Calibration - Radiometer Analytical MeterLab PHM220 Mode D'emploi

A pH meter must be calibrated against standards, i.e. buffer solutions, to
make the pH reading correspond to the individual characteristics of the
electrode(s) used. During a two-point calibration, the two important para-
meters the electrode's zero pH and sensitivity are determined and stored for
the sample measurements which follow. During a one-point calibration, the
zero pH is determined while the sensitivity found in the previous two-point
calibration is used. With the PHM220 two types of calibration procedures are
available:
AUTO: Automatic recognition of the pH buffer solutions. Three different
types of buffer solutions are recognised: IUPAC certified pH stan-
dards (pH 1.679, 4.005, 7.000, 10.012, or 12.45), Technical buffers
(pH 1.09, 4.65 or 9.23) or 4-7-10 Series. Buffers of different types
cannot be used in one calibration (e.g. using a technical buffer fol-
lowed by a IUPAC buffer for the calibration will lead to erroneous
results).
FIXED: Calibration using 1, 2 or 3 buffers from a list of fixed buffer values:
IUPAC pH standards (pH 1.679, 4.005, 6.865, 7.000, 7.413, 9.180,
10.012, or 12.45), Technical buffers (pH 1.09, 4.65 or 9.23) or 4-7-10
Series.
In both AUTO and FIXED, the actual pH value of the buffer at the current
temperature is used for the calibration calculation. The temperature is adjust-
ed using the arrow keys (in steps of 1°C) or read automatically when a tem-
perature sensor is connected (temperature displayed to 1 decimal place e.g.
23.4°C).
A glass electrode's sensitivity varies with time. Therefore, in order to obtain
the high degree of accuracy which the PHM220 has been designed to pro-
vide, a two-point calibration must be carried out frequently. If the glass elec-
trode is new, a calibration should be performed often during the first few days.
Single buffer calibrations are adequate when the sensitivity of the electrode
remains stable. Nevertheless, frequent two-point calibrations are
recommended to ensure optimal accuracy.
The PHM220 can automatically remind you that a new calibration is needed.
In the section, Editing a Calibration, which starts on page 33, you can read
about how to enter the maximum delay before a new calibration has to be
performed. Radiometer Analytical has selected 24 hours as the default value.
To provide you with a practical overview of the two calibration procedures, the
displays shown during each type of calibration are illustrated on the next
page. A step-by-step description of the various calibration procedures starts
on page 15.
12
Chapter 2

Calibration