Experiment 176

Objective

To analyze and identify the basic and acidic components in an unknown salt sample, hypothesized as ferric nitrate (Fe(NO₃)₃).

Introduction

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

Physical Properties of Sample

·       Color: Colorless

·       Physical State: Crystal

·       Solubility: Soluble in cold water

Materials

Chemicals

·       Sample (presumed as Fe(NO₃)₃)

·       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: Brown, jelly-like precipitate forms, suggesting Fe³⁺ presence.

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. No sample presence, suggesting Na⁺ and K⁺ are absent.

7.    Confirmatory Tests for Ferric Ion (Fe³⁺)

·       Dissolve the brown precipitate in water and heat; it dissolves, confirming Fe³⁺ presence.

·       First Test: Add potassium ferricyanide to the solution. Formation of concentrated blue precipitate confirms Fe³⁺.

·       Second Test: Add ammonium thiocyanate (NH₄SCN). Blood-red color forms, further confirming Fe³⁺ presence.

Analysis of Acidic Group (Anion) in Salt

1.    Preliminary Test

Add nitric acid and silver nitrate to the sample solution.

Observation: No white precipitate, suggesting absence of Cl⁻, Br⁻, or I⁻ ions.

2.    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. Brown ring formation between the layers confirms NO₃⁻ presence.

Observations

·       Cation (Basic Group): Ferric ion (Fe³⁺) is present.

·       Anion (Acidic Group): Nitrate ion (NO₃⁻) is present.

Conclusion:  The sample is identified as ferric nitrate (Fe(NO₃)₃).

Questions

1.    What is ferric nitrate?

2.    What occurs when H₂S is added to a ferric salt solution?

3.    What color forms when potassium ferricyanide is added to ferric nitrate solution?

4.    What happens when NH₄SCN is added to ferric nitrate solution?