Keeping your pH measurements
reliable and accurate.
Reliable pH measurements are essential to a variety of processes and industries. From agriculture,
to wastewater and plating, to food processing,
measuring pH is a crucial step in determining
the efficiency of a process or quality of a product.
Vital steps in achieving accurate pH results
include calibrating a pH meter and ensuring the
electrode is working properly. But how do we
determine when a calibration is necessary or how
to troubleshoot a faulty electrode? Hanna Instruments has compiled six tips to consider when
calibrating a pH meter.
1Always read the manual Calibration procedures change based on which meter is used. In order to best
understand your specific meter and perform
calibration correctly, it’s important to read the
2Ensure your calibration points bracket the pH range of your sample
When performing a pH calibration it’s recommended to calibrate to at least two points.
When choosing calibration points, it’s important
to consider the pH range of the samples being
measured. pH calibration is most effective when
calibration points bracket the sample pH.
3Use best practices Ensure the glass bulb of the pH electrode is hydrated before performing a calibration.
Calibrating a dry pH electrode will result in the
need for frequent calibration.
Use an adequate volume of pH buffer to fully
submerge the pH bulb and the reference junc-
tion, which is typically a small ceramic frit on the
side of the probe.
Open or loosen the fill hole on the electrode to
ensure flow or reference electrolyte into the sample. This is crucial for fast electrode response and
stable electrode readings.
During pH calibration and measurement, the
solution should always be stirring. Wait for the
electrode reading to stabilize before accepting
each calibration point.
Always use fresh buffer solution for calibration, this ensures the buffer being used is not
Check the expiration date of all buffers and
keep track of how long the buffer has been
opened. Buffer solutions may expire much sooner than the specified expiration dates once they
are opened due to evaporation and influx of
atmospheric carbon dioxide.
4Calibrate often For the most accurate measurements, it’s uggested to calibrate a pH meter each
time a new set of measurements is performed.
Calibration can also help identify a problem with
5Be aware of your offset and slope Offset and slope have a significant impact on the condition of a pH meter’s electrode.
The offset is the mV reading of the electrode when
it’s submerged in pH 7 buffer. The theoretical off-
set is 0 mV; however, the offset shouldn’t exceed
±30 mV. The slope of the electrode is calculated by
determining the mV change between two different
pH buffers. By dividing the mV difference by the
change in pH units, users can get the actual slope
of the electrode. If users then divide the slope of
the electrode by the theoretical slope of 59.16 mV
and multiply by 100, they can get an overall per-
centage condition of the electrode.
If there’s an offset outside of the ±30 mV
range, this could mean either a clogged junction
or the probe requires cleaning. If this occurs, try
soaking the probe in cleaning solution, rehydrat-ing the bulb and repeat the process of checking
offset. If the slope is too far from the ideal slope
of 59.16 mV, this could mean expired or contaminated buffers have been used. If this is the case,
repeat the calibration, ensuring fresh buffers are
used. Following calibration, many Hanna Instruments meters will display both the slope and offset in the GLP data screen. Some Hanna meters
also come equipped with CAL Check; this feature
provides indicators to potential problems during
the calibration process based on previous offset
and slope values, allowing users to quickly solve
6Electrode care pH calibration and measurements are only as good as the condition of pH electrode.
pH electrodes should never be stored in distilled
or deionized water, as this can dehydrate the
sensing glass of the pH electrode and deplete reference electrolyte. Instead, storing the electrode
in storage solution will provide stable and highly
responsive readings due to a properly hydrated
and responsive bulb.
By following these few simple tips, pH meter
users can obtain accurate calibrations and pH
Ann Arbor, Mich.
to Use When
Your pH Meter