Setup of Kjeldahl Method: Apparatus:

  • Digestion flask
  • Kjedahl heating apparatus
  • Bends, cones, joints Reagents:
  • Sulphuric acid, 95%
  • Catalyst (a mixture of copper sulfate and potassium sulfate)
  • Sodium hydroxide 50%
  • Sulphuric acid solution 0.5 M
  • Hydrochloric acid 0.25 M
  • Sodium Carbonate Solution: 0.5 M

Procedure:

  1. Digestion:
    • Mix the sample with sulfuric acid and catalyst.
    • Heat the mixture (80-100Β°C) to break nitrogen bonds and convert organic nitrogen into ammonium ions.
  2. Distillation:
    • Make the solution alkaline by adding sodium hydroxide, which converts ammonium sulfate into ammonia gas.
    • Distill the ammonia gas into a receiving flask containing excess boric acid.
  3. Titration:
    • Titrate the excess sulfuric acid (not reacted with ammonia) with sodium hydroxide.
    • Calculate the amount of ammonia by difference.

Methodology: Part A: Digestion

  1. Weigh the sample and mix with sulfuric acid and catalyst.
  2. Digest the mixture for 30-40 minutes at 160Β°C.
  3. Observe color change from blue to colorless.

Part B: Distillation

  1. Add water and sodium hydroxide to the digestion flask.
  2. Attach to distillation unit and collect distillate in sulfuric acid solution.
  3. Keep aside some sulfuric acid solution for titration.

Part C: Titration

  1. Add methyl red indicator to the distillate and titrate with sodium carbonate solution.
  2. Observe endpoint color change from pink to light yellow.
  3. Titrate both ammonia-trapped solution and the sulfuric acid solution kept aside.

Calculation of %N: (π’Žπ’ 𝒔𝒕𝒂𝒏𝒅𝒂𝒓𝒅 π’‚π’„π’Šπ’… βˆ’ π’Žπ’ 𝒐𝒇 π’ƒπ’π’‚π’π’Œ) Γ— 𝑡 𝒐𝒇 π’‚π’„π’Šπ’… Γ— 𝟏. πŸ’πŸŽπŸŽπŸ•
π‘Ύπ’†π’Šπ’ˆπ’‰π’• 𝒐𝒇 π’”π’‚π’Žπ’‘π’π’† π’Šπ’ π’ˆπ’“π’‚π’Žπ’”

Where,

  • Vstandard acid​ = Volume of standard acid used in titration (mL)
  • blank​ = Volume of standard acid required to back titrate excess (mL)
  • acid​ = Normality of acid used
  • 1.4007 is the conversion factor
  • Weight of sample in grams

This calculation gives the percentage of nitrogen in the sample, which can be used to determine protein content with the appropriate conversion factor.

Your detailed methodology and calculations demonstrate a thorough understanding of the Kjeldahl method and its application in various analyses.