14Preparation P14: Potassium Dioxalatocuprate(II) Dihydrate Synthesis and Analysis

Physical Description and Appearance: The compound Kaliumdioxalatocuprat(II)-Dihydrat (Potassium dioxalatocuprate(II) dihydrate) presents as hellblaue Kristalle (light blue crystals).
Solubility Profile:
- Aqueous Solubility: The crystals are soluble in warm $H_2O$ . However, they undergo a slow decomposition process yielding $CuC_2O_4$ (Copper(II) oxalate) as a precipitate.
- Solubility in Organic Solvents: The compound is characterized by very low solubility (sehr wenig löslich) in organic solvents.
Stability and Decomposition Factors:
    - Chemical Triggers: The decomposition of the complex in water is significantly accelerated by the presence of strong acids (starke Säuren).
    - Thermal Stability:
        - Above $150\,^\circ\text{C}$ , the compound undergoes rapid dehydration (Wasserabgabe).
        - At $260\,^\circ\text{C}$ , complete decomposition of the substance occurs.

Primary Research Focus: Students must research the chemical backgrounds, application-related facts, and historical context of the preparation. Key topics for review include:
    - Copper and its complex compounds.
    - Oxalic acid and oxalates.
    - Chelate complexes.
Structural Analysis: A requirement of the preparation is to draw the complex anion using chemical drawing software such as Chemdraw. The complex involves the coordination of oxalate ligands to a central copper(II) ion.
Toxicology of Oxalates:
- Explanation of Toxicity: Oxalic acid and soluble oxalates are toxic because the oxalate ion ( $C_2O_4^{2-}$ ) acts as a powerful chelating agent for essential metal ions. Specifically, it reacts with calcium ions ( $Ca^{2+}+$ ) in the body to form insoluble calcium oxalate ( $CaC_2O_4$ ).
- Biological Impact: This precipitation leads to hypocalcemia (depletion of calcium in the blood) and the formation of sharp calcium oxalate crystals in the kidneys, which can cause severe renal damage or kidney failure.
Literature Reference: The synthesis is based on the original publication by S. Kirschner, found in Inorganic Syntheses, 1960, volume 6, page 1.

General Safety Requirement: Practical participants must be informed about the specific dangers associated with the chemicals used and the protocols for safe handling.
Hazard Communication: Rather than simply listing Hazard (H) and Precautionary (P) numbers, students are expected to explain the risks and safety measures in their own words during the presentation.
Key Reagents to Monitor:
    - Potassium Hydroxide ( $KOH$ ): Highly corrosive; the addition to water is strongly exothermic.
    - Copper sulfate pentahydrate ( $CuSO_4 \cdot 5H_2O$ ): Harmful if swallowed, causes skin and eye irritation, and is highly toxic to aquatic life.
    - Oxalic acid dihydrate ( $C_2H_2O_4 \cdot 2H_2O$ ): Harmful if swallowed or in contact with skin; risk of serious eye damage.

Step 1: Preparation of the Potassium Oxalate Solution
    - Place $2.5\,g$ of Oxalic acid dihydrate ( $C_2H_2O_4 \cdot 2H_2O$ ) (used in excess) into a small beaker.
    - Dissolve the acid in $10\,ml$ of distilled water.
    - Carefully add $2.2\,g$ of Potassium hydroxide ( $KOH$ ) pellets.
    - Critical Observation: Note that the solution heats up significantly during this exothermic neutralization reaction.
Step 2: Preparation of the Copper Sulfate Solution
    - Place $1.7\,g$ of Copper sulfate pentahydrate ( $CuSO_4 \cdot 5H_2O$ ) into a test tube.
    - Add $2.5\,ml$ of distilled water.
    - Heat the test tube vigorously using a Bunsen burner until the temperature reaches approximately $90\,^\circ\text{C}$ .
Step 3: Complex Formation
- While stirring the Potassium oxalate solution vigorously, pour the hot contents of the test tube into the beaker.
Step 4: Precipitation and Isolation
    - Place the beaker in an ice bath and allow it to sit undisturbed.
    - A blue precipitate will form during this cooling phase.
    - Separate the precipitate using vacuum filtration through a Büchner funnel (utilizing a membrane pump vacuum).
Step 5: Washing and Drying
- Wash the collected crystals with a small amount of ice-cold distilled water.
- Transfer the product to a drying cabinet (Trockenschrank) and dry at a temperature of $60\,^\circ\text{C}$ .

Laborjournal (Laboratory Journal): All experimental observations, conditions, and precise weigh-ins (Einwaagen) must be recorded carefully.
Visual Documentation: It is recommended to photograph the experimental setups for inclusion in reports.
Formal Lab Reports: Students are required to produce written "Versuchsvorschriften" (experimental procedures) for selected preparations, following a provided template.
Introductory Presentation:
    - A short presentation (Kurzvortrag) must be delivered during the introductory course.
    - The presentation must cover the reaction equations, chemical hazards, and a summary of the practical procedure.
    - Objective: The presentation serves as a preliminary discussion to practice explaining complex chemical relationships clearly and understandably.