The technology of obtaining gold, silver and platinum at home.
Gold is obtained by amalgamation or leaching. When amalgated, gold dissolves in mercury (Hg), forming an amalgam, which becomes hard when the Au content exceeds 15%. From the obtained amalgam, mercury is distilled in condensing devices, leaving as concretions Au and Ag (silver). Purification of Au from silver is discussed below.
For leaching, a solution of sodium cyanide or potassium cyanide - NaCN or KCN (concentrated solution in distilled water) is used in which Au dissolves in the presence of oxygen (by blowing with air) to form complex anions [Au (CN) 2] - dicycous-potassium hydroxide: 4Au + 8CN- + O2 + 2H2O = 4 [Au (CN) 2] - + 4OH-.
From the obtained solution, gold is isolated with metallic zinc (sawdust Zn) into a colloidal precipitate: 2 [Au (CN) 2] - + Zn = [Zn (CN) 4] 2- + 2Au. The precipitated gold is treated to separate zinc from it with dilute sulfuric acid; washed and dried.
Further purification of Au from impurities (mainly from silver) is carried out by treatment with hot concentrated sulfuric acid H2SO4 or electrolysis is needed. Au also readily dissolves in "royal vodka" (a mixture of hydrochloric HCl and nitric HNO3 acids 1: 4), forming a complex gold chloride H [AuCl4], which crystallizes upon evaporation in the form of light yellow needles H [AuCl4] • 4H2O.
It is also easy for Au to dissolve in "chlorine water" and in aerated, i.e. air purged, solutions of alkali metal cyanides.
All Au compounds are easily decomposed upon heating with the release of metallic Au. Because of its softness, Au is used in alloys, usually with silver or copper. These alloys are also used for electrical contacts. Working with these reagents requires BIG CAUTION, as they are all deadly poisonous. The presence of Au in radio components is determined by the passport details and by the characteristic color of the coatings.
Silver is obtained by amalgamation or leaching. In leaching, silver dissolves under the action of a concentrated solution of potassium cyanide KCN or NaCN in distilled water. From the obtained complex cyanide compound K [Ag (CN) 2], Na [Ag (CN) 2], metallic silver precipitates by passing through the electrocurrent solution [Ag (CN) 2] - Ag + 2CN-), a dense cermet crystal layer on cathode.
Salt and dilute sulfuric acid on Ag does not work. Silver is highly soluble in nitric acid HNO3, therefore nitric acid is used for the separation of Ag from Au gold, which does not dissolve in HNO3: Ag + 2HNO3 = AgNO3 + NO2 + H2O.
All compounds of Ag under the action of reducing agents (for example, formalin, hydrosine) are easily reduced with the liberation of metallic Ag. The process of amalgamation is analogous to gold Au. A large amount of Ag is used for electrical contacts and in silver-zinc batteries.
Allocation of platinum is similar to gold. When platinum is dissolved in the "royal vodka", hexo-chloroplatinic acid H2 [PtCl6] is obtained, which upon evaporation of the solution is released as red-brown crystals of the composition H2 [PtCl6] • 6H2O, which decompose on further heating with the release of Pt.