Question

Not washing the precipitate in part III-2 that is used in part III-3 will have no effect on the identification of iron and manganese.

True False

When adding NaOH to the supernatent in part III-2, if a pH of only 7 was reached instead of 8, it would potentially give a false negative for iron and manganese.

True False

Procedure III-2 Separation of Fe3+ and Mn2+ from Al3+ and Zn2:

Treatment of Precipitate from III-1: Add 15 drops of 3M HNO3. Place the test tube in boiling water bath and stir for about 2 minutes until the only remaining solid is elemental sulfur. Cool and centrifuge. Decant the supernatant into another 10-cm test tube; discard the precipitate. Add 8M NaOH until the supernatant is made barely basic, ~8-- test with pH paper-- then add 6 drops 8M NaOH excess. Fe(OH)3 and Mn(OH)2 will precipitate if Fe3+ and Mn2+ are present. Centrifuge. Carefully decant the supernatant into another 10-cm test tube and save it for treatment by Procedure III-4. Wash the precipitate once with 20 drops of hot water, centrifuge, and discard the wash. Analyze the precipitate as described in Procedure III-3.

Procedure III-3 Identification of Fe3+ and Mn2+ :

Transfer a small quantity of the precipitate from Procedure III-2 to a 10-cm test tube b means of a story rod or pipet. Dissolve it in 3 to 4 drops of 2M HCL. Add 3 to 4 drops of 0.2 M KSCN. A dark red color due to the formation of the complex [Fe(SCN)]2+ identifies Fe3+. To the rest of the precipitate from Procedure III-2 add 7 drops of 16M HNO3 and 3 drops of water and stir about 30 seconds. Most of the precipitate should dissolve. Centrifuge and transfer the clear supernatant to an evaporating dish. Without stirring add a very small amount of sodium bismuthate, NaBiO3. If Mn2+ is present it will be oxidized to HMnO4 which will be evident as a reddish-purple color developing in the solution in the vicinity of the solid NaBiO3.

Answer 1

The ions Fe³⁺, Mn²⁺, Al³⁺ and Zn²⁺ all form precipitates with NaOH; however, the precipitates of Al³⁺ and Zn²⁺ dissolve in excess NaOH and are discarded. The precipitates of Fe³⁺ and Mn²⁺ must be washed with water to remove excess NaOH. This is because if excess NaOH is not removed, it will react with the acids (HCl or HNO₃) during the identification steps and thus, all the Fe³⁺ and Mn²⁺ will not be quantitatively converted to the identifiable forms. Hence, the given statement is false.

The precipitates of group III metals appear in strongly basic medium (NaOH) for which the pH must be higher than 8.0. If the pH is around 7.0 (neutral), then the precipitates will not be formed and thus, the ions will be considered absent. Thus, the given statement is true.