AOAC Final Action Methods in 2018

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

C ( a ), for the sample portion. The titrant consumption of the blank test obtained at the end point should be less than 0.05 mLwhen using the standardized silver nitrate solution, D ( a ), and 0.2 mL when using the 0.025 mol/L standardized silver nitrate solution, D ( g ). Otherwise, check the reagents and water involved in the procedures, and perform the blank test again until the criteria is achieved. H. System Suitability Test Prior to use, transfer exactly 5 mL NaCl solution, C ( h ) or D ( b ), into a 120 mL sample beaker. If 0.025 mol/L AgNO 3 titrant is required, then use 1 mL NaCl solution. Add 5 mL nitric acid solution, D ( e ), and 50 mL water. Place the wash solution, D ( f ), in the washing position of the autosampler. Titrate using an automatic, semiautomatic, or manual titrator. Repeat in triplicate. Calculate concentration of the silver nitrate solution according to I ( Calculations section). 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 Calculate the concentration of silver nitrate, c snc , in mol/L, for system suitability verification and report to four decimal places. If using the prepared sodium chloride solution, D ( b ), use equation 1:

m

c

V u u 1 1 5 844 10 ,

snc

(1)

where m 1 = mass of sodium chloride in 5 or 1 mL sodium chloride solution, D ( b ), (in mg); V 1 = volume of 0.1 or 0.025 mol/LAgNO 3 solution consumed at titration end point (in mL); 5.844 = sodium chloride mass (in mg) corresponding to 1 mL of 0.1 mol/LAgNO 3 ; and 10 = mass conversion from titer to the concentration of titrant. Or if using a commercially available standard grade 0.10 mol/L sodium chloride solution, C ( h ), use equation 2: c V V snc u 0 1 3 1 , (2) where V 3 = volume of purchased standard grade 0.10 mol/L sodium chloride added (in mL); and V 1 = volume of 0.1 or 0.025 mol/L AgNO 3 solution consumed at titration end point (in mL). Calculate the chloride mass fraction, w cl , in mg/100 g in the sample and report to three significant digits, using equation 3:

c V V f m 2 0

u u

u u

35 45 ,

100

(3)

w

cl

where m = sample mass (in g); c = certified concentration of silver nitrate titrant (0.1000 mol/L or standardized concentration equation 1; V 2 = volume of AgNO 3 solution consumed at titration end point (in mL); V 0 = volume of AgNO 3 solution consumed at titration end point for blank, G ( d ), (in mL); and f = dilution factor

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