Experiment 134

Objective:

To identify the chemical structure and formula of an unknown salt sample (NiSO₄) through the analysis of its basic and acidic groups.

Introduction:

The unknown salt, suspected to be nickel sulfate (NiSO₄), is identified based on a series of tests targeting its cation (basic group) and anion (acidic group). These tests will confirm the chemical composition and properties of the salt.

Sample Physical Characteristics:

●      Color: Blue-green

●      Physical State: Crystalline

●      Solubility: Soluble in cool water

Materials:

Chemicals:

●       Unknown salt sample (NiSO₄)

●       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 holder

●       Bunsen burner

●       Thermometer

●       500 mL flask

●       Test tube stand

●       Pipette

Reagents Preparation:

1.    Dissolve 1.275 g of the unknown salt in 50 mL of distilled water using a 500 mL flask.

2.    Prepare 5% HCl and ammonium hydroxide solutions in separate beakers.

3.    Generate hydrogen sulfide gas using a round-bottom flask.

4.    Prepare 5% ferrous sulfate solution in a 250 mL flask.

Procedure:

1.    Basic Group (Cation) Tests:

●       Step 1: Add dilute HCl to the salt sample in a test tube. No precipitation indicates the absence of Pb²⁺ ions.

●       Step 2: Heat the solution and pass H₂S gas through it. No black precipitation indicates the absence of Pb²⁺ and Cu²⁺ ions.

●       Step 3: Heat the solution to expel H₂S gas. Add NH₄Cl and NH₄OH to make the solution basic. The formation of black precipitation indicates the presence of Ni²⁺, Fe²⁺, or Al³⁺ ions.

●       Step 4: Flow H₂S through the basic solution again. No further precipitation suggests the absence of additional Ni²⁺, Co²⁺, or Zn²⁺ ions.

●       Step 5: Reheat the solution to remove H₂S gas. Add NH₄OH, NH₄Cl, and (NH₄)₂CO₃. No precipitation indicates Ba²⁺ and Ca²⁺ are absent.

●       Step 6: Divide the solution into two portions:

●       Portion 1: Heat and add NH₄OH and Na₂HPO₄. No precipitation indicates Mg²⁺ is absent.

●       Portion 2: Heat and evaporate the solution. The absence of any solid residue suggests Na⁺ and K⁺ are not present.

2.    NH₄⁺ Radical Test:

●      Add Nelson reagent to a test tube containing the sample solution. The absence of brown precipitation confirms that NH₄⁺ is not present.

3.    Ni²⁺ Confirmation Test:

●      Dissolve the black precipitate in water and heat. Divide the solution into two portions:

●      Portion 1: Add NH₄OH and NH₄Cl, followed by di-methyl glyoxime. Red precipitate confirms the presence of Ni²⁺.

●      Portion 2: Add NH₄SCN and amyl alcohol. The absence of a blue layer indicates Co²⁺ is not present.

4.    Acidic Group (Anion) Tests:

●      Step 1: Add nitric acid followed by silver nitrate to the sample solution. No precipitation indicates the absence of Cl⁻, Br⁻, and I⁻ ions.

●      Step 2: Add sodium nitroprusside to another portion. No violet color indicates the absence of S²⁻ ions.

●      Step 3: Add dilute HCl and barium chloride. White precipitate formation confirms the presence of SO₄²⁻ ions.

●      Step 4: Ring Test: Add concentrated sulfuric acid to the solution. Cool it and carefully layer ferrous sulfate on top. The absence of a brown ring confirms that NO₃⁻ is not present.

Observations:

●       Cation Identified: Ni²⁺ (nickel ion)

●       Anion Identified: SO₄²⁻ (sulfate ion)

Thus, the unknown salt is identified as Nickel Sulfate (NiSO₄).

Discussion Questions:

1.    What is nickel sulfate?

2.    What happens when H₂S is added to a basic solution of nickel salt?

3.    What color forms when di-methyl glyoxime is added to a basic solution of nickel salt?