Analytica Chimica Acta, 209 (1988) 333-338 Elsevier Science Publishers BV., Amsterdam - Printed in The Netherlands

Short Communication

SPECTROPHOTOMETRIC DETERMINATION OF GERMANIUM IN ROCKS AFTER SELECTIVE ADSORPTION ON SEPHADEX GEL

ATSUSHI HARADA” and TOSHIKAZU TARUTANI

Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashiku, Fukuoka 812 (Japan)

KAZUHISA YOSHIMURA*

Chemistry Laboratory, College of General Education, Kyushu University, Ropponmatsu, Chuo- ku, Fukuoka 810 (Japan)

(Received 4th January 1988)

Summary. Germanium (IV) is preconcentrated on Sephadex G-25 from a carbonate solution (pH 12 ) , and desorbed into 0.1 mol dm -’ nitric acid. Iron (III) and tin (IV) in eluate were removed by a small cation-exchange column. The combination of the two columns made it possible to deter- mine germanium in rocks at mg kg-’ levels by spectrophotometry with phenylfluorone.

Germanium is usually distributed in silicate minerals at levels of a few ,ug g -’ and in sea and river water at extremely low levels. An atomic absorption spectrometric method for germanium has been based on hydride generation; it is highly sensitive and has been applied for the determination of germanium in natural waters [l] and silicate rocks [ 21. Solution spectrophotometry has been frequently applied but as it is less sensitive and selective, germanium has to be separated and preconcentrated, for example, by liquid/liquid extraction of germanium tetrachloride [3] or coprecipitation with hydrated lanthanum or iron oxides [4].

Selective preconcentration methods for boron (III), vanadium (V) , molyb- denum (VI ) and tungsten (VI) have been developed [ 5-81 by taking advantage of the reversible adsorption of their 0x0 anions on Sephadex G-25 gel. Ger- manium(IV) is also adsorbed reversibly on Sephadex G-25 from an alkaline solution. Thus, when a rock sample is fused with sodium carbonate and the germanium in the resulting material is extracted into water, the solution can be directly loaded onto the column. In this communication, a selective precon-

“Present address: Toyo Soda Manufacturing Corporation, 4560 Tonda, Shin Nan-yo 746, Japan.

0003-2670/88/$03.50 0 1988 Elsevier Science Publishers B.V.

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centration method for germanium, especially in rocks, is described. After con- centration on a Sephadex gel column, the germanium is determined spectrophotometrically with phenylfluorone [ 91.

Experimental Chemicals. Deionized/distilled water and analytical-grade chemicals were

used. The phenylfluorone solution (0.04% ) was prepared by dissolving 0.04 g of the compound in 70 cm3 of ethanol, adding 5 cm3 of 3 mol dme3 sulfuric acid and filtering through a Toyo No. 5A filter paper. The filtrate was diluted to 100 cm3 with ethanol. The poly (vinyl alcohol) solution (0.25% ) was prepared by dissolving 0.25 g of poly (vinyl alcohol) (polymerization degree ca. 2000) in 80 cm3 of water, filtering and diluting to 100 cm3 with water. The cetyltrime- thylammonium bromide solution (1.14%) was prepared by mixing 1.14 g of compound with 80 cm3 of water, filtering and diluting to 100 cm3 with water. The standard germanium (IV) solution (1000 ,ug cmm3) was supplied by Wako Pure Chemicals (germanium dioxide solution). The Sephadex G-25 (medium) was from Pharmacia.

Procedure. Fuse a powdered rock sample (0.1-0.2 g) containing 0.1-2 pug germanium with a lo-fold amount of sodium carbonate. Suspend the cooled melt in about 60 cm3 of hot water to extract the germanium. Adjust the pH to 12, if necessary, with solid sodium hydroxide. Filter through a 0.45-pm mem- brane filter (Millipore). At 5 cm3 min-‘, pass the filtrate through a column (16 mm i.d., 75 mm long) packed with 3 g of Sephadex G-25 gel, previously conditioned with 0.01 mol dmb3 sodium hydroxide. Wash with 25 cm3 of 0.01 mol dmm3 sodium hydroxide and desorb the germanium with 25 cm3 of 0.15 mol dmm3 nitric acid at about 80” C. Reject the first 10 cm3 of effluent and add the next 15 cm3 to a column (10 mm i.d., 30 mm long) packed with a cation- exchanger (AG 5OW-X8, H+ form, 100-200 mesh). Wash this column with 10 cm3 of water. Collect the effluent in a 25-cm3 beaker and evaporate it to dry- ness. Wash the Sephadex gel column with 50 cm3 of 0.2 mol dmm3 oxalic acid before using again.

Dissolve the residue with 1 cm3 of 0.5 mol dmm3 sodium hydroxide and wash it into a lo-cm3 volumetric flask with a little water. Add 1 cm3 of concentrated hydrochloric acid, 1 cm3 of the poly(viny1 alcohol) solution, 1 cm3 of the ce- tyltrimethylammonium bromide solution and 2 cm3 of the phenylfluorone so- lution to the sample containing 0.1-2 pg of germanium in the lo-cm3 flask. Leave at room temperature for 30 min, then measure the absorbance of the solution in a 2-cm cell at 505 nm against a reagent blank (a Hitachi spectro- photometer, Model 100-50, was used). Calibrate with solutions up to 0.2 pug cmm3 germanium, using the phenylfluorone method without preconcentration.

Distribution measurements. To investigate the adsorption of germa- nium(IV) at a concentration of 1 pug cm -3 (1.38~ 10m5 mol dmm3) on the Sephadex G-25 gel, the distribution ratio was measuredby the batch technique.

The ionic strength of the solution was maintained at 0.1 mol dmT3 with sodium chloride and the pH was adjusted with dilute sodium hydroxide or hydrochloric acid solution. Then 0.2 g of gel and 50 cm3 of test solution were stirred for 24 h. The pH of the equilibrated solution was measured and germanium remain- ing in the solution was determined by the phenylfluorone method.

Results and discussion The effect of pH on the distribution ratio (D) is shown in Fig. 1, Germa-

nium (IV) is strongly adsorbed on Sephadex G-25 gel only from alkaline solu- tion, but at pH > 12, D was decreased. This behavior is similar to that of boron(II1) [5], though the interaction of germanium with the gel is much stronger than that of boron. The mole fraction diagram of germanate species (Ge (OH) 4, GeO (OH) 3 , and GeOz (OH); ) shown in Fig. 1, was calculated by using the acidity constants of Ge (OH),, pK, = 9.1 and p& = 12.1 [lo]. The plot of D vs. pH seems to be intermediate between the mole fraction diagrams for GeOz (OH):- and GeO (OH), . Polyols are known to form complexes with germanium(W) [ 11,12 1, as well as with other 0x0 anions such as those of boron (III) [ 131, molybdenum (VI) and tungsten (VI) [ 141. The decrease of the D value above pH 12 may be due to dissociation of hydroxyl groups on the gel matrix.

The strong adsorbability of germanium on Sephadex G-25 at pH 11-12 is applicable for the separation and preconcentration of germanium by the col- umn techique. After adsorption, quantitative elution of germanium was ob- tained with 0.15 mol dme3 nitric acid, the germanium appearing in the effluent between 12 and 22 cm3. Therefore, the first 10 cm3 of the effluent was rejected and the fraction from 10 to 25 cm3 was used for germanium determination. The precision was measured with 100-cm3 solutions containing 0.30 ,ug of germa-

1DOOr

D

500 -

0 2 4 6 8 10 12 14O

PH

Fig. 1. pH dependence of the adsorption of 1.4 X 10m5 mol dme3 germanium(W) on 0.2 g of Seph- adex G-25. Experimental points (0 ) relate to 50 cm3 of 0.1 mol dm-3 NaCl medium. Dotted lines indicate the mole fraction diagram of germanate species.

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nium. For 5 determinations, the absorbances obtained were 0.130 with a stan- dard deviation of a result of 0.004.

When high concentrations of tungsten(V1) were investigated [8], low re- coveries were obtained, because the isopolytungstates formed are adsorbed to a lesser extent than the mononuclear species present in dilute solutions. Ger- manium (IV), however, exists as a mononuclear species at pH 12 at all concen- trations investigated. The recovery was complete for as much as 1 mg of germanium. The column was found to have sufficient capacity to remove ger- manium completely from 3 dm3 of solution. Germanium is present in geother- mal waters in the range 2-30 pg dmb3 [ 151, so the present method could be applicable to such waters if a large sample was used.

Effects of other ions. 0x0 anions such as those of vanadium(V), molybde- num( VI), and tungsten (VI) are adsorbed on Sephadex G-25 gel from acidic solution, whereas they are not adsorbed at pH 12 [6-81, so that these 0x0 anions can easily be removed from the column whilst germanium is retained. The presence of a large amount of some other salts and ions, except for tin (IV), did not affect the recovery (Table 1).

It has been reported that tin (IV) forms complexes with polyols [ 161; there- fore tin (IV) could be desorbed with germanium. A small amount of hydrated iron oxide was also found to remain at the top of the gel column, and was desorbed with germanium when warm nitric acid (0.15 mol dmm3) was used as eluent. Because tin (IV) and iron (III) interfere in the phenylfluorone method, these ions were removed by passing the effluent from the Sephadex column through a small cation-exchange column. The recovery of germanium became incomplete if a Sephadex column was used that had previously been used for a sample containing a large amount of tin(IV). This problem was solved by washing the column with an oxalic acid solution for the desorption of tin (IV) before reuse.

Analysis of rock samples. Large amounts of silicon and aluminium did not

TABLE 1

Effect of other ions on the recovery of germanium

Species added Mole ratio (ion/Ge)” Ge found (pg) Error, (% )

Al(II1) 1 x105 1.46 -2.5

Si(IV) 1 x106 1.43 -4.7

B(II1) 4 x104 1.50 0 Sn(IV) 10 3.71 + 148

10 1.56 +4.0b NaCl 1.5x 107 1.55 +3.3 Na&03 6 x106 1.50 0

“lOO-cm3 sample containing 1.50 M of Ge and the other ion loaded onto the Sephadex column. bSolution passed through a cation-exchange column after Sephadex treatment.

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TABLE 2

Recovery of germanium

JB-1 (0.20 g) Ge added (pg) Absorbance Ge found (pug)

JG-1 (0.20 g) Ge added (pg) Absorbance Ge found (pg)

“See Table 3.

0 0.10 0.20 0.30 0.40 0.056 0.102 0.131 0.194 0.231 0.183 0.256 0.344 0.459 0.556

0 0.10 0.20 0.30 0.40 0.114 0.166 0.224 0.242 0.301 0.277 0.403 0.544 0.692 0.731

TABLE 3

Determination of germanium in some rocks

Sample Ge Sample Ge Sample Ge content’ content” content” (mg kg-‘) (mg kg-‘) (mg kg-‘)

JA-1 (andesite) 1.2 kO.2 JB-3 (basalt) 0.72 f0.16 JG-lb 1.7 +0.2 JB-lb (basalt) 0.80 kO.07 JR-l (rhyolite) 2.4 f0.08 (granodiorite) JB-2 (basalt) 1.2 +0.1 JR-2 (rhyolite) 2.4 +0.3 JGb-1 (gabbro) 0.84f0.19

“Meanfsd. (n=4or 5). bCertifiedvalues [17]: JB-1,1.3 mg kg-‘; JG-1,1.3 mg kg-‘.

affect the recovery of germanium, thus the present method is appropriate for the determination of germanium in rocks. The rock sample was fused with sodium carbonate, and the germanium was quantitatively extracted into water; most of the iron and titanium remained behind. The pH of the solution was about 12, so that it could be applied directly to the Sephadex column. When the method was applied to determining germanium in standard rock samples from the Geological Survey of Japan, the results for JG-1 and JB-1 by the standard addition method agreed with literature values; the germanium added was confirmed to be quantitatively recovered by the present method (Table 2 ) .The germanium contents found for various rocks are listed in Table 3. Ex- cept for rhyolite (JR-l and JR-2; 2.4 mg kg-‘), these igneous rocks contained about 1 mg kg-l germanium.

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