Effect of Ionic Strength on Ion Dissociation

This experiment demonstrates the effect of ionic strength on the dissociation of the red iron(III) thiocyanate complex  

Prepare a solution of 1 mM FeCl3 by dissolving 0.27 g of FeCl3 6H2O in 1 L of water containing 3 drops of 15 M (concentrated) HNO3. The acid slows the precipitation of Fe(OH)3, which occurs in a few days and necessitates the preparation of fresh solution for this demonstration. 

To demonstrate the effect of ionic strength on the dissociation reaction, mix 300 mL of the 1 mM FeCl3 solution with 300 mL of 1.5 mM NH4SCN or KSCN. Divide the pale red solution into two equal portions and add 12 g of KNO3 to one of them to increase the ionic strength to 0.4 M. As the KNO3 dissolves, the red FeCSCN) complex dissociates and the color fades noticeably (Color Plate 6). 

Add a few crystals of NH4SCN or KSCN to either solution to drive the reaction toward formation of Fe(SCN), thereby intensifying the red color. This reaction demonstrates Le Chatelier s principle—adding a product creates more reactant. 

Test Yourself Find the ionic strength of 1.0 mM Ca(C104)2. (Answer 3.0 mM) 

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