Chapter 2

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    Physical Properties of Gold   Gold is a soft yellow metal, with the highest ductility and malleability of any metal. Gold crystallizes in the cubic system, although crystals of gold are very rare (it is usually found as irregular plates or grains). Gold has high thermal and electrical conductivities. The only natural isotope of gold  is ' 7 Au; however, 19 isotopes  —  ranging from I 8 5Au to 2 0 Au  —  have beeti   produced artificially. Those isotopes are radioactive, with half-lives rang- ing from a few seconds to 199 days (see Table 2-1). Pure gold and many gold alloys are nonmagnetic. An alloy of gold and manganese is somewhat magnetic, and alloys of gold with iron, nickel, or cobalt are ferromagnetic. The equilibria of numerous binary gold alloys are described by Hansen and Anderko (1958). Except for white golds (Au-Ag), the carat golds, used mainly in jewelry, are alloys of gold, silver, and copper. The carat is used to express the proportion of gold contained: 24 carats are pure gold, 18 carats are 75% gold, and so on. Gold forms alloys with a number of metals (see Table 2-2). Mercury wets gold particles, forming amalgams, and it is used in gold extraction operations to selectively remove gold from ground ores. Gold has a very low solubility(0.13%) in mercury. Mercury forms a solid solution with gold up to about 169c Hg. Larger contents of mercury form intermetallic com-  pounds like Au3Hg and Au2Hg. Molten lead is a good solvent for gold, and is used as such in fire assay and in some secondary smelting operations. 11   CHAPTER 2 Physical and Chemical Properties of Gold  12 PHYSICAL AND CHEMICAL PROPERTIES OF GOLD 193 17.5 hr 194 39.5 hr 195m 31.0 sec 195   183 day 196m 9.7 hr 196   6.2 day 197m 7.5 sec 197   stable 198 2.70 day 199 3.15 day 200 48.4 min 201   26 min 202   30 sec 203   55 sec 204 4.05 sec k m = metastable.    b ‹x   = alpha emission, EC = electron capture, J3 + = position emission, g = beta particle emission, = gamma radiation. EC,j   EC,p+,y   M+.v EC,y   ]   EC, d, j   Reprinted by permission. R.J. Puddephatt, The Chemistry of Gold. Elsevier Science Pub- lishers, Physical Science and Engineering Division (New York), 1978, p. 8. (Originally pub- lished in L. Myerscough, Gold Bull., (6): 62, 1973.)   Chemical Properties of Gold   Gold is the most inert, or the noblest, of the metallic elements. It exhibits great stability and resistance to corrosion. Simple mineral acids, with the exception of selenic acid, do not dissolve gold. Hydrochloric acid in the  presence of oxidants (such as nitric acid, oxygen, cupric or ferric ions, and manganese dioxide) dissolves gold. The combination of hydrochloric and nitric acids, aqua regia, vigorously attacks gold.   An + 4HCl + HNOL --+ H[AuCl4] + 2H2 + NO TABLE 2-1. The isotopes of gold. Wass No.ã  half life  Mode of'decal    177 1.35 sec 178 2.65 sec 179 7.25 sec   181 11.55 sec ‹x, EC  183 45.5 sec 185 4.3 min 186 12 min 187 8 min   n, EC  188 8 min  EC, 9 189 29.7 min EC, 9  189m 4.7 min EC, q 190 39 min EC, 9 191 3.2 hr EC, j 192 5.0 hr EC, p + , y 193m 3.9 sec 7  
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