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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

82