OMB Meeting Book - Jan. 11, 2018

1116 J audzems : J ournal of AOAC I nternational V ol . 99, N o . 4, 2016

Figure 2016.03B. Example of titration curve from a Mettler autotitrator.

(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.

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

( SNC mol L / )

=

3

V

1

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 .—

I. Calculations

) c V V f m 2 - 0

× × 35.45 (

× ×

100

( / 100 ) CL mg g

=

(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: 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

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