|
|
|
Analytical Instrumentation |
|
pH/ORP
Technology |
|
 The
pH value is one of the fundamental and most frequently
used process variables for controlling and monitoring
chemical processes in industry - such as, for example,
production control, quality assurance or waste water control.
The pH value indicates the concentration of the smallest
ions known - the H+ hydrogen ions. An aqueous solution
will be acidic, neutral or alkaline depending on its pH
value.
|
|
pH
Measurement: |
PH
measurement is performed almost exclusively with glass electrodes.
The membrane glass supplies an electric potential registered
via the internal electrolyte and the silver/silver chloride
electrode. The measuring transmitter then converts the potential
to the corresponding pH value according to the Nernst equation.
In principle, a pH electrode system consists of a pH-sensitive
electrode and a reference electrode. For over 40 years,
glass electrodes have been principally used in industrial
pH measurement. They exhibit several useful advantages compared
with other electrode types, such as good linearity over
a broad pH range (approx. pH 0 to 14), sufficient chemical
stability, high selectivity, and they are independent of
redox systems. Because of their easy manufacturing and very
good reproducibility, silver/silver chloride electrodes
are the most frequently used reference electrodes. Moreover,
this reference system is non-poisonous and thus usable in
food technology as well. Today, pH electrodes are frequently
used which consist of a combination of glass electrode and
reference electrode in a single body. Special designs (e.
g. with a pressure-compensated reference electrode) also
permit pH measurements in pressurized media. Gel-filled
electrodes whose reference electrolyte is usually solidified
by means of a synthetic polymer are also finding increasing
use. These electrodes do not require a supply of reference
electrolyte and, depending on the design, are resistant
to pressure. In addition, the reference system is not dependent
on the position for storage and use.
|
|
pH
Measurement and Temperature: |
In
addition to the Nernst dependence of the measurement, which
is usually compensated for, the temperature dependence of
the acid/base equilibrium in aqueous systems can lead to
significant changes of the pH value. Particularly when measuring
ultrapure water in power plants, where traces of ammonia
determine the pH value, measurements at higher temperatures
differ by up to two pH units from measurements at 25 °C.
Therefore, these pH values must be related to a standard
temperature, for example to allow comparison of measurement
before and after a heat exchanger.
|
|
pH
Analyzers: |
pH
analyzers convert the voltages supplied by the sensor (pH
electrode) into pH values and standard signals (output).
The input circuits are designed for an extremely high resistance
so that the voltage drop at the input does not cause intolerably
great measured value deviations. Apart from the electrode
voltage, the temperature of the process medium is detected
for the temperature compensation of the pH voltage, the
basis being provided by the theoretical temperature coefficient
of the Nernst voltage. The temperature sensors that are
used most often in industry are the Pt 100 / Pt 1000 precision
resistors. Measured value processing and control of auxiliary
functions are performed by means of microprocessor technology.
|
|
pH
Probes
|
|
Aysix
Replacement Probes
|
|
Probe
Make & Model
|
Equivalent
Aysix Probe Model
|
|
Rosemount
Model 396P
|
Aysix
Model M21 (M17) & Aysix Model M22 (M18)
|
|
Rosemount
Model 399
|
Aysix
Model M21( M17)
|
|
|
