Experiment 4: Synthesis and Analysis of an Iron(III)-Oxalate Complex Study Guide

Introduction to Synthetic Chemistry: Iron(III)-Oxalate Complex

• Objective: The primary goal is to synthesize a coordination compound (an iron-oxalate complex) and subsequently analyze its iron composition using spectrophotometry to determine its empirical formula and percent yield.

• The Ligand: The anionic compound oxalate ($C_2O_4^{2-}$) acts as the ligand. It is a Lewis base that coordinates with the iron ion.

• Oxalate Chemistry:

  • The oxalate ion is derived from the deprotonation of oxalic acid ($H_2C_2O_4$).

  • It is described as a bidentate ligand because it possesses two coordination sites (electron lone pairs) capable of binding to a single metal ion.

  • Ligands binding at more than one site are generally referred to as chelates (derived from the Greek word for "claw").

• Target Complex: The synthesis involves complexing oxalate with iron in the $+3$ oxidation state, referred to as $Fe(III)$. The resulting complex is a potassium salt with the general formula $K_w[Fe_x(C_2O_4)_y] \cdot zH_2O$.

Principles of Spectrophotometry and Beer's Law

• Definition: Spectrophotometry measures the interaction of light with matter. An analyte absorbs specific wavelengths of light relative to its properties.

• Physical Observation: When a sample absorbs visible light, the eye perceives the complementary colors that are not absorbed.

  • Example: The green $Fe(III)$-oxalate complex absorbs red light.

• Quantitative Metrics:

  • Transmittance ($T$): The ratio of the intensity of light transmitted through the sample ($I_t$) to the initial incident light intensity ($I_{in}$):         $T = \frac{I_t}{I_{in}}$

  • Percent Transmittance ($\%T$): Defined as $T \times 100\%$.

  • Absorbance ($A$): Defined mathematically as:         $A = -\log(T) = \log\left(\frac{I_{in}}{I_t}\right)$

• Beer-Lambert Law (Beer's Law): Relates absorbance to concentration ($c$), pathlength ($b$), and molar absorptivity ($\epsilon$):     $A = \epsilon_{\lambda}bc$

  • Molar Absorptivity ($\epsilon$): A constant specific to the molecule, solvent, and wavelength ($\lambda$), typically measured in $M^{-1} cm^{-1}$.

  • Pathlength ($b$): The distance light travels through the sample, usually in $cm$.

  • Concentration ($c$): The molarity of the analyte ($M$).

• Instrumentation: A spectrophotometer or spectrometer is used. In practice at UCSD CHEM 7L, it is recommended to record $\%T$ (because the instrument response to $\%T$ is linear) and then convert it to $A$.

Calibration Curves and Sensitivity

• Calibration Curve: A plot of absorbance ($A$) versus concentration ($c$) at a fixed wavelength.

  • The slope of the resulting line equals $\epsilon_{\lambda}b$.

  • This curve allows for the determination of unknown concentrations by comparing their absorbance to the standard plot under identical conditions.

• Wavelength Selection: For maximum sensitivity, measurements are taken at the wavelength of maximum absorbance ($\lambda_{max}$).

  • For the $Fe(II)$-bipyridine complex used in this lab, $\lambda_{max} = 520\,nm$.

• Measurement Range: Accurate readings typically fall between $10\%$ and $90\%$ transmittance. Solutions outside this range should be diluted or concentrated.

Part I Procedure: Synthesis of the Fe(III)-Oxalate Complex

• Step 1: Water Bath Preparation: Prepare a boiling water bath in a $250\text{-mL}$ beaker with $4\text{-}5$ boiling chips. Set hotplate to $350\,^{\circ}C$. Heat a second beaker of DI water for washing.

• Step 2: Iron Solution: Weigh $1.0$ to $1.4\,g$ of $Fe(NH_4)_2(SO_4)_2 \cdot 6H_2O$. Dissolve in $3\,mL$ of DI water with $2$ drops of $6\,M$ $H_2SO_4$ (Corrosive). Warm slightly if solids do not dissolve.

• Step 3: Initial Precipitate Formation:

  • Add approximately $6\,mL$ of $1\,M$ oxalic acid.

  • A yellow precipitate of $Fe(C_2O_4) \cdot 2H_2O$ forms.

  • Reaction: $Fe^{2+}(NH_4)_2(SO_4)_2 \cdot 6H_2O + H_2C_2O_4 \rightarrow Fe^{2+}(C_2O_4) \cdot 2H_2O + H_2SO_4 + (NH_4)_2SO_4 + 4H_2O$

  • Heat in water bath for $~5$ minutes. Decant supernatant. Wash the precipitate three times with $3\text{-}4\,mL$ of hot DI water ($80\text{-}90\,^{\circ}C$).

• Step 4: Oxidation to Fe(III):

  • Add $3.0\,mL$ of $1.7\,M$ $K_2C_2O_4$ to the yellow precipitate.

  • Stir and maintain temperature at $40\,^{\circ}C$ ($38\text{-}44\,^{\circ}C$ range).

  • Add $10\,mL$ of $6\%$ $H_2O_2$ dropwise. This reaction is exothermic. Bubbles of $O_2$ will form.

• Step 5: Final Complex Formation:

  • The resulting brown solid is $Fe(OH)_3$.

  • Heat to boiling for $10$ seconds, then add $2.0\,mL$ of $1\,M$ oxalic acid.

  • Stir, and add an additional $0.8\,mL$ of oxalic acid until a clear yellow-green solution of $Fe_x(C_2O_4)_y$ is obtained.

• Step 6: Crystallization: Add $10\,mL$ of $95\%$ ethanol (EtOH). If no precipitate forms, add $2\,mL$ more EtOH and place in an ice bath.

• Step 7: Recrystallization: Heat the solution to redissolve crystals into a clear green solution. Boil for $10$ seconds. Add small amounts of DI water if it fails to dissolve ($0.25\,mL$ increments). Allow to cool gradually to room temperature. Wrap in aluminum foil to protect from light (photosensitive) and store.

• Step 8: Isolation: Perform vacuum filtration using a crucible and filter paper. Wet paper only with $95\%$ EtOH (do not use water). Wash twice with $5\,mL$ of $95\%$ EtOH. Air dry for $30$ minutes.

• Step 9: Yield: A minimum of $0.15\,g$ of crystals is required.

  • Quality Check: Good crystals are shiny and green (plate-like or block-like gem morphologies). Poor crystals are sand-like grains, fine powders, or mixed with clear oxalic acid crystals.

Parts II-IV Procedure: Spectrophotometric Analysis

• The Conversion Strategy: The $Fe(III)$-oxalate complex is a weak absorber of light. For better analysis, it is converted to the intensely red $Fe(II)$-bipyridine complex ($Fe(bpy)_3^{2+}$).

• Chemical Steps:

  1. Oxalate Removal: Calcium chloride ($CaCl_2$) is added to precipitate oxalate as white $CaC_2O_4$.

  2. Centrifugation: The mixture is spun at $2000\,rpm$ for $2$ minutes. A counterbalance tube within $\pm 0.5\,g$ must be used.

  3. Reduction: Ascorbic acid (Vitamin C) reduces $Fe(III)$ to $Fe(II)$.

  4. Complexation: Bipyridine (bpy) ligand is added to form the red complex.

  5. Buffering: Acetate buffer maintains a pH of approximately $4.7$ (optimal range is $2\text{-}8$).

• Standard Preparation (Part II):

  • Dissolve $~0.12\,g$ $Fe(NH_4)_2(SO_4)_2 \cdot 6H_2O$ in a $100\text{-mL}$ volumetric flask (Primary Stock).

  • Combine $~80\,mg$ ascorbic acid, $2.00\,mL$ primary stock, $15\,mL$ of $0.0020\,M$ bipyridine, and acetate buffer in a $50\text{-mL}$ flask (Secondary Stock).

• Unknown Preparation (Part III):

  • Dissolve $~0.12\,g$ of synthesized green crystals in DI water with $6\,mL$ of $6\,M$ $H_2SO_4$.

  • Add $15\,mL$ of $0.5\,M$ $CaCl_2$, centrifuge, and collect supernatant.

  • Add $2.00\,mL$ of this supernatant to $~80\,mg$ ascorbic acid, $20\,mL$ bipyridine, and acetate buffer ($pH \approx 5$).

• Measurement (Part IV):

  • Prepare serial dilutions (suggested ratios $1:10$, $1:5$, $1:3.3$, $1:2$, and $1:1.4$).

  • Zero the instrument with a DI water blank ($100\% T$).

  • Measure standard samples in order of increasing concentration to minimize carryover.

  • Ensure unknown absorbance values fall within the range of the standards.

Calculations and Empirical Formula

• Molar Ratio Determination:

  • The mass percent of oxalate ($C_2O_4$) is given as $53.75\%$.

  • The mass percent of iron ($Fe$) is determined experimentally from the spectrophotometric analysis.

  • The ratio of these mass percentages (converted to moles) gives the $x:y$ ratio in the formula $K_w[Fe_x(C_2O_4)_y] \cdot zH_2O$.

• Integer Values: Values for $w, x, y,$ and $z$ must be scaled and rounded to the nearest integer.

  • $w$ is determined via charge balance.

  • $z$ is determined via mass balance.

• Percent Yield: Calculated based on the limiting reagent from the synthesis and the final integer-value empirical formula.