OMB Meeting Book - Jan. 11, 2018

(d)  Blank test: determination of reagent background content of chloride .—Perform a blank test using reagents, substituting water, C ( a ), for the sample portion. The titrant consumption of the blank test obtained at the end point should be less than 0.05 mL when using the 0.1 M standardized silver nitrate, and less than 0.2 mL when using the 0.025 M standardized silver nitrate. Otherwise, check the reagents and water involved into the procedures and then perform the blank test again until the criterion is achieved. H. System Suitability Test Perform a system suitability test prior to use. (a)  Transfer 5 mL NaCl solution, C ( h ) or D ( b ), into a 120 mL sample beaker. If 0.025 MAgNO 3 titrant is required, then use 1 mL NaCl solution. (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 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 MAgNO 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.

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

© 2016 AOAC INTERNATIONAL

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