93 / 363
200
B
olong
et al
.:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
1, 2016
H. Calculations
Calculate silver nitrate concentration (SNC) in moles per liter
for system suitability verification and report to 4 decimal places:
m m
m V
SNC mol L
5.844
1 1
10
1
2
3
1
(
)
=
× × ×
where
m
1
= weight in milligrams of NaCl SS,
m
2
= weight in
milligrams of sodium chloride used to prepare the standard
solution,
m
3
= total weight in milligrams of prepared NaCl
SS,
V
1
= silver nitrate consumption volume in milliliters
up to titration end point, 5.844 = sodium chloride weight in
micrograms corresponding to 1 mL of 0.1 mol/L silver nitrate,
and 10 = mass conversion from titer to the concentration of
titrant.
Calculate chloride content in sample (CL) and report to 3
significant digits:
(
)
=
× × × ×
c V f
m
CL mg 100g
35.5
100
2
4
where
m
4
= sample weight in grams,
c
= certified concentration
in moles per liter of silver nitrate titrant,
V
2
= silver nitrate
consumption volume in milliliters up to titration end point,
f
= dilution factor, 35.5 = chloride weight in micrograms
corresponding to 1 mL of 1 mol/L silver nitrate, and 100 = mass
conversion to milligrams per 100 g.
See refs. 2–4 for more detail.
Results and Discussion
System Suitability
NaCl SS was prepared as in Table 2 and used to determine the
SNC of the silver nitrate solution as described in the method.
Titrations were carried out on 3 days in duplicate, and results
are shown in Table 3. The calculated concentrations were all
within 5% of the certified value of 0.1004 mol/L as required by
the method. Thus, system suitability was achieved.
Precision
Two samples each of 10 SPIFAN matrixes from previous
multilaboratory studies, including National Institute of
Standards and Technology Standard Reference Material (SRM)
1849a, were tested on 6 different days. The results are presented
in Table 4. Average within-day repeatability (RSD
r
) values for
the 10 matrixes varied from 0.43 to 1.34%, meeting the SMPR
criterion of ≤2%. Interday intermediate reproducibility varied
from 0.80 to 3.04% across the 10 matrixes, in agreement with
the reproducibility requirement of ≤4%, suggesting that the
method may meet the criterion in a multilaboratory validation.
Table 2015.07. Titration conditions for system suitability
test (dynamic equivalence-point titration U mode) and
sample analysis
Module and parameter
Condition
Start conditions
Pause
15 s
Titration parameters
Measure point density
4
Minimum increment
10.0 μL
Dosing rate
Maximum mL/min
Signal drift
50 mV/min
Equilibrium time
26 s
Measure input
1
Stirrer rate
10
Stop conditions
Stop volume
10 mL
Stop measure value
120 mV
Stop equivalence point
1
Volume after equivalence point
1 mL
Evaluation
Equivalence-point recognition
Greatest
Reports
PC/LIMS
a
On
Automation
Dripping time
3 s
Rinsing time
15 s
Stirring rate
10
a
PC/LIMS = Personal computer/laboratory information management
system.
Table 2. NaCl stock solution preparation
Substance
Amount
NaCl, mg
255.43
NaCl SS
Total weight, mg
25049.0
Concentration, mg NaCl/g solution
10.20
Table 3. System suitability results for silver nitrate solution with a certified concentration of 0.1004 mol/L
Test round
Duplicate
Mass of NaCl SS, mg
NaCl equivalent, mg
Silver nitrate, mL
Calculated SNC, mol/L
1
1
1007.6
10.27
1.7480
0.1006
2
1043.2
10.64
1.8133
0.1004
2
1
1024.2
10.44
1.7818
0.1003
2
1205.6
12.29
2.0930
0.1005
3
1
1013.3
10.33
1.7568
0.1006
2
1071.5
10.93
1.8628
0.1004
Candidates for 2016 Method of the Year
92