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B
olong
et al
.:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
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 ×
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.
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
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.
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
a
,
b
LOQ
≤5
a
,
b
Accuracy
95–105%
Repeatability (RSD
r
)
≤2%
Reproducibility (RSD
R
)
≤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.
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