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Showing posts with label SURFACE CHEMISTRY. Show all posts
Showing posts with label SURFACE CHEMISTRY. Show all posts

Sunday, August 6, 2023

Find the Gold Number if 700 ml of standard gold solution is just prevented from coagulation by adding 0.35 g of starch before adding 1 ml of 10% NaCl solution?

We know that Gold number is the minimum amount of protective (lyophilic) colloid in milligrams which prevents the  coagulation or flocculation of 10 ml of standard hydro gold sol by the addition of 1 ml of 10% NaCl solution
The concept of gold number was given by Zsigmondy.

As par Questions 700 ml of gold solution requires (0.35gm ) or 350 mg of starch.
Hence the amount of lyophilic colloid in mg to protect 10 ml of gold solution is  starch =(350×10)÷ 700=5 mg 
"Gold number is 5"

Monday, June 22, 2020

What is water in oil emulsions ? explain with examples.

Water in oil emulsions: In this type of emulsions, water acts as dispersed phase and oil (organic solvent) acts as dispersion medium. Cold cream, butter etc, are examples of oil emulsions.

What are Oil in water emulsions ? explain with examples.

Oil in water emulsions: In this type of emulsions, oil acts as (organic solvent) dispersed phase and water acts as dispersion medium. Some examples of this type of emulsions are milk, vanishing cream, etc. In milk, liquid fat is dispersed in water.

Thursday, June 18, 2020

What is "EMULSIONS"

Emulsions are colloids in which both dispersed phase and dispersion medium are liquids. 
Emulsion can be broadly classified into two types.
(i) Oil in water emulsions: In this type of emulsions, oil acts as (organic solvent) dispersed phase and water acts as dispersion medium. Some examples of this type of emulsions are milk, vanishing cream, etc. In milk, liquid fat is dispersed in water.
(ii) Water in oil emulsions: In this type of emulsions, water acts as dispersed phase and oil (organic solvent) acts as dispersion medium. Cold cream, butter etc, are examples of oil emulsions.
Identification of Emulsion 
The following tests may be employed to distinguish between the two types of emulsions:
(i) Dye test: Some oil soluble dye is added to the emulsion. If the background becomes coloured, the emulsion is water – in – oil type and if the coloured droplets are seen, the emulsion is oil – in – water type.
(ii) Dilution test: If the emulsion can be diluted with water, this indicates that water is the dispersion medium and the emulsion is of oil – in – water type. In case the added water forms a separate layer, the emulsion is water – in – oil type.
Preparation of Emulsion:
The process of making an emulsion is known as emulsification. Emulsion may be obtained by vigorously mixing both the liquids. The dispersed drops at once come together and form separate layers. To stabilize an emulsion, the addition of a small quantity of a third substance known as emulsifying agent or emulsifier is essential. Soaps and detergents are most frequently used as 
emulsifiers.
The other common stabilizing agents are proteins, gum and agar-agar.

What is application of colloids ?


Colloids including emulsions find a number of uses in our daily life and industry. Some of the uses are given below:
(i) Rubber plating: Latex is a colloidal solution of negatively charged rubber particles. Rubber plated articles are prepared by depositing negative charged particles over the article to be rubber plated by making that article an anode in a rubber plating bath.
(ii) Medicines: Medicines in colloidal form are easily absorbed by the body tissues and hence are more effective.
(iii) Sewage disposal: Colloidal particles of dirt, mud etc. carry electric charge. Hence when sewage water is passed through the plates kept at a high potential, the colloidal particles are 
coagulated due to electrophoresis and the suspended matter gets removed.
(iv) Purification of water: The precipitation of colloidal impurities present in water can be done by adding certain electrolytes like alum etc. The negatively charged colloidal particles of 
impurities get neutralized by the Al3+ ions and settle down and pure water can be decanted off.
(v) Formation of delta: River water contains charged colloidal particles of clay, sand and many other materials. Sea water is a very big store – house of a variety of electrolytes dissolved in it. As soon as river water comes in contact with sea water, the electrolytes present in sea water coagulate the suspended colloidal particles which ultimately settle down at the point of contact and thus the level of the river bed rises. As a result, water adopts a different course and delta is formed in due course of time.
(vi) Smoke screen: In warfare, smoke screens are used which are colloidal dispersion of certain 
substance in the air.

What is "peptization" ?


Peptization may be defined as the process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of a small amount of electrolyte. The electrolyte used for this purpose is called peptizing agent. This method is applied, generally, to convert a freshly prepared precipitate into a colloidal sol. 
For example, when freshly precipitated Fe(OH)3 is shaken with aqueous solution of FeCl3(peptizing agent) it adsorbs Fe+3 ions and thereby breaks up into small – sized particles.

Why medicines are more effective in colloidal state?

A colloidal state has larger surface area of sol particles and this shows more effective adsorption. Thus medicines in colloidal state are more effectively absorbed and give better results.

What is Associated Colloids (Micelles) ?

There are some substances which at low concentrations behaves as normal, strong electrolytes but at higher concentrations exhibit colloidal  behaviour due to the formation of aggregated particles. The aggregated particles thus formed are called micelles. These are also known as associated colloids. 
The formation of micelles take place only above a particular temperature called kraft temperature (Tk) and above a particular concentration called critical micelle concentration (CMC). On dilution, these colloids revert back to individual ions. Surface active agents such as soaps and synthetic detergents belong to this class. For soaps, the CMC is -10-4
to 10-3 molperL. 
These colloids have both lyophobic and lyophilic parts. Micelles may contains as many as 100 molecules or more.


Acetyl chloride on reduction with H2 in presence of Pd as catalyst gives ethanol, whereas in presence of Pd-BaSO4, it gives ethanal.

Solutions are homogenous systems e.g. sugar solution. If sand is stirred in water, it slowly settles down and is called a suspension. Between the extremes of suspensions and solutions a large group of systems called colloidal dispersion or simply colloids exist.
A colloidal is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called dispersion medium. The essential difference between a solution and a colloidal is one of particle size. In a solution, the particles or ions are small molecules. In a colloid, the dispersed phase may consist a particles of a simple macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. They have a range of a diameter between 1 and 1000 nm.

Comment on the statement that “colloid is not a substance but a state of substance”.


Any substance (solid, liquid or gas) using special method can be brought into colloidal state. For example, NaCl in water forms true solution but in benzene forms colloidal solutions in alcohol but colloidal in water.

Friday, May 22, 2020

What is Gold number of protective colloidal solution?


Lyophilic sols are more stable than lyophobic sols.this is due to the fact that lyophilic colloids are extensively solvated, i.e. colloidal particles are covered by a sheath of the liquid in which they are dispersed.

Lyophilic colloids have a unique property of protecting lyophobic colloids. When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and, thus, protect the latter from electrolytes.

Lyophilic colloids used for this purpose are called protective colloids.

The lyophilic colloids differ in their protective power. The protective power is measured in terms of Gold Number and is defined as the number of milligrams of a lyophilic colloid that will just prevent the precipitation
of 10 ml of a gold sol on the addition of 1 ml of 10% sodium chloride solution.

Lyophilic sols are more stable than lyophobic sols. Why ?

Lyophilic sols are more stable than lyophobic sols.this is due to the fact that lyophilic colloids are extensively solvated, i.e. colloidal particles are covered by a sheath of the liquid in which they are dispersed.

Lyophilic colloids have a unique property of protecting lyophobic colloids. When a lyophilic sol is added to the lyophobic sol, the lyophilic particles form a layer around lyophobic particles and, thus, protect the latter from electrolytes.
Lyophilic colloids used for this purpose are called protective colloids.

Give some examples of common negatively charged colloidal solution?

The common negatively charged colloids are given below:

As2S3 sol,
Sb2S3 sol, 
CdS sol, 
Au sol, 
Cu sol, 
Ag sol 
And acid dyes like
congo red.

Give some examples of common positively charged colloidal solution?

The common positively charged colloids are given below:
Fe(OH)3 sol, 
Cr(OH)3 sol,
Al(OH)3 sol, 
Ca(OH)2 sol, 
And also dyes like methylene blue and haemoglobin.