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J
audzems
:
J
ournal of
AOAC I
nternational
V
ol
.
99, N
o
.
4, 2016
(e)
One-mark volumetric flasks
.—50, 100, 500, and
1000 mL, glass, Class A.
(f)
Graduated cylinders
.—25, 100, and 500 mL, glass.
(g)
Autosampler beaker
.—e.g., 120 mL, depending on the
titrator used.
(h)
pH Meter/mV meter
.—With a scale covering ±700 mV,
and a 20 or 25 mL buret (Mettler-Toledo, Columbus, OH), or
equivalent.
(i)
Automatic titrator
.—Autosampler (Mettler-Toledo Rondo
Tower) and motorized piston buret (Mettler-Toledo T50) with
remote-control dispensing and filling (Mettler-Toledo LabX 3.1
software) or the Metrohm 862 Compact Titrosampler with 800
Dosino and 10mLExchangeUnit (Riverview, FL), or equivalent.
Alternatively, a semiautomated (e.g., Metrohm Titrado 905/907,
with Metrohm
tiamo
TM
software, or equivalent) or a manual
titrator (using a buret with accuracy of 0.01 mL) may be used.
(j)
Combined ring silver electrode
.—e.g., Mettler DM
141 or DMi145-SC, Metrohm Ag Titrode No. 6.0430.100S,
or equivalent. Alternatively, a silver electrode with reference
electrode may be used.
(k)
Magnetic stirrer
.—Heidolph MR 3000 (Schwabach,
Germany) or Metrohm 804Ti Stand with 802 Rod Stirrer, or
equivalent.
(l)
Water bath
.—Capable of warming water to 40°C, or
equivalent.
(m)
Ultrasonic cleaner
.—Model AS2060B (Tianjin
Automatic Science Instrument Co. Ltd, Nanyang, China), or
equivalent.
(n)
Disposable syringe
.—3 mL, with handspike and 0.45 μm
disposable syringe filter.
C. Chemicals and Reagents
(a)
Water, purified
.—Greater than 18MΩ (EMD Millipore
Corp., Billerica, MA), or equivalent.
(b)
Sodium chloride (NaCl)
.—Certified reference material
for titrimetry, ≥99.5%, certified by the Federal Institute for
Materials Research and Testing (Berlin, Germany) according to
ISO 17025 (Cat. No. 71387; Sigma-Aldrich, St. Louis, MO), or
equivalent.
(c)
Silver nitrate (AgNO
3
)
.—Meets analytical reagent
specification of the European Pharmacopoeia (Reag. Ph. Eur.),
British Pharmacopoeia, and the United States Pharmacopeia
(USP); assay 99.8–100.5% (Cat. No. 10220; Sigma-Aldrich);
or equivalent.
(d)
Potassium
ferrocyanide
trihydrate
[K
4
Fe(CN)
6
3H
2
O]
.—Grade puriss p.a., American Chemical Society (ACS),
International Organization for Standardization (ISO), and Reag.
Ph. Eur.; ≥99% (Cat. No. 31524; Sigma-Aldrich); or equivalent.
(e)
Zinc acetate dihydrate [Zn(CH
3
COO)
2
2H
2
O]
.—
Grade ACS and puriss p.a., ≥99.0% (Cat. No. 96459; Sigma-
Aldrich), or equivalent.
(f)
Nitric acid (HNO
3
)
.—Minimum 65% puriss p.a. (Cat. No.
100452; Merck, Darmstadt, Germany), or equivalent.
(g)
Standardized AgNO
3
solution
.—0.1 mol/L (0.1 N)
Titripur
®
grade Reag. Ph. Eur. and USP (Cat. No. 1.09081.1000
or EM3214-1; Merck, Darmstadt, Germany) or ready-to-use
standardized titrant prepared according to GB/T 601-2002 (4),
or equivalent.
(h)
Sodium chloride
(
NaCl) standardized solution, 0.1 M
.—
Cat. No. 35616 (Alfa Aesar, Ward Hill, MA), or equivalent.
(i)
Glacial acetic acid, 100%
.
—
Anhydrous for analysis;
EMSURE
®
grade ACS, ISO, and Reag. Ph. Eur. (Cat. No.
100063; Merck); or equivalent
(j)
Potassium nitrate (KNO
3
)
.—For analysis, EMSURE
®
grade ISO and Reag. Ph. Eur. (Cat. No. 105063; Merck), or
equivalent.
(k)
Acetone.—
For cleaning of the electrode (Cat. No. 010-4;
Honeywell, Muskegon, MI), or equivalent.
(l)
Dimethylpolysiloxane
.—Defoaming agent (Cat. No.
DMPS2C; Sigma-Aldrich), or equivalent.
D. Preparation of Solutions
(a)
Standardized AgNO
3
solution, 0.1 M.
—If ready-to-use
AgNO
3
standard solution [
C
(
c
)] is not available, then weigh
16.9890 ± 0.0005 g AgNO
3
previously dried for 2 h at 120 ± 2°C.
Dissolve in water and dilute to volume in a 1000 mL volumetric
flask. Store in a brown reagent bottle.
Note 1
: After preparation, check the titer by titration of
5.0 mLwith exactly 0.1 M NaCl solution. For either commercial
or in-house solution, verify the titer on a regular basis.
Note 2
: The standardized AgNO
3
solution must be protected
from light, and can be stored for up to 2 months.
(b)
Sodium chloride solution, 0.1 M
.—If ready-to-use NaCl
standard solution is not available, weigh 5.8440 ± 0.0005 g
NaCl [
C
(
b
)] previously dried for 2 h at 110 ± 2°C.
Dissolve in water and dilute to volume in a 1000 mL volumetric
flask.
Note
: This solution is stable for up to 1 month.
(c)
Precipitating solution (Carrez) I
.—Weigh 106 g
potassium ferrocyanide trihydrate [
C
(
d
)], dissolve in an
appropriate amount of water, and transfer to a 1000 mL
volumetric flask. Dilute to volume using water. Mix well.
(d)
Precipitating solution (Carrez) II
.—Weigh 220 g zinc
acetate dihydrate [
C
(
e
)] and transfer to a 1000 mL volumetric
flask. Dissolve with an appropriate amount of water and add
30 mL glacial acetic acid [
C
(
i
)]. Dilute to volume using water.
Mix well.
(e)
Nitric acid solution
.—With care, add 100 mL
concentrated nitric acid [
C
(
f
)] to 300 mL water. Mix well.
(f)
Wash solution
.—According to autosampler/titrator
manufacturer’s instructions [e.g., acetone or nitric acid solution
(
see
e
, above)], or other.
(g)
AgNO
3
solution, 0.025M (optional)
.—Into a 1000 mL
volumetric flask, pipet 250 mL 0.1 M AgNO
3
solution [
C
(
g
) or
D
(
a
)]. Dilute to volume with water.
Note
: Prepare freshly before use, and then check the titer by
titration of 25 mL against 0.025 M NaCl solution.
(h)
NaCl solution, 0.025M (optional)
.—Into a 100 mL
volumetric flask, pipet 25 mL 0.1 M NaCl solution [
D
(
b
)].
Dilute to volume with water.
Note
: Prepare freshly before use.
(i)
KNO
3
solution, 1 M.
—Weigh 10.11 g potassium nitrate
[
C
(
j
)] into a 100 mL volumetric flask. Add about 80 mLwater and
place the flask in an ultrasonic cleaner [
B
(
m
)] to dissolve with
ultrasound and heating until dissolved thoroughly. Cool down
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