In the Laboratory
Metallurgy in the Laboratory: Preparation of Pure Antimony Brooke L. O’Klatner and Daniel Rabinovich* Department of Chemistry, The University of North Carolina at Charlotte, Charlotte, NC 28223; *
[email protected] In its pure form, antimony is a grayish, crystalline, hard, and brittle metalloid, often regarded as a metal despite its low electrical conductivity. Although it is relatively rare in Earth’s crust (only 0.2 ppm), there are more than a hundred minerals known to contain antimony. The main source of the element by far is the mineral stibnite (antimony trisulfide), known since the times of the early Egyptians, who used it as a pigment to darken women’s eyebrows (1). It was also used by the Greeks and the Romans, who called it stibium, the name from which the modern symbol of the element (Sb) was derived. Later, in the Middle Ages, antimony compounds played a central role in the alchemists’ quest to transmutate base metals into gold (2). Antimony is prepared today on a large industrial scale; some 25,000 metric tons were produced in 1998 in the USA alone (3). Several methods exist for the isolation of antimony, depending on the nature of the mineral ore available (4, 5). For example, ores containing 25–40% antimony in the form of mixed oxides and sulfides are smelted in a blast furnace. Remaining impurities such as lead, arsenic, sulfur, iron, and copper are reduced or eliminated by treatment with strong alkalis, and extremely pure antimony can be obtained if necessary by electrolysis or zone refining. The pure element has limited applications, but its alloys with tin and lead are used in storage batteries, bearings, and type metal; and those with aluminum, gallium, or indium, in semiconductor devices. On a small laboratory scale, pure antimony can be easily prepared by reducing antimony trioxide with potassium cyanide at high temperature (eq 1). Sb2O3
+
3 KCN
high temp
2 Sb
+
3 KOCN
(1)
The experiment described herein is a modification of the original procedure involving the reduction of antimony pentoxide with potassium cyanide, as reported by Schenk (6 ). It is noteworthy that very few metals can be readily prepared in the laboratory. The production of iron in the thermite reaction (7, 8), arguably the best known pyrometallurgical demonstration, can hardly be considered an experiment
Figure 1. Antimony pellet or “regulus”.
suitable for individual students in spite of its visual flare. The experiment that follows is a simple, brief, and inexpensive procedure for the preparation of small samples of pure antimony in almost quantitative yield. Thus, this experiment serves to illustrate the basic principles of metallurgy and also introduces students to an element whose chemistry is seldom discussed in general or inorganic chemistry classes. The experiment should be particularly interesting for undergraduate students taking an introductory inorganic chemistry laboratory course, or perhaps as a component of a more advanced class in synthetic methods or inorganic chemistry laboratory. Experimental Method
Equipment and Reagents In addition to standard laboratory equipment (see below), this experiment requires the use of a Meker burner, which is a special burner with a grid top and a short, uniform, hightemperature flame. Meker burners are available from a number of vendors—for example Fisher Scientific (catalog #03-902). Commercially available reagents, antimony trioxide (Sb2O3) and potassium cyanide (KCN), were used as received. p CAUTION: Potassium cyanide is an extremely toxic substance if ingested, inhaled, or absorbed through the skin (9), and should be handled with due care (use of gloves, goggles, etc.). The entire experimental procedure should be carried out in a well-ventilated hood and all waste should be kept away from acids (to avoid generation of gaseous HCN) and disposed of properly. There are several straightforward procedures available for the destruction and disposal of inorganic cyanide wastes (10–12).
Procedure A clean 30-mL porcelain crucible is loaded with Sb2O3 (5.0 g) and KCN (5.0 g), and the white solids are thoroughly mixed with a spatula. The crucible, placed on a wire triangle supported by a metal ring attached to a support stand, is heated with a Meker burner. Within 10 minutes, if strong heating is maintained, a brick-red melt with a shiny globule of molten antimony at the bottom is formed. The crucible is allowed to cool to room temperature and the reaction mixture solidifies. The shiny antimony pellet or “regulus” (Fig. 1) is liberated from the white salts and other impurities (including KOCN and excess KCN) by washing several times with small portions of water, heating gently if necessary. Weights of the clean and dry antimony pellets are typically in the range of 3.6–4.1 g (86–98% yield). Concluding Remarks The antimony pellets obtained are usually used for further studies. For example, the reactivity of antimony towards concentrated hydrochloric, nitric, and sulfuric acids, with or
JChemEd.chem.wisc.edu • Vol. 77 No. 2 February 2000 • Journal of Chemical Education
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In the Laboratory
without external heating, is investigated using small fragments splintered away from the pellet. A pulverized pellet could also be dissolved in aqua regia or in a mixture of nitric and tartaric acids to perform one or more of several qualitative tests available for the detection of Sb(III) or Sb(V) in solution (13). Alternatively, antimony powder could be used to prepare antimony(III) iodide, SbI3, starting from the elements (14). Literature Cited 1. Stwertka, A. A Guide to the Elements; Oxford University Press: New York, 1996; p 135. 2. Dufrenoy, M. L.; Dufrenoy, J. J. Chem. Educ. 1950, 27, 595– 597. 3. United States Geological Survey. Minerals Information: Antimony Statistics and Information; http://minerals.er.usgs.gov/minerals/pubs/ commodity/antimony/ (accessed Oct 1999). 4. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. I., Ed.; Wiley: New York, 1992; Vol. 3, pp 367–381.
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5. Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.; Butterworth-Heinemann: Oxford, 1997; Chapter 13, pp 548–550. 6. Schenk, P. W. In Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic: New York, 1963; p 606. 7. Bozzelli, J. W.; Barat, R. B. J. Chem. Educ. 1979, 56, 675–676. 8. Shakhashiri, B. Z. Chemical Demonstrations, A Handbook for Teachers of Chemistry; The University of Wisconsin Press: Madison, WI, 1983; Vol. 1, pp 85–89. 9. Labianca, D. A. J. Chem. Educ. 1979, 56, 788–791. 10. Lunn, G.; Sansone, E. B. Destruction of Hazardous Chemicals in the Laboratory; Wiley: New York, 1990; pp 77–82. 11. NRC Committee on Hazardous Substances in the Laboratory. Prudent Practices for Disposal of Chemicals from Laboratories; National Academy Press: Washington, DC, 1983; pp 86–87. 12. Armour, M.-A. J. Chem. Educ. 1988, 65, A64–A68. 13. Vogel’s Qualitative Inorganic Analysis, 6th ed.; Svehla, G., Ed.; Longman Scientific and Technical: Essex, UK, 1987; pp 91–96. 14. Roesky, H. W.; Möckel, K. Chemical Curiosities; VCH: New York, 1996; pp 60–61.
Journal of Chemical Education • Vol. 77 No. 2 February 2000 • JChemEd.chem.wisc.edu
