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MODEL 6081-P pH/ORP SECTION 10.0
CALIBRATION – PH
SECTION 10.0
CALIBRATION — pH AND ORP
10.1 INTRODUCTION
For pH sensors, two-point buffer calibration is standard. Both automatic calibration and manual calibration are
available. Auto calibration avoids common pitfalls and reduces errors. Its use is recommended. In auto calibration
the 6081 calculates the actual pH of the buffer from the nominal value entered by the user and does not accept
calibration data until readings are stable. In manual calibration the user enters buffer values and judges when read-
ings are stable. The pH reading can also be standardized, that is, forced to match the reading from a referee instru-
ment. Finally, if the user knows the electrode slope (at 25°C), he can enter it directly.
The ORP calibration is a single-point calibration against an ORP standard.
A new pH sensor must be calibrated before use. Regular recalibration is also necessary.
A pH measurement cell (pH sensor and the solution to be measured) can be pictured as a battery with an extreme-
ly high internal resistance. The voltage of the battery depends on the pH of the solution. The pH meter, which is
basically a voltmeter with a very high input impedance, measures the cell voltage and calculates pH using a con-
version factor. The actual value of the voltage-to-pH conversion factor depends on the sensitivity of the pH sens-
ing element (and the temperature). The sensing element is a thin, glass membrane at the end of the sensor. As
the glass membrane ages, the sensitivity drops. Regular recalibration corrects for the loss of sensitivity. pH cali-
bration standards, also called buffers, are readily available.
In automatic calibration the transmitter recognizes the buffer and uses temperature-corrected pH values in the cal-
ibration. The table below lists the standard buffers the controller recognizes. The controller also recognizes sever-
al technical buffers: Merck, Ingold, and DIN 19267. Temperature-pH data stored in the controller are valid between
at least 0 and 60°C.
10.1 Introduction
10.2 Procedure – Auto Buffer Calibration
10.3 Procedure – Manual Two-Point Buffer Calibration
10.4 Procedure – Standardization
10.5 Procedure – Entering a Known Slope Value
10.6 ORP Calibration
pH at 25°C Standard(s)
(nominal pH)
1.68 NIST, DIN 19266, JSI 8802, BSI (see note 1)
3.56 NIST, BSI
3.78 NIST
4.01 NIST, DIN 19266, JSI 8802, BSI
6.86 NIST, DIN 19266, JSI 8802, BSI
7.00 (see note 2)
7.41 NIST
9.18 NIST, DIN 19266, JSI 8802, BSI
10.01 NIST, JSI 8802, BSI
12.45 NIST, DIN 19266
Note 1: NIST is National Institute of Standards,
DIN is Deutsche Institute für Normung, JSI is
Japan Standards Institute, and BSI is British
Standards Institute.
Note 2: pH 7 buffer is not a standard buffer. It is
a popular commercial buffer in the United
States.