Volume 5: Copper, Silver, Gold, Zinc, Cadmium and Mercury
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Chapter 2 Silver and its Compounds
Wolfgang Hillera and Christian Erich Zybillb a Anorganisch-chemisches Institut der Technischen Universität München, Lichtenbergstraße 4, D-85474 Garching bei München, Germany b Physik Department E16, Technischen Universität München, James-Franck-Straße 1, D-85474 Garching bei München, Germany 2.1 Elemental Silver 2.2 Inorganic Compounds of Silver 2.3 Organometallic Compounds of Silver 2.4 Silver Compounds Described in Other Volumes 2.1 Elemental Silver
Highly Pure Silver Metal Procedure Crude silver (for instance from reaction residues) is dissolved in concentrated nitric acid and the solution is diluted. Then silver chloride is precipitated at 0 °C by addition of a pure NaCl solution. The solid is washed several times with cold water and then dissolved again in a freshly prepared ammonia solution. After a few hours for settling, the solution is filtered and the AgCl is precipitated by addition of pure nitric acid. The solid is washed free of nitrate with water (decantation) and reduced to Ag in a silver vessel by addition of fructose/NaOH (from sodium metal) at 60 °C or by addition of saccharose/NaOH or with a hot alkaline formaldehyde solution. The formaldehyde should be distilled prior to use, the sugar is dissolved, filtered through charcoal, and recrystallized. The formed Ag sediment is isolated by filtration, washed free of chloride with hot water, dried, and smelted to a regulus on pure CaO (960 °C). Provided that heating is only for a brief period, and the reguli are cooled down, e.g., in the reducing part of the flame, the obtained Ag is very pure. It contains about 0.001% S, traces of C, AgCl, and O2. The obtained Ag is subjected to further electrolytic purification. The major amount of the silver is formed into a foil and used as anode (against a fine silver cathode). A 10% AgNO3 solution is used as electrolyte which has been prepared from Ag and pure HNO3 As anode, a glass tube filled with Ag and with a sintered glass filter plate at the bottom is used. The electrolysis requires a potential of 1.39 V. The Ag formed at the cathode is collected from time to time, dried, placed in a CaO boat and melted to a regulus (H2 or Ar atmosphere). For the synthesis of finely divided Ag see below. Properties Mr : 107.87, mp 960.8 °C, bp 2212 °C, D (20 °C): 10.494 g · cm-3. References 1 O. Hönigschmid, R. Sachtleben, Z. Anorg. Allgem. Chem. 195, 207 (1931). 2 T. W. Richards, R. C. Wells, Z. Anorg. Allgem. Chem. 47, 56 (1905). 3 O. Hönigschmid, E. Zintl, M. Linhard, Z Anorg. Allgem. Chem. 136, 263 (1924). Recovery of Silver from Waste Material Procedures The collected material is treated with concentrated HCl/H2O (1:1). After settling of the insolubles, the solution is decanted. The solid is washed several times with HCl/water until it is free of any iron. It is separated by suction filtration, placed in a large porcelain bowl, and concentrated HCl/H2O (1:1) together with some pieces of Zn are added. With agitation, the material is reduced to Ag. When all white particles of AgCl have been consumed, the formed Ag sediment is washed with hot water until it is free of acid and Zn. The obtained Ag sediment can be either converted into pure Ag (a, see below) or transformed into AgNO3 (b, see below). When only pure AgCl has to be worked up, the material is conveniently dissolved in NH3 and Ag is precipitated by addition of a 20% solution of hydrazine hydrate.1, 2 The Ag mass is dried, placed in a porcelain crucible and heated with some borax until it melts (960 °C). The liquid Ag is poured into water to form granules of solid Ag. The Ag sediment is dissolved in concentrated HNO3/H2O (1:1), the solution is filtered and the liquid is evaporated carefully on a steam bath until crystals of AgNO3 have formed. The salt is further dried under vacuum or melted and recrystallized. Very finely divided Ag for organometallic synthesis is prepared according to a procedure described by Gomberg and Cone:3 Carefully purified AgCl is placed in a beaker and covered with water. A porous ceramic cylinder is fitted inside the beaker and filled with water (slightly acidified with HCl). Inside the cylinder are now placed some Zn pieces and into the AgCl a Pt foil is dipped. Both metals are connected with a Pt wire. The level of liquid in the ceramic cylinder should be below the level in the beaker to avoid excessive ion diffusion. The AgCl converts into a gray Ag powder (for 250 g of AgCl, a few days are required) which is washed with water, dilute ammonia, alcohol, and diethyl ether and dried under vacuum at 150 °C. Alternative Procedure4 The Ag obtained by reduction with Zn is dissolved in dilute HNO3. The solution is filtered, and AgCl is precipitated with dilute HCl from a hot solution. The precipitate is filtered off, carefully washed with warm water and dried. 1 part of AgCl is mixed with 0.5 parts of Na2CO3 and 0.15 parts of KNO3 in a mortar and heated to red heat (960 °C in a porcelain crucible). Ag is formed according the equation: The AgCl can also be added portionwise into the hot salt melt. The Ag immediately melts. A regulus forms upon cooling which is treated with boiling water containing H2SO4 to remove any salts. References 1 Handbuch für das Eisenhüttenlaboratorium, Verlag Stahleisen, Düsseldorf, Vol. 1, p. 317 (1939). 2 F. Specht, Quantitative anorganische Analyse in der Technik, 1953. 3 G. Gomber, L. H. Cone, Ber. Dtsch. Chem. Ges. 39, 3287 (1906). 4 G. Bomemann. Anorg. Präparate, Leipzig, 1926, p. 160. Recovery of Silver from Photographic Processes Procedure1 The respective solution is treated with ammonium hydroxide, and then a slight excess of ammonium sulfide is added. The mixture is allowed to settle overnight and the liquid is removed. The residue is washed with water, separated by filtration, placed in a porcelain crucible, dried and finally heated (in the presence of borax) to 960 °C. The produced Ag regulus is treated with boiling H2O. Alternative Procedure In a consumed photographic bath, Zn pieces are placed for about one week with frequent agitation. An Ag sediment forms which is filtered off and subjected to a coupling procedure with Pb.2 This process involves heating of Ag and Pb in a stream of air until a melt of Ag and PbO has formed. The PbO takes up the impurities. Alternatively, the solution is brought to pH 6.9 – 7.2 by addition of sodium carbonate. Then CuSO4 and Al2(SO4)3 are added. The silver is precipitated together with the hydroxide. When the silver has not been introduced as AgCl, FeCl3 is added. The voluminous hydroxide is allowed to precipitate for 3 – 4 days and is then treated with H2SO4 of increasing concentration (96%). Hydroxides, gelatin and other components are dissolved and Ag is enriched to 20 – 50 %. The Ag is isolated and melted in a crucible (960 °C) with borax to form a regulus. References 1 Handbuch für das Eisenhüttenlaboratorium, Verlag Stahleisen, Düsseldorf, Vol. 1, p. 318 (1939). 2 H. Gubitsch, Anorg. Präparative Chemie, Wien, 1950, p. 454. 3 Eastman Kodak Co., C. S. Patent 2131045 (27.9.1938), Chem. Abstr. 32, 89724. Colloidal Silver Procedure To 200 mL of a 10% AgNO3 solution 200 mL of a 30% FeSO4 solution are added. Furthermore, 280 mL of a 40% sodium citrate solution and about 50 mL of a 10% NaOH solution are added. A colloidal Ag precipitate forms which is allowed to settle, washed 4 – 5 times with 10% NH4NO3 solution (decantation), and finally 2 times with 96% alcohol. The Ag is isolated by centrifugation and dried carefully on a water bath or in a desiccator. Alternative Procedures A 0.001 M AgNO3 solution (100 mL) is treated with a few drops of a freshly prepared tannin solution and one drop of 1% Na2CO3. Warming of the mixture affords a silver sol. A hot 0.001 M AgNO3 solution is reduced by dropwise addition of hydrazine hydrate.1 Ag-sol by electrospray method: Two rods of Ag (2 – 3 mm diameter) are taken as electrodes. At one end, about 2 cm of the rod is bent through 90°. The electrodes are mounted in a beaker filled with distilled water with their ends facing each other. If the two ends are shorted, a potential of 110 V and a current of 4 – 6 amps are measured (with a resistance connected in front of them). An arc is formed between the two electrodes in which clouds of colloidal Ag appear. The addition of a few drops of a 2% Na2CO3 solution is recommended.2 Properties A black grainy powder containing about 97% Ag is obtained. It is soluble in water with formation of a red-brown to black, finely divided...
