Question

Propose a simple method for separating a mixture of a solid consisting of Al(OH)3 and Fe(OH)3. Draw a flowchart for the separation. Include the relevant confirmation tests for each ion

Answer 1

I will put the procedure to separate and identify each ion, and at the end I’ll draw the flowchart:

Before you can develop a flow scheme for analyzing an unknown, you need to know how each of the four ions to be tested (Al3+, Fe3+) will react with each of the three reagents to be used (6 M HCl, 6 M NH3, and 6 M NaOH).

Step 1. Prepare a set of 2 test tubes, each test tube containing a few drops of one of the four ions to be tested.

Step 2. Add one drop of one of the reagents, 6 M NaOH, for example, to each of the four test tubes. Observe the result. If no precipitate has formed, add 4-5 more drops of the reagent. This reagent (NaOH, HCl, or NH3) will lead to precipitates, such as Fe(OH)3, with some or all of the ions.

Fe3+(aq) + 3 OH- (aq) ? Fe(OH)3(s)

Step 3. If an ion has formed a precipitate with the reagent added in Step 2, you have to know what will dissolve this precipitate. Therefore, you should centrifuge to collect the precipitate, decant the supernatant liquid, and wash the precipitate with a few drops of distilled water. (NOTE: If no precipitate has formed, you may proceed to another ion.)

Step 4. Now you want to see what reagent will dissolve the precipitate formed in Step 2 (and washed in Step 3). Because there are three reagents that can possibly dissolve the precipitate, you will need 3 test tubes, each containing some of the precipitate. Therefore, if you have not already done so, prepare 3 batches of washed precipitate as described in Steps 2 and 3.

As we know, we already have the compounds as OH so Step 1 and 2, can be omitted.

Step 5. To each of the three test tubes containing one of the precipitates from Steps 3 and 4, say Fe(OH)3(s), add a few drops of one of the three reagents. For example, to test tube 1 add 6 M HCl, to test tube 2 add 6 M NH3, and to test tube 3 add 6 M NaOH. Some or all of the added reagents will dissolve the Fe(OH)3. You should try as hard as possible to dissolve each precipitate. It may take excess reagent or heating. Remember to stir thoroughly in any event.

CONFIRMATORY TESTS

After you have carried out the separation of a mixture of ions in solution, you must be able to confirm the presence or absence of a single ion in a solution. To do this, you must carry out confirmatory tests on individual ions. As you carry out the tests outlined below, be sure to record your observations in your notebook.

Test for aluminum, Al3+. Using a few drops of the solution containing Al3+, make it distinctly acidic with 6 M HCl. Add 1 drop of aluminon dye, and then add 6 M NH3 dropwise until the solution is basic to litmus paper. (Avoid adding an excess of NH3.) If present, Al3+ will form a gelatinous precipitate of Al(OH)3 that absorbs the red dye to give what is commonly called an "aluminum lake."

Test for iron(III), Fe3+. Make the solution acidic with 6 M HCl. Add 1 drop of K4Fe(CN)6 solution. A blue precipitate of Fe4[Fe(CN)6] (usually called "Prussian blue") confirms the presence of iron(III).

The flow chart: