6.11 - Qualitative Analysis

Purpose and Physical Observations

Qualitative analysis identifies the cation and anion in a salt from its physical and chemical properties.
Use evidence in sequence: colour and solubility → effect of heat and gas tests → anion and cation tests → confirmatory tests.
Solubility narrows possibilities but does not itself confirm an ion.
White soluble salts form colourless solutions. Transition-metal ions often give coloured salts or solutions:

Ion	Typical solid/solution colour
$F e^{2 +}$	Green solid; green or pale-green solution
$F e^{3 +}$	Brown solid; brown or yellowish-brown solution
$C u^{2 +}$	Blue nitrate/sulphate solution; green copper(II) chloride solid forming blue solution; green insoluble carbonate

A black residue suggests $C u^{2 +}$ ; brown suggests $F e^{3 +}$ ; yellow hot/white cold suggests $Z n^{2 +}$ ; brown hot/yellow cold suggests $P b^{2 +}$ .
Cloudy limewater after heating suggests carbonate; brown acidic gas plus oxygen suggests nitrate; pungent alkaline gas suggests ammonium.
These are preliminary inferences and should be confirmed chemically.
A gas that rekindles a glowing splinter may indicate a nitrate or $A g_{2} C O_{3}$ ; oxygen alone does not confirm nitrate.

Experiment 6.9 — identifying anions:

Aim: identify anions in aqueous salt solutions.
Hypothesis: anions can be identified from chemical-test observations.
Variables: anion type (manipulated), observations (responding), salt-solution volume (fixed).
Materials: sodium carbonate, sodium chloride, sodium sulphate and sodium nitrate; distilled water, limewater and the test reagents listed below.
Apparatus: test tubes, holder and rack, glass rod, dropper, stopper with delivery tube, spatula, $100 c m^{3}$ beaker and $10 c m^{3}$ measuring cylinder.
Procedure:
1. Dissolve each salt separately to make $20 c m^{3}$ solution.
2. Put $2 c m^{3}$ portions in four separate test tubes for the four tests below.
3. For carbonate, add $2 c m^{3}$ of $2.0 mol d m^{- 3}$ $HCl$ and pass any gas into limewater.
4. For chloride, add excess $2.0 mol d m^{- 3}$ $HN O_{3}$ , then $2 c m^{3}$ of $0.1 mol d m^{- 3}$ $AgN O_{3}$ .
5. For sulphate, add excess $2.0 mol d m^{- 3}$ $HCl$ , then $2 c m^{3}$ of $1.0 mol d m^{- 3}$ $BaC l_{2}$ .
6. For nitrate, add $2 c m^{3}$ each of $1.0 mol d m^{- 3}$ $H_{2} S O_{4}$ and $FeS O_{4}$ ; shake. Tilt the tube, slowly add a few drops of concentrated $H_{2} S O_{4}$ along its wall and gently return it upright.
Precautions: add concentrated acid carefully; silver nitrate stains skin brown.
Conclusion: the characteristic observations identify the anions, as summarised below.

Anion	Test / Positive observation / equation
$C O_{3}^{2 -}$	Add dilute acid; pass gas into limewater; Effervescence; limewater turns cloudy. $2 H^{+} + C O_{3}^{2 -} \to C O_{2} + H_{2} O$
$C l^{-}$	Acidify with excess dilute $HN O_{3}$ , then add $AgN O_{3}$ ; White $AgCl$ precipitate. $A g^{+} + C l^{-} \to AgCl$
$S O_{4}^{2 -}$	Acidify with excess dilute $HCl$ , then add $BaC l_{2}$ ; White $BaS O_{4}$ precipitate. $B a^{2 +} + S O_{4}^{2 -} \to BaS O_{4}$
$N O_{3}^{-}$	Add dilute $H_{2} S O_{4}$ and $FeS O_{4}$ ; carefully add concentrated $H_{2} S O_{4}$ ; Brown ring of nitrosyliron(II) sulphate at the boundary

Acidification removes interfering carbonate ions before chloride or sulphate precipitation tests.

Experiment 6.10 — identifying cations:

Aim: identify cations in aqueous solutions.
Hypothesis: cations can be identified from chemical-test observations.
Variables: cation type (manipulated), observations (responding), salt-solution volume (fixed).
Materials: $2.0 mol d m^{- 3}$ sodium hydroxide and ammonia; $1.0 mol d m^{- 3}$ solutions of calcium, magnesium, aluminium, zinc and lead(II) nitrates, iron(II) sulphate, iron(III) chloride, copper(II) sulphate and ammonium nitrate.
Apparatus: test tubes, holder and rack, dropper, $100 c m^{3}$ beaker, red litmus paper, Bunsen burner and $10 c m^{3}$ measuring cylinder.
Use $2 c m^{3}$ salt-solution portions. For the sodium-hydroxide test with no precipitate, heat and test the gas with moist red litmus; use a fresh portion for ammonia.
Conclusion: compare precipitate colours and solubility in excess reagent to identify possible cations; use confirmatory tests where observations overlap.

Add a few drops of reagent, note precipitate colour, then add reagent in excess and note solubility.

Cation	With $NaOH$ / With $N H_{3}$
$C a^{2 +}$	NaOH: White precipitate, insoluble in excess; NH₃: No precipitate
$M g^{2 +}$	NaOH: White precipitate, insoluble in excess; NH₃: White precipitate, insoluble in excess
$A l^{3 +}$	NaOH: White precipitate, soluble in excess; NH₃: White precipitate, insoluble in excess
$Z n^{2 +}$	NaOH: White precipitate, soluble in excess; NH₃: White precipitate, soluble in excess to colourless solution
$P b^{2 +}$	NaOH: White precipitate, soluble in excess; NH₃: White precipitate, insoluble in excess
$F e^{2 +}$	NaOH: Green precipitate, insoluble in excess; NH₃: Green precipitate, insoluble in excess
$F e^{3 +}$	NaOH: Brown precipitate, insoluble in excess; NH₃: Brown precipitate, insoluble in excess
$C u^{2 +}$	NaOH: Blue precipitate, insoluble in excess; NH₃: Blue precipitate, soluble in excess to dark-blue solution
$N H_{4}^{+}$	NaOH: No precipitate; on warming, released $N H_{3}$ turns moist red litmus blue. NH₃: No precipitate

$Z n^{2 +} (aq) + 2O H^{-} (aq) \to Zn (OH)_{2} (s)$

Experiment 6.11 — confirming cations: the aim is to confirm $N H_{4}^{+}$ , $F e^{2 +}$ , $F e^{3 +}$ and $P b^{2 +}$ in aqueous solutions.

To $2 c m^{3}$ ammonium chloride solution, add a few drops of Nessler reagent.
To $2 c m^{3}$ iron(II) sulphate solution, add a few drops of potassium hexacyanoferrate(III), $K_{3} Fe (CN)_{6}$ .
To separate $2 c m^{3}$ iron(III) chloride portions, add potassium hexacyanoferrate(II), $K_{4} Fe (CN)_{6}$ , or potassium thiocyanate, $KSCN$ .
Mix $2 c m^{3}$ lead(II) nitrate solution with $1 c m^{3}$ potassium iodide solution. Add $3 c m^{3}$ distilled water, heat, then cool.

Conclusion: the characteristic observations below confirm the corresponding ions.

Ion	Confirmatory reagent / Observation
$N H_{4}^{+}$	Nessler reagent; Brown precipitate
$F e^{2 +}$	$K_{3} Fe (CN)_{6}$ ; Dark-blue precipitate
$F e^{3 +}$	$K_{4} Fe (CN)_{6}$ ; Dark-blue precipitate
$F e^{3 +}$	$KSCN$ ; Blood-red solution
$P b^{2 +}$	$KI$ ; Yellow $Pb I_{2}$ precipitate; dissolves in hot water and recrystallises on cooling

$KI$ distinguishes $P b^{2 +}$ from $A l^{3 +}$ : lead gives the yellow precipitate, aluminium gives none.
$K_{4} Fe (CN)_{6}$ with $F e^{2 +}$ gives a light-blue precipitate; $K_{3} Fe (CN)_{6}$ with $F e^{3 +}$ gives a greenish-brown colour. These differ from the dark-blue positive tests above.

Observe the solid’s colour.
Heat a small portion and test any gas; observe residue colours while hot and cold.
Add distilled water to another portion and test solubility and solution colour.
If soluble, divide the solution into fresh portions for cation and anion tests.
If insoluble in water, add dilute nitric acid to obtain an aqueous solution for ion tests.
Use preliminary results to select confirmatory tests; never identify the salt from one ambiguous observation.