PHYSICAL CHEMISTRY OF MILK

Module 7. Electrical conductivity

Lesson 14
ELECTROLYTES AND NON ELECTROLYTES, IONIC MOBILITY

14.1 Introduction

It is very interesting to note that the electrical current will pass through the medium when certain substances are dissolved in it but in certain situations it will not allow the current to pass through. To understand such phenomena we need to study the nature of these substances.

14.2 Definition

Substances which cause increase in electrical conductivity in solutions and which dissociate in the process of conducting the electric current are known as electrolytes. In other words, electrolyte is an electrical conductor in which current is carried by ions rather than by free electrons (as in a metal).

A non-electrolyte, however, is a compound composed of molecules that does not conduct electricity when molten or in aqueous solutions. If the substances which give the expected result to osmotic pressure, freezing point and boiling point but do not significantly increase the electrical conductivity such substances are known as non electrolytes.

14.3 Electrolytes Dissociation

Non electrolytes behave in a definite manner with respect to osmotic pressure, freezing point and boiling point and this behavior can be calculated and predicted with a fair degree of accuracy.

But the electrolytes when subjected to similar calculations, the predicted results were always observed to be less than their effect as observed by actual trial. In case of very dilute solutions, their effect comes close to being even 2, 3, or 4 times greater than as predicted. As an explanation to such behaviour and to explain electrical conductivity of solutions and the facts of electrolysis, Arrhenius advanced the theory of electrolytic dissociation or ionization was proposed.

14.4 Arrhenius Theory

According to this theory, molecules of an electrolyte on going into solution dissociate into ions. The ions are dissimilar as to the elements or elements represented in them and as to the electrical charge which they carry. In each case there are ions that carry positive charge and ions that carry negative charges in such numbers that the two types just compensate each other so that the solution is electrically neutral or balanced. This is illustrated by giving a few chemical dissociation equations

The above reactions are in equilibrium and the dissociation is more complete, the more dilute the solutions are made. At complete dissociation each molecule gives rise to 2, 3 or 4 ions as the case may be and this accounts for the fact that in very dilute solutions the effect on the osmotic pressure, freezing point and boiling point is nearly 2, 3 or 4 times as great as anticipated on the basis of calculations that holds for non electrolytes. According to the modern views we consider the electric current to be a stream of electrons.

The electron is the ultimate unit of electricity its charge is negative its mass is approximately 1/1000 the mass of the hydrogen atom. A positive ion has 1, 2 or 3 positive charges because it lacks 1, 2 or 3 electrons. A negative ion has 1, 2 or 3 negative charges because it contains an excess of 1, 2 or 3 electrons (A limited number of ions with a still larger number positive or negative charges are also there) to understand the difference between atoms and ions more fully e.g. Na (sodium atoms) as compared to Na+ (sodium ion) it would be necessary to consider the structure of atoms. The atom consists of a positive nucleus surrounded by electrons in such number that the positive nucleus is just satisfied. The atoms of some elements have a strong tendency to lose electrons such elements yield positively charged ions. Other elements have a strong tendency to take on additional electrons such elements yield negatively charged ions.

When an electric current is passed through a solution it is conducted by the migration of ions. The positive ions migrate to the negative electrode and then take on the requisite number of electrons to neutralize the charge. The discharged ion is then an atom (if the ion contained only one element) and is deposited on the electrode (electro-plating) or it may escape as a gas (two atoms combining to form molecule) or it may react with water of the solution, depending upon the chemical nature of the element involved. Similarly negative ions migrate to the positive electrode and there give up the excess electrons.

14.5 The Theory of Electrolytic Dissociation

The theory of electrolyte dissociation explains

· The abnormal behaviors of electrolytes with respect to osmotic pressure, freezing point and boiling point.

· The electrical conductivity of solutions, and

· The facts of electrolysis.

Since ions play such an important role part in chemical behavior let us examine the extent of dissociation and the equilibrium conditions more closely. The degree of dissociation can be measured in various ways. The extent, to which electrolytes deviate from the accepted rules for osmotic pressure, freezing point, and boiling point, can be a basis for computing the degree of dissociation. Since electrical conductivity is dependent upon ions which can also be used to measure the dissociation.

The equilibrium conditions follow the law of mass action only in a qualitative sense in the case of electrolytes that dissociate very readily, but in strict mathematical sense in the case of electrolytes they dissociate only to a slight extent. Weak acids and bases fall under the latter class, and since we are dealing with such substances in milk it is necessary to consider the law of mass action if we are to develop a full understanding of milk acidity and buffering reaction.

14.6 Ionic Mobility

In the absence of external electrical field, the ions of a solution are in random motion since all directions are equivalent. When an external electrical field is applied although the randomness of motion will basically remain, one of the directions becomes preferred, and more so the higher the potential gradient i.e. the greater drop of potential per cm. The velocity of an ion is the value of this preferential migration towards one of the electrode expressed in cm/s. The potential gradient for the purpose of comparing the ion velocity is 1 volt/cm and these mobilities are known as absolute mobilities. Since the mobilities are taken as proportional to the potential gradient, the absolute mobility is expressed in cm²/ volt. The mobilities are very low having values of only 0.0005 – 0.003 cm²/ volt. Ionic conductance is the contribution of a given type of ion to the total equivalent conductance in the limit of infinite dilution.

Ionic mobility = Ionic conductance/ 96500