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© 2015 AOAC INTERNATIONAL
AOAC Official Method 2015.08
Chloride in Infant Formula
and Adult/Pediatric Nutritional Formula
Potentiometry
First Action 2015
[Applicable to the determination of chloride in ready-to-feed
(RTF) liquid concentrate and powder products from levels of 1.4 to
1060 mg/100 g reconstituted product or RTF liquids.]
See
Tables
2015.08 A
–
C
for the results of the single-laboratory
validation (SLV) study supporting acceptance of the method. The
method was evaluated against
Standard Method Performance
Requirements
AOAC SMPR
®
2014.015.
A. Principle
Reconstitute powder samples by dissolving 25 g powder
sample in 200 g warm water (40°C). Add 50 mL of 2% (v/v) nitric
acid solution. Stir with a magnetic stirrer until mixed or finely
suspended. Ensure pH is <1.5. Titrate potentiometrically against
standardized silver nitrate (AgNO
3
) solution, 0.1 M, using a silver
electrode to detect the end point.
B. Apparatus
Common laboratory equipment and, in particular, the following:
(
a
)
Analytical balance.—
Precision 0.1 mg.
(
b
)
Class A volumetric flasks.—
100 and 1000 mL.
(
c
)
pH meter/mV meter with a scale covering ±700 mV, and
buret, 20 or 25 mL.—
Mettler-Toledo (Columbus, OH, USA) or
equivalent.
(
d
)
Automatic titrator.
—Autosampler, motorized piston buret,
with remote-control dispensing and filling (Mettler Toledo T50 Rondo
Tower autosampler, Mettler LabX 3.1 software, or equivalent).
(
e
)
Combined ring silver electrode.—
e.g., Mettler Toledo DM
141 or DMi145-SC, or equivalent; alternatively, a silver electrode
with reference electrode can be used.
(
f
)
Magnetic stirrer.—
Heidolph MR 3000 or equivalent (Sigma-
Aldrich, St. Louis, MO, USA).
(
g
)
Water bath
.—Capable of warming water to 40°C.
(
h
)
Laboratory oven.—
Capable of heating to 120°C.
(
i
)
Pipets (1, 20, 50, and 100 mL)
.—Class A glass volumetric or
automatic (Eppendorf or equivalent).
(
j
)
Buret.—
10 mL.
C. Chemicals and Reagents
(
a
)
Acetone.—
p.a. (Merck, Darmstadt, Germany, or equivalent).
(
b
)
Water, purified.—
Greater than 18MΩ (EMD Millipore
Corp., Billerica, MA, USA, or equivalent).
(
c
)
Sodium chloride (NaCl)
,
crystal
.—Fluka 71387
(Sigma-Aldrich, or equivalent).
(
d
)
AgNO
3
.—Sigma-Aldrich 10220, or equivalent.
D. Solutions
(
a
)
Nitric acid.—
Minimum 65% p.a. (Merck, or equivalent).
(
b
)
Standardized AgNO
3
solution, 0.1 M.—
Merck TitriPUR, or
equivalent.
(
c
)
NaCl solution, 0.1 M
.—Alfa Aesar (Ward Hill, MA, USA),
or equivalent.
E. Preparation of Solutions
(
a
)
Dilute nitric acid solution, 2% (v/v).—
Into a 1000 mL
volumetric flask, add about 800 mL water. Carefully pipet 20 mL
concentrated nitric acid (65%). Make up to 1000 mL with water.
Stopper the volumetric flask and mix well.
Table 2015.08A. Precision of results expressed on reconstituted product
Matrix
a
Chloride mean, mg/100 g
RSD
r
, %
b
RSD
R
, %
c
NIST SRM 1849a
68.5
0.42
1.59
Child formula powder
46.1
0.26
0.32
Infant elemental powder
36.8
0.10
0.12
Adult nutritional RTF, high protein
38.2
0.64
2.76
Adult nutritional RTF, high fat
30.8
2.77
10.09
d
Infant formula RTF, milk based (SPIFAN blank milk formula)
20.0
0.22
0.18
Adult nutritional powder milk, protein based
36.4
0.45
0.47
Infant formula powder partially hydrolyzed, milk based
42.4
0.09
0.13
Infant formula powder partially hydrolyzed, soy based
46.2
0.15
0.16
Adult nutritional powder low fat
40.3
0.20
0.18
Child formula powder
38.6
0.67
0.81
Infant elemental powder
39.3
0.21
0.23
Infant formula powder, milk based
46.5
0.88
0.85
Infant formula powder, soy based
56.1
0.29
0.33
Infant formula RTF, milk based (SPIFAN control milk formula)
44.2
0.34
0.24
Adult nutritional RTF, high protein
154.5
0.44
0.40
Adult nutritional RTF, high fat
162.2
1.01
0.78
a
Samples provided in the SPIFAN I Test Kit. SRM 1849a was reconstituted 10 g to 100 g, and all other powders were reconstituted 25 g to 225 g.
b
RSD
r
= Repeatability RSD.
c
RSD
R
= Reproducibility RSD.
d
Higher repeatability and intermediate reproducibility were observed in this nonfortified RTF sample compared to the rest of the samples. This is most likely
due to sample heterogeneity, since the sample was beyond its expiration date. The sample contents separated even after 15 min shaking prior to opening.
These results were not included in the evaluation. Additional high-fat sample was within the SMPR requirements.
Candidates for 2016 Method of the Year
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