Lesson Plan: Salt Analysis

Practical Chemistry

Salt Analysis (Qualitative)

The detective work of chemistry. Identifying Cations (Basic radicals) and Anions (Acid radicals) in an inorganic salt. The key is Systematic Analysis—skipping steps or group reagents leads to incorrect results.

🔎

⚠️ Prerequisites

Solubility Product ($K_{sp}$): Principle behind precipitation of groups.
Common Ion Effect: Why HCl is added before $H_2S$ in Group 2.
Redox: Reactions involved in confirmatory tests (e.g., Chromyl Chloride).

🧠 Study Approach

Elimination Method: You identify ions by ruling them out group by group. Memorize the Group Reagents and the Color of Precipitates. Without this table, you are guessing.

Study Sequence

1. Preliminary Tests (Dry Tests)

Physical State: Colour and Odour ($NH_3$ smell = Ammonium).
Dry Heating: Gas evolved ($NO_2$ brown gas, $CO_2$ lime water).
Flame Test: Paste with Conc HCl. $Na$ (Golden Yellow), $Sr$ (Crimson), $Ba$ (Apple Green), $Cu$ (Bluish Green).

2. Anions: Dilute Acid Group

Add Dilute $H_2SO_4$.
Carbonate ($CO_3^{2-}$): Brisk effervescence ($CO_2$).
Sulphide ($S^{2-}$): Rotten egg smell ($H_2S$).
Nitrite ($NO_2^-$): Brown fumes ($NO_2$).

3. Anions: Conc. Acid Group

Add Conc $H_2SO_4$.
Chloride ($Cl^-$): White fumes ($HCl$). Confirmatory: Chromyl Chloride Test.
Bromide ($Br^-$): Reddish brown fumes ($Br_2$).
Nitrate ($NO_3^-$): Brown fumes (intensifies with Cu turnings). Confirmatory: Brown Ring Test.

4. Anions: Independent Group

Do not react with acids.
Sulphate ($SO_4^{2-}$): Add $BaCl_2 \to$ White ppt ($BaSO_4$) insoluble in mineral acids.
Phosphate ($PO_4^{3-}$): Ammonium Molybdate test $\to$ Canary Yellow ppt.

5. Cations: Group 0 to 2

Group 0 ($NH_4^+$): Heat with NaOH $\to$ Ammonia smell. Nessler's Reagent $\to$ Brown ppt.
Group 1 ($Pb^{2+}$): Dil HCl $\to$ White ppt ($PbCl_2$). Soluble in hot water.
Group 2 ($Cu^{2+}, As^{3+}$): $H_2S$ in Acidic medium. Black ppt ($CuS$) or Yellow ppt ($As_2S_3$).

6. Cations: Group 3 & 4

Group 3 ($Fe^{3+}, Al^{3+}$): $NH_4OH$ in presence of $NH_4Cl$. Brown ppt ($Fe$) or Gelatinous White ($Al$).
Group 4 ($Zn^{2+}, Mn^{2+}, Ni^{2+}, Co^{2+}$): $H_2S$ in Basic medium. Flesh/Buff ppt ($Mn$), Dirty White ($Zn$), Black ($Ni/Co$).

7. Cations: Group 5 & 6

Group 5 ($Ba^{2+}, Sr^{2+}, Ca^{2+}$): $(NH_4)_2CO_3$ in basic medium $\to$ White carbonate ppt. Flame test distinguishes them.
Group 6 ($Mg^{2+}$): Disodium Hydrogen Phosphate $\to$ White ppt.

🎯 How to Practice

Memorize Group Reagents: Write out the table: Group 1 (Dil HCl), Group 2 (H2S + HCl), etc. ten times. If you mess up the reagent, you mess up the analysis.

Reaction Logic: Understand the Brown Ring Test: $NO_3^- + 3Fe^{2+} + 4H^+ \to NO + 3Fe^{3+} + 2H_2O$. The ring is $[Fe(H_2O)_5NO]^{2+}$. This is a favourite viva question.

Color Codes: Memorize unique colors. Blue Solution $\to$ $Cu^{2+}$. Green/Brown residue $\to$ $Fe/Cr$. Pink $\to$ $Co/Mn$.

📝 Reagent Table (Cations)

Group 0 ($NH_4^+$)

None / Nessler's

Group 1 ($Pb^{2+}$)

Dilute HCl

Group 2 ($Cu^{2+}$)

$H_2S$ + Dil HCl

Group 3 ($Fe, Al$)

$NH_4OH + NH_4Cl$

Group 4 ($Zn, Mn$)

$H_2S + NH_4OH$

Group 5 ($Ba, Ca$)

$(NH_4)_2CO_3 + NH_4OH$

Group 6 ($Mg^{2+}$)

Disodium Hydrogen Phosphate

Search This Blog