Umkristallisation von Benzoesäure

Reinigung eines Feststoffs durch Lösen in heißem Lösungsmittel und Abkühlen

Crystallization Intermediate (High School) 45 Min. ~$10,00

Ziel

Unreine Benzoesäure durch Umkristallisation aus heißem Wasser reinigen und dabei die Standardreinigungsmethode für organische Feststoffe demonstrieren.

Hintergrund

Recrystallization exploits the difference in solubility of a substance at different temperatures. Benzoic acid is very soluble in hot water but nearly insoluble in cold water. By dissolving impure benzoic acid in hot water and cooling slowly, pure crystals form while impurities remain in solution.

Sicherheitswarnungen

  • Handle hot water and hot glassware with care
  • Benzoic acid dust can irritate airways
  • Use heat-resistant gloves when handling hot flasks

Erforderliche PSA

goggles lab_coat

Materialien

  • Impure benzoic acid (3 g)
    Mixed with charcoal or sand
  • Distilled water (200 mL)
  • Activated charcoal (0.5 g)
    Optional, for decolorization

Ausrüstung

250 mL Erlenmeyer flask Hot plate Buchner funnel Filter flask Vacuum pump Melting point apparatus Ice bath Watch glass

Durchführung

1

Weigh 3 g of impure benzoic acid and record the mass. Note the appearance (color, particle size).

3 Min.
2

Add the impure sample to 100 mL of distilled water in the Erlenmeyer flask. Heat to boiling while stirring.

8 Min. Handle hot flask with care
3

If the solution is colored, add 0.5 g activated charcoal, swirl, and filter hot through fluted filter paper.

5 Min.
4

Allow the clear filtrate to cool slowly to room temperature. Do not disturb.

15 Min.
5

Cool further in an ice bath to maximize crystal yield.

5 Min.
6

Collect the crystals by vacuum filtration. Wash with a small amount of ice-cold water.

5 Min.
7

Dry the crystals on a watch glass. Weigh and calculate percent recovery.

3 Min.
8

Determine the melting point (pure benzoic acid: 122°C) and compare with the impure sample.

5 Min.

Erwartete Ergebnisse

White, needle-like crystals with a sharp melting point at 121-123°C. Typical recovery is 60-80%. The purified crystals should be whiter and have a sharper melting point than the crude material.

Aufräumen

Dispose of filtrate down the drain (benzoic acid is food-grade). Rinse glassware. Dispose of charcoal in solid waste.

Details

Kategorie
Crystallization
Schwierigkeit
Intermediate (High School)
Dauer
45 Min.
Geschätzte Kosten
$10,00
Schritte
8
Materialien
3

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Frequently Asked Questions

What is the objective of Umkristallisation von Benzoesäure?
Unreine Benzoesäure durch Umkristallisation aus heißem Wasser reinigen und dabei die Standardreinigungsmethode für organische Feststoffe demonstrieren.
How difficult is Umkristallisation von Benzoesäure?
This experiment is rated as Intermediate (High School). It takes approximately 45 minutes to complete.
What safety precautions are needed for Umkristallisation von Benzoesäure?
Key safety precautions include: Handle hot water and hot glassware with care; Benzoic acid dust can irritate airways; Use heat-resistant gloves when handling hot flasks.
What materials are needed for Umkristallisation von Benzoesäure?
The main materials required are: Impure benzoic acid, Distilled water, Activated charcoal.
What results should I expect from Umkristallisation von Benzoesäure?
White, needle-like crystals with a sharp melting point at 121-123°C. Typical recovery is 60-80%. The purified crystals should be whiter and have a sharper melting point than the crude material.