Inorganic Chemistry Lab Notes: Transition Metals and Ammonium Sulfide Separation Scheme

General Lab Conduct and Safety Prerequisities

• Laboratory Schedule: The introductory course takes place on days specified in the schedule from $13:00$ to $18:00$. It begins with the workstation assignment and a mandatory safety briefing (signature required).

• Attendance: Punctuality is mandatory for all appointments.

• Required Materials: Students must bring the specific scripts for each section:

  • Part 1: Fundamentals (Grundlagen).

  • Part 2: Quantitative Analysis (Quantitative Analytik).

  • Part 3: Preparative Chemistry (Präparative Chemie).

• Recommended Textbooks:

  • 1) G. Jander and E. Blasius: Lehrbuch der analytischen und präparativen anorganischen Chemie, S. Hirzel Verlag Stuttgart.

  • 2) E. Riedel, C. Janiak: Anorganische Chemie, de Gruyter Verlag Berlin, New York.

General Behavioral Rules in the Chemical Laboratory

• No working without an assistant present.

• Protective goggles and closed lab coats must be worn at all times.

• Long hair must be tied back.

• Eating (including chewing gum), drinking, and smoking are strictly forbidden.

• Maintain cleanliness at the workstation.

• Experiments should be conducted under the fume hood (Abzug) as much as possible.

• Never touch door handles while wearing gloves.

• No chemicals are allowed on the window sills in the lab hall.

• Be economical with the consumption of chemicals.

• Carefully observe the labeling of all chemicals.

Day 7: Transition Metals – Topics and General Tasks

• Core Topics: Transition metals position in the Periodic Table of Elements (PSE), electron configurations, redox chemistry, and amphoterism.

• Coordination Chemistry: Basic concepts and nomenclature of complexes.

• Detection Reactions: Identification of Chromium, Manganese, Iron, Cobalt, Nickel, and Aluminium. (Copper, Silver, and Gold may follow on Day 8).

• Theoretical References: Acid-base reactions, redox reactions, and complex formation.

• General Assignment:

  • Name all complex compounds occurring in experiments 7.2–7.6 and 7.10–7.13.

  • Define "Chelate Ligand": Provide two examples with structural formulas and identify the "teeth" of the ligand.

Experiment 7.1: Phosphorus Salt Beads as Preliminary Tests

• Safety Warning: Nickel and Cobalt salts are toxic and can cause cancer. Perform under the fume hood.

• Substances Used: $FeCl_3 \times 6 H_2O$, $MnSO_4 \times H_2O$, $NiSO_4 \times 6 H_2O$, $Ni(NO_3)_2 \times 6 H_2O$, $CoCl_2 \times 6 H_2O$, $CoSO_4 \times 7 H_2O$, $Co(NO_3)_2 \times 6 H_2O$, $KCr(SO_4)_2 \times 12 H_2O$, $CrCl_3 \times 6 H_2O$, and $KAl(SO_4)_2 \times 12 H_2O$. Also used: $FeSO_4 \times 6 H_2O$.

• Bead Preparation:

  1. Heat a magnesia rod in the burner flame and dip the hot end into ammonium sodium hydrogen phosphate ($(NH_4)NaHPO_4 \times 4 H_2O$, "phosphorus salt").

  2. Heat until a clear melt forms and gas bubbles ($H_2O$ and $NH_3$) stop appearing.

  3. Target bead size is $2\text{ to }3\text{ mm}$.

  4. Pick up a tiny amount of substance and melt again.

• Chemical Principle: Heating converts salts into oxides, which dissolve into the glassy metaphosphate $(NaPO_3)_x$ formed from the phosphorus salt.

• Redox Environments: Oxidation beads cool in air; reduction beads cool inside the reduction cone of the flame.

Key Detection Reactions for Transition Metals

Experiment 7.2: Aqua-Complexes of $Cr^{3+}$

• Procedure: Dissolve powdered $KCr(SO_4)_2 \times 12 H_2O$ in distilled water and heat in the flame.

• Concept: Note how $Cr^{3+}$ is coordinated in aqueous solution and the state of the 12 water molecules in the solid salt.

Experiment 7.3: Amphoterism of $Zn(II)$

• Procedure: Add $2\text{ mol/l }NaOH$ dropwise to $ZnSO_4$ solution until turbidity appears, then continue adding dropwise until a change occurs.

• Observations: Initial formation of white $Zn(OH)_2$ followed by dissolution into the tetrahydroxozincate complex $[Zn(OH)_4]^{2-}$.

Experiment 7.4: Nickel(II) Detection

• Procedure: Add $10\%$ acetic acid to Nickel(II) chloride solution, then $3.5\%$ ammonia until weakly basic. Add ethanolic Dimethylglyoxim (DMG) solution.

• Observation: Formation of a pink-red precipitate: Bis(dimethylglyoximato)nickel(II).

• Disposal: Dissolve the precipitate with $24\%$ $HCl$ before disposal in heavy metal waste.

Experiment 7.5: Cobalt(II) Detection

• Procedure: Add $10\%$ acetic acid and solid ammonium thiocyanate ($NH_4SCN$) to Cobalt(II) nitrate solution. Layer with diethyl ether and add isoamyl alcohol.

• Observation: The ether phase turns blue due to the formation of the tetrakis(thiocyanato)cobaltate(II) complex.

Experiment 7.6: Iron(III) Detection

• 7.6 a) Thiocyanate: $Fe^{3+}$ reacts with $NH_4SCN$ to form a blood-red complex. Adding sodium fluoride ($NaF$) decolorizes the phases by forming the more stable, colorless hexafluoridoferrate(III) complex.

• 7.6 b) Berlin Blue: Acidify $Fe^{3+}$ with dilute $HCl$ and add potassium hexacyanoferrate(II) ($K_4[Fe(CN)_6]$). A deep blue precipitate (Berlin Blue) forms.

Experiment 7.7: Manganese(II) Detection

• Procedure: Add concentrated $HNO_3$ and lead dioxide ($PbO_2$) to $Mn^{2+}$ solution; heat carefully.

• Observation: A deep violet Permanganate ($MnO_4^-$) solution forms.

• Interferences: Chloride ions interfere but are oxidized by excess $PbO_2$. High Manganese concentrations require more oxidant.

• Advanced Property: Unlike Chromate(VI), Permanganate does not condense into higher molecular aggregates in acidic solution. However, concentrated $H_2SO_4$ with solid $KMnO_4$ forms highly explosive, volatile $Mn_2O_7$.

Experiment 7.8: Aluminium(III) Detection

• Procedure: Add glacial acetic acid and Morin solution to potassium aluminium sulfate. Inspect under UV light.

• Observation: Green fluorescence of the aluminium-morin complex.

• Constraints: High concentration of $HCl$ prevents complex formation.

Experiment 7.9: Chromium(III) Detection

• Oxidation to Chromate: Mix $30\%$ $H_2O_2$ with $6\text{ mol/l }NaOH$, then add $Cr^{3+}$ solution. Boil until yellow (removal of excess $H_2O_2$).

• Chromium Peroxide Test: Acidify the cold yellow solution with $9\%$ $H_2SO_4$, layer with ether, and add $3\%$ $H_2O_2$. A temporary blue color in the ether phase indicates chromium peroxide ($CrO(O_2)_2$– or $CrO_5$).

Experiment 7.10 & 7.11: Copper Identification

• Dissolution: Dissolve copper powder in $65\%$ $HNO_3$. This produces toxic brown $NO_2$ gas and a blue solution.

• Tetraammine Complex: Neutralize the solution with $6\text{ M }NaOH$, then add concentrated ammonia. The formation of a deep "ink-blue" solution ($[Cu(NH_3)_4(H_2O)_2]^{2+}$) signifies Copper(II).

Experiment 7.12: Silver Mirror (Tollens' Probe)

• Procedure: Add $3.5\%$ ammonia to silver nitrate until the grey precipitate ($Ag_2O$) dissolves. Add $2\text{ M }NaOH$ and saturated glucose solution. Heat gently (do not boil).

• Observation: A silver mirror deposits on the glass wall.

• Safety: Silver nitrate causes black spots on skin; nitric acid dissolves the mirror afterward.

Experiment 7.13: Gold(III) and Gold Purpure

• Gold Dissolution: Gold dissolves in "Aqua Regia" (Königswasser), a mixture of concentrated $HNO_3$ and three parts concentrated $HCl$.

• Cassius’s Gold Purpure: Add a centrifuge-cleared Tin(II) chloride ($SnCl_2$) solution to the gold solution. A purple/ruby-red colloidal gold suspension forms.

Polyoxometallates: Vanadium(V) Demonstration

Oxometallates of Group 5 and 6 elements form higher aggregated polyoxometallates via condensation depending on pH:

• $2 [VO_4]^{3-} + 2 H^+ ⅄ [V_2O_7]^{4-} + H_2O$ (Colorless)

• $2 [V_2O_7]^{4-} + 4 H^+ ⅄ [V_4O_{12}]^{4-} + H_2O$

• $5 [V_4O_{12}]^{4-} + 8 H^+ ⅄ 2 [V_{10}O_{28}]^{6-} + H_2O$ (Orange-red)

• $[V_{10}O_{28}]^{6-} + 6 H^+ ⅄ 5 [V_2O_5 \times aq] + H_2O$

• $[V_2O_5 \times aq] + 2 H^+ ⅄ 2 [VO_2]^+ + H_2O$ (Pale yellow)

• Redox Step: Adding sodium sulfite ($Na_2SO_3$) reduces the solution to blue Vanadium(IV).

Day 8: Ammonium Sulfide Separation Scheme (Trennungsgang)

This separation is used for $Cr$, $Mn$, $Fe$, $Co$, $Ni$, and $Al$.

I. Sample Preparation

• Buffer the sample with $3.5\%$ $NH_3$ and solid $NH_4Cl$ to $pH\text{ }8\text{ to }9$.

• Solid samples: Dissolve in water or $7\%$ $HCl$ with heating before buffering.

II. Sulfide Precipitation

• Add $7\%$ $Na_2S$ solution and heat in a water bath. Repeatedly centrifuge and test for completeness of precipitation.

• Precipitates: $NiS$, $FeS$, $CoS$ (black); $MnS$ (pink); $Cr(OH)_3$ (green); $Al(OH)_3$ (white).

III. & IV. Washing and Acid Separation

• Wash the residue with ammoniacal water.

• Add $7\%$ $HCl$ and boil until $H_2S$ gas is gone.

• Centrifugation:

  • Residue: Contains black $NiS$ and $CoS$.

  • Centrifugate: Contains $Fe$, $Mn$, $Cr$, and $Al$.

V. Nickel and Cobalt Processing

• Dissolve the black residue in $10\%$ acetic acid and $1\text{ drop }30\% H_2O_2$. Boil extensively to remove excess $H_2O_2$.

• Adjust $pH$ to $8$ with $NH_3$. Split into two parts for Nickel (DMG) and Cobalt (Thiocyanate) tests.

VI. & VII. Alkaline Sturz (Separation of Fe/Mn from Cr/Al)

• Add concentrated $HNO_3$ to the centrifugate from step IV and evaporate to a small remainder. Neutralize with $NaHCO_3$ until orange/brown.

• Alkaline Sturz: Mix $6\text{ M }NaOH$ and $3\% H_2O_2$ ($1:1$ ratio). Pour the neutralized sample into this alkaline mixture (never vice-versa).

• Boil until bubbles stop.

• Result: Dark brown residue (Iron(III) hydroxide and Manganese(IV) oxide) vs. yellow liquid (Chromate and Aluminate).

VIII. & IX. Final Identification

• Fe/Mn: Dissolve the dark residue in $7\%$ $HCl$; test for Iron (Thiocyanate/Berlin Blue) and Manganese ($PbO_2/HNO_3$).

• Cr/Al: Acidify centrifugate with $NH_4Cl$ until $pH\text{ }8$. $Al(OH)_3$ precipitates. Centrifuge, dissolve in acetic acid, and test with Morin. Identify Chromium in the remaining liquid as Chromium peroxide.

Special Procedure: Mono-Chemistry Liquid Analysis

If the sample contains $Mn^{2+}$, $Fe^{3+}$, $Co^{2+}$, $Ni^{2+}$, $Li^+$, $Ca^{2+}$, $Sr^{2+}$, and $Ba^{2+}$:

1. Take a $2\text{ to }3\text{ ml}$ portion.

2. Buffer to $pH\text{ }8\text{ to }9$ with $NH_3/NH_4Cl$.

3. Precipitate transition metals with $Na_2S$. Centrifuge.

4. Treat the centrifugate (containing Alkaline Earths/Lithium) with $7\%$ $HCl$ to remove $H_2S$.

5. Add $Na_2CO_3$ to precipitate alkaline earth carbonates.

6. Wash and dissolve the carbonate residue in $10\%$ acetic acid and proceed with the Chromate-Sulfate separation method.

Chemical Inventory Summary

• E1-E9 (Einer-Satz): Frequently used reagents like $NH_3$, $NH_4Cl$, $CH_3COOH$, $NaOH$, and mineral acids.

• F1-F17 (Fünfer-Satz): Specialized salts and reagents (e.g., $AgNO_3$, $SODA$, $NATRON$, concentrated acids like $96\% H_2SO_4$).

• V1-V12 (Vergleichs-Satz): Standardized chloride and sulfate solutions ($AlCl_3$, $KI$, $ZnSO_4$, etc.) for comparative testing.

• Abzüge (Fume Hoods): Concentrated $HCl$, $HClO_4$, $H_2O_2$, and organic solvents (Ether, Ethanol, DMG, Morin).