Why do we not prefer to prepare original solution in conc. `H_(2)SO_(4)` or conc. `HNO_(3)`?

Explanation

it is not advisable to prepare original solution in conc. `HNO_(3)` or conc. `H_(2)SO_(4)` because these are oxidisng agents and they oxidise `H_(2)S` in group II to produce a yellow ppt.
of sulphur
`H_(2)SO_(4)toH_(2)O+SO_(2)+[O]`
`HNO_(3)toH_(2)O+NO_(2)+[O]`
`H_(2)S+OtoH_(2)+underset(("yellow"))(S(s))`
Also `Pb^(2+),Ba^(2+),Sr^(2+),Ca^(2+)` are precipitated as their insoluble sulphates in presence of conc. `H_(2)SO_(4)`
`BaCl_(2)+H_(2)SO_(4)tounderset(("Insoluble"))(BaSO_(4)(s))+2HCl`