Archive for the ‘Solutions and Colloidal Systems’ Category
Properties of Colloidal Systems
Some of the important properties of colloids are due to the fact that the total surface area of the micelles is very large. If a cube having each surface 1 cm is cut into cubes 0.001 µ on an edge (the approximate size of the smallest colloidal particle), the total exposed surface of the cubes will be 6000 sq. meters. This is an increase of 10,000,000 times over the exposed surface area of 1 cm cube. The contact surfaces between micelles of dispersed phase and dispersion medium are called interfaces. The interfacial area of a colloidal system being very large, a large number of other substances can be adsorbed on the interfaces. Adsorption is also helped by electric charges present on the surface. Some properties of colloidal systems are given below:
Classes of Colloidal Systems
The commonly accepted types of colloidal system are:
- Gas in liquid (foam)
- Gas in solid
- Liquid in gas (fog)
- Liquid in liquid (emulsoid)
- Liquid in solid
- Solid in gas (smoke)
- Solid in liquid (suspensoid)
- Solid in solid
Colloidal System
Thomas Graham (1861) pointed out that soluble substances could be divided into two classes, the Crystalloids and the Colloids. He designated as crystalloids all those substances, such as sugar, urea, common salt etc. which could be obtained in a crystalline form and could diffuse through animal or parchment membrane. The colloids, on the other hand, were substances, like protein, starch, gum, clay etc. which could not diffuse through animal membrane, and were amorphous.
Buffer Solutions
These are the solutions which put a considerable check on changes in hydrogen ion concentrations, upon the addition of acid or base to a solution. In some biological investigations, such as, the culture of micro organisms, it is essential to keep the acidity of the culture medium constant. Buffer solutions are used in such experiments. They are usually prepared by mixing a week (poorly dissociating) acid with one of its strong (strongly dissociating) salts, as for example, the mixture of acetic acid (CH3COOH) and sodium acetate (CH3COONa). If an acid, such as, hydrochloric acid is, added to such a buffer solution, sodium acetate reacts with it and sodium chloride and acetic acid are formed as expressed below:
Hydrogen Ion Concentration
Total acidity refers to the normality of an acid solution. The normality of an acid solution indicates the total replaceable hydrogen, whether, in the form of ions or in the form of molecules. The hydrogen ion concentration, in strict sense, means the concentration of ions of hydrogen in the solution, not of the molecules. A normal solution of hydrogen ions is the one which contains 1.008 gm of hydrogen ions in a litre of solution or 1 mole of H+ per litre of solution at 20°C. A solution with N/10 concentration of hydrogen ions is a highly acidic solution.
Normal Solution
Solutions of acids, bases and their salts are referred to in terms of normal concentration or normality. A normal solution (1N) contains as many grams of the dissolved substance as the equivalent weight of the substance in a litre of the solution. A normal solution of an acid contains sufficient acid to provide one equivalent or 1 mole of hydrogen ion (H+) per litre of the solution at 20°C. Similarly, 1N base solution contains sufficient amount of the base to provide one equivalent or 1 mole of hydroxyl ion (OH-) per litre of the solution at 20°C.
Saturated, Unsaturated and Supersaturated Solutions
When a solute is added to a given volume of solvent, in small quantities, at a certain temperature a limit is reached when no more of the solute is dissolved such a solution, which can not hold any more of the solute, is said to be saturated. An unsaturated solution is the one, which contains in its given volume, less amount of the solute than is required to saturated. When a solution, saturated at a given temperature, is heated, it can dissolve more of the solute. When a hot saturated solution is cooled some of the solute separated out. But under certain conditions, When a hot saturated solution is allowed to cool, no solid separates out, although the solution contains more amount of solid than is required to saturate it at lower temperature. Such a solution is termed as supersaturated solution.
Solutions and Colloidal Systems
A metabolic cell contains a large amount of water. About 90 percent of the bulk of a living cell is made of water. Water does not occur in pure state in the cell, but contain inorganic and organic substances. Some of these substances are dissolved in it. Thus, these substances, either form true solutions, or colloidal systems. In order to understand the state of particles of such material in the aqueous system of the cell, it is essential to know the nature and characteristics of a solutions and colloidal systems.