SILVER

Silver is the most conducting metal with atomic number 47 and represented with the symbol ‘Ag’ in the periodic table.

Silver is abundant in mineral-rich soils. It is available in the mixed form, generally in crystal form.

Plants absorb silver and measured levels in the soil come around 0.03 – 0.5 ppm.

Occurrence of silver

It occurs in both free and combined state. In combined state it occurs as:

     1.       Argentite or silver glance (Ag₂S)

      2.       Horn silver (AgCl)

      3.       Ruby silver (Ag₂SbS₃)

      4.       Argentiferrous galena (PbS) which contains 0.01 to 0.1 % Ag.

Physical properties of Silver

Silver ( Ag ) is a white, soft, lustrous, very ductile and malleable metal.

It is a very good conductor of electricity and heat.

It has the highest electrical conductivity of all metals, but the high cost of it has restricted us from using it in all electrical devices.

                                     Uses of Silver

The principle use of this metal is a precious metal, including jewelry and decorative items.

The other applications include:

·         Currency – still in some countries silver coins are used as currency.

·         Jewelry and silverware.

·         It is used in the manufacturing of solar panels.

·         Air conditioning – It is used in the manufacturing of typical air conditioners.

·         Water purification – It is used in water purifiers to prevent the growth of algae and bacteria in filters.

·         Photography and electronic devices.

·         Used as an antibiotic coating on medical devices.

·         Thermal or infrared coatings use silver as it reflects some wavelengths better than aluminum.

EXTRACTION OF SILVER

 The two important methods of extraction of silver are:

                  1.       Cyanide process

2.       Parke’s Process

Cyanide process

Cyanide process is also called as Macarthur-forest Process. It is the process of extracting gold or silver from the ores by dissolving in a dilute solution of potassium cyanide or sodium cyanide. This process was introduced in the year 1887 by the Scottish chemists naming Robert W. Forrest, John S. MacArthur, and William Forrest.

This method involves three steps-

 a.       Concentration:

In the first step, the finely ground ore is contacted with the solution containing the cyanide, the second step involves the separation of solids from the clear solution and the third step involves the recovery of precious metals from solution by precipitating with the zinc dust.

Argentite being sulphide ore and is concentrated by froth flotation process. The pulverized ore is kept in large tank containing water and pine oil. The mixture is disturbed by passing compressed air where ore forms froth with pine oil & comes to the surface while impurities are left in water.

 b. Treatment with sodium cyanide:

The concentrated are is treated with 0.4% to 0.7% aqueous solution of sodium cyanide and the current of air pass through it. The argentite ore dissolves in sodium cyanide forming sodium argentocyanide.

 Ag₂S + 4NaCN                  →                2Na [Ag(CN)₂] + Na₂S

The reaction is reversible so is passed to oxidize Na2S to Na2SO4 such that the equilibrium shifts towards product.

Na₂S + O₂               →                Na₂SO₄

The solution is filtered and the filtrate containing sodium argento cyanide is used to recover silver metal.

c. Precipitation of Silver:

 The solution obtained is treated with Zn scrap where Zn displaces silver from its complex.

 Zn + 2Na [Ag (CN)₂]             →             Na₂[Zn (CN)₄] + 2Ag

The ppt is collected, washed and fused to get compact mass of silver.

d. Refining:

 The impure silver is purified by electrolytic method. A block of impure metal is anode while a thin strip of pure silver is cathode. A mixture of AgNO3 solution is electrolyte. On passing current impure silver dissolves and equivalent amount of pure silver is deposited at cathode.

AgNO₃(aq)          →                    Ag+ + NO^–3

At cathode: Ag⁺ + e               →             Ag

At anode: Ag                     →                 Ag⁺ +e

Purity of Silver

The purity of silver is expressed in terms of fitness. Fitness of silver is mass of pure silver in grams present in every 1000 gram of silver sample.

For eg. The fitness of silver in 900 means 900 gm of pure silver is present in 1000 gram of sample of silver.

In case of gold, purity is expressed in both fitness as well as carat. A 100% pure gold simple has purity of 24 carat. For eg. 18 carat gold is (18/24 × 100%) = 75% pure and has fitness 750.

Properties

                    ·         It is a white lustrous metal having mpt 9600c and bpt 22120c.

·         It is specific gravity 10.5 and is highly malleable and ductile.

·         Silver is not affected by water, alkali and non-oxidizing acids.

Action of HNO3

Silver dissolves in both dilute and conc. HNO3 having silver nitrate.

3Ag + 4HNO₃                  →       3AgNO₃ + NO + 2H₂O

Ag + Conc.2HNO₃         →        AgNO₃ + NO₂ + H₂O

Silver is readily attacked by sulphur and H₂S to form a black stain of silver sulphide. (Ag₂S)

SILVER NITRATE (AgNO₃)

Silver nitrate is a chemical compound with the formula AgNO3. It consists of an ionic bond between the silver cation (Ag+) and the nitrate anion (NO3–). Due to the ionic nature of this compound, it readily dissolves in water and dissociates into its constituent ions.

 Silver nitrate is a precursor to many compounds of silver, including the silver compounds used in photography. When compared to silver halides, which are used in photography due to their sensitivity to light, AgNO₃ is quite stable when exposed to light.

Properties of Silver Nitrate

Some important physical and chemical properties of silver nitrate are listed in this subsection.

 Physical Properties

·         The molar mass of silver nitrate is 169.872 grams per mole.

·         AgNO3 as a colorless appearance in its solid state and is odorless.

·         In its solid state, it has a density of 4.35 grams per cubic centimeter. Its density in the liquid state at a temperature of 210oC corresponds to 3.97 g/cm3.

·         The melting and boiling points of silver nitrate are 482.8 K and 713 K respectively. However, this compound tends to decompose at temperatures approaching its boiling point.

·         Silver nitrate, like most ionic compounds, dissolves readily in water. Its solubility in water corresponds to 122g/100mL at 0oC and 256g/100mL at a temperature of 25o

·         The crystal structure of AgNO₃ is orthorhombic.

 Chemical Properties

·         The hazards of AgNO3 include its toxic and corrosive nature.

·         The reaction between silver nitrate and ethanol is explosive.

·         The silver present in this compound is displaced by copper, which forms copper nitrate. The chemical equation for this reaction is given by :

·         2AgNO₃ + Cu → Cu(NO₃)₂ + 2Ag

·         When heated to 440oC, this compound completely decomposes to give oxygen, nitrogen dioxide, and silver.

·         It can be noted that even though metal nitrates generally decompose to yield metal oxides, the decomposition reaction of silver nitrate gives rise to elemental silver because silver oxide decomposes at an even lower temperature than AgNO3.

         Uses of Silver Nitrate

·         Silver nitrate has a wide range of applications in many fields such as biology, chemical synthesis, and medicine. Some of these uses of AgNO3 are listed below.

Silver chloride

 It occurs in nature as horn silver as ore.

 Preparation

AgCl is obtained by adding any soluble chloride to silver nitrate solution.

AgNO₃ + KCl            →             AgCl (White ppt)   + KNO₃

The ppt is washed, collected and then dried.

 Properties

                 ·         It is white amorphous solid insoluble in water.

·         AgCl is light sensitive compound and turns violet and finally to black on exposure to light.

 Action of NH3 solution

AgCl dissolves in NH₃ solution by forming a soluble diamine complex.

AgCl + NH₄OH             →         [Ag (NH₃)₂] Cl + 2H₂O

 Action of KCN solution:

AgCl dissolves in KCN solution forming a soluble potassium argento cyanide complex.

AgCl + KCN                →           K [Ag (CN)₂]  +   KCl

                                  (  Potassium argento cyanide)

Uses of AgCl

            ·         The most effective form of water- activated battery uses magnesium as the anode and silver chloride as the positive electrode.

·         Used in electroplating and polishing mirrors and in making alloys.

·         Used as an antidote that reacts with the poison to produce a harmless chemical compound.

·         Used in medicines and silver salts are used in photographic films.