Measurement of pH in wastewater
has always posed several challenges. Of the many problems one might face under
such circumstances, the most common would probably be fouling of the reference
electrode system.
A conventional combination pH
electrode is made up of two parts. The first is the glass electrode and the second, the reference. The glass electrode is made of a special glass bulb attached to one end of a glass stem. This is usually done by a glass blowing process. This bulb is a pH sensitive membrane. The inside of the bulb is filled with a conductive buffered solution. A silver wire is immersed in this solution. This
configuration is usually referred to as the glass or measuring electrode. Refer
to figure below (Diagram 1):
It can be seen that the only part
of the glass electrode system that comes into contact with the sample is the
glass bulb itself. The inner fill solution and the silver wire are physically
isolated from the sample. Therefore very rarely does one see any problems
associated with these components. The problems to look for however would be related to exposure of the bulb to harsh chemicals like hydrofluoric acid that may attack glass.
The reference electrode is much
simpler in construction. In its simplest form, it consists of a reference
chamber, a reference junction and silver wire. The reference junction is
usually made of porous material. Its function is to allow the reference
electrolyte to flow from the chamber through to the sample and thus establish
an electrical connection. Refer to figure below (Diagram 2):
It can be seen from the diagram
above that unlike the glass electrode, the reference is prone to contamination or
fouling from outside. The problems generally seen are reference junction
blocks, contaminants entering the reference chamber and thus reacting with the
reference electrolyte and / or the silver wire. In wastewater measurements this
could happen easily due to pressure heads. In either case, the performance of
the pH electrode as a whole would be compromised.
In cases where a reference
contamination is likely, one option that may be available is to use a double
junction electrode. In this type of electrode, a second junction is introduced
between the first junction and the reference silver wire. This serves to delay
the contamination process but does not eliminate it completely.
We notice that the glass
electrode system is less prone to such problems. With this in mind, can we not think
of a reference electrode system built very similar to that of the glass electrode?
The answer is the differential pH
electrode.
The difference in construction
between a simple combination pH electrode and a differential pH electrode can
be easily understood from figure shown below (Diagram 3):
Diagram 3
The reference part of the
differential electrode is very similar in construction to that of the glass
electrode. The reference chamber can be filled with a conducting salt solution.
This electrolyte can be emptied and refilled if necessary.
In order to make the electrode
more superior, the reference junction can be replaceable type. The advantage
would be that in the event the junction clogs; it could be easily replaced thus
extending the life of the electrode.
Problems associated with ground
loops potentials can be eliminated by having a liquid ground built into the
electrode. The pH and reference voltages will be measured by the instrument
with respect to the ground pin potential and therefore be free from the effects
of the ground loop potential as shown in the simple electrical model of the
differential electrode in the figure below (Diagram 4):
Diagram 4
The final signal of interest
would be
(E1 – E3) – (E2
–E3) which is E1 – E2
Advantages of the differential pH
electrode can be summarized as follows:
- Longer sensor life in a wide range of measurement
environments.
- The reference electrode solution is buffered and less
susceptible to contamination than the standard KCL reference solutions
used in combination electrode systems.
- Any buffer standard from 2 pH to 10 pH can be used as
a reference. This is very useful for applications that require adjustable
isopotential points.
- The reference electrode is a pH glass bulb and the
inner fill and silver wire are not easily contaminated. For standard combination electrode
systems silver/silver chloride wires are used as a reference and can react
with some process chemicals over time.
- Reference solutions and salt bridges can be easily
replaced when fouling does occur.
- Integral electronic amplifiers measure potential
difference of the process electrode and the reference electrode with
respect to solution ground. This provides a low impedance output from the sensor which results in the following benefits:
- Extended cable lengths of up to several hundred feet
- Low noise and expensive coaxial cable is not needed.
- Hand capacitance is dramatically reduced – readings do not fluctuate when sensors are held.
- Electrical interference from motors, pumps and transformers are reduced.
- Ground currents are essentially eliminated.
It is therefore evident that the
differential pH electrodes can last much longer than the conventional electrodes
even in a harsh wastewater environment.
Differential pH electrodes need
special input electronics and cannot be connected to conventional pH
transmitters and controllers. A good example of a differential pH system is the
Alpha pH 2000D Controller (Diagram 7) and the Differential pH Electrode
(Diagram 5) from Eutech Instruments.
Article is submitted by Mr.
Bhaskar Narayanan (Managing Director) and Mr. Subramanian Raghuraman (General Manager - Technology) of Eutech
Instruments, a leading ISO9001 certified water quality analysis
instrumentation manufacturer that markets their comprehensive product ranges
worldwide through an extensive distributor network. For more information,
please contact our distributors or visit our corporate website at www.eutechinst.com
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