Methods to be Reviewed by OMB in 2016

olong

et al

ournal of

AOAC I

nternational

99, N

1, 2016

199

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.

B. Apparatus

(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.

C. Reagents

(a)

Water

.—Reagent grade.

(b)

Silver nitrate solution

.—0.1 M standard titrant with

certified concentration to 4 significant figures.

(c)

Potassium ferrocyanide trihydrate

.—Analytical reagent.

(d)

Zinc acetate dehydrate

.—Analytical reagent.

(e)

Nitric acid

.—Analytical reagent.

(f)

Ethanol

.—Analytical reagent.

(g)

Sodium chloride

.—Reference reagent.

D. Preparation of Solutions

(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.

E. Sample Preparation

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 ×

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.

F. System Suitability

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

. 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.

G. Analysis

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

Table 1. Criteria of SMPR 2014.015

Parameter

Minimum acceptable criteria

Analytical range

5–500

LOQ

≤5

Accuracy

95–105%

Repeatability (RSD

)

≤2%

Reproducibility (RSD

)

≤4%

Concentrations apply to (

) “ready-to-feed” liquids “as-is”;

(

) reconstituted powders (25 g into 200 g water); and (

) liquid

concentrate diluted 1:1 by weight.

Milligrams per 100 g in reconstituted final product.

Candidates for 2016 Method of the Year