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J
audzems
:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
4, 2016
1115
to room temperature and dilute to volume with water. Filter
using a 0.45 μm membrane disposable syringe before use.
E. Sample Preparation
(a)
Milk product, infant formula, and adult/pediatric
nutritional.
—Mix well to ensure that sample is homogeneous.
(b)
Powder samples.
—Reconstitute by dissolving 25 g
powder sample in 200 g warm water (40°C).
F. Extraction
For high-protein samples requiring additional protein
precipitation beyond that accomplished by addition of nitric
acid solution, perform steps
F
(
a
–
c
), below. Otherwise, begin
with step
F
(
d
).
(a)
Weigh an appropriate aliquot of RTF or reconstituted
powder (e.g., 25 g, accurate to 0.1 mg) into a 50 mL
centrifuge tube.
Note
: For samples with a high chloride
content, weigh a smaller test portion, e.g., 5 g reconstituted
or RTF product.
(b)
Transfer 2.5 mL precipitating solution I [
D
(
c
)] and
2.5 mL precipitating solution II [
D
(
d
)] into the tube. Dilute
to 50 mL with water. Mix well. If foam impacts the constant
volume, then one or two drops of defoaming agent [
C
(
l
)] should
be added.
(c)
Centrifuge at 12000 ×
g
for 5 min at 4°C, and then
equilibrate to room temperature.
(d)
Accurately transfer either 10 mL supernatant from steps
F
(
a
–
c
) or weigh an appropriate aliquot of RTF or reconstituted
powder (e.g., 25 g, accurate to 0.1 mg).
Note
: For samples with
a high chloride content, weigh a smaller test portion, e.g., 5 g
reconstituted or RTF product.
Into a 120 mL sample beaker or autosampler cup, add 5 mL
nitric acid solution [
D
(
e
)] and 50 mL water before titration.
Add a magnetic stirring rod (if the titrator does not have a
built-in rod stirrer). Place the autosampler cup or beaker onto
a magnetic stirrer and stir until dissolved or finely suspended.
(e)
The pH of the test solution must be below 1.5. If in doubt,
check pH by means of a pH meter and, if necessary, add a little
more nitric acid solution [
D
(
e
)].
G. Instrument Operating Conditions
(a)
Check and maintenance of the combined silver
electrode
.—Rinse electrode with deionized water and wipe
before use. Renew the electrolyte with 1 M KNO
3
[
D
(
i
)]
periodically per the manufacturer’s recommendations.
If fat sticks to the electrodes during a series of analyses, then
eliminate it by briefly immersing the electrode in acetone.
The silver electrode must be stored in 1 M KNO
3
[
D
(
i
)]
after appropriate cleaning.
Note
: Instead of the combined silver
electrode, separate silver and reference electrodes may also be
used.
(b)
Titration
.—Connect the combined silver electrode to the
titration apparatus according to the manufacturer’s indications.
Ensure that the titration vessels are correctly placed on the
autosampler and that there are enough reagents: both nitric acid
solution {if added automatically [
D
(
e
)]} and 0.1 M AgNO
3
[
C
(
g
) or
D
(
a
)]. If no autosampler is available, then place the
sample solutions manually under the titration equipment.
Put the wash solution [
D
(
f
)] in the washing position if an
auto sampler used. Ensure that the volume of wash solution is
adequate.
Under continuous stirring and without touching the electrode,
titrate the sample solution automatically with 0.1M standardized
silver nitrate solution [
C
(
g
) or
D
(
a
)] up to the end potential. The
consumption of 0.1 M of silver nitrate solution [
C
(
g
) or
D
(
a
)]
should be recorded automatically and can be read from the
titrator software or documented in the titrator operating records.
For manual titration, using a buret, add 0.1 M standardized
silver nitrate solution [
C
(
g
) or
D
(
a
)] until the end potential has
nearly been met. Continue to titrate slowly until the end point
is met, as observed by the two small additions (about 0.05 mL)
of silver nitrate solution. See Figures
201603A
and
2016.03B
.
(c)
Special case: determination of very low amounts of
chloride.
—When determining low chloride concentrations such
as found in desalted whey powder, for greater precision, it is
preferable to use a standardized 0.025 mol/L AgNO
3
[
D
(
g
)]
solution for the titration.
(d)
Blank test: determination of reagent background content
of chloride
.—Perform a blank test using reagents, substituting
water [
C
(
a
)] for the sample portion. The titrant consumption
of the blank test obtained at the end point should be less than
0.05 mL when using the 0.1 M standardized silver nitrate, and
less than 0.2 mL when using the 0.025 M standardized silver
nitrate. Otherwise, check the reagents and water involved into
the procedures and then perform the blank test again until the
criterion is achieved.
H. System Suitability Test
Perform a system suitability test prior to use.
(a)
Transfer 5 mL NaCl solution [
C
(
h
) or
D
(
b
)] into a
120 mL sample beaker. If 0.025 M AgNO
3
titrant is required,
then use 1 mL NaCl solution.
Figure 2016.03A. Automatic titration end point recognition using
the dynamic titration mode on a Methohm Titrodo 905 titrator.
U[mV], voltage of Ag electrode detected during titration; V[mV],
volume of consumption of the standardized AgNO
3
titrant during
titration; ERC, first derivative of the titration curve drawn by voltage
of electrode versus volume of titrant consumption.
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