22Synthesis and Analysis of Mohr's Salt: Diammonium Iron(II) Sulfate Hexahydrate

General Information on Mohr's Salt

• Chemical Name: Diammonium iron(II) sulfate hexahydrate.
• Empirical Formula: $(NH_4)_2Fe(SO_4)_2 \cdot 6H_2O$
• Physical Appearance: The substance consists of weak bluish-green crystals.
• Solubility and Stability:
      * It is easily soluble in water.
      * It is significantly more air-stable than iron(II) sulfate heptahydrate ($FeSO_4 \cdot 7H_2O$), making it a preferred standard in various analytical applications.
• Research Topics for Preparation:
      * Iron and its various oxidation states in aqueous media.
      * Redox reactions.
      * Comprehensive review of the original publication (e.g., via SciFinder, library, or Internet databases) to ensure conceptual understanding of the synthesis.

Synthesis Procedure and Practical Methodology

• Initial Dissolution of Iron:
      * Weigh exactly $1\,g$ of iron powder into a $100\,ml$ beaker.
      * Calculate the required stoichiometric amount of $9\%$ sulfuric acid ($H_2SO_4$) necessary to dissolve $1\,g$ of iron.
      * Dissolve the iron powder in the calculated volume of $9\%$ sulfuric acid under a fume hood while heating using a tripod/ceramic plate/Bunsen burner setup.
• Safety Precautions During Dissolution:
      * Warning: The mixture must not be allowed to evaporate to dryness. If the mixture dries out, sulfur trioxide ($SO_3$) may be released, which is an irritant to the respiratory tract.
      * Stability Hint: A very small residual amount of metallic iron should remain in the solution to prevent the atmospheric oxygen from oxidizing the dissolved $Fe(II)$ to $Fe(III)$.
• Filtration and Concentration:
      * Separate the undissolved iron from the solution via filtration using a Büchner funnel under membrane pump vacuum.
      * Important: The desired product is contained in the filtrate within the suction flask.
      * The filtrate is then transferred to a porcelain dish and concentrated on a water bath until a "crystal skin" (Kristallhaut) begins to form on the surface.
• Preparation of Ammonium Sulfate Solution:
      * In a separate beaker, dissolve the stoichiometric amount of ammonium sulfate ($(NH_4)_2SO_4$) in $4\,ml$ of distilled water while heating.
      * Concentrate this solution on the water bath until a crystal skin just begins to form.
• Final Crystallization:
      * Combine the two hot, saturated solutions (iron(II) sulfate and ammonium sulfate) in a porcelain dish.
      * Allow the mixture to crystallize.
      * Filter the resulting pale bluish-green crystals using a Büchner funnel.
      * Wash the crystals with approximately $1\,ml$ of ice-cold distilled water.
      * Dry the product in the air (typically overnight under a fume hood).

Chemical Analysis: Permanganometry

• Objective: Determine the actual iron content of the prepared Mohr's salt via titration with potassium permanganate ($KMnO_4$) and compare it against the theoretical mass fraction.
• Sample Preparation:
      * Weigh a maximum of $2\,g$ of the product using an analytical balance.
      * Note: Always ensure at least half of the synthesized substance remains as a reserve for potential repetition of the analysis.
      * Dissolve the sample to create $100\,ml$ of sample solution in a volumetric flask.
      * The solution must be absolutely clear before filling to the mark. If turbidity occurs, add one drop of concentrated sulfuric acid and shake well.
• Titration Protocol:
      * Use an aliquot of either $20\,ml$ or $25\,ml$ for each titration.
      * Calculate the theoretical consumption of the titrant beforehand based on a strictly pure compound to guide the process.
      * Add either $6\,ml$ of phosphoric acid or $20\,ml$ of Reinhard-Zimmermann solution to each aliquot to sharpen the endpoint detection.
      * Note: Since chloride ions should be absent from this preparation, the specific components of the Reinhard-Zimmermann solution intended to prevent chloride oxidation are not strictly necessary, but the solution aids in visual clarity.
• Data Accuracy:
      * Account for the specific titer of the potassium permanganate standard solution (determined previously in quantitative analysis modules).
      * If the titer is unknown, it must be re-determined.

Chemical Reactions and Mathematical Considerations

• Primary Dissolution Reaction:
  $Fe + H_2SO_4 \rightarrow FeSO_4 + H_2$
• Synthesis Reaction:
  $FeSO_4 + (NH_4)_2SO_4 + 6H_2O \rightarrow (NH_4)_2Fe(SO_4)_2 \cdot 6H_2O$
• Redox Titration Reaction (Ionic Form):
  $MnO_4^- + 8H^+ + 5Fe^{2+} \rightarrow Mn^{2+} + 5Fe^{3+} + 4H_2O$
• Documentation Requirements:
      * Maintain a precise laboratory journal capturing all observations, experimental conditions, and exact weights.
      * Photographically document experimental setups.
      * The resulting data will be used to produce a formal written report ("Versuchsvorschrift") following a provided template.

Safety and Hazards

• Chemical Hazards:
      * Sulfuric Acid ($H_2SO_4$): Handled as a corrosive substance; requires knowledge of H- and P-phrases regarding skin and eye contact.
      * Sulfur Trioxide ($SO_3$): Gaseous byproduct of overheating; acts as a severe respiratory irritant.
      * Potassium Permanganate ($KMnO_4$): Strong oxidant.
• Presentation Requirements:
      * Students must be able to verbalize hazards and safe handling procedures in their own words rather than simply reading lists of hazard codes.
      * The synthesis and theoretical background must be summarized in a short presentation during the introductory course.