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Jaudzems:
J
ournal of
AOAC I
nternational
V
ol.
98, N
o.
5, 2015
1393
(a)
Check and maintenance of the combined silver
electrode
.—Rinse electrode with deionized water and wipe
before use. Renew the electrolyte periodically permanufacturer’s
recommendations. If fat sticks to the electrodes during a series
of analyses, then eliminate it by briefly immersing the electrode
in acetone. Follow manufacturer’s recommendations for the
storage of electrodes when not in use.
(
Note
: In place of the combined silver electrode, separate
silver and reference electrodes may also be used.)
(b)
Automated titration
.—Prior to first use, check the
system linearity by use of a range of required volumes
(e.g., 0.1–15 mL) of 0.1 M NaCl solution. Additionally, prior
to each use check the system suitability by preparing three
calibration check samples using 5.0 mL 0.1 M NaCl solution.
H. Extraction and Analysis
(a)
Weigh an appropriate aliquot RTF or reconstituted
powder (e.g., 25 g) into a suitable beaker (e.g., 150 mL,
manual or semiautomatic procedure) or the autosampler
titrator cups (automatic titration). For adult nutritionals with
a high chloride content, weigh a smaller test portion, e.g., 5 g
reconstituted or RTF product.
(b)
Add 50 mL 2% (v/v) nitric acid solution as well as
a magnetic stirring rod. Place the autosampler cup on a
magnetic stirrer and stir until mixed or finely suspended.
(c)
The pH of the test solution should be below 1.5. In case
of doubt, check by means of a pH meter and, if necessary, add
more 2% (v/v) nitric acid solution.
(d)
Under continuous stirring, titrate the sample solution
automatically with 0.1 M AgNO
3
solution up to the end
potential. Record the volume of 0.1 M AgNO
3
solution
consumed. If performing manual titrations, plot a graph
of the variation of potential difference as a function of the
quantity of the titrant added, continuing the addition of the
titrant beyond the presumed equivalence point. The end point
of the titration corresponds to the point at which the potential
changes most rapidly (
see
Figure
2015.08
).
(e)
Special case: determination of very low amounts
of chloride
.—When determining chloride amounts below
20 mg/100 g, for greater accuracy and precision it is preferable
to use a 0.01 MAgNO
3
solution for the titration. Determine the
titer of this solution by means of a 0.01 M NaCl solution.
I. Calculations
Calculate chloride content (w) in mg/100 g RTF or
reconstituted sample using the equation:
w
= A × Mw ×
Cm
x
F
× 100
m
where A = volume (mL) of 0.1 M or 0.01 M AgNO
3
solution used for titration; Mw = atomic weight of chloride
(= 35.45 g/mol);
Cm
= exact molar concentration of the AgNO
3
solution (0.1000 or 0.0100); m = mass of the test portion, in g;
and F = dilution factor for preparation of reconstituted powder
or concentrate.
Results
The validation study was conducted in accordance with the
SPIFAN SLV guidelines (3).
Figure 2015.08. Typical titration curve obtained from an automatic titrator.
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