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© 2015 AOAC INTERNATIONAL
Figure 2015.08. Typical titration curve obtained from an automatic titrator.
(
a
)
Check and maintenance of the combined silver electrode
.—
Rinse electrode with deionized water and wipe before use. Renew
the electrolyte periodically per manufacturer’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 MAgNO
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 M AgNO
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.
References:
J. AOAC Int . 98 , 1390(2015)DOI: 10.5740/jaoacint.15-136
AOAC SMPR 2014.015
J. AOAC Int . 98 , 1079(2015)DOI: 10.5740/jaoacint.SMPR2014.016
Posted: October 12, 2015
Candidates for 2016 Method of the Year
98