INTRODUCTION:

It is known, the Nernst’s distribution equation is only valid when an any dissolved solute present in the normal state or a same molecular form in the non-miscible solvents i.e. the solute does not associate or dissociate in any one of the immiscible solvents, in this case distribution constant can be measured^1,2. The distribution law properly stated as ‘a dissolved solute, irrespective of its amount, distributes⁴ itself between two non-miscible solvents⁶ in such a way that to attained an equilibrium, the observed ratio of the concentration of the solutes in the two immiscible layers is remains constant⁷, at any given temperature.’

If the solute undergoes dissociation or association⁸ in any one of the immiscible solvents the distribution law³ is not applicable and the concentration ratio of the solute in the two immiscible solvents is not constant then modified Nernst’s distribution equation is preferable. The Nernst distribution law can be valid for any solute when itself distributes between two miscible solvents and also existence of distribution ratio to miscible solvents. The theoretical approximation is the extraction of a solute i.e. solvent extraction¹⁰ and solute associates or dissociates⁹ in any one of the miscible solvents may be valid. Considering a solute XA is itself distributing between x (non-polar organic solvent), y (polar solvent) and z (non-polar organic solvent). C₁, C₂ & C₃ is the concentration of the solute (XY) in the solvents x, y & z respectively. Distribution constant equation for the two miscible solvents can be obtained but the Solute XY present in the normal state in solvents.

According to the Zeroth Law of Thermodynamics, states that if the two systems are each in thermal equilibrium with other third system, then it is also equilibrium with each other’s. the following general expression is based on the Zeroth law of thermodynamics,

Applying law of mass action to the equilibrium condition as follows,

the solute present in normal state and the equilibrium constant as well as distribution ratio as,

by using the equation (1) and (2), we get,

K₃ is the distribution ratio or distribution constant for the miscible solvents x and z.

MATERIAL AND METHODS:

Chemicals and reagents:

Cyclohexane (10ml), Iodine crystals, Potassium iodide (0.5 N KI)¹¹, Carbon tetrachloride (10ml) and Separating funnel

Method:

1. The two small crystals of iodine of the same size were put into each of the two separating funnels. 10ml of cyclohexane and carbon tetrachloride were added separately into two separating funnels. Corked the separating funnel and was shaking until the iodine dissolves. We observed the different colors in the two solvents. 5ml of 0.5 N KI solution was added into each separating funnel with shaking, after some time two layers was formed in two separating funnels.

2. After one hour slowly added the potassium iodide and cyclohexane solvents layers from one of the funnels into over another separating funnel which contain potassium iodide and carbon tetrachloride solvents layers.

3. After one day we observed that the three layers, at bottom carbon tetrachloride, middle aqueous potassium iodide and upper cyclohexane layers in its iodide distribute itself until to attaining equilibrium state, the distribution ratio of the miscible solvents (Carbon tetrachloride and cyclohexane) remains constants.

4. The different color developing in the newly added solvents in each layer as shown in figure (a), after one day watching for changes in color intensity in each layer as shown in figure (b), The situation we should be achieved is called an equilibrium.

RESULT AND DISCUSSION:

Iodine exists as normal state in solvent Cyclohexane, water and carbon tetrachloride, cyclohexane and carbon tetrachloride are miscible solvents. The following expression based on the Zeroth law of thermodynamics as,

Applying law of mass action concept to the above equilibrium system that exists as

Keq = [I₂]_H2O /[I₂]_cyclohexane,

K₁= 𝐶₂/𝐶₁ --------------------------------------------------- (1)

Keq = [I₂]_CCl4 / [I₂]_H2O

K₂ = 𝐶₃/𝐶₂ -------------------------------------------------- (2)

Here, the solute present in normal state in three solvents and the equilibrium constant and distribution ratio are the same for this system.

By using equation (1) and (2), we get,

K₃ = K₁ x K₂ = 𝐶₂ /𝐶₁ x 𝐶₃/𝐶₂

K₃ = 𝐶₃/𝐶₁,

K₃ is the distribution coefficients for miscible (Cyclohexane and carbon tetrachloride) solvents. Using an iodine crystal dissolved in 10ml of cyclohexane and carbon tetrachloride solvents, here the used of three solvent system, it may create an equilibrium distribution. It is found that iodine crystals dissolving in identical 10 ml of miscible solvents, cyclohexane and carbon tetrachloride and an immiscible intermediate aqueous solution of 10ml 0.5N potassium iodide. The distribution of the iodine from one-layer cyclohexane to the aqueous potassium iodide and additionally to the carbon tetrachloride until an equilibrium distribution is reached, at the equilibrium point we found different colors as shown in figure (a) and (b). Using 10 ml volumes of the three solvents, they could exhibit that it is far possible to reap almost whole extraction from one solvent to the other solvents.

Figure: (a) Three layers: Upper cyclohexane (pink), Middle aqueous KI (orange) and bottom carbon tetrachloride (bright violet) After one hour)

Figure: (b) Three layers: Upper cyclohexane (purple), middle aqueous KI (yellow brown) and bottom (bright violet) (after one day, color intensity changes)

CONCLUSION:

It will be possible to calculate the distribution constant for miscible solvents, using the three-component system, whenever a solute present in normal state in the miscible solvents. The varying color intensity of iodine in two miscible solvents is an intriguing phenomenon. The solubility of iodine in cyclohexane produces a purple and in carbon tetrachloride gives purple (Bright violet) color similar in coloration to iodine vapors, in which non-polar iodine molecules have been separated from each other in the crystal structure by non-polar solvent molecules. Iodine is slightly soluble in water. Its solubility in aqueous potassium iodide, to give a yellow-brown color solution is due to the formation of a stable I₃⁻ ion. This technique is useful for determination of a stability constant or Dissociation constant or an equilibrium constant of complexes, in terms of understanding the thermodynamic stability of a complexes. It is useful in reaction mechanism of an inorganic complexes.

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Received on 14.03.2020 Modified on 23.03.2020

Asian J. Research Chem. 2020; 13(3):225-227.

DOI: 10.5958/0974-4150.2020.00043.7