Why AgSCN is selectively precipitated before the formation of FeSCN2+?
given that Ksp of AgSCN = 1x10^-12
and Kinstab of FeSCN2+ = 7.1x10^-3
To understand why AgSCN is selectively precipitated before the formation of FeSCN2+, we need to consider the solubility products (Ksp) and the instability constants (Kinstab) of both compounds.
Solubility product (Ksp) is a measure of the solubility of a compound in a solution. It represents the equilibrium constant between the dissociated ions of a sparingly soluble salt. The lower the value of Ksp, the less soluble the compound is in the solution.
Instability constant (Kinstab) measures the tendency of a complex ion to dissociate back into its constituent ions. Higher values of Kinstab indicate greater stability of the complex ion.
In this case, we are comparing the solubility of AgSCN with the stability of FeSCN2+.
Given that Ksp of AgSCN is 1x10^-12, we know that AgSCN has a very low solubility in solution. It means that most of the Ag+ ions (from AgSCN) will precipitate and form a solid AgSCN.
On the other hand, the Kinstab of FeSCN2+ is 7.1x10^-3, indicating that FeSCN2+ is relatively stable in solution and will not easily dissociate into its constituent ions.
When both silver nitrate (AgNO3) and thiocyanate (NaSCN) are added to the solution containing Fe2+, SCN- ions react with Fe2+ ions to form a red complex ion, FeSCN2+. However, when AgNO3 is added first, the silver ions (Ag+) combine with the thiocyanate ions (SCN-) to form insoluble AgSCN, which precipitates out of the solution.
The selective precipitation of AgSCN ensures that most of the silver ions are removed from the solution before the formation of FeSCN2+. This selective removal of silver ions prevents interference or contamination of the red color of FeSCN2+. As a result, the red color produced by the FeSCN2+ complex is more intense and accurate, allowing for better quantification.
In summary, AgSCN is selectively precipitated before the formation of FeSCN2+ due to its low solubility (low Ksp) and the desire to prevent interference or contamination of the FeSCN2+ complex ion.