8. Procedure
8.1 Select the appropriate weight of sample from the following table:
8.2 Weigh the sample into a 250-mL iodine flask that has been flushed with nitrogen or carbon dioxide. Add 25 mL of 1,1,2-trichloro-1,2,2,trifluoroethane (Warning - See A1.1). Pass a vigorous flow of nitrogen or carbon dioxide through the solvent for at least 1 min; then, without stopping the gas flow, add 20 mL of acetic acid solution and reduce the flow of gas so that the rate is one bubble per second. Add 2 mL of KI solution and mix vigorously for 30 +/- 1 s. Set the flask aside to stand for 5 min +/- 3 s.
8.3 At the end of the reaction period, add 100 mL of water and stop the gas flow. Add 5 mL of starch solution. Titrate with 0.005 N Na2S2O3 solution to the disappearance of the blue color.
NOTE 1 - This end point is of the returning type. The end point intended in this method is the disappearance of the blue color for 30 s or longer.
8.4 Titrate a reagent blank just prior to each set of unknown determinations, following the same procedure as described in 8.1-8.3.
9. Calculation
9.1 Calculate the peroxide number as follows:
Peroxide number, mg/kg = [(A - B)N x 1000 x 8]/S
where:
A = millilitres of Na2S2O3 solution required for titration of the sample,
B = millilitres of Na2S2O3 solution required for titration of the blank,
N = normality of the Na2S2O3 solution, and
S = grams of sample used.