometry. Potassium at very high levels is used extensively as an ionization suppressor and sodium contamination is also difficult to avoid in the laboratory. As the present work was designed to avoid contamination with alkali metals as far as possible in a standard analytical laboratory and to remove the sample matrix, the low values obtained (Tables 2 and 3) can be regarded as a closer approximation to the true values than the higher literature values. All values represent the average of two to three separate analyses. There were no batches in the lithium hydroxide extraction as only one reaction vessel was available. Values above 0.7% N&O and above 0.1% KzO show good agreement with values that have established common ground: usable values [8], recommended values [lo], consensus values [ 111 and certified values [ 121. In contrast to sodium and potassium, the rarer alkali metals rubidium and cesium have been determined by only a few analysts. Acceptable values cannot be derived statistically but single measurements obtained by isotopedilution mass spectrometry [14, 231 may be regarded as the most reliable results. In the case of cesium, radiochemical [24] or epithermal neutron activation also provides sufficiently strong and selective analytical signals. At higher levels of cesium, concurrent studies [l, 211 of the flame emission signals of cesium in the presence of the entire rock matrix except silica
Origin
A.N.R.T. A.N.R.T. B.A.S. B.A.S. B.A.S. U.S.G.S. S.A.B.S. A.N.R.T. U.S.G.S. XIGMR XIGMR XIGMR XIGMR CCRMP CCRMP CCRMP U.S.G.S. U.S.G.S. A.N.R.T. CCRMP S.A.B.S. S.A.B.S. U.S.G.S.
Symbol
AI-1 AN-G BCS 267 BCS 302 BCS 375 BHVO-1 NIM-D DT-N DTS-2 DZ-Cr-1 DZ-Cr-2 DZ-1 DZ-2 FeR-1 FeR-3 FeR-4 G-2 GSP- 1 IF-G MRG-1 NIM-N NIM-P PCC-1
Albite Anorthosite Silica brick Iron ore Soda feldspar Basalt Dunite Kyanite Dunite Chromite Chromite Ultrabasic Ultrabasic iron ore Iron ore Iron ore Granite Granodiorite Iron ore Gabbro Norite Pyroxenite Peridotite
Rock type
0.017 0.004 0.002 0.019 0.007 0.003 0.026 4.00 2.65 0.028
0.051
11.15 10.89 1.62 0.051 0.088 10.58 2.23 0.0072 0.027 0.004 0.0173 0.0046 0.0023 0.0190 0.0043 0.0056 0.027 4.02 2.71 0.025 0.711 2.45 0.344 0.0067 0.005-0.4 0.002-0.02 0.01-0.13 2-4.87 2.3-5.8 0.003-0.24 0.63-1.33 2.20-2.72 0.27-0.51 0.002-0.05
10.4 [ 151 2.21 [ll]
10.24-10.48 2.10-2.35 0.01-0.08 0.01-0.15 DTS-1: 0.04 [22] 0.007 [lo] 0.025 [12] 0.009 [12] 0.008 [12] 0.028 [12] 0.03 [25] 0.03 [25] 0.05 [25] 4.07 [9] 2.80 [ 91 0.032 [ 271 0.71 [S] 2.46 [8] 0.37 [8 ] 0.01 [8]
10.6 [27] 1.63 [13] 0.06 [15]
9.5-11.8 1.1-2.2 0.05-0.07
0.116 0.004 0.005 0.005 0.011 0.017 0.247 4.44 5.50 0.007
0.641
0.132
HF 0.133 0.135 0.124 0.633 0.763 0.507 0.0048 0.125 0.0009 0.116 0.0057 0.0046 0.0073 0.0101 0.0154 0.248 4.53 5.44 0.0058 0.179 0.242 0.084 0.0012
LiOH
Range
HF Common
This work
This work LiOH
K,G (%) Literature
Na,O (%)
Results obtained for alkali metals in standard rocks
TABLE 2
0.0006-0.06 0.004-0.06 0.17-0.34 2.2-7.8 4.2-7.1 0.001-0.18 0.16-0.22 0.19-0.31 0.06-0.12 0.0003-0.1
0.72-0.85 0.45-0.68 0.00-0.03 0.05-0.21 DTS-1:
0.06-0.20 0.04-0.24 0.12-0.18
Range
Literature
0.12 [22] 0.0012 [lo] 0.11 [12] 0.010 [12] 0.010 [12] 0.009 [12] 0.02 [25 J 0.03 [25] 0.29 125 J 4.49 [ 91 5.49 [ 91 0.012 [27] 0.18 [8] 0.25 [8] 0.09 [8] -
0.79 [15] 0.54 [8 3
0.14 [27] 0.13 [13] 0.14 [15]
Common
A.N.R.T. A.N.R.T. B.A.S. B.A.S. B.A.S. U.S.G.S. S.A.B.S. A.N.R.T. U.S.G.S. XIGMR XIGMR XIGMR XIGMR CCRMP CCRMP CCRMP U.S.G.S. U.S.G.S. A.N.R.T. CCRMP S.A.B.S. S.A.B.S. U.S.G.S.
Al-I
AN-G BCS267 BCS302 RCS375 BHVG-1 NIM-D DT-N DTS-2 DZCr1 DZCr-2 DZ-1 DZ-2 FeR-1 FeR-3 FeR-4 G-2 GSP-1 IF-G MRG NIM-N NIM-P PCC-1
Origin
Symbol
25
1.1
6.6
5.5 [271 6.05 0.84 0.85 ~141 3.65 23.3 48 ~151 46.1 a [161,8.7 ~171.10.9 cl81 8.61 0.218 0.2 c201 4.45 5 c221 0.04-0.06 (DTS-1 1231) 0.041 5.76 0.168 0.172 0.359 0.443 0.491 13-27 1251 16.1 171 1141.168 C231 169 254 [Sl 254 0.308 0.4 [271 7.17 1261 7.10 2.4-7 [20] 3.46 2-a 1201 2.49 0.062 1231 0.061 0.61 0.23 0.14
1.03 <0.05 <0.05 0.0s co.3 0.32 0.70 1.35 0.98
0.08 0.13
1.07
0.37 0.07
0.34 0.046 0.527 1.12 1.01 0.082 0.083 0.114 0.0028 1.05 0.019 0.039 0.068 0.117 0.159 0.668 1.33 0.97 0.056 0.594 0.242 0.140 0.0051
LiOH
HF
Lit..
I-IF LiOH
Ce (mgkg+)
Rb (mgkg-')
6.2 0.21 0.43 Ultrabasic 0.42 Ultrabesic 0.14 Iron ore 0.50 Ironore 16.1 Ironore Granite 165 Grenodiorite 248 Ironore 7.41 Gebbro Norite Pyroxeaite Peridotite
Albite Anorthosite Silicabrick Iron o*e SodaFeldspar Basalt DUIlite Kyenite DUllita Chromite
RocktyPa
Results obtained for alkali metals in standard rocks
TABLE 3
0.5-3 [251 1.33 191 0.95 c91 0.06 1271 0.55 p.211 0.24 L1.211 0.12 Cl.211 0.0053--0.0055 C241
0.08 [161.o.oa clsi,o.o9 cl91 0.05 [l,211 0.21 [l.211 0.005 (DTS-1 C241)
0.34 1271 0.043 1141
Lit.
480 2920 620 490 300 1040 400 490 400 370 620 490 370 410 510 320 486 389 340 450 1170 610 360
rati0
R/Rb atomic
192
produced a useful set of data. In the case of rubidium, x-ray fluorescence signals are sufficiently strong to allow accurate determination at sufficiently high levels, e.g., 8.7 mg kg-’ for BHVO-1 [17]. There is good agreement between the rubidium and cesium data obtained by reaction with lithium hydroxide and those literature values obtained by other methods that are especially suitable for rubidium or cesium determinations in rocks at low levels. For eight of the reference samples analysed for these two metals, no literature values were available for comparison. A set of K/Rb ratios was calculated from the lithium hydroxide data, as K/Rb ratios are used routinely for the study of rock differentiation. The highest ratios are found in rocks with high anorthite content, e.g., basalts and anorthosites. Ultrabasic rocks that are low in feldspar show a relatively low K/Rb ratio. Precision Values for KzO and NazO above 0.1% (1000 mg kg-‘) obtained from different runs for the same sample did not vary by more than 3% (range). The variations are attributed partly to the combined errors of the factors fl, f2, and partly to flame emission fluctuations. Relative standard deviations (RSD) based on six runs of the in-house basalt containing 2% Na,O, 0.55% KzO, 10 mg kg-’ Rb and 0.09 mg kg-’ Cs were 1.2,1.4, 2.5 and 3.8%, respectively. Near the detection limits (0.0005% NazO, 0.003 mg kg-’ Rb and 0.001 mg kg-’ Cs), which are only approached in the case of cesium in DTS-2 and PCC-1, the noise in the flame emission signal of 0.0003 mg kg-’ cesium determines the possible error. For greater precision and accuracy of potassium or sodium alone, as needed in K/Ar age determination, a simple extraction method requiring less sample gave excellent reproducibility of the results (0.4% RSD). These small variations (e.g., 0.511 +- 0.002% KzO in BHVO-1) are of the same magnitude as the noise of the flame emission signal. By atomic absorption measurements in the hydrogen-air flame, precision was further improved (e.g., 0.513 +-0.001% K,O in BHVO-1). Conclusions Lack of a satisfactory method for the quantitative and selective extraction and preconcentration of rubidium and cesium from rocks led to the development of the proposed technique. Data at very low natural levels can be obtained by flame emission spectrometry. Such data could hitherto be obtained only by using isotope-dilution mass spectrometry or neutron activation analysis. The selective extraction of alkali metals from rocks by lithium hydroxide provides not only a very good solution for the preconcentration of rubidium and cesium but also for the direct determination of the more concentrated alkali metals, potassium and sodium. Other cations and anions are present in too low concentrations to produce any interferences.
193
I am grateful to Mr. R. G. Ditchbum, Institute of Nuclear Sciences, for his help with 134Cstracer work. REFERENCES 1 2 3 4 5 6
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