FSc Notes Part 2 Chemistry Important Chemicals Tin Plating, Pigments and Epsom Salt
Tin Plating
Definition
The art of coating a metal with tin is called Tin-Plating.
Those metals which are coated with tin are called tin-plated metals.
Purpose And Examples of Tin Plating
The purpose of tin plating is to protect metals from corrosion and food poisoning. Iron is often tin-plated to protect it from rusting. The common cooking oil containers are made of tin-plated iron. The household utensils of copper and brass are tarnished in moist air due to the formation of thin layer of oxides and carbonates of copper. These are poisonous, due to these problems, utensils are coated with tin.
Method of Tin Plating
Tin plating is carried out by the following methods.
1. Hot Dipping or Mechanical Method
In this method, clean iron or steel sheets are dipped in the bath of molten tin. A layer of tin accumulates on the iron sheet and it gets coated.
2. Electrolytic Method or Electro-Plating
This method is based on electrolytic process. An electrolytic cell is developed, which contains metals to be tin-plated as cathode and pure tin as anode. The electrolytic solution consists of salt of tin such as tin chloride or tin sulphate and an acid such as hydrochloric acid. On passing electric current through the electrolytic cell tin deposits on the metal sheet. Through this method a uniform layer of tin is coated on zinc.
3. Classical Method
In this method, the clean hot surface of a utensil is polished with tin metal with a rag. Copper and Brass utensils are tin-plated by this method. The utensils are heated and rubbed with ammonium chloride before they are tin plated. This is done to remove the oxide from the utensils
Pigments
Definition
Pigments are the substances which are used to give the proper colour to paints.
Types of Pigments
Lead forms various types of pigments. Some of them are given below.
- White lead Pigment
- Red lead Pigment
- Chrome Yellow Pigment
- Chrome Red Pigment
- Turner’s Yellow Pigment
1. White Lead Pigment
White lead Pigment is a basic lead carbonate.
Molecular Formula
[2PbCO3. Pb(OH)2] or [Pb3(OH)2.(CO3)2]
Colour
This lead pigment is white in colour.
Properties
The white colour gradually darken due to the formation of Pbs with Atmospheric H2S. It is also poisonous.
2. Red Lead Pigment (Sandhur)
Molecular Formula
Pb3O4 – Triplumbic Tetra Oxide
2PbO.PbO2 – Lead-Sesqui Oxide
Colour
It is used as red coloured pigment, which varies from orange red to brick red dur to particle size and impurities.
Properties
It is soluble in water but soluble in acids.
3. Chrome Yellow Pigment
Molecular Formula
PbCrO4 – Lead Chromate
It occurs in nature as crocoite.
Colour
It is used as yellow coloured pigment.
Properties
It is insoluble in water but soluble in nitric acid and caustic alkalis.
4. Chrome Red Pigment
Molecular Formula
PbCrO4.PbO – Basic Lead Chromate
Pb2CrO5 – Basic Lead Chromate
Colour
It is used as dark red pigment in paints.
5. Truner’s Yellow Pigment
Molecular Formula
PbCl2.4PbO
Other Pigments
Except above pigments, yellow lead monoxide (massicot) and red lead monoxide (litharge or Murda-sang) are also used in paints
Epsom Salt
Definition
The hepta hydrate from of magnesium sulphate is known as Epsom Salt.
Molecular Formula
The molecular formula of Epsom Salt is
MgSO4. 7H2O
Occurrence
Magnesium sulphate occurs in nature as Kleserite (MgSO4.H2O)
Preparation of Epsom Salt
Magnesium sulphate is prepared by the reaction of sulphuric acids on Magnesium metal, its oxide, hydroxide or carbonate.
Mg + H2SO4 —-> MgSO4 + H2
MgO + H2SO4 —-> MgSO4 + H2O
Mg(OH)2 + H2SO4 —-> MgSO4 + 2H2O
Mg(OH)2 + H2SO4 —-> MgSO4 + 2H2O
MgCO3 + H2SO4 —-> MgSO4 + H2O + CO2
Properties of Epsom Salt
- It is soluble in water
- Crystalline Epsom loses water on heating forming anhydrous magnesium sulphate.
Use of Epsom Salt
- Epsom salt is used as a mild purgative.
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