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(b) Transfer 2.5mL precipitating agent I and 2.5mL precipitating agent II into the tube, bring
to 50mL with water and mix well.
(c) Centrifuge at 12500 x
g
for 5min at 4°C (6min for skim milk powder) and equilibrate to
room temperature.
(d) Accurately transfer 10 mL supernatant (20mL for desalted whey powder D90) into a 120
mL sample beaker, add 5mL nitric acid solution and 50mL water before titration.
F.
System Suitability
(a) Weigh 1000 - 1500mg (accurate to 0.1mg) NaCl SS into a 120 mL sample beaker.
(b) Add 5mL nitric acid solution and 50mL water.
(c) Put the washing solution in the washing position of the auto sampler, and replace with
fresh washing solution after every 10 or 11 single titration tests.
(d) The titration conditions for system suitability analysis are presented in Table
2015.07
.
(e) Titrate using the titrator.
(f) Calculate concentration of the silver nitrate solution according to section
H
(a). The
difference between the calculated concentration and the certified value should be within 0.5% .
If outside the acceptance value, check the experimental procedures and titration system. If issue
is not resolved, use fresh silver nitrate. If fresh silver nitrate cannot be able to gain a acceptable
range, replace the electrolyte of the electrode and check the condition of dosing unit.
G.
Analysis
(a) Titrate the prepared sample solution on the titrator. Put the washing solution in special
washing position of the auto sampler, and use change freshwashing solution every 10 or 11
single titration tests. The sample titration conditions are the same as the system suitability
analysis in Table
2015.07
.
Candidates for 2016 Method of the Year
87