2.Ammonia can be determined spectrophotometrically by the reaction with phenol i
ID: 871590 • Letter: 2
Question
2.Ammonia can be determined spectrophotometrically by the reaction with phenol in the presence of hypochlorite to give the blue indophenol absorbing at Lambadmax = 625 nm:
A 4.370 mg sample of a protein was chemically digested to convert all of its nitrogen to ammonia. After this treatment, the volume of the sample was 100.00 mL. Then a 10.00 mL aliquot of the ammonia solution was placed in a 50.00-mL volumetric flask and treated with 5 mL phenol solution plus 2 mL of sodium hypochlorite solution. The sample was diluted to 50.00 mL, allowed to stand for 30 min., and its absorbance was measured at 625 nm in a 1.00 cm cell. A standard solution was prepared by dissolving 0.0001 g NH4Cl in 10.00 mL of water, placed in a 50.00-mL volumetric flask and treated in the same manner as the unknown sample. A reagent blank was also prepared using distilled water in place of the unknown. The absorbance readings were as follows: Blank 0.140 Standard 0.308 Unknown Sample 0.592 (a) Calculate the weight percent of nitrogen in the protein sample.
(b) Calculate the molar absorptivity (?) of the blue indophenol product.
Explanation / Answer
NH4Cl standard 0.0001g/10 mL
0.0001g/58.5 g/mol NH4Cl = 1.71
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