Experiment 116

Objective:

To analyze and identify the basic and acidic components in an unknown salt sample, hypothesized as potassium iodide (KI).

Introduction:

Physical Properties of Sample

●      Color: White

●      Physical State: Crystal

●      Solubility: Soluble in cold water

The salt is identified through tests for basic and acidic groups, enabling determination of its chemical structure and formula.

Materials

●      Sample (presumed as KI)

●      Distilled water

●      Hydrochloric acid (HCl)

●      Hydrogen sulfide gas (H₂S)

●      Ammonium chloride (NH₄Cl)

●      Ammonium hydroxide (NH₄OH)

●      Ammonium carbonate ((NH₄)₂CO₃)

●      Sodium hydrogen phosphate (Na₂HPO₄)

●      Freshly prepared ferrous sulfate (FeSO₄)

Apparatus

●       Test tubes and test tube holder

●       Bunsen burner

●       Thermometer

●       500 mL flask

●       Test tube stand

●       Pipette

Reagents Preparation

●       Sample Solution: Dissolve 1.275 g of the supplied sample in 50 mL of distilled water.

●       5% HCl Solution: Prepared in a beaker.

●       Ammonium Hydroxide Solution: Prepared in a beaker.

●       Ferrous Sulfate Solution: 5% solution prepared in a 250 mL flask.

Procedure

Analysis of Basic Group (Cation) in Salt

1.    Preliminary Test

●      Place the sample in a test tube and add dilute HCl.

●      Observation: No precipitate; indicates absence of Pb²⁺ ions.

2.    Hydrogen Sulfide Test

●      Heat the sample and introduce H₂S gas.

●      Observation: No precipitate; confirms absence of Pb²⁺ and Cu²⁺ ions.

3.    Ammonium Hydroxide and Ammonium Chloride Test

●      Add NH₄Cl and NH₄OH to make a basic solution.

●      Observation: No precipitate; suggests Fe²⁺ and Al³⁺ ions are absent.

4.    Secondary Hydrogen Sulfide Test

●      Introduce H₂S again in the basic solution.

●      Observation: No precipitate; indicates absence of Ni²⁺, Co²⁺, and Zn²⁺ ions.

5.    Carbonate Test

●      Heat the solution to remove H₂S, then add NH₄OH, NH₄Cl, and (NH₄)₂CO₃.

●      Observation: No precipitate; confirms absence of Ba²⁺ and Ca²⁺ ions.

6.    Division of Solution

●      Portion 1: Heat, add NH₄OH and Na₂HPO₄. No precipitate, indicating Mg²⁺ is absent.

●      Portion 2: Vaporize and dry. Sample presence suggests potential Na⁺ or K⁺ ions.

7.    Confirmatory Tests for Cations

●      Potassium: Add potassium pyroantimonate. No precipitate; Na⁺ is absent.

●      Potassium: Add tartaric acid and alcohol; white precipitate confirms K⁺.

●      Potassium: Add sodium cobalty nitrite solution; yellow precipitate further confirms K⁺.

Analysis of Acidic Group (Anion) in Salt

1.    Preliminary Test

●      Add nitric acid and silver nitrate to the sample solution.

●      Observation: Yellow precipitate suggests Cl⁻, Br⁻, or I⁻ presence.

2.    Ammonium Hydroxide Solubility Test

●      The yellow precipitate is insoluble, indicating I⁻ presence.

3.    Additional Tests

●      Sulfide Test: Add sodium nitroprusside; no violet color indicates absence of S²⁻.

●      Sulfate Test: Add dilute HCl and barium chloride; no precipitate, confirming SO₄²⁻ absence.

●      Nitrate Ring Test: Add concentrated H₂SO₄, then ferrous sulfate to form a layer. Absence of a brown ring confirms NO₃⁻ absence.

Observations:

●       Cation (Basic Group): Potassium ion (K⁺) is present.

●       Anion (Acidic Group): Iodide ion (I⁻) is present.

Conclusion:  The sample is identified as potassium iodide (KI).

Questions

What is potassium iodide salt?What occurs when tartaric acid and alcohol are added to a potassium salt solution?What occurs when sodium cobalty nitrite is added to a potassium salt solution?