Emerson 54e pH/ORP instruction manual Section Theory of Operation, Continuous Sensor Diagnostics

MODEL 54e pH/ORP	SECTION 6.0
	THEORY OF OPERATION

SECTION 6.0

THEORY OF OPERATION

6.1 THE pH SENSOR ASSEMBLY

The pH measurement is accomplished by means of a measuring electrode (usually made of glass) which develops a potential directly related to the hydrogen ion concentration (pH) of the solution in which the elec- trode is immersed. A second electrode, called the ref- erence electrode, is necessary to complete the electri- cal circuit and to serve as a constant reference poten- tial against which the potential of the glass electrode can be compared. Together, the two electrodes com-

prise the pH sensor.

designed to avoid blocking of the internal liquid junc- tion. It is essentially a complete electrode within an electrode outer body, using two liquid junctions.

A non-reactive electrolyte (potassium chloride gel) is the filling solution in the outer body, and only this solu- tion is in contact with the process. Clogging of the outer junction is minimized, since neither potassium ions nor chloride ions form insoluble compounds with the majority of materials found in process streams. Contamination of the inner junction is not likely with this configuration.

The Glass Electrode

The glass electrode is a thin-walled bulb of glass con- taining a pH-buffered solution and the elements of a half-cell (typically silver/silver chloride). The glass membrane provides a means of collecting hydrogen ions so the potential can be measured. The hydrogen ions on the surface of the glass membrane are in equi- librium with the hydrogen ions in the solution being measured. As the hydrogen ion concentration in the solution changes, the concentration of hydrogen ions on the electrode changes, creating a change in poten- tial on the glass electrode.

The Reference Cell

The purpose of the reference cell is to maintain a sta- ble reference potential regardless of a change in sam- ple pH. The cell is comprised of the reference elec- trode, reference fill solution and porous reference junc- tion. The reference cell also serves to complete the voltage measuring circuit. Within the reference elec- trode, a silver chloride element is surrounded by a con- centrated potassium chloride solution. The internal ele- ment maintains electrical contact with the pH sensitive glass electrode through the porous liquid junction (a multi-capillary hardwood plug) of the internal reference electrode.

The external wood reference junction and the gelled reference fill solution provide protection for the internal reference electrode to prevent contamination from harsh chemical environments. The internal liquid junc- tion can become blocked if the process contains any material that reacts with the filling solution to form a precipitate. The double-junction reference electrode is

The Temperature Compensation Element

In addition to the glass electrode and reference cell, a third element required in the pH measurement system is the temperature compensation element. Its purpose is to compensate for "apparent" pH changes due to the pH response of a glass electrode increasing with tem- perature. At values around 7 pH, the variation with tem- perature is zero. Thus, no compensation is required at 7 pH. The error caused by varying temperatures is greater at high/low pH values; in other words, as you move away from a neutral pH of 7. This compensation is done automatically by the Model 54e pH/ORP unless specifically disabled.

The Preamp

Because of the high resistance of the glass measuring electrode, a preamplifier must be used to transform the extremely low level, high impedance signal to a low- impedance signal. The Model 54e pH/ORP includes a switchable preamplifier that may be used if the dis- tance to the pH sensor is less than 15 feet. When longer runs of cable are necessary, the preamp can be located in the pH sensor itself or in a remote junction box. Either design allows transmission of the pH signal long distances without suffering degradation or inter- ference from outside voltage sources.

6.2 CONTINUOUS SENSOR DIAGNOSTICS

The Model 54e pH/ORP verifies the integrity of the glass and reference segments of the pH sensor by con- tinuously measuring the impedance between each segment and the solution ground.

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