Emerson 6081-P instruction manual Section, Calibration pH and ORP

MODEL 6081-P pH/ORP	SECTION 10.0
	CALIBRATION – PH

SECTION 10.0

CALIBRATION — pH AND ORP

10.1Introduction

10.2Procedure – Auto Buffer Calibration

10.3Procedure – Manual Two-Point Buffer Calibration

10.4Procedure – Standardization

10.5Procedure – Entering a Known Slope Value

10.6ORP Calibration

10.1INTRODUCTION

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.

	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.

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