PROCEDURES

Write net ionic equations on Page 16-14 for the separation and identification of all seven cations. The number of equations for each cation is shown in parentheses.

The test for NH₄⁺ must be performed before doing any other tests. A separate portion of the sample will be used to test for the other cations.

Prepare a Boiling Distilled Water Bath
Mount the iron ring on the ring stand and use wire gauze to support a 400 mL beaker containing approximately 250 - 300 mL of distilled water. Light the Bunsen burner and position it beneath the beaker so that the cone of the flame just touches the bottom of the beaker.

Note: You will need to check the water level in your hot water bath from time to time during the experiment. Do not let the water level drop below 200 mL.

Ammonium Ion
The original unknown solution must be used to test for the ammonium ion.

Wet a piece of red litmus paper and stick it to the underside of a watch glass. Add 5 drops of 50% NaOH solution to a casserole and add 1 drop of the solution to be tested. Have the watch glass ready. Cover the casserole with the watch glass with the red litmus paper stuck to the bottom. If NH₄⁺ is present, the red litmus will turn blue within 30-45 seconds.

Wet Litmus	Positive Test

If 50% NaOH is splattered onto the litmus paper or if the sides of the casserole are contaminated with base, the litmus will turn blue even if NH₄⁺ is not present.

Precipitation of Group III
The known solution contains all 7 cations and will be a pale blue solution. Unknowns will contain at least 2 of the 7 cations and will be a variety of colors depending on the ions present.

Place 9 drops of the solution being tested in a test rube and add 3 drops of 0.1 M NH₄Cl. Add 6 M NH₃ until the solution is just basic to litmus. To test the pH of the solution, use a stirring rod to place a single drop of solution on a piece of litmus paper and observe the appropriate color changes (red to blue = basic, blue to red = acidic). Once the solution is basic, add 2 additional drops of 6 M NH₃ until the surface turns green and you get a precipitate that persists. Mix the solution thoroughly with a stirring rod and centrifuge.

Note: Remember to balance the centrifuge with a test tube containing distilled H₂O.

Test for completeness of precipitation by adding 1 drop of 6 M NH₃ then centrifuge. There will be a blue solution on top and a brown precipitate in the bottom of the test tube. Pour off the supernatant liquid into a test tube labeled Decantate 1. Wash the precipitate twice with a mixture of 5 drops of distilled water, 1 drop of 6 M NH₃ and 3 drops of 0.1 M NH₄CL. Stir, centrifuge and add the washings to Decantate 1. Precipitate 1 contains the Group III cations Fe^2+/3+ and Cr³⁺, Decantate 1 contains the Group IV cations Ca²⁺ and Ba²⁺ and the Group V cation Ni²⁺.

Note: If the precipitate is not sufficiently washed Ba²⁺ and Ca²⁺ can be lost.

Separation of Iron (III) and Chromium (III): Precipitate 1
Precipitate 1 contains the hydroxides of iron and chromium if they are present. To the solid add 3 drops of 6 M HCl, 1 drop of 6 M HNO₃ and 2 drops of water. Mix thoroughly and heat until all of the precipitate is dissolved and a translucent solution is obtained.

When all of the precipitate has dissolved, add 50% NaOH drop-wise until the solution is basic to litmus. Once the solution is basic, add 2 additional drops of 50% NaOH and 6 drops of water. Add 6 drops of 3% H₂O₂ and mix the solution thoroughly. The evolution of gas will be indicated by the formation of small bubbles within the liquid. Once the evolution of gas has slowed, boil the mixture gently for about 2 minutes in the hot water bath.

Centrifuge the mixture and pour the liquid into a test tube labeled Decantate 2. This solution will be tested for the presence of Cr³⁺. The precipitate (Precipitate 2) will be used for the confirmation of Fe³⁺.

Confirmation of Fe (III): Precipitate 2
Precipitate 2 should contain iron (III) hydroxide, Fe(OH)₃. Dissolve the precipitate by adding 6 drops of 6 M HCl. Add 6 drops of water and 5 drops of 0.1 M KSCN, potassium thiocyanate, and mix thoroughly. The appearance of a blood red color indicates the formation of the ion [FeSCN]²⁺ and confirms the presence of Fe³⁺.

Confirmation of Cr (III): Decantate 2
Add 6 M CH₃COOH, acetic acid, to Decantate 2 until the solution is just acidic. (This may take several drops of 6 M CH₃COOH. Be sure to mix the solution well before testing for acidity.) Add 5 drops of Pb(CH₃COO)₂, lead acetate. The formation of a bright yellow color confirms the presence of chromium (III).

Separation of Group V from Group IV: Decantate 1
Decantate 1 will contain Ni²⁺ (Group V) and the Group IV cations Ca²⁺ and Ba²⁺ if they are present. If any solids are present in Decantate 1 centrifuge and discard the solid before proceeding. Test to be sure the solution is basic, if not, add 6 M NH₃ until the solution is basic. Add 12 drops of 3 M (NH₄)₂CO₃, ammonium carbonate, to the solution and mix thoroughly. Centrifuge and pour the liquid into a test tube labeled Decantate 3. Decantate 3 will be blue and the precipitate will be white. The precipitate (Precipitate 3) will contain the Group IV cations.

Confirmation of Ni (II): Decantate 3
Decantate 3 contains the complex ion Ni(NH₃)₆²⁺. Add 15 drops of 1% dimethylglyoxime and mix the solution thoroughly. Allow the test tube to stand for several minutes. The formation of a strawberry red precipitate confirms the presence of nickel (II).

Note: The precipitate can be slow to form, allow time before determining a test to be negative. It is unusual for only a small amount of precipitate to form during a positive test. If only a small amount of precipitate formed, the test should be repeated.

Separation and Confirmation of Ba²⁺: Precipitate 3
Precipitate 3 will contain barium carbonate and calcium carbonate if both cations are present. Add 6 drops of 6 M CH₃COOH and mix thoroughly to dissolve the precipitate. Heat the solution in the hot water bath for up to 2 minutes if the precipitate does not dissolve completely. If the precipitate has not dissolved after heating, add an additional drop of 6 M CH₃COOH. After the precipitate has been dissolved completely, add an additional 3 drops of 6 M CH₃COOH.

Add 6 drops of 0.1 M K₂CrO₄, potassium chromate, and mix thoroughly. Warm the mixture in the boiling water bath and centrifuge immediately.

Note: In this step barium is removed from the solution through precipitation as BaCrO₄. If the solution is not kept hot, CaCrO₄ may precipitate as well and the separation of Ba²⁺ from Ca²⁺ will not be achieved.

The supernatant liquid should be deep yellow after centrifuging (essentially the same color as the K₂CrO₄ solution which was added), if not, add 2 more drops of acetic acid and 4 more drops of K₂CrO₄, heat and centrifuge again. Pour the liquid (while hot) into a test tube labeled Decantate 4 to be tested for calcium.

The precipitate should be yellow BaCrO₄, which confirms the presence of barium.

Confirmation of Ca²⁺: Decantate 4
Add 10 drops of 0.2 M (NH₄)₂C₂O₄, ammonium oxalate, to the yellow Decantate 4. Mix thoroughly and centrifuge. The formation of a white precipitate, CaC₂O₄, confirms the presence of calcium.

 

Note: It may be necessary to discard the yellow supernatant liquid to confirm that the solid formed is a white precipitate.

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