Estimate the crude protein content of the given food sample by the micro-Kjeldahl method.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Aim. To estimate the total nitrogen and crude protein of the given food sample by the micro-Kjeldahl method.
2 · Principle.
Digestion: conc. H2SO4 with K2SO4 and CuSO4 turns organic N into ammonium sulphate.
Distillation: 40% NaOH frees ammonia, trapped in boric acid as ammonium borate.
Titration: with 0.01 N HCl; 1 mL ≡ 0.14 mg N.
Protein = N × 6.25 (proteins are about 16% nitrogen).
3 · Requirements. Food sample, conc. H2SO4, catalyst mixture, 40% NaOH, 2% boric acid with mixed indicator, 0.01 N HCl; micro-Kjeldahl flasks, digestion rack in a fume hood, Markham still, volumetric flask, microburette.
4 · Procedure.
Digest about 200 mg sample with catalyst and 3 mL acid until clear; run a blank.
Make up to 50 mL; distil a 5 mL aliquot with 10 mL 40% NaOH into boric acid.
Titrate with 0.01 N HCl from green back to pink.
5 · Observation. [Table: titre for sample (S) and blank (B)]
6 · Calculation. (5.70 − 0.20) × 0.01 × 14 = 0.770 mg N in 5 mL; × 50 ÷ 5 = 7.70 mg; ÷ 200 mg × 100 = 3.85% N; × 6.25 = 24.06 g protein per 100 g.
7 · Result. Nitrogen ____ g/100 g; crude protein ____ g/100 g.
8 · Precautions. Fume hood, goggles, gloves; digest until clear; subtract a blank; avoid back-suction; never pipette acid or alkali by mouth.
Scoring tip: Write the three stages as three short reactions, then the formula with units. The factor 6.25 with its reason (16% nitrogen) is almost always asked.
