Hach AutoCAT 9000, Instruction Manual

Page: 57 / 176

PAO/5.64 mN
PAO/5.64 mN
50081_PAONaThioCalibration.fm
Page 55
Safety
Use good safety practices and laboratory techniques throughout the procedure. Consult
the Material Safety Data Sheet (MSDS) for specific reagent(s) information.
Summary of Method
The 0.00564 N reductant titrants, phenylarsine oxide (PAO) and sodium thiosulfate
(NaThio), are standardized by titrating them against a know volume of 0.0282 N Iodine
Standard Solution. The PAO/NaThio calibration procedure embedded in the
AutoCAT 9000 is adapted from procedure 4500-Cl C.
a . in “Standard Methods for
Examination of Water and Wastewater“(20th Edition).
A calibration sample is prepared by adding 0.50 mL of 0.0282 N Iodine Standard
Solution to 200 mL of chlorine-free deionized water. Potassium iodide (KI) and pH 4
acetate buffer are then added to the calibration sample. The calibration sample is then
titrated using the desired reductant titrant. The embedded procedure calls for a 2.2-mL
predose prior to titration. The titration end point is around 2.5 mL using the embedded
method as defined. If the concentration of the Iodine Standard Solution is significantly
less than 0.0282 N, it may be necessary to reduce the predose volume prior to titration.
Titrate to an amperometric end point.
25.
Press
1
to select
New Test and run a replicate
calibration. Proceed to
step 26 .
Press
3
to select
End Analysis and store the
titrant calibration value and
return to the Methods menu.
Note: If subsequent replicate
calibrations are run, the final
titrant concentration will
automatically be stored as the
mean value from all the
replicate titrant calibrations.
26.
The replicate analysis is
identical to the first
calibration.
Upon completion of the
analysis, and acceptance of
the data, subsequent replicate
analyses can be performed if
desired.
Press
1
to continue.
Press
2
to select
More Details to view
analysis statistics specific to
the current calibration.
27.
The mean and standard
deviation are calculated
from the replicate analyses.
Press the
CHECK MARK
to
accept the calibration value
and return to the
Titrants menu.
In the Titrants menu, the
accepted calibration has
caused the
Titrant tab icon
to become sunny. This
indicates that the titrant
concentration has been
properly calibrated and
analyses may resume.