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BACK TO BASICS
Understanding pH measurement
In
the process world, pH is an important parameter to be measured
and controlled. The pH of a solution indicates how acidic
or basic (alkaline) it is. The formal mathematical definition
of pH is the negative logarithm of hydrogen ion activity.
In most cases, hydrogen ion activity can be approximated by
the hydrogen ion concentration, and the formula becomes pH
= - log10 [H+]. On the pH scale, which varies from
0-14, a very acidic solution has a low pH value, a very basic
solution has a high pH value, and a neutral solution has a
pH of approximately 7.
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A pH measurement loop is
made up of three components, the pH sensor, which includes
a measuring electrode, a reference electrode, and a
temperature sensor; a preamplifier; and an analyzer
or transmitter. A pH measurement loop is essentially
a battery where the positive terminal is the measuring
electrode and the negative terminal is the reference
electrode. The measuring electrode, which is sensitive
to the hydrogen ion, develops a potential (voltage)
directly related to the hydrogen ion concentration of
the solution. The reference electrode provides a stable
potential against which the measuring electrode can
be compared.
When immersed in the solution,
the reference electrode potential does not change with
the changing hydrogen ion concentration. A solution
in the reference electrode also makes contact with the
sample solution and the measuring electrode through
a junction, completing the circuit. Output of the measuring
electrode changes with temperature (even though the
process remains at a constant pH), so a temperature
sensor is necessary to correct for this change in output.
This is done in the analyzer or transmitter software.
The pH sensor components are usually combined into one
device called a combination pH electrode. The measuring
electrode is usually glass and quite fragile. Recent
developments have replaced the glass with more durable
solid-state sensors. The preamplifier is a signal-conditioning
device. It takes the high-impedance pH electrode signal
and changes it into a low impedance signal which the
analyzer or transmitter can accept. The preamplifier
also strengthens and stabilizes the signal, making it
less susceptible to electrical noise.
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Keeping
the system up and running
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A
system's pH electrodes require periodic maintenance
to clean and calibrate them. The length of time
between cleaning and calibration depends on process
conditions and the user's accuracy and stability
expectations. Over time, electrical properties
of the measuring and reference electrode change.
Calibration in known-value pH solutions called
buffers will correct for some of these changes.
Cleaning of the measuring sensor and reference
junction will also help. However, just as batteries
have a limited life, a pH electrode's lifetime
is also finite. Even in the "friendliest" environments,
pH electrodes have to be replaced eventually.
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The sensor's electrical signal
is then displayed. This is commonly done in a 120/240 V ac-powered
analyzer or in a 24 V dc loop-powered transmitter. Additionally,
the analyzer or transmitter has a human machine interface
for calibrating the sensor and configuring outputs and alarms,
if pH control is being done.
Keep in mind, application requirements
should be carefully considered when choosing a pH electrode.
Accurate pH measurement and the resulting precise control
that it can allow, can go a long way toward process optimization
and result in increased product quality and consistency. Accurate,
stable pH measurement also controls and often lowers chemical
usage, minimizing system maintenance and expense.
| pH |
| 0 |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
| 100 |
10-1 |
10-2 |
10-3 |
10-4 |
10-5 |
10-6 |
10-7 |
10-8 |
10-9 |
10-10 |
10-11 |
10-12 |
10-13 |
10-14 |
| [H+] |
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Tom
Griffiths is product manager, pH measurement at Honeywell
in Ft.Washington, Pa. He holds a BS in Chemical Engineering
from Clarkson University and an MBA in Marketing from
Temple University. Contact him at thomas.griffiths@honeywell.com
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