Why is ammonium chloride added before addition of ammoniun hydroxide in preciptn of grp 3 cations?

Asked by  | 26th Jan, 2009, 03:27: PM

Expert Answer:

This is to prevent the precipitation of hydroxides of higher groups.

Ammonium chloride is a strong electrolyte. SO, it dissociates completely.

NH4Cl NH4+ + Cl-

Now when NH4OH is added, due to common ion effect its dissociation is supressed.

NH4OH NH4+ + OH-

The concentration of hydroxide ion is less. Hence, the ionic product of hydroxide ion and cation only exceeds the solubility product of hydroxides of group 3 cations and they are precipitaed. The solubility product of higher group cations is higher than their ionic product and hence they are not precipitated.

 

Answered by  | 6th Feb, 2009, 04:47: PM

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