Preparation of colloidal solution and its purification
Preparation of colloidal solution includes techniques to prepare both lyophilic and lyophobic colloids. Preparation of lyophobic colloids includes two types of methods which are condensation and dispersion methods. Condensation method are discussed under following headings: i) Reduction ii) Oxidation iii) Double decomposition iv) Hydrolysis v) Exchange of solvent The topics which constitute the dispersion methods are considered below: i) Mechanical dispersion ii) Electrodispersion iii) Peptization The preparation procedures of lyophilic colloids involves simply mixing the dispersed phase and dispersion medium under ordinary conditons and then the mixture is gently warmed and allowed to stand for some duration. Purification of colloidal solution is exremely imporatant for the removal of impurities and the most common ways for purification are given below: i) Dialysis ii) Electro-dialysis iii) Hot-dialysis iv) Ultra-filtration
Summary
Preparation of colloidal solution includes techniques to prepare both lyophilic and lyophobic colloids. Preparation of lyophobic colloids includes two types of methods which are condensation and dispersion methods. Condensation method are discussed under following headings: i) Reduction ii) Oxidation iii) Double decomposition iv) Hydrolysis v) Exchange of solvent The topics which constitute the dispersion methods are considered below: i) Mechanical dispersion ii) Electrodispersion iii) Peptization The preparation procedures of lyophilic colloids involves simply mixing the dispersed phase and dispersion medium under ordinary conditons and then the mixture is gently warmed and allowed to stand for some duration. Purification of colloidal solution is exremely imporatant for the removal of impurities and the most common ways for purification are given below: i) Dialysis ii) Electro-dialysis iii) Hot-dialysis iv) Ultra-filtration
Things to Remember
Some of the points to remember are given below:
- Substances like colloidal graphite ie a lubricant and printing inks can be prepared bt mechanical dispersion method.
- The process of Electro-dispersion is also known as Bredig's arc method
- Electro-dialysis is not meant for the removal of non-electrolytes like carbohydrates, organic substances. etc.
- The vessel in which the dilaysis is carried out is known as dialyzer.
- Ultra-filters with pores larger than that used in ordinary filters is applied in case of ultra-filtration process.
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Subjective Questions
Q1:
Define Preliminary/Statutory General Meeting.
Type: Short Difficulty: Easy
<p>A preliminary report is to be sent to all the shareholders certified by at least two directors. It shows the amount of preliminary expenses, names, address, of directors, auditors and managers, of the company.</p>
<p>ed by at least two directors and that report should be sent to all the shareholders. The report must be certified by the authority and also be sent to the Company Register. This report must contain following details:</p>
<ul>
<li>Total number of share allotted</li>
<li>Number of paid-up shares</li>
<li>Total amount received from shares</li>
<li>Report of income and expenditure till 35 days before meeting</li>
<li>Name and address of directors, managers, company secretary, accountants and auditors and their appointment and terms and condition of services.</li>
<li>Details of agreements which are required for meetings</li>
<li>Report of underwriting of shares</li>
<li>Share money due from directors</li>
<li>Other necessary information</li>
</ul>
Q2:
Define Annual General Meeting.
Type: Short Difficulty: Easy
<p>The matter to be presented by the chairman of the board in the annual general meeting are as follows:</p>
<ul>
<li>Auditing financial statement consisting balance sheet, income statements, profit and loss and annual report</li>
<li>Appointment and remuneration of directors and auditors</li>
<li>Report of directors and auditors</li>
<li>Declaration of rate of dividend</li>
<li>Other particulars presented by shareholders representing a minimum 5% of total votes</li>
<li>Other particulars</li>
</ul>
<p> </p>
Q3:
Explain about Preliminary and Annual General Meeting.
Type: Long Difficulty: Easy
<p> This is the first official meeting of shareholder after its incorporation. The main aim of this meeting is to give details of a company to all shareholders about the number of shares issued and to provide information about the prospectus to its members. According to the Act, it must be conducted within a year after receiving the certificate of commencement of business. Shareholders must be pre-noticed about the place, time, date and agenda in advance of 21 days of conducting the meeting.</p>
<p>A preliminary report is to be sent to all the shareholders certified by at least two directors. It shows the amount of preliminary expenses, names, address, of directors, auditors, and managers, of the company.</p>
<p>ed by at least two directors and that report should be sent to all the shareholders. The report must be certified by the authority and also be sent to the Company Register. This report must contain following details:</p>
<ul>
<li>Total number of share allotted</li>
<li>Number of paid-up shares</li>
<li>Total amount received from shares</li>
<li>Report of income and expenditure till 35 days before meeting</li>
<li>Name and address of directors, managers, company secretary, accountants and auditors and their appointment and terms and condition of services.</li>
<li>Details of agreements which are required for meetings</li>
<li>Report of underwriting of shares</li>
<li>Share money due from directors</li>
<li>Other necessary information<br /><br /><br /></li>
</ul>
<p><strong>Annual General Meeting</strong></p>
<p>Annual General Meeting is the meeting of a shareholder of the company which is held every year. The main objective of this meeting is to inform shareholder about the progress of the company, about the ongoing performance and future plans of the company. The annual general meeting should be held each year within six months from the expiry date. The shareholders must be pre-noticed about the place, time, date and agenda about the meeting in advance of 21 days of conducting a meeting. In the case of public company, the notice must be published in a popular national newspaper by inviting all the shareholder to attend the meeting.</p>
<p>The matter to be presented by the chairman of the board in the annual general meeting are as follows:</p>
<ul>
<li>Auditing financial statement consisting balance sheet, income statements, profit and loss and annual report</li>
<li>Appointment and remuneration of directors and auditors</li>
<li>Report of directors and auditors</li>
<li>Declaration of rate of dividend</li>
<li>Other particulars presented by shareholders representing a minimum 5% of total votes</li>
<li>Other particulars</li>
</ul>
Q4:
Explain the Special/Extraordinary General Meeting
Type: Long Difficulty: Easy
<p>All the general meetings other than preliminary and annual general meeting of the company are called special annual general meeting. If any important matter arise and it needs to be discussed sooner and it cannot wait until the annual general meeting then this meeting is held. It is also called special circumstances and is held when special and urgent decisions have to be made. The shareholder must be pre-noticed about the place, time, date and agenda about the meeting in advance of 15 days of conducting a meeting. According to the Company Act 2053, this meeting may be called by board of directors, shareholders, </p>
<p>The following matter is discussed in the special general meeting:</p>
<ul>
<li>Alteration in the Memorandum and the Articles of association</li>
<li>Change in the name or main objective of the company</li>
<li>Increase in authorized capital</li>
<li>Reduction of share capital</li>
<li>Issue of bonus shares</li>
<li>Conversion of a private limited company into a public limited company</li>
</ul>
<p>The special general meeting is conducted by the following authorities:</p>
<ol>
<li><strong>By board of directors:</strong></li>
</ol>
<p>In the case of emergency, if the board of director feels necessary to call special general meeting then it is held. The board of directors held this meeting in an urgent matter. However, it cannot be conducted before the preliminary general meeting.</p>
<ol start="2">
<li><strong>By auditors:</strong></li>
</ol>
<p>In auditing the account there can be need of shareholder's idea and so auditor calls this meeting with a reasonable cause. The auditors request the board of directors to call the meeting and if the board of director fails to call then the auditor can request to the office of Company Registrar for the same. In such situation, the office of Company Registrar calls the special general meeting.</p>
<ol start="3">
<li><strong>By shareholders:</strong></li>
</ol>
<p>The shareholders with 10% paid-up capital or at least 25% of total shareholders demand to call this meeting with reasonable cause then they can call the meeting. They request to the board of director and submit an application to call the meeting. If the board of director is unable to call this meeting then the shareholders can complain to Company Registrar with reasonable reasons. Then the Office of Company Registrar will call the special general meeting.</p>
<ol start="4">
<li><strong>By Office of Company Registrar:</strong></li>
</ol>
<p>If the office of Company Registrar finds any reasonable cause then it can call the special general meeting through inspecting the company.</p>
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Preparation of colloidal solution and its purification
We discuss the methods of preparation of colloidal solution under separate headings because methods employed for the preparation of lyophobic and lyophilic colloids are generally not similar.
Preparation of Lyophobic Colloids
The lyophobic colloids can be prepared using following methods :
a) Condensation methods
b) Dispersion methods
1. Condensation methods : These methods include conversion of group of ions or molecules into larger aggregates of insoluble particles of colloidal size. This is achieved my means of chemical reactions. Some of the chemical reactions that have been used are discussed below.
i) Reduction : Sols of metals are usually obtained by reduction of their salt solutions in water by using certain reducing agents. For example, gold sol is prepared by reduction of auric chloride. A few drops of 1% AuCl3 solution are added to tube containing distilled water. This solution is heated to boiling and 2-3 ml of very solution of formalin is added. This liquid converts into a beautiful purple colour due to the formation of colloidal particles of gold.
2AuCl3 + 3HCHO + 3H2O \(\longrightarrow\) 2Au + 6HCl + 3HCOOH
Similarly, colloidal silver can be prepared by the reduction of AgNO3 solution by tannin.
ii) Oxidation : Sols of non metals are usually obtained by oxidation. For example, when oxygen is bubbled through an aqueous solution of H2S, it is oxidized and colloidal sulphur is obtained.
2H2S + O2 \(\longrightarrow\) 2H2O + 2S
Likewise, selenium sol is obtained by the oxidation of hydrogen selenide.
2H2Se + O2 \(\longrightarrow\) 2H2O + 2Se
iii) Double decomposition : This method is generally used for the preparation of sols from insoluble salts. For example for the preparation of arsenoius sulphide sol, 1-2 grams of pure arsenoius oxide is dissolved in 500 ml of boiling distilled water. After prolonged boiling, the solution is cooled to room temperature and filtered if necessary. The resulting saturated solution is diluted with distilled water. H2S gas, carefully purified by slow bubbling is now passeed through this solution. An orange coloured colloidal solution of arsenious sulphide is obtained.
As2O3 + 3H2S \(\longrightarrow\) As2S3 + 3H2O
Excess of H2S is removed by passing a current of hydrogen through the sol.
iv) Hydrolysis : This method is generally used for the preparation of hydroxide sols. For example, for the preparation of ferric hydroxide sols, ferric hydroxide solution is poured into boiling distilled water. The hydolysis of ferric chloride occurs instaneously and a beautiful deep red sol of ferric hydoxide is formed.
FeCl3 + 3H2O \(\longrightarrow\) Fe(OH)3 + 3HCl
Red sol
v) Exchange of solvent : There are a number of substances whose colloidal solutions can be prepared by taking a solution of the substance in a suitable solvent and then pouring it into another solvent in which the substances is insoluble is insoluble. For example, colloidal solution of phosphorous can be prepared by pouring its alcoholic solution in water.
2. Dispersion methods : In these methods , bigger particles are disintegrated into particles of colloidal dimensions by suitable methods. The particles so obtained remain in the dispersed state for some time and have a tendency to reform bigger particles. The system, is however, stabilized by the addition of small amount of a suitable electrolyte. Some of the dispersion methods are given below:
i) Mechanical dispersion : The solid along with the liqiud dispersion medium is fed into a colloidal mill. The mill consists of two steel plates nearly touching each other and rotating in opposite directions with high speed. The solid partcles are crushed down to colloidal size and are then dispersed in the liquid to give the sol. Colloidal graphite, a lubricant and printing inks are made by this method.
ii) Electrodispersion ( Bredig's arc method) : This method is generally applied for preparing colloidal solutions of metals, ie Ag, Au, Cu, Pt, etc. Two rods of metals to be dispersed are kept immersed in cold water containing some KOH and direct electric arc is struck between them. The vapours of the metal condense to colloidal particles. The traces of alkali act as stabilizing agent.
iii) Peptization : The conversion of a freshly formed precipitate into a sol by the addition of a small of a siutable electrolyte is called peptization. For example, if freshly prepared Fe(OH)3 precipitate is treated with a small quantity of FeCl3solution it immediately forms a dark reddish brown colloidal solution of Fe(OH)3. Similarly, a colloidal solution of Al(OH)3 is obtained when freshly prepared Al(OH)3 is treated with a small quantity of dil HCl, the amount of acid added is insufficient to convert the hydroxide completely into chloride. Substances like FeCl3 and HCl are known as peptizing agents. Many freshly precipitated metallic sulphides suspended in water can be peptidized by the passage of H2S to form colloidal solutions.
Preparation of Lyophilic colloids
The lyophilic colloids have robust type affinity between particles of dispersed phase and dispersion medium. therefore, these colloidal solutions are readily prepared by simply mixing the dispersed phase and dispersion medium under ordinary conditions. Some substances of complex composition and large molecules such as gelatin, gum, starch, egg, albumin, etc form colloidal solutions if merely left in contact with water for a sufficient length of time. To prepare their colloidal solutions, 1-2 gm of the substance are added to about 100 ml of distilled water. The mixture is stirred, gently warmed and then allowed to stand for some time.
Purification of Colloidal Solutions
Impurities have to be removed in order to obtain the pure collidal solution and some of the prominent purification techniques are described below:
1) Dialysis : The purification of colloidal solution by this method is based on the inability of the sol particles to pass through an animal membrane or a parchment paper which allows only the molecules or the ions to pass an animal membrane or a parchment paper which allows only the molecules of the ion to pass through. The vessel in which the dialysis process is carried out is called dialyzer. A dialyzer consists of a special type vessel opened at both the ends. To, the lower end a membrane is stretched. This membrane allows only the solvent and other molecules to pass through it but is impermeable to the colloidal particles. The dialyzer is then suspended in a larger vessel containing pure distilled vessel and removing it by means of a syphon. Dilaysis is carried out and continued till no further contamination of the washing occurs. The impurities diffuse in water through the membrane whereas the colloids remain in the bag.
Hence, the process of separating impurities ( ions or molecules) from a sol by diffusion method with the help of a membrane is known as dialysis.
2) Electro- diaysis : The removal of ionic impurities from a sol can be greatly facilated under the influence of an electric field. In this case, the outer vessel is fitted with two electrodes. The application of the electric field allows the passage of the ions towards oppositely charged electrodes. Electro- dialysis is not meant for the removal of non- electrolytes like carbohydrates, organic substances, etc.
3) Hot dailysis : In hot dailysis, a temperature of 65-900C increases the rate of dialysis by about three times. Heating is done by placing an electric unit in the dialyzer. All hydrophobic sols cannot withstand the high temperature and the membrane may give off undesired colloidal impurities.
4) Ultra-filtration : In this method, ordinary filter paper have pores larger than 2000A0 ( 10-5 cm), so that the colloidal particles having smaller size can pass through them. But if the pores are made smaller, then the colloidal particles may be retained in the filter paper. This method is also applied for concentration of sol besides its purification. In ultra-filtration, special type of filter papers called ultra-filters are used, which are effective thoughout the range of colloidal and even molecular sizes. In this method, the colloidal particles are removed not only from the liqiud dispersion medium but also from the substance in the true solution. Pressure is applied to the solution, so that the passage to the liquid through the very small pores of ultra-filters may be accelerated.
Reference :
Glasstone, Samuel. Textbook of Physical Chemistry. New Delhi: Macmillan India Ltd, 1996
Maron, S.H. and Prutton C.F.,. Principles of Physical Chemistry. Oxford and IBH publication Company, 1992.
Moore, Walter J.,. Physical Chemistry. New Delhi: Orient Langman Ltd, 1999.
Lesson
Colloidal State
Subject
Chemistry
Grade
Bachelor of Science
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