AOAC Methods for Review in Codex STAN 234_11-2018

AOAC Official Methods Listed in CXS 234 for Milk and Milk Products

13

9.2.09

dilution. Add initial volume HNO 3

to equal ca 2 times dry sample

AOAC Official Method 960.40 Copper in Food Colorimetric Method

weight and 5 mLH 2

SO 4

, or as many mLH 2

SO 4

as g dry sample, but

³ 5 mL. Digest as in 963.21 C ( see 9.2.02). When sample contains large amount of fat, make partial digestion with HNO 3 until only fat is undissolved. Cool, filter free of solid fat, wash residue with H 2 O, add H 2 SO 4 to filtrate, and complete digestion as above. After digestion, cool, add 25 mL H 2 O, and remove nitrosylsulfuric acid by heating to fumes. Repeat addition of 25 mL H 2 O and fuming. If after cooling and diluting, insoluble matter is present, filter through acid-washed paper, rinse paper with H 2 O, and dilute to 100 mL. Prepare reagent blank similarly. E. Isolation and Determination of Copper Pipet 25 mL sample solution into 100 or 250 mL short-stem separator and add 10 mLcitrate–EDTAreagent. Add 2 drops thymol blue indicator, C ( e ), and 6 N NH 4 OH dropwise until solution turns green or blue-green. Cool, and add 1 mL carbamate solution and 15 mL CCl 4 . Shake vigorously 2 min. Let layers separate and drain CCl 4 through cotton pledget into glass-stoppered tube or flask. Determine A or T in suitable instrument at ca 400 nm. If >50 m g Cu is present in 25 mL aliquot, use smaller aliquot and dilute to 25 mLwith 2.0 NH 2 SO 4 . Highest accuracy is obtained at ca 25 m g Cu level ( A ca 0.3 in 1 cm cell). To test for Bi and Te, return CCl 4 solution to separator, add 10 mL 5%KCN solution, and shake 1min. If CCl 4 layer becomes colorless, Bi and Te are absent. If test is positive, develop color in another 25 mL aliquot as above (without KCN). Drain CCl 4 layer into second separator, add 10 mL 1 N NaOH, and shake 1 min. Let layers separate and drain CCl 4 into third separator. Again wash CCl 4 extract with 10 mL 1 N NaOH. Determine A or T of CCl 4 layer and convert to m g Cu. F. Preparation of Standards and Calibration Curves Transfer 0, 1, 2.5, 5, 10, 15, 20, and 25 mL Cu standard solution (2 m g/mL) to separators and add 2.0 N H 2 SO 4 to make total volume of 25 mL. Add 10 mL citrate–EDTA reagent and proceed as in E , beginning “Add 2 drops thymol blue indicator, ¼ ”. Plot A against m g Cu on ordinary graph paper. If readings are in % T , use semilog paper, and plot T on log scale. Since there is usually some deviation from linearity, read sample values from smoothed curve. Reference: JAOAC 43 , 695(1960).

First Action 1960 Final Action 1965

International Union of Pure and Applied Chemistry– AOAC Method

A. Principle Sample is digested with HNO 3 . Cu is isolated and determined colorimetrically at pH 8.5 as diethyl dithiocarbamate in presence of chelating agent, EDTA. Bi and Te also give colored carbamates at pH 8.5 but are decomposed to colorless compounds with 1N NaOH. Cu complex is stable. Range of color development is 0–50 m g. Blank is ca 1 m g Cu. B. Precautions Clean glassware with hot HNO 3 . Use white petrolatum to lubricate stopcocks of separators, and do not use brass chains. Purify H 2 O and HNO 3 by distillation in Pyrex. C. Reagents ( a ) S o d i um d i e t h y l d i t h i o c a r b ama t e ( c a r b ama t e solution) .—Dissolve 1 g of the salt in H 2 O, dilute to 100 mL, and filter. Store in refrigerator and prepare weekly. ( b ) Citrate–EDTA solution .—Dissolve 20 g dibasic ammonium citrate and 5 g Na 2 EDTA (Eastman Kodak Co.) in H 2 O and dilute to 100 mL. Remove traces of Cu by adding 0.1 mL carbamate solution and extracting with 10mLCCl 4 . Repeat extraction until CCl 4 extract is colorless. ( c ) Copper standard solutions .—( 1 ) Stock solution .—1 mg/mL. Place 0.2000 g Cu wire or foil into 125 mL Erlenmeyer. Add 15 mL HNO 3 (1 + 4), cover flask with watch glass, and let Cu dissolve, warming to complete solution. Boil to expel fumes, cool, and dilute to 200 mL. (2) Intermediate solution .—100 m g/mL. Dilute 20 mL stock solution to 200 mL. (3) Working solution .—2 m g/mL. Prepare daily by diluting 5 mL intermediate standard solution to 250 mL with 2.0 N H 2 SO 4 . ( d ) Ammonium hydroxide .—6 N. Purify as in ( b ). ( e ) Thymol blue indicator .—0.1%. Dissolve 0.1 g thymol blue in H 2 O, add enough 0.1N NaOH to change color to blue, and dilute to 100 mL. D. Preparation of Sample ( Caution : See safety notes on nitric acid and sulfuric acid.) Weigh sample containing £ 20 g solids, depending upon expected Cu content. If sample contains <75% H 2 O, add H 2 O to obtain this and H 2 SO 4

CAS-7440-50-8 (copper)

ã 2006 AOAC INTERNATIONAL

10/9/2018