Chlorine Dioxide gas detection: A comprehensive guide
Introduction
Sensor technology – what is on the market?
Calibration
Introduction
Some years ago, the bleaching process in the paper industry was changed from
using Chlorine (Cl2) as bleaching agent to Chlorine dioxide (ClO2). This posed a
major challenge to industrial hygienists, as there were no sensors or detectors
available that were specific to Chlorine dioxide.
As a temporary solution, chlorine detectors were calibrated to chlorine dioxide. The
sensor output signal created by ClO2 is lower compared to the signal created by
Chlorine. Looking at the alarm thresholds of 1 ppm for Cl2, (OSHA ceiling) compared
to 0.1 ppm for ClO2, it is obvious that such technology would not really do the job.
Sensor technology
The most common type of sensors used for toxic gas detection are electrochemical
cells. They work like a gas-operated battery:
Gas in  current out.
By choosing the right elements of such a battery, the sensor can be designed to
respond specifically to certain gases. One of the easiest examples to illustrate and
explain this technology is the electro-chemical sensor used for carbon monoxide:
The working electrode
This is the electrode exposed to the gas to be measured; in this case, it consists of
Platinum – Pt acting as a catalyst.
The electrolyte
It makes the interior of the sensor conductive and takes part in the chemical reaction
at the working electrode. For a CO sensor, the electrolyte consists of Sulphuric acid
(H2SO4).
The counter electrode
It keeps the sensor electrically neutral and compensates for the electrons produced
or consumed by the working electrode.
In the acidic environment at the working electrode, CO will be oxidised to CO2:
CO + H2O  CO2 + 2 H+ + 2 eNow two electrons are available to be conducted through an amplifier to the counter
electrode. This electrode must be designed to react with these two electrons to keep
the overall sensor assembly electrically neutral:
2 e- + ½ O2  H2O
The overall equation is:
2 CO + O2  CO2
What does this tell us?
1. The sensor takes carbon monoxide in.
2. The sensor is emitting CO2.
3. Each molecule will create two electrons, which can be conducted through a
sensitive amplifier.
4. Current = Charge / Time
5. Therefore, the sensor can be defined as a “molecule counter” with I ~ c (I =
current, c = concentration)
6. Knowing the way a sensor works makes it easier to predict which substances
it will respond to. In this case, it will be every substance that can be oxidised
by sulphuric acid and a catalyst. Examples: H2  H2O, H2S  H2SO4 etc. Any
other gases, other than the target gas being detected, which also create a
signal are called interfering gases or cross sensitivities.
Sensor design criteria
The illustration above shows which parameters can be used to develop a new gas
sensor:
Working electrode,
Electrolyte,
Counter electrode.
There are effectual ways to fine-tune and optimise a sensor, such as adding a third of
fourth electrode. Such electrodes can be used to apply a voltage to the working
electrode to force oxidation or reduce the number of substances that would not react
with the assembly as is. They can also contribute to stabilising the counter electrode
voltage and thus maintain a proper zero. The problem is that the more you move
away from the straightforward “battery design”, the more dependent the gas monitor
becomes on critical electrical parameters. This can be a problem with portable
instruments when the battery is discharged, or under extreme temperature
conditions.
Another way to fine-tune a gas sensor is with the use of interference filters. They are
designed to take away interfering gases, but let the target gas pass through. This
may be an appropriate tool to make a gas sensor more selective, but will certainly
increase response time and decrease sensitivity by a certain amount. The major
problem with filters is that you never know if they are saturated or polluted. Therefore,
the performance of the gas sensor does not depend on the sensor itself but on status
of the filter, which is usually unknown.
How to choose a gas sensor
The first criterion when choosing a gas detection system is the measuring range. Will
the sensor provide a strong enough signal to reliably monitor the gas concentration at
the alarm level? Another important criterion in selecting a sensor technology is to
check if the sensor may be exposed to interference gases, in the environment where
it will ultimately be used. If so, check if these gases are also dangerous: A false alarm
created by another dangerous gas may be acceptable. Alarms created by nondangerous gases are not. Check if the response at the alarm threshold of the
interfering gas is similar to the target gas. If it is the same or lower it may be
acceptable. If it is much higher, it is not.
Instruments which respond to harmless gas concentrations, will lead to unnecessary
production interruptions and will soon be ignored.
Non-selective gas detectors can be expensive and dangerous!
Electrochemical sensors for chlorine dioxide
Now let us look at chlorine dioxide gas detection. In the early times after converting
the bleaching process from Cl2 to ClO2, chlorine sensors were used to detect ClO2.
Cl2 + 2 H+ + 2 e-  2 HCl
Other than a CO sensor, this sensor will not oxidise, but reduce the gas to be
detected. The same effect can be applied to detect ClO2:
ClO2 + 4 H+ + 5 e-  Cl- + 2 H2O
This reaction delivers enough electrons to reliably detect chlorine, but there is a
major problem. The TWA of chlorine dioxide is at 0.1 ppm. Instruments equipped with
such sensors would not allow the user to set the alarm to TWA. The sensor current is
too close to zero noise.
In many plants, chlorine is still used. The high sensitivity of such sensor technology to
chlorine could, and usually does, create false alarms.
An even more dangerous aspect is the negative interference to H2S which is very
likely to be present in paper mills. H2S will cause a ClO2 sensor to dive negative. A
ClO2 sensor exposed to 15 ppm H2S would have an indicated reading of “-1.5 ppm”.
Hydrogen sulphide has a TLV of 10 ppm.
Imagine if H2S and ClO2 are present at the same time! The readings could
compensate to zero. This is the perfect example how dangerous non-selective
sensors can be.
New sensor technology
Now ClO2 specific sensors are on the market. They have proven to operate reliably
even in the challenging atmosphere of paper mills.
The new sensor generation is using the same overall equation:
ClO2 + 4 H+ + 5 e-  Cl- + 2 H2O
Even though the equation is the same, everything about the new generation sensor
is different.:
The working electrode is now gold and the counter electrode is carbon. Most
important: The electrolyte is not acid! This reduces the interference to chlorine and
hydrogen sulphide to zero. This sensor can be used without extra tools such as
additional electrodes or interference filters. Important too: The electrolyte is not liquid!
It is a special high-performance gel developed for gas sensors. Why is this
important? A gel needs no extra membrane to be kept inside the sensor body. This
allows the chlorine dioxide gas to access the working electrode without passing
through any additional material. The result is a strong and stable sensor signal.
Gas detection systems using this technology can be specified to a measuring range 0
to 0.5 ppm with a detectable limit of 0.02 ppm. Even in this low range, they will
operate free of false alarms in the corrosive environment of a paper mill.
Calibration
Having good sensor technology on hand is one thing; ease of use and calibration is
another. Since ClO2 span gas is not available in gas cylinders, how can a ClO2 gas
detector accurately be calibrated?
No matter which brand or which application: Each gas detection system must be
calibrated in a certain interval. This interval is best specified by the user, depending
on the conditions the instrument is used in. Many manufacturers help by giving
recommendations.
As gas in cylinders is not available, people have tried to help themselves by using
electrochemical gas generators. These commonly use a NaClO2 / Na Cl solution or
reduction of chlorate in oxalic acid. As of the writing of this reference guide, the
author doesn’t know of any of the calibration devices that can generate a low enough
concentration to be used to calibrate a state of the art chlorine dioxide gas detector.
Another issue is stability: None of the calibrators on the market has proven to be
stable enough over more than a few months. Some even fail to generate
reproducible gas generations after replacement of the electrolyte.
Therefore, some paper manufacturers even have regulations in place on how to
calibrate the calibrator. This calibration method is a wet chemistry method requiring a
spectrophotometer, chemicals and laboratory glassware. It can be done by skilled
personnel only.
There is no question that this strategy is not cost effective at all.
So after all, a reliable surrogate reference calibration gas was needed. Chlorine could
not be used, as the interference of state of the art ClO2 gas detectors to chlorine is
zero.
Intensive research has proven that NO2 mixtures in air are the best method to obtain
repeatable results.
With this new method, the ClO2 sensor is calibrated with ClO2 at TLV level in the
factory and a second time with NO2. The signal ratio ClO2 / NO2 is calculated and
indicated on a label on the sensor as a “reference factor”.
This factor is constant over the lifetime of the sensor. This allows the user to easily
perform calibrations with a reasonably priced gas mixture in the field.
This has been a breakthrough for reliable Chlorine Dioxide gas detection.