ChemLab - Chemistry 3/5 - Qualitative Analysis of Anions

Qualitative Analysis of Anions

Overview

Getting Started

Techniques

Procedure

FAQ

Full Lab Manual

Introduction & Goals

Chemistry & Background

Key Questions

Prelab Problems

Safety

Procedure

In Your Write-up

Experiments Index

ChemLab Home

The Carbonate Ion, CO₃^-2 and Hydrogen Carbonate Ion, HCO₃^-
Place a pea-sized amount of solid baking soda in a small clean test tube that has been rinsed with distilled water. Add to it 1 or 2 drops of 18 M sulfuric acid (H₂SO₄) (CAUTION! STRONG ACID) and record your observations. Confirm the presence of CO₂ by dripping a drop of Ba(OH)₂ solution down the side of the test tube, as the gas bubbles are forming. Watch carefully for precipitate in the drop of solution running down the side of the test tube. This delicate procedure is illustrated on the ChemLab website, in the Procedure Overview section. Write a balanced chemical equation and describe your observations.

Repeat this experiment using vinegar in place of the sulfuric acid. Vinegar contains acetic acid, CH₃CO₂H. Write balanced chemical equations to describe your observations.

Test common blackboard chalk for this ion as follows: place a small piece of chalk in a dry test tube and add a few drops of 2 M HCl. Test the escaping gas by carefully running a drop of barium hydroxide solution from an eye dropper down the inner wall of the tube. Record your observations and write balanced chemical equations.

The Chloride Ion Cl^-
Put an amount of sodium chloride the size of a small pea in a small, clean test tube that has been rinsed with distilled water. Take the tube to the fume hood and carefully add 1 or 2 drops of 18 M sulfuric acid. (CAUTION, avoid breathing the gas formed). Carefully lower a piece of moist pH indicator paper into the tube, as the gas is evolved, to get more information about the gas. Indicator paper is impregnated with a colored compound which is sensitive to acids and bases. It is used to categorize solutions and gases as strongly acidic (red), neutral (no significant color change), or basic (blue). Record your observations and write balanced chemical equations.

Using a similar amount of NaCl in a new small test tube, add 15 drops distilled water and one drop of 3 M nitric acid (CAUTION! STRONG ACID). Then add 3-4 drops of 0.1 M silver nitrate and mix the contents. What happens? Answer with a balanced chemical equation. The purpose of the nitric acid, HNO₃, in this reaction is to prevent the precipitation of undesired silver salts, like AgOH, which can occur with non-acidic conditions. It is not a reactant in the balanced net ionic equation, for this reaction of chloride ion and silver nitrate.

Now test Hanover tap water for the presence of chloride ion. Place about 2 mL of tap water (without NaCl) into a clean test tube and add 1 drop of 3 M HNO₃. Then add the silver nitrate. Look carefully for precipitate. You may wish to look through the test tube the long way, through the opening, and compare to a test tube containing only distilled water. What can you conclude about the presence of chloride ions in tap water? Write a balanced chemical equation to describe your observations.

The Sulfate Ion SO₄^-2
Place a pea-sized amount of Epsom salts in a small, clean test tube that has been rinsed with distilled water and add 1-2 drops of 18 M sulfuric acid. Compare your observations with those made above for the carbonate ion and record them in your notebook. Write a chemical equation to describe your observations.

Dissolve a similar amount of Epsom salts in about 1 mL of distilled water in a new test tube. Add 1 drop of 3 M nitric acid and then 1-2 drops of 0.2 M BaCl₂. Compare the result of this experiment to those seen above for the carbonate ion. Write balanced chemical equations for your observations.

The Iodide Ion I^-
To test for the iodide ion, dissolve a pea-sized amount of sodium iodide (NaI) in a test tube in 1 mL of distilled water; then add 5 drops of bleach (CAUTION, avoid getting bleach on your skin). Write a balanced chemical equation to match your observations. Confirm the presence of I₂ and I^- by observing the reaction between a corn starch packing peanut and the reaction mixture in your test tube. Tear off a piece of a packing peanut and push it into the solution in your test tube with a stirring rod. Record your observations.

As a second test, dissolve a small amount of NaI in 1 mL of distilled water and add a drop of 3 M HNO₃, then 3-4 drops of 0.1 M silver nitrate solution. Record your observations and write a balanced chemical equation.

For a third test, place a very small amount of NaI in a dry test tube. In the fume hood, carefully add 1-2 drops of 18 M sulfuric acid. (CAUTION, avoid breathing the gases formed). Again, record your observations and write a balanced chemical equation. Confirm the presence of I₂ and I^- by observing the reaction between a corn starch packing peanut and the reaction mixture in your test tube. First add about 1 mL of distilled water. Then tear off a piece of a packing peanut and push it into the solution in your test tube with a stirring rod. Record your observations.

The Unknown Ion
You will get a sample of one of the ions studied above. Make sure to write down the number of your unknown in your notebook! Before testing your unknown sample, check your prelab problem flowchart with your TA.

With the appropriate safety precautions, treat a small amount of the unknown with sulfuric acid as above. Carry out further tests to confirm the identity your unknown ion. Record your observations in your notebook.