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200 B olong et al .: J ournal of AOAC I nternational V ol . 99, N o . 1, 2016

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 = 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. where m 4 = sample weight in grams, c = certified concentration in moles per liter of silver nitrate titrant, V 2

H. Calculations

Calculate silver nitrate concentration (SNC) in moles per liter for system suitability verification and report to 4 decimal places:

(

) =

m m

1 1 10

1

2

× × ×

SNC mol L

m V

5.844

3

1

where m 1 = 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, 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 = weight in milligrams of NaCl SS, m 2

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.

Stop conditions  Stop volume

10 mL

 Stop measure value  Stop equivalence point

120 mV

1

 Volume after equivalence point Evaluation  Equivalence-point recognition

1 mL

Greatest

Reports  PC/LIMS a

Table 2. NaCl stock solution preparation Substance

On

Automation  Dripping time  Rinsing time  Stirring rate

Amount

3 s

NaCl, mg

255.43

15 s

NaCl SS  Total weight, mg

10 a  PC/LIMS = Personal computer/laboratory information management system.

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 2 1 2 1 2

1007.6 1043.2 1024.2 1205.6 1013.3 1071.5

10.27 10.64 10.44 12.29 10.33 10.93

1.7480 1.8133 1.7818 2.0930 1.7568 1.8628

0.1006 0.1004 0.1003 0.1005 0.1006 0.1004

1

2

3

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