Can NaCl or NH4Cl be used instead of HCl as group reagent for the detection of group I cations? Justify your answer
Answers
The hydroxides of Group IIIA have a very low value of solubility product ( Ksp ) compared to the hydroxides of the subsequent groups. Only a certain concentration of OH− should be enough to precipitate Group IIIA; if the concentration exceeds this then the cations of subsequent groups may get precipitated and give a false positive.
NaOH is a strong electrolyte so it dissociates almost completely in solution. Hence, even a small amount of NaOH would supply a huge concentration of OH− ions and may give a false positive.
NH4OH is used with NH4Cl for testing Group IIIA cations. First a pinch of solid NH4Cl is added to the test tube containing a small amount of the original solution (O. S.). To check whether NH4Cl has dissolved, hold the lower part of the test tube between your fingers, it should feel cooler than the surroundings. This is because the dissolution of NH4Cl is endothermic. So, this means that now there are NH+4 ions in the solution. Now NH4OH is added and it’s dissociation is limited because NH+4 ion is already present in the solution in sufficient concentration. This is called common-ion effect. As dissociation of NH4OH is limited, the concentration of OH− in the solution is also limited and just enough to precipitate Group IIIA cations but not the cations of the subsequent groups.
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