27Synthesis and Preparation of Tetraamminecopper(II) sulfate monohydrate

Overview of Tetraamminecopper(II) sulfate monohydrate

• Chemical Formula: $[Cu(NH_3)_4]SO_4 \cdot H_2O$
• Physical Appearance: The complex appears as deep dark blue crystals.
• Stability and Reactivity: The product is unstable when exposed to air and will undergo decomposition over time.
• Explanation for Decomposition: The complex decomposes slowly when standing in air because the ligand, ammonia ($NH_3$), is a gas under normal conditions (room temperature and atmospheric pressure). As the gas escapes the crystal lattice, the complex breaks down.

Theoretical Background and Research Tasks

• Chemical Context for Research:
      - Copper Chemistry: Investigation focuses primarily on the oxidation states of copper in aqueous media.
      - Coordination Chemistry: Analysis of copper complex compounds, specifically the transition between aqua complexes (where water is the ligand) and ammine complexes (where ammonia is the ligand).
• Literature and Databases:
      - Students are required to use databases such as SciFinder, university libraries, or the internet to locate original publications and literature related to this synthesis.
      - Reference Literature: The primary procedure follows J. Strähle, E. Schweda, Jander-Blasius – Lehrbuch der analytischen und präparativen anorganischen Chemie, S. Hirzel Verlag, 16th Edition, 2006, page 229.
• Reaction Equations: Students must formulate and understand all relevant chemical equations, including the formation of the complex and any intermediate precipitates, to explain them during an oral presentation.

Safety and Handling of Chemicals

• Hazard Communication:
      - Students must be familiar with the H- and P-phrases (Hazard and Precautionary statements) for all chemicals used.
      - It is a requirement to be able to explain these hazards in one's own words rather than simply listing code numbers.
• Specific Chemical Risks:
      - Ammonia ($NH_3$): $25\%$ Ammonia solution must be handled with extreme caution and exclusively under a fume hood (Abzug) due to its volatility and corrosiveness.
      - Diethylether: Used in the washing step; noted as being highly flammable (Hochentzündlich).
      - Ethanol: Used for layering and washing; requires handling in a ventilated environment.

Synthesis Procedure: Step-by-Step

• Initial Dissolution:
      - Weigh out $2.5\,g$ of Copper sulfate pentahydrate ($CuSO_4 \cdot 5H_2O$).
      - Dissolve the salt in approximately $3\,ml$ of distilled water in a small, tall beaker or a suitable test tube.
      - Apply gentle heating to facilitate the dissolution process.
• Complex Formation:
      - Add $25\%$ ammonia solution dropwise and with great caution while working in a fume hood.
      - Continue adding the ammonia until the initial precipitate (basic copper salts) is completely redissolved, resulting in a clear, deep dark blue solution.
• Layering for Crystallization:
      - Allow the solution to cool to room temperature.
      - First Layer: Carefully layer $2-3\,ml$ of an ethanol-water mixture ($1:1$) over the solution. This should be done by letting the liquid flow slowly down the inner wall of the beaker using a pipette.
      - Second Layer: In the same manner, layer $2-3\,ml$ of $96\%$ ethanol over the ethanol-water layer (performed under a fume hood).
• Crystallization Phase:
      - Cover the beaker with a watch glass or a small crystallizing dish (ensuring it is stable and will not tip over).
      - Let the setup stand undisturbed in a protected area of the fume hood for $30\text{ to }60$ minutes.
• Filtration and Purification:
      - Filter the resulting deep dark blue crystals using a Büchner funnel supported by a membrane pump vacuum.
      - Washing: Wash the crystals twice with a small amount of $96\%$ ethanol and once with a small amount of diethylether.
      - Drying: Allow the final product to air dry within the fume hood.

Presentation and Documentation Requirements

• Oral Presentation:
      - Students must summarize their findings in a short presentation during the introductory course.
      - The presentation must cover the theoretical background, the practical steps of the synthesis, and the chemistry involved.
      - This serves as a preliminary discussion and an exercise in clearly explaining chemical concepts.
• Laboratory Journal:
      - All observations and conditions must be meticulously recorded.
      - Documentation must include exact weights (initial samples/"Einwaagen") and experimental setups.
      - Photographic documentation of experimental setups is highly recommended.
• Written Reports:
      - Students are required to prepare formal written reports ("Versuchsvorschriften") for selected experiments.
      - A sample template for the report is provided by the Institute for Chemistry and Biochemistry.