In Project 6, we will use chemical analysis techniques to identify unknown salts. This process is termed qualitative analysis.

• In Project 1 of this laboratory program, you learned hot to determine physical characteristics of a material such as solubility in water and in a non-polar solvents, density, the ability to conduct an electric current as a solid, liquid, and/or when dissolved in water, the melting point, and the boiling point.
• Solubility in water, one of the techniques from Project 1, will be a key characteristic in identifying the salts used in these exercises. A general solubility chart is found in Appendix A in the laboratory manual.

Project 6, introduces additional tests for behavior of salts that can be used to help identify the ions of a salt. Each exercise investigates the following behavior of salts and how the experimental results can be used to develop a method for identifying the ions of a salt.

1. Exercise A: Acidity or basicity of an aqueous solution of a salt, i.e. hydrolysis of ions.
2. Exercise B: Behavior of a solid salt on the addition of acid
3. Exercise C: Precipitate formation between ions of a salt and selected reagents
4. Exercise D: Complex ion formation in solution
   
5. Exercise E: Oxidation-reduction reactions
   
6. Exercise F: Two miscellaneous tests

   The cations to be tested are: Ag⁺, Pb²⁺, K⁺, Na⁺, Zn²⁺, Cu²⁺, Ba⁺, Al³⁺, Fe³⁺, NH⁴⁺ and Ni²⁺.

   The anions to be tested are: MnO₄^-, Cl^-, Br^-, I^-, NO₃^-, SO₄^2-, S^2-, CH₃CO₂^- and CO₃^2-.

IMPORTANT NOTE: Please complete the tables in the self-test portion of the lab manual which are associated with this project.

1. Enter the information as the labs are completed.
2. Check the information when the graded labs are returned.
3. Redo any tests for the information was incorrect.

Exercise A: Acidity or Basicity of The Water Solution of A Salt

Cations and anions can react with water to form an acid or basic solution. To learn how these reactions take place, please review Ion Hydrolysis. Litmus paper can be used to determine whether an ion undergoes hydrolysis.

• The test does NOT provide positive proof of the identity of the ions.
• It can often be used to determine an ion if two possibilities are given.

  Example: When a solution of either NaCl or AlCl₃ was placed on litmus paper, the paper turned red. What compound is in the solution?

  1. Neither NA⁺ nor CL ions hydrolyze, so if the solution contains these ions, the litmus test will indicate a neutral solution
  2. The chloride ion of AlCl₃ will not hydrolyze. However, the Al³⁺, a small, highly charged ion, will hydrolyze to yield an acidic solution. Therefore, a solution of AlCl₃ will be acidic.
  3. The test confirms that the solution is AlCl₃

   

  BEFORE COMING TO LAB: Use Project 5 Exercise C-4, General Rules for Ion Hydrolysis and the text book for the class, make a list of:

  1. anions that DO hydrolyze.
  2. cations that DO hydrolyze.
  3. anions that DO NOT hydrolyze.
  4. cations that DO NOT hydrolyze.
  5. Which cations in this project will hydrolyze in water?
  6. Which anions in this project will hydrolyze in water?
  7. Write equations for reactions cations and anions which do hydrolyze.

Exercise B: Behavior of Salts With Acids

The hydronium ion (H₃O⁺) from a strong acid solution can react readily with anions whose conjugate acids are weak acids. For example:

2H₃O⁺(aq) + CO₃^2- (aq) H₂CO₃(aq) + 2H₂O(l)

2H₃O⁺(aq) + S^2-(aq) H₂S (aq) + 2H₂O(l)

H₃O⁺(aq) + CH₃CO₂^-(aq) CH₃CO₂H (aq) + 2H₂O(l)

H₃O⁺(aq) + OH^-(aq) 2H₂O(l)

These reactions can be used to:

1. Dissolve salts that have low solubilities, if the anion of the salt is the anion of a weak acid.
• A general rule for affect of addition of H₃O⁺ to a salt is: If the anion of an insoluble salt is from a weak acid, the solubility of the salt will be enhanced by the addition of H₃O⁺. If the anion of an insoluble salt is from a strong acid, the solubility will not be enhanced by the addition of H₃O⁺.
2. Identify the anion of an unknown salt, if the products of the reaction are readily detected.
   • Here are two ways that to determine if a reaction has occurred:
     1. Evolution of a gas (the solution fizzed)
     2. Smell

Exercise C: Precipitation Formation Between Ions and Selected Reagents

The formation of precipitates with the ions of an unknown salt in aqueous solution and other reagents can be used to help identify and separate the ions of the unknown salt.

1. Chloride Precipitation: Chloride ions, CL, can be added to a solution of an unknown salt. Inspection of the general solubility table (Appendix A) indicates the chloride salts of Ag⁺, Pb²⁺, and Hg₂²⁺ are insoluble in water. Therefore, it is possible to detect the presence of Ag⁺ and Pb²⁺ by adding CL ions. To differentiate between Ag⁺ and Pb²⁺ additional tests (see complex ions section of Project 6) can be carried out.

2. Sulfide Precipitation: The sulfide ion, S^2-, can be added to a solution of an unknown salt. Examination of the general solubility table shows that many sulfides are insoluble, i.e. they have very low Ksp's. Using the solubility table, it is possible to classify cations into two categories

   (a) those that form soluble sulfides
   (b) those that form insoluble sulfides.

   The insoluble sulfides can be divided into two groups,

(b1) those which are insoluble

(b2) those that are very insoluble, by the controlling the acidity and the sulfide ion of the salt solution.

In order to differentiate between the very insoluble and the insoluble ions, we must control the sulfide ion concentrations. At very low sulfide ion concentrations, only the very insoluble sulfides will precipitate. At high concentrations of sulfide ion, all the insoluble sulfides will precipitate.

A low sulfide ion concentration is produced by adding the organic compound thioacetamide to the solution. When the thioacetamide mixture is heated, hydrogen sulfide, H₂S is released.

Because the H₂S is a weak acid, it undergoes acid hydrolysis with water. The hydrolysis reactions can be seen below. At very high concentrations of acid, the equilibrium of the second reaction will shift to the left (Le Chatelier's Principle). This causes the concentration of the sulfide ion to be very low. And thus, only the very insoluble sulfides will precipitate.

H₂S(aq) + H₂O(l) HS^-(aq) + H₃O⁺(aq)
HS^-(aq) + H₂O(l) S^2-(aq) + H₃O⁺(aq)

If the sulfide ion concentration is increased, the all the insoluble sulfides will precipitate. Adding a base, such as ammonia, will reduce the concentration of H₃O⁺ by forming NH₄⁺ and water. The H₂S reactions above will shift to the right producing additional S^2- ion. With additional sulfide ion present, the insoluble sulfides will precipitate. However, the soluble sulfides will not precipitate even in basic H₂S solution.

Therefore, by governing the pH of the solution with thioacetamide present, it is possible to classify the soluble and insoluble sulfides into three categories:

1. Soluble sulfides: Those that do not form precipitates in either acidic or basic H₂S.
2. Insoluble sulfides: Those cations that form precipitates in basic H₂S.
3. Very insoluble sulfides: Those cations that form precipitates in acidic H₂S.

3. Sulfate Precipitation: Another precipitation reaction that will prove useful in the identification of ions in solution is the reaction between cations in solution and the sulfate ion. Sulfate ion may be provide by addition of 0.5M H₂SO₄.

Exercise D: Complex Ions

Please visit the above link to learn about complex ions.

Exercise E: Oxidation-Reduction Reactions

Please visit the above link to learn about redox reactions.

Exercise F: Miscellaneous Tests and Information

Two tests that may be useful in identifying ions include:

1. Addition of OH^- to a solution thought to contain NH₄⁺ will result in the reaction:

NH₄⁺(aq) + OH^-(aq) NH₃(aq) + H₂O(l)

Therefore, the odor of ammonia after the addition of OH^- will indicate the presence of the NH₄⁺ cation.

2. Flame tests are useful in the identification of some cations. Since K⁺ and NA⁺ ions do not form insoluble precipitates with common reagents, it is necessary to find another means of identification.
   1. A solution of potassium ions will burn lavender in an otherwise colorless flame.
   2. A solution of sodium ions will burn yellow.