Precipitation and Coprecipitation

Of the numerous published reactions for the precipitation of germanium, only a few are practical for use in radiochemistry. These reactions include precipitation in the form of sulfide, coprecipitation with acid-insoluble sulfides, and coprecipitation with hydroxides of Group III, especially with Fe(OH)3.

a. Reaction with Hydro.qen Sulfide

White germanium disulfide is formed in a strong acidic medium (3-4 M HCI or 5-6 M H2SO4) , and it thus differs from sulfides of most elements of the hydrogen sulfide group. The sulfides of other elements are precipitated first from a less acidic solution, and then the filtrate is acidified to precipitate GeS 2. The only other white sulfide is ZnS, which precipitates only in basic solutions. To further increase the selectivity, GeS 2 is precipitated after the separation of Ge by distillation from acidic chloride solutions in the presence of the oxidizing agents (see preceding section). In strongly acidic solutions, only germanium forms the yellow-orange precipitate of GeSe 2. Therefore, the reaction of germanium with hydrogen selenide is a selective method for the detection of germanium.

S. MIRZADEH, R. M. LAMBRECHT: RADIOCHEMISTRY OF GERMANIUM

The solubility of GeS 2 is 4.45 mg/ml in water at 20~ and only 0.1 pg/ml in 2 M H2SO 4 containing >0.02 M_M_ H2S; 163'164 thus GeS 2 precipitate should be washed with an H2S-saturated acid solution. The GeS 2 precipitate typically contains occluded sulfur which cannot be completely eliminated even by heating under CO 2 to sublime sulfur.

To complete the determination, GeS 2 should be oxidized to GeO 2 by dissolving the GeS 2 in 10 M NH4OH followed by addition of 3% H202.

b. Coprecipitation with Acid-Insoluble Sulfides

It is known that germanium, in concentrations greater than 10 .6 M, can be coprecipitated with the sulfides which are insoluble in acidic or weakly acidic solutions (the sulfides of copper, arsenic, zinc, cadmium, etc.). Quantitative coprecipitation of macro amounts of Ge (0.25-20 mg/I) with copper and As s+ sulfides has been reported by Shevyakina 185 and Baraboshkin.166 Carrier-free 68Ge also can be coprecipitated with acid-insoluble sulfides. 128 The results of coprecipitation experiments performed with the sulfides of Sb 3+, As 3+, Bi, Cu, Hg and Pb formed in the presence of carrier-free 68Ge are summarized in Table 6.3. In general, the fraction of 68Ge coprecipitated with these sulfides increases with increasing cation concentration. Quantitative coprecipitation occurs with sulfides of Bi and Hg, when the initial concentration of these cations is 10 -4 M, in 1.8 M__ H2SO 4 after allowing the solution to stand for 4 hours.

Table 6.3 Coprecipitation of carrier-free S8Ge with various acid-insoluble sulfides in 1.8 M H2SO4 .12s

Germanium-68 coprecipitated, % Initial concentration of cation, M__

10.6 10 .5 10 -4

Precipitate

As2S 3 12 41 60

Bi2S 3 56 89 ~100

Sb2S 3 47 60 84

CuS 69 82 94

HgS 69 79 -100

PbS 7.8 13 55

After the addition of 68Ge, H2S was passed through the solutions for 5 minutes. After 4 hours the solutions were centrifuged for 30 minutes at 2500 g.

S. MIRZADEH, R. M. LAMI3RECHT: RADIOCHEMISTRY OF GERMANIUM

A comparison of the solubility products of the sulfides used (Bi2S 3, 1097i Sb2S 3, 1095; PbS, 1028; CuS, 1048; HgS, 10 "54) shows nearly the same molar solubility for the Group I sulfides Bi2S3, Sb2S 3 and PbS, and the Group II sulfides CuS and HgS; and gives no indication that carrier-free 68Ge would be carried so completely with Bi2S 3.

These results are consistent with the results of the adsorption experiments, in which the highest adsorption of carrier-free 68Ge was observed on the sulfides of bismuth and mercury (see Sect. 4). 128

c. Coprecipitation with Hydroxides of_Group III Elements

The hydroxides of the Group III elements are known to coprecipitate microgram quantities Of germanium and, under certain conditions, the process is quantitative. 167173 Among the possible hydroxides, the most commonly used is Fe(OH)3, where quantitative coprecipitation occurs at pH>6 and the process is rather independent of the temperature and presence of foreign ions. The presence of ammonium salts improves the coprecipitation. A study of the effects of various factors is given by Agakova et aL 157 The required ratio of Fe:Ge for complete coprecipitation depends on the Ge concentration in the solution. In a solution containing 0.01 pg/ml of germanium, the required ratio is ~103:1.158 According to Novikov et aL, 169 the sequential separations of Zn, Ga, G e, As, and Se are also possible by coprecipitation with Fe(OH)3.

The separation of carrier-free 68Ge was briefly reported by Sewastjanow. 172 Accordingly, carrier-free 68Ge produced in a Ga target by [p,2n] reaction was separated from target solution by coprecipitation with Fe(OH)3 [~50 mg, added as Fe(NO3)3] at pH 11-12. Separation from Co and Ni impurities was achieved by the addition of hold-back carriers.

Quantitative coprecipitation of Ge with AI(OH)3 occurs only in neutral solutions (pH 6-8). 168'174 In a solution containing 0.01 pg/ml of germanium, the required ratio of Al:Ge is -10,000.168 The efficiency of the process decreases slightly at higher temperatures.174

The isolation of Ge from aqueous solutions by coprecipitation with copper gallate and tannate* is described by Andrianov et aL 175 The Ge gallates and tannates are more stable than the corresponding copper complexes and, hence, copper is readily replaced

* Gallic acid: 3,4,5-trihydroxybenzoic acid. Tannic acid: 076H52046

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by Ge in these complexes. The optimum pH ranges for quantitative coprecipitation of Ge with copper gallate is 6.5-6.0 and 3-11 for Cu tannate. The gallate and tannate precipitates can be dissolved in dilute mineral acids or in >6 M HCI + BuOH, respectively. It is also shown that freshly prepared copper tannate adsorbs Ge strongly (50 mg of Gel1 g adsorbent) and apparently Cu 2+, Zn 2+, Cd 2+, AI 3+, Mn 2'4+, As 3+, Sn 2+, and Sb 3+ are not adsorbed.