How do we differentiate between `Fe^(3+)` and `Cr^(3+)` in group III ?

Options

A. By adding excess of `NH_(4)OH`
B. by increasing `NH_(4)^(+)` ions concentration
C. by decreaseing `OH^(-)` ion concentration
D. Both (b) and (C)

Explanation

`OH^(-)` ions precipitates both `Fe^(3+)` and `Cr^(3+)` as their hydroxides. However, `Cr(OH)_(3)` is soluble in excess of `NH_(4)OH`
`Fe^(3+) +3OH^(-) to Fe(OH)_(3) darr`
`Cr^(3+) +3OH^(-) to Cr(OH)_(3) darr`
`Cr(OH)_(3)(s) +6NH_(3) (aq) to [Cr(NH_(3))_(6)](OH)_(3) (aq)`