Ionic Strength

Ionic Strength Ionic Strength The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts. One of the main characteristics of a solution with dissolved ions is the ionic strength.

The ionic strength plays a central role in the Debye–Hßckel theory that describes the strong deviations from ideality typically encountered in ionic solutions. It is also important for the theory of double layer and related electrokinetic phenomena and electroacoustic phenomena in colloids and other heterogeneous systems.

That is, the Debye length, which is the inverse of the Debye parameter (Îş), is inversely proportional to the square root of the ionic strength. Debye length is characteristic of the Double layer thickness. Increasing the concentration or valence of the counterions compresses the double layer and increases the electrical potential gradient.

Media of high ionic strength are used in stability constant determination in order to minimize changes, during a titration, in the activity quotient of solutes at lower concentrations. Natural waters such as seawater have a non-zero ionic strength due to the presence of dissolved salts which significantly affects their properties.

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Standard State Conditions Either it is available in the system itself or it has to be supplied by the surroundings referred to as the standard stated conditions. Certain reactions are instantaneous or spontaneous and will stop only when the reaction is complete. In other words the entire reactants get converted into products if they are in molecular proportions. Certain reactions take place slowly and they will end in equilibrium.

The activity of the substances involved can be measured in these later reactions. Vapour pressure is one of the bases for all measurements of chemical equilibria in solutions. In a solution there is a relationship between the vapour pressure of the solute and the vapour pressure of the solvent. Henry’s law and Raoult’s law describe these relationships, which form the basis of all measurements of chemical equilibria in solutions.

Raoult's Law Henry’s law applies to the solute in solution, activity of the solute and the standard states of solutes. Raoult’s law deals with the solvent in a solution. He proposed that the vapour pressure of the solvent is reduced by the presence of solute and the reduction in vapour pressure is proportional to the mole fraction of the solvent.

Ionic Activity It is necessary to count the calculations based upon the activity and activity coefficient and not on concentration. In the measurement of physiological parameters like the serum electrolyte calculations the activities and activity coefficients are required.

In clinical medicines more accurate measurements of body constituents can be obtained by taking into consideration the activity and activity coefficient. Dealing with ions the activity is termed as IONIC ACTIVITY and certain ion specific electrodes will give most accurate ionic activity.

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Ionic concentration is measured in milli equivalents per litre. This is a measure of number of ions in one litre of solution. Ionic activity reflects also the interaction with its environment. Ionic activity ‘a’ is thus a product of activity coefficient (Îł I ) and the concentration (c).

Ionic Strength There are two types of electrolytes : 1. Strong electrode 2. Weak electrode Strong electrolytes dissociate completely in a solution while weak electrolyte dissociates only partly. When ionic activity is measured with ion selective electrodes, the ionic concentration decides the value. Strong electrolytes because of their dissociation show greater ionic concentration than the weak electrolytes.

The hydrogen ion concentration which is measured as pH value is a common example of this ionic concentration. Hydrogen ion will not form bonds with surrounding ions and hence they are completely dissociated. This results in the maximum ionic activity per the given ionic concentration.

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