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Ensuring maximum accuracy in
pH measurement
Calibration, adjustment and verification

A variety of tests can be carried out in the laboratory to ensure the accuracy of pH sensors.

When it comes to pH
measurements, a reliable measured
value is essential. This is because
fluctuating pH values or pH values
outside of a specified range canhave serious consequences for
product and plant safety. Various
procedures can be used to test anddocument whether the connected
pH sensor is reliable at recording
measured values. But which
procedure is used when? When do
you calibrate, when do you adjust
and when is verification useful oreven necessary?
Calibration
The purpose of calibration is to findout if the measured value of a pH
sensor compares to that of anotherdevice. This calibration represents
a snapshot, which explains why theprocedure needs to be repeated atdefined intervals. It requires the use ofpH buffer solutions with a specific pH
value. As a traceable standard, the pH
buffer offers accredited reliability andsafety. With regard to the calibration
process itself, a distinction is madebetween a 1-point calibration and a
2-point calibration. With a 1-pointcalibration, the pH sensor is immersed
in a buffer solution with a known
pH value. A transmitter is then used
to read the measured value. If, forexample, a buffer solution with pH 7
is used and the transmitter displays avalue of pH 7.2, this represents a clear
deviation from the set point.
With a 2-point calibration, however,two different buffer solutions are used
to record both the zero point and theslope. Depending on the requirementsthat need to be met within the plant,
certain limit values may exist for theslope, for example. This is because thelower the slope of the sensor, the lessaccurate the measurement.
If no discrepancies are detectedduring calibration, the pH sensor canbe reinstalled in the process and will
provide reliable measured values.
However, if there is a deviation fromthe set point or from the previous
adjustment, a pH adjustment must be
carried out in the second step.
Adjustment
The aim of adjustment is tocompensate for the deviation
established during calibration byentering an offset. The focus is on

adjusting the pH measuring system
so that it is as accurate as possible.
With a 1-point adjustment, as withcalibration, the sensor is immersed
in a buffer solution and the deviationfrom the set point is identified.
An offset, also called a pH buffer
set point, is then stored in thetransmitter, which corrects the value
accordingly.
With a 2-point adjustment, the zero
point and the slope are determined
and the deviation from the previous
adjustment is calculated. The newly
recorded values for the slope andzero point are subsequently replaced.
Unlike calibration, adjustmentinvolves direct intervention in themeasuring system.
Verification
After the sensor has been adjusted,
an optional verification can beperformed. This is used to verify theresult and prove that an adjustment
was successful and that the pH valuesof the buffer solutions are recorded
correctly. In some sectors, such as thepharmaceutical industry, verification
is often included in the standard

Overview
Calibration• Detects deviations• Snapshot• Can be performed using one
or two buffer solutions• 1-point calibration: detects
deviation from set point• 2-point calibration: records
zero point and slopeAdjustment• Direct intervention in themeasuring system• 1-point adjustment:compensates for deviation byentering an offset• 2-point adjustment: zero
point and slope values ofprevious adjustment are
replaced
Verification• Verifies an adjustment• Checks if deviations arewithin specified limit values

An offset value is stored during adjustment to ensure that the measurement is accurate.

procedure and is part of the Standard
Operating Procedure (SOP).
Following a 1-point adjustment, thepH sensor is immersed in a buffer
solution that is different from theadjustment buffer solution previously
used. With the desired, optimallinear behavior, the deviation of themeasured value should not exceed
0.02 pH (pH 2 – pH 9) or 0.05 pH(pH 10 – pH 12). If verification
is carried out following a 2-pointadjustment, the pH sensor isimmersed in a buffer solution whose
pH values are situated between thetwo adjustment buffers. With thistype of verification, statistical error

propagation based on the two pH
buffer solutions is taken into account:In the pH 2 to pH 9 range, thedeviation should not exceed 0.028 pH.
In the pH 10 to pH 12 range, the value
is 0.053 pH.
All three steps ensure the accuracy ofthe sensor and yet each has different
objectives and results. Deviations aredetected during calibration, which
are rectified by means of adjustment
and cross-checked during verification.
These steps reliably ensure optimal
recording and mapping of themeasured value by the pH sensor.

WP01154C/07/EN/01.21

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