Experiment 20

Write net ionic equations on Page 17-15 for the identification of all nine anions. The equations are to be prepared outside of lab and are required as part of the lab report.

Part I. Analysis of Known Samples

Elimination Tests
Group I Anions
In a 75 mm test tube dissolve a small quantity (about the size of an uncooked grain of rice, See Figure) of solid Na₂SO₄ in 10 drops of distilled water. Add 5 drops of 0.1 M BaCl₂ solution. Shake the tube to assure complete mixing and record all observations on the data sheet. If a precipitate has formed, add 3 to 6 drops of 6 M HNO₃ and record all observations.

Repeat this procedure using solid Na₂CO₃, Na₃PO₄, NaCl, NaBr, NaI, NaNO₃, NaNO₂ and CH₃COONa. Do not add nitric acid if no precipitate has formed. In each case, record all observations on the data sheet.

Group II Anions
In a 75 mm test tube dissolve a small quantity (about the size of an uncooked grain of rice, See Figure) of solid Na2SO4 in 10 drops of distilled water. Add 5 drops of 0.1 M AgNO₃ solution. Shake the tube to assure complete mixing and record all observations on the data sheet. If a precipitate has formed, add 3 to 6 drops of 6 M HNO₃ and record all observations.

Group III Anions
Anions that did not form a precipitate with barium ion and did not form an acid insoluble silver salt are classified as Group III anions. No group test is necessary to classify these ions further; instead individual tests are used to identify each anion.

Confirmatory Tests
Sulfate Ion (SO₄^2-)
In a clean 75 mm test tube dissolve a small quantity of solid Na₂MSO₄ in 10 drops of distilled water. Add 5 drops of 0.1 M BaCl₂ solution. Shake the tube to assure complete mixing and record all observations on the data sheet. If a precipitate has formed, add 3 to 6 drops of 6 M HNO₃ and record all observations

Carbonate Ion (CO₃^2-)
Place a small quantity of Na₂CO₃ in a 75 mm test tube. Add 5 drops of 3 M H₂SO₄M|. Note the color and rate of any gas evolved. Carbonates will produce a vigorous reaction with a resulting "fizz."

Phosphate Ion (PO₄^3-)
Dissolve a small quantity of Na₃PO₄ in 10 drops of distilled water in a 75 mm test tube. Add 5 drops of 6 M HNO₃ and 5 drops of 0.5 M (NH₄)₂MoO₄ solution. Record all observations on the data sheet.

Chloride (Cl-), Bromide (Br-) and Iodide (I-) Ions
Place a small quantity of NaCl in a clean 75 mm test tube. To the solid sample, add 10 drops of 3% hydrogen peroxide (H₂O₂) and 2 drops of 3 M H₂SO₄. Stopper the tube and shake the contents thoroughly. Add 5 drops of toluene and again stopper and shake the contents thoroughly. The contents of the tube will separate into two layers. Note the color of the toluene (upper) layer.

Save the test tube for comparison with the tests for Br- and I-. Repeat the procedure with NaBr and NaI. In each case, record all observations on the data sheet. Compare the color of the toluene layer for the three anions. When testing an unknown, you may wish to prepare test tubes containing the three known samples for direct comparison.

Nitrate Ion (NO₃-)
Brown Ring Test: Dissolve a small quantity of NaNO₃ in about 10 drops of distilled water and add 5 drops of 1 M FeSO₄ solution. Hold the test tube at an angle and carefully run 5 drops of concentrated (18 M) H₂SO₄ down the side of the test tube (3 M H₂SO₄ won’t work!). Do not mix the contents! Two layers will appear. A brown color will appear at the interface of the two solutions indicating the presence of nitrate ion. Record all observations on the data sheet.Nitrite Ion (NO₂-)

This test must be performed under the hood at your workstation as noxious gases are produced. Place a small quantity of NaNO₂ in a 75 mm test tube. Carefully add 5 drops of 3 M H₂SO₄. Record all observations on the data sheet including reaction rate and the color of any gas produced. When nitrite is present, a reddish-brown gas should be evolved near the mouth of the test tube.

Acetate Ion (CH₃COO-)
Basic Ferric Acetate Test: Dissolve a small quantity of CH₃COONa in 3 drops of distilled water. Add 3 drops of 0.5 M FeCl₃ solution. If no color change is observed, add one drop of 6 M nitric acid. Record all observations on the data sheet.

Part II: Analysis of Unknown Anions
Each anion unknown contains only one anion. You will be required to identify three unknowns. Follow the procedures for the Elimination Tests given in Part I to classify your unknown as a Group I, Group II or Group III anion. Once the classification has been established, proceed with the Confirmatory Tests pertinent to that particular group in order to identify your anion.