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ELECTRICAL MEASURING INSTRUMENTS For the detection or measurement of electric current, potential difference, and resistance certain instruments have been devised viz. The galvanometer for the detection of small currents measurements of small currents of the order of micro amperes or mili amperes the voltmeter or potentiometer for the measurement of potential difference (and voltage) between two points of a circuit are the EMF of a source the ammeter for the measurement of large currents the wheatstone bridge the meter Bridge the post office box and the ohmmeter for the measurement of resistance. THE MOVING COIL GALVANOMETER The moving coil galvanometer is a basic electrical instrument it is used for the detection (and Measurement) of small currents. Its underlying principle is the fact that when a current flows in a rectangular coil placed in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional t...
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LEAD PIGMENT

Lead forms various types of pigments which are used to give the proper color to paints etc.Some of them are described below:

  1. White Lead pigment
  2. Red Lead Pigment
  3. Chrome Yellow Pigment
  4. Chrome Red Pigment
  5. Turner's Yellow Pigment

(i) White Lead Pigment :

It is a basic lead carbonate and has approximately composition as [ 2PbCO3.Pb(OH)2] or [Pb3.(CO3)2]

Preparation :

Chamber Process or Duch Process :

When a mixture of the vapours of acetic acid and steam is passed into the chambers containing lead sheets (buckets) about 2-3 days,until the corrosion of lead is complete.By passing the mixture of CO2 and vapours of acetic acid through this corroded lead for about 3-4 weeks,the White Lead is formed on the surface of lead sheets.This product is then scratched,dried and packed.

2Pb +2CH3COOH + 2H20 ---------> [Pb(OH)2.Pb(CH3COO)2] + 2H2
3[Pb(OH).Pb(CH3COO)2] + 2H2O + 2CO2 -------------> 2[2PbCO3.Pb(OH)2] + 6CH3COOH

A better "White Lead" is obtained by passing CO2 into the suspension of lead oxide in water containing little amount of lead acetate.

2PbO2 + 2CO2 + 2H2O + Pb(CH3COO)2 ----------> [2PbCO3.Pb(OH)2] + 2CH3COOH + O2

It is used as white pigment for paints, but it gradually darkens due to the formation of PbS with atmospheric H2S.It is also poisonous hence in recent years, white-lead has been replaced by titanium-dioxide.

(ii) Red Lead Pigment :

It is also called triplumbic tetra oxide or lead sesqui and its composition is Pb3O4 or 2PbO.PbO2

Preparation :

It is prepared by heating lead monoxide in a revolving with excess of air at about 450 C

6PbO + O2 -----------> 2Pb3O4

It is used a red colored pigment.Actually the color of this pigment varies from orange red to brick red due to particle size and impurities,It is insoluble in water but soluble in acids.

(iii) Chrome Yellow Pigment :


It is lead chromate with composition.It occur in nature as crocoite.

Preparation :

It is prepared by adding the solution of potassium chromate to the solution of lead nitrate,then fine yellow precipitates of lead chromate (chrome yellow) are obtained.

Pb(NO3)2 + K2CrO4 ------------> PbCrO4 + 2KNO2

It is also prepared by the action of K2CrO4 on lead acetate.

Pb(CH3COO) + K2CrO4 ------------> PbCrO4 + 2CH3COOK

It is yellow colored pigment, insoluble in water but soluble in nitric acid and caustic alkalis.

(iv)| Chrome Red Pigment :

It is basic red chromate and its composition is PbCrO4.PbO or PbCrO5.

Preparation :

It is prepared by digesting lead chromate with NaOH.

2PbCrO4 + 2NaOH --------> Na2CrO4 + Pb2CrO5 + H2O

It is dark red pigment and is used in paints.

(v) Turner's Yellow Pigment :

Its Composition is PbCl2.4PbO

Preparation :

It is prepared by boiling the solution of sodium chloride with litharge (PbO) and then heating to solid product.

5PbO + H2O +2NaCl -------------> 2NaOH + PbCl2.4PbO

It is also used as lead pigment in paints.

Except above pigments,yellow lead monoxide (massicot) and red lead monoxide also is paints.








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