42. Explain cyanide electrolyte solution forelectro deposition of silver.

When exposed to solutions of potassium or sodium cyanide, into simple salts of silver, a white curdy precipitate of silver cyanide, which easily dissolves in excess cyanide salts to form a complex. These reactions can be expressed by the following equations:

AgNO_s+ KCN= AgCN+ KN0₃;

AgCN+KCN=KAg (CN)₂orAgCl+2KCN = KAg (CN)₂ + KC1.

The resulting complex salt KAg (CN)2 is the foundation of cyanide silver plating electrolyte. In the manufacture of pure salt electrolyte AgN03 sufficiently mixed in equivalent amounts AgNO₃ solutions and KCN, and to introduce an additional electrolyte obtained potassium nitrate in an amount of 100-150 g / l. When dissolving metallic silver or silver alloys in a nitric acid AgCl precipitation operation is inevitable. It should only take into account that the freshly precipitated salt dissolved much faster and more completely than cooked in advance. Dissolution of silver potassium cyanide salts occur in accordance with the following equations:AgN0₃+ 2KCN = KAg (CN), + KN0₃

170 + 2-65 = 199 + 101

или

AgCl + 2KCN = KAg (CN), + KC1

143,5 + 2-65 - 199 + 74,5.

From the equations it follows that for every 170 grams AgN03 or 143.5 g AgCl theoretically necessary for 130 grams of potassium cyanide. Primary coating of silver produced in a special electrolyte. electrolyte composition and mode of operation are as follows: Cyanic complex silver salt (based nametallicheskoe Silver) --- 0.8-1.5 Cyanic complex salt of copper (based on metallic copper) -6.0-7.5 Potassium cyanide KCN (free) -------------50-60 Operating temperature in ° C -------- 15-25 DK Cathodic current density in A / dm? ----------- 0.1-0.2 Aging in 5-10 minutes ----------------------------------- Further growth of the silver layer to a predetermined thickness to produce an electrolyte with high concentrations of silver salts and a relatively low content of potassium cyanide. For this purpose, the following composition (in g / l) and electrolyte mode may be applied: Cyanic silver salt (in terms of metal) -------- 30-40 Potassium cyanide KCN ------------------- --- 35-45 Operating temperature in ° C --------------------- 12-25 Dc cathodic current density in A / dm2, 0.1-0.2 ----

The current efficiency in%% ------------------- 99-100 Deposition of lead with silver anodes.

43. Explain cyanide electrolyte solution for electro deposition of gold.

In the absence of finished gold salts they are prepared directly in the shop in two ways: the anode and the chemical dissolution of metallic gold. The first method is suitable only for the preparation of cyanide electrolytes and is technologically simpler and more economical than chemical dissolution. When using the method of anodic dissolution working gilding bath filled with electrolyte composed of potassium cyanide solution in distilled water to a concentration of 20-30 g / l, and heated to a temperature of 70 ° C. As used anodes anodes of gold dissolution with the greatest possible area. The second method for preparing gold salts suitable for preparation of both cyanide and ferrocyanide electrolytes and is chemically dissolving gold in aqua regia. Cyanide electrolytes are characterized by high throwing power, the fine structure of the precipitated metal and satisfactory current efficiency. Among the various cyanide electrolyte composition (in g\l) And the mode of operation of one of the most common: Complex salt of KAu (CN) 2 (in terms of metallic gold) --- 3-5 Potassium cyanide KCN (general) --------------------- 15-25 Potassium cyanide KCN (free) ----------- 3-6 Operating temperature in ° C 60-70 ------------------------------ Cathodic density in DK and / dm1 -------------------------- 0.2-0.3 Current efficiency% to 60-80% --------------------------------------- sometimes used more concentrated electrolytes to produce a large thickness. It is one of the compositions (in g / l) of the electrolyte and the operating mode: Cyanic gold complex salt (in terms of metallic gold) 15-25 Potassium cyanide KCN (free) ----------- 8-10 Potash K2C03 ------------------------------------------- To 100 Potassium hydroxide KOH ---------------- 1 Cathodic current density DK --------------- 2-4 A / dm2 The anode current density Da ------------------ 1 The electrolyte temperature in the ---------------55-60 ° C Anodes - gold. In order to avoid dissolution of the metal products and clogging electrolyte zavesku products produced under the current.