6. AOACSPIFANMethods-2018Awards

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B olong et al .: J ournal of AOAC I nternational V ol . 99, N o . 1, 2016  199

(c)  Potassium ferrocyanide trihydrate .—Analytical reagent. (d)  Zinc acetate dehydrate .—Analytical reagent.

Table 1. Criteria of SMPR 2014.015 Parameter

Minimum acceptable criteria

(e)  Nitric acid .—Analytical reagent. (f)  Ethanol .—Analytical reagent. (g)  Sodium chloride .—Reference reagent.

5–500 a , b

Analytical range

≤5 a , b

LOQ

Accuracy

95–105%

Repeatability (RSD r ) Reproducibility (RSD R )

≤2%

D. Preparation of Solutions

≤4% a  Concentrations apply to ( 1 ) “ready-to-feed” liquids “as-is”; ( 2 ) reconstituted powders (25 g into 200 g water); and ( 3 ) liquid concentrate diluted 1:1 by weight. b  Milligrams per 100 g in reconstituted final product.

(a)  Precipitating agent I .—Dissolve 106 g potassium ferrocyanide trihydrate and dilute to 1000 mL using water. (b)  Precipitating agent II .—Dissolve 220 g zinc acetate dihydrate and dilute to 1000 mL using water. (c)  Nitric acid solution .—Add 100 mL nitric acid to 300 mL water and mix well. (d)  Washing solution .—Add 75 mL ethanol to 25 mL water and mix well. (e)  Sodium chloride standard solution (NaCl SS) .—Weigh 250 mg (accurate to 0.01 mg) NaCl and dissolve in water to total solution weight of 25 g (accurate to 0.1 mg). Mix well. Prepare fresh before the titer check. Weigh 5 g (accurate to 0.1 mg) powder sample (2 g for skim milk powder) or 20 g (accurate to 0.1 mg) liquid sample in 50 mL centrifuge tube. Add 25 mL 40°C water for powder sample and dissolve thoroughly. Transfer 2.5 mL precipitating agent I and 2.5 mL precipitating agent II into the tube, dilute to 50 mL with water, and mix well. Centrifuge at 12 500 × g for 5 min at 4°C (6 min for skim milk powder) and equilibrate to room temperature. Accurately transfer 10 mL supernatant (20 mL for desalted whey powder D90) into 120 mL sample beaker and add 5 mL nitric acid solution and 50 mL water before titration. Weigh 1000 to 1500 mg (accurate to 0.1 mg) NaCl SS into 120 mL sample beaker. Add 5 mL nitric acid solution and 50 mL water. Place the washing solution in the washing position of the autosampler and replace with fresh washing solution after every 10 or 11 single titration tests. The titration conditions for system suitability analysis are presented in Table 2015.07 . Titrate using the titrator. Calculate concentration of the silver nitrate solution according to H . The difference between the calculated concentration and the certified value should be within 0.5%. If outside the acceptance value, check the experimental procedures and titration system. If the issue is not resolved, use fresh silver nitrate. If fresh silver nitrate is not able to gain an acceptable range, replace the electrolyte of the electrode and check the condition of the dosing unit. E. Sample Preparation F. System Suitability

AOAC Official Method 2015.07 Chloride in Infant Formula and Adult/Pediatric Nutritional Formula Potentiometric Titration Method First Action 2015

(Applicable for determination of chloride in all forms of infant, adult, and/or pediatric formula, including powders, ready-to-feed liquids, and liquid concentrates.) Caution : Refer to Material Safety Data Sheets prior to use of chemicals. Use appropriate personal protective equipment when performing testing.

A. Principle

Samples are treated with potassium ferrocyanide and zinc acetate to precipitate protein, acidified with nitric acid, and titrated using silver nitrate as titrant. The end point of titration is determined by the potentiometric method on a silver titrode with a silver sulfide-coated electrode. Chloride content is calculated in the sample via the titrant volume at the end point. (a)  Analytical balances .—Accurate to 0.1 and 0.01 mg. (b)  Centrifuge .—Tabletop with rotor to fit 50 mL conical tubes. (c)  Centrifuge tubes .—50 mL, conical, polypropylene. (d)  Pipet .—10 mL, grade A. (e)  Volumetric flasks .—50, 100, 500, and 1000 mL, glass. (f)  Graduated cylinders. —25, 100, and 500 mL, glass. (g)  Beaker .—120 mL sample beaker (Metrohm, Herisau, Switzerland 6.1459.300 or equivalent) (Metrohm). (h)  Silver titrode .—Ag titrode (Metrohm 6.0430.100 or equivalent). (i)  Automatic potentiometric titration system .—Metrohm 862 Compact Titrosampler (2.862.0010) equipped with 800 Dosino (2.800.0010), 10 mL dosing unit (6.3032.210), and automatic propeller blending device or equivalent. B. Apparatus

G. Analysis

C. Reagents

Titrate the prepared sample solution on the titrator. Place the washing solution in special washing position of the auto sampler, and use fresh washing solution every 10 or 11 single titration tests. The sample titration conditions are the same as the system suitability analysis in Table 2015.07 .

(a)  Water .—Reagent grade. (b)  Silver nitrate solution .—0.1 M standard titrant with certified concentration to 4 significant figures.