Gravimetric Analysis Notes

Precipitating Agents

• Specific: Reacts with only one substance.
• Selective: Reacts with a limited number of substances.

Properties of Precipitating Agents

• Easily Filtered and Washed: Facilitates contaminant removal.
• Low Solubility: Minimizes loss of precipitate during washing.
• Unreactive: Prevents unwanted side reactions.
• Known Composition: Essential for accurate gravimetric calculations.

Types of Precipitates

• Colloidal Particles:
  • Usually too small to be filtered directly.
  • Heating, stirring, and adding electrolytes cause colloidal particles to bind together (coagulate) to become filterable.
• Crystalline:
  • More easily filtered due to larger particle size.

Digestion of Colloidal Precipitates

• The precipitate is heated for about 1 hour in the solution from which it was formed (mother liquor).
• $H_2O$ molecules are lost, making the precipitate more dense and easier to filter.
• Involves continuous recrystallization, which increases purity and filterability.

Digestion of Crystalline Precipitates

• Increases filterability and purity by allowing imperfections and impurities to dissolve and reprecipitate more slowly and perfectly.
• Involves continuous recrystallization.

Gravimetric Calculations

• Concentration in Percent:
  • Expressed as the weight of analyte A divided by the weight of the sample, multiplied by 100%.
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Gravimetric Factor (G.F.)

• The gravimetric factor is used to convert the weight of a compound to the weight of an element or another compound.
• Calculated as: $G.F.= \frac{a}{b} \times \frac{\text{formula weight of substance sought}}{\text{formula weight of substance weighed}}$
  • Where:
    • a is the number of moles of the substance sought.
    • b is the number of moles of the substance weighed.

Example 1: Gravimetric Factor

• Problem: Assume 0.5263g $BaSO<em>4$ are precipitated. Express this in terms of S, $SO</em>2$, and $Al<em>2(SO</em>4)<em>3 \cdot K</em>2(SO<em>4) \cdot 24H</em>2O$.

Example 2: Gravimetric Factor

• Problem: A 0.3516g sample of a commercial phosphate detergent was ignited to destroy organic matter. The residue was treated with hot HCl, converting P to $H<em>3PO</em>4$. Phosphate was precipitated as $MgNH<em>4PO</em>4 \cdot 6H<em>2O$ by adding $Mg^{2+}$ followed by aqueous $NH</em>3$. After filtering and washing, the precipitate was converted to $Mg<em>2P</em>2O_7$ (fw=222.57) by ignition at 1000°C. This residue weighed 0.2161g. Calculate the percent P (fw=30.974) in the sample.

Example 2: Solution

• (Solution to example not provided in transcript)

Example 3: Gravimetric Factor

• Problem: An iron ore was analyzed by dissolving a 1.1324g sample in concentrated HCl. The resulting solution was diluted, and iron(III) was precipitated as the hydrous oxide $Fe<em>2O</em>3 \cdot xH<em>2O$ by adding $NH</em>3$. After filtration and washing, the residue was ignited to give 0.5394g of pure $Fe<em>2O</em>3$ (fw=159.69). Calculate the percent of Fe (fw= 55.847).

Example 3: Solution

• (Solution to example not provided in transcript)

Example 4: Percent Purity

• Problem: At elevated temperatures, $NaHCO<em>3$ is converted quantitatively to $Na</em>2CO<em>3$: $2NaHCO</em>3(s) \rightarrow Na<em>2CO</em>3(s) + CO<em>2(g) + H</em>2O(g)$
  Ignition of a 0.3592g sample containing $NaHCO_3$ and nonvolatile impurities yielded a residue weighing 0.2362g. Calculate the percent purity of the sample.

Example 4: Solution

• The difference in weight before and after ignition represents the amount of $CO<em>2$ and $H</em>2O$ evolved from the $NaHCO_3$ in the sample.
• Residue weighing 0.2362g = $Na<em>2CO</em>3$ and nonvolatile impurities.
• Therefore, the calculation must be based on $CO<em>2$ and $H</em>2O$ loss.
• Mass of $CO<em>2$ & $H</em>2O$ = initial weight - residue weight = 0.3592g - 0.2362g = 0.1230g.

Example 4: Solution (Final Result)

• Final result: 92.76%

Gravimetric Method Example: Determining $NaHCO_3$ Content of Antacid Tablets

• Reaction: $NaHCO<em>3(aq) + H</em>2SO<em>4(aq) \rightarrow CO</em>2(g) + H<em>2O(l) + NaHSO</em>4(aq)$
• Gas collected by an absorbent (ascarite II – NaOH on nonfibrous silicate) in a weighed collecting tube.
• Amount of substance volatilized = difference in weight of the tube before and after collection.

Conversion Review

• Problem: A 0.8378g sample of calcium oxalate is heated to 1000°C according to the following equation:
  $CaC<em>2O</em>4(s) \rightarrow CaO(s) + CO(g) + CO<em>2(g)$ Calculate the moles of CaO remaining after ignition and the weight of $CO</em>2$ produced.

Conversion Review

• Problem: The Rammelsberg reaction involves the thermal decomposition of an alkaline-earth iodate to the corresponding paraperiodate. With strontium iodate, for example, the reaction is: $5Sr(IO<em>3)</em>2(s) \rightarrow Sr<em>5(IO</em>6)<em>2(s) + 4I</em>2(g) + 9O<em>2(g)$ When a 0.6961g sample of $Sr(IO</em>3)_2$ (fw=437.4) undergoes this reaction:
  • What weight of $Sr<em>5(IO</em>6)_2$ (fw=883.9) is produced?
  • How many mmoles of $O_2$ are produced?
  • How many moles of $I_2$ are produced?