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6.4 CALIBRATION – pH
6.4.1 Purpose
A pH sensor consists of a glass and reference electrode. Usually, the two electrodes are combined into a single
body, called a combination pH sensor. When the sensor is placed in an aqueous solution, it produces a voltage
proportional to pH. An ideal pH sensor has a potential of 0 mV in pH 7 solution and a slope of -59.16 mV/pH at
25ºC, that is, a unit increase in pH causes the potential to drop 59.16 mV. However, even in a new pH sensor
the slope and offset are rarely equal to the ideal values. And, as the sensor ages, the offset typically increases
and the slope decreases. For these reasons, a new pH sensor should be calibrated before use, and the sensor
should be recalibrated at regular intervals. A pH sensor is calibrated by exposing it to standard solutions having
known pH values. The standard solutions are called buffers.
6.4.2 Definitions
1. AUTOMATIC BUFFER CALIBRATION. In automatic buffer calibration, the analyzer recognizes the buffer and
uses the temperature-corrected pH value in the calibration. The table lists the buffers the analyzer
recognizes. Temperature-pH data are valid between at least 0 and 60ºC.
Buffer list Buffer pH
standard (note) 1.68, 3.56, 3.78, 4.01, 4.64, 6.86, 7.01, 7.41, 9.18, 10.01, 12.45
DIN19267 1.09, 3.06, 4.65, 6.79, 9.23, 12.75
Ingold 1.993, 4.005, 7.002, 9.206
Merck 2.002, 4.014, 7.003, 9.004, 12.009
Fisher 1.00, 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, 10.00, 11.00
4. STANDARDIZATION. The pH measured by the analyzer can be changed to match the reading from a
second or referee instrument. The process of making the two readings agree is called standardization.
During standardization the difference between the two pH values is converted to the equivalent voltage. The
voltage, called the reference offset, is added to all subsequent measured sensor voltages before they are
converted to pH. If a pH sensor is buffered, then standardized and placed back in the buffer solution, the meas-
ured pH will differ from the buffer pH by an amount equivalent to the standardization offset.
The analyzer also measures noise and drift and does not
accept calibration data until readings are stable. Stability
criteria are user-programmable.
The use of automatic buffer calibration minimizes errors
and its use is strongly recommended.
2. MANUAL BUFFER CALIBRATION. In manual calibration,
the user must enter the pH value of the buffer at the
temperature of the buffer. In addition, the user must judge
when pH readings are stable.
3. SLOPE AND OFFSET. Once the analyzer successfully
completes the calibration, it calculates and displays the
calibration slope and offset. The slope is reported as the
slope at 25ºC. Figure 6-2 defines the terms.
FIGURE 6-2. Calibration Slope and Offset
Note: With the exception of pH 7.01 buffer, the standard buffers are NIST buffers.
MODEL FCL-1056 SECTION 6.0
CALIBRATION