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J
audzems
:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
4, 2016
(b)
Add 5 mL nitric acid solution [
D
(
e
)] and 50 mL water.
(c)
Place the washing solution [
D
(
f
)] in the washing position
of the auto sampler.
(d)
Titrate with 0.1 M standardized silver nitrate solution
[
C
(
g
) or
D
(
a
)] up to the end potential using an automatic,
semiautomatic, or manual titrator.
(e)
Repeat in quadruplicate.
(f)
Calculate concentration of the silver nitrate solution
according to section
I
(
a
). The difference between the calculated
concentration and the certified value should be within 0.5%.
If it is outside the acceptance value, check the experimental
procedures and titration system. If the issue is not resolved,
then use fresh standardized silver nitrate. If fresh standardized
silver nitrate does not provide an acceptable result, replace the
electrolyte of the electrode and check the operating condition of
the dosing unit.
I. Calculations
(a)
Calculate silver nitrate concentration for system
suitability verification, and report to four decimal places
.—If
using in-house made standardized AgNO
3
solution [
D
(
a
)],
=
× ×
V
(
/ )
5.844
10
1
1
SNC mol L
m
where
SNC
is the silver nitrate concentration (mol/L),
m
1
is
the weight (mg) of sodium chloride in 5 mL or 1 mL standard
solution [
D
(
b
)],
V
1
is the volume (mL) of 0.1 M or 0.02 M
AgNO
3
consumed at titration end point, 5.844 is the sodium
chloride weight (μg) corresponding to 1 mL of 0.1 mol/L
AgNO
3
, and 10 is the mass conversion from titer to the
concentration of titrant.
Or, if using purchased standard grade 0.10 M NaCl [
C
(
g
)],
=
×
V
(
/ )
0.1
3
1
SNC mol L
V
where
SNC
is the silver nitrate concentration (mol/L),
V
3
is
the volume (mL) of purchased standard grade 0.10 M sodium
chloride added, and
V
1
is the volume (mL) of 0.1 M or 0.025 M
AgNO
3
consumed at titration end point.
(b)
Calculate chloride content in the sample, and report to
three significant digits
.—
=
× ×
× ×
( / 100 ) 35.45 (
)
100
2 - 0
CL mg g
c V V f
m
where:
CL
is the chloride content (mg/g);
m
is the sample
weight (g);
c
is the certified concentration of silver nitrate titrant
[0.1000 mol/L or standardized concentration;
I
(
a
)];
V
2
is the
AgNO
3
volume (mL) consumed at titration end point;
V
0
is the
Figure 2016.03B. Example of titration curve from a Mettler autotitrator.
9