Thermo Orion 410Aplus, Instruction Manual

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Orion Benchtop pH/ISE Meter Instruction Manual
Calibration and Measurement of pH 15
Chapter V
Calibration and Measurement of pH
A. General Information on pH Calibration
A one, two or multipoint (where available) calibration should be performed
using fresh buffers before pH is measured. It is recommended that a two
buffer calibration, using buffers that bracket the expected sample range, be
performed at the beginning of each day to determine the slope of the
electrode. This serves a dual purpose, determining if the electrode is
working properly and storing the slope value in memory. Perform a one
buffer calibration every two hours to compensate for electrode drift,
using a fresh aliquot from one of the calibration buffers used in the
initial calibration.
The instruments use a point-to-point calibration scheme, i.e. the meter
stores in memory the different electrode slopes for each portion of the
calibration curve. When measuring in a particular region of the curve, the
electrode slope for that region is employed in the calculation of sample pH.
After calibration, the average electrode slope for all the segments of the
entire calibration curve is displayed. Use of this scheme increases accuracy
in the different regions of the calibration curve. However, the electrode
slope may be lower than normal, especially if buffers from the pH extremes
< 2.00 or > 12.00 are used. See Appendix C.
There are two ways of calibrating Orion Benchtop Meters, autocalibration
or manual calibration. The following are descriptions and instructions of
each method, for each model.
For Best Results
It is recommended that an ATC probe be used. If an ATC probe is not used,
all samples and standards should be at the same temperature or manual
temperature compensation should be used. Stir all buffers and samples with
a magnetic stirrer while a measurement is being made.
NOTE: Some magnetic stirrers generate enough heat to
change solution temperature. To avoid this, place a piece
of cardboard, foam rubber or other insulating material
between the stir plate and beaker.
Always use fresh aliquots of buffers whenever calibrating.