AOAC Official Method 935
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D1CEBB538D7C4A1DA0F0E240C843B21D |
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日期： |
2024-7-30 |
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39.1.20,AOAC Official Method 935.48?,Nitrates and Nitrites in Meat,Xylenol Method,First Action 1935,Surplus 1994,Final Action 1996,A. Apparatus,Use simple distillation apparatus, including distillation bulb.,Type of glass condenser utilizing thin, rapidly moving H2O film as,cooling medium (West type) is recommended.,B. Reagents,(a) m-Xylenol.—2,4-Dimethylphenol. Eastman Kodak Co. No.,1150, or equivalent.,(b) Silver–ammonium hydroxide solution.—Dissolve 5 g nitrate-,free Ag2SO4 in 60 mL NH4OH. Heat to bp, concentrate to ca,30 mL, cool, and dilute to 100 mL with H2O.,(c) Bromocresol green indicator.—Dissolve 0.1 g bromocresol,green in 1.5 mL 0.1M NaOH, and dilute to 100 mL with H2O.,(d) Nitrate standard solution.—Dissolve 0.1805 g recrystallized,KNO3 in H2O and dilute to 1 L, or dilute 17.85 mL 0.1M HNO3 to,1 L; 10 mL contains 0.25 mg nitrate N.,C. Determination,Mix 5–10 g finely comminuted and thoroughly mixed test sample,with 80 mL warm H2O. Break up all lumps and heat on steam bath,1 h, stirring occasionally. Transfer to 100mLvolumetric flask, cool,dilute to volume, and mix. Filter, or let settle, and pipet 40 mL filtrate,or supernate, into 50 mL volumetric flask. (No correction for,volume occupied by meat is necessary.) Add 3 drops bromocresol,green indicator. AddH2SO4 (1 + 10) dropwise until color changes to,yellow. Oxidize nitrites to nitrates by adding 0.2M KMnO4 solution,dropwise with shaking until faint pink remains ca 1 min. Add 1 mL,H2SO4 (1 + 10) and 1 mL phosphotungstic acid solution,(20 g/100 mL). Dilute to volume, mix, and filter.,Into 500 mL flask measure 20 mL containing 0.025–0.25 mg nitrate,N. (If >20 mL is required, make slightly alkaline and concentrate,by evaporation.) Add enough Ag–NH4OH solution to,precipitate all chlorides and most of excess phosphotungstic acid.,(Slight excess of Ag reagent is not harmful; 1–2 mL is usually,enough.) Without decanting or filtering, add volume of H2SO4,(3 + 1) ca 3 times volume of liquid in flask. Stopper flask, mix, cool,to ca 35°C, add 0.05 mL (1–2 drops) of the m-xylenol, stopper,shake, and hold 30 min at 30–40°C.,(Yellow to brownish yellow color, indicative of nitrates, appears.,Bright red precipitate, due to incomplete removal of,phosphotungstic acid, may also appear. Slight excess of,phosphotungstic acid causes no interference but large excess may.),After nitration is complete, add 150 mL H2O, taking care to wash,off stopper, and distil 40–50 mL into receiver containing 5 mL,NaOH (10 g/L). Quickly remove any nitroxylenol solidifying in,condenser by stopping H2O flow and letting condenser become,warm. Transfer distillate to 100 mL volumetric flask, dilute to volume,with H2O, and determine nitrate N by comparing reading of,color of suitable aliquot with standard curve prepared at ca 450 nm.,Prepare color standard from 10 mL nitrate standard solution, using,0.05 mLm-xylenol and 30 mLH2SO4 (3 + 1), and diluting distillate,to 500 mL.,References: JAOAC 18, 459(1935); 22, 596(1939).,Revised: March 1997,. 2000 AOAC INTERNATIONAL……
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