Chlorine Dioxide gas detection: A comprehensive guide

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Courtesy of Compur Monitors GmbH & Co. KG

Introduction
Some years agoago, the bleaching process in the paper industry was changed from using Chlorine (Cl2) as bleaching agent to Chlorine dioxide (ClO2). This was a challenge forThis posed a major challenge to industrial hygienists, as there were no sensors or detectors available that were specific to Chlorine dioxide: There was no sensor technology for gas detectors available that would be specific to chlorine dioxide.

As a temporary solutionsolution, 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 Ssensors used for toxic gas detection are are most commonly so called electrochemical cells. They work like a gas operatedgas-operated battery:

Gas in current out.
By choosing the right elements of such a batterybattery, the sensor can be designed to respond specifically to certain gases. One of the most easy to understandeasiest examples to illustrate and explain for this technology is the electro-chemical sensor used for carbon monoxide:

The working electrode
This is the electrode exposed to the gas to be measuredmeasured; in this casecase, 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, In this case it 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 e-
Now 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. So 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 easy easier to predict to which substances it will respond to. In this casecase, 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 Such alternative gases creating also 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. To fine tune an existing sensor, additional parameters can be used to optimise it: Introduce a third or even a fourth electrode. Such electrodes can be used can be used to apply a voltage to the working electrode to force oxidation or reduction reduce the number of substances that would not react with the assembly as is. They can also contribute to stabilisinge the counter electrode voltage and thus stabilise maintain a proper zero. The problem is that the more you move away from the straightforward “battery design”, the more the properties of the gas detector depend on the performance ofdependent 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 tunefine-tune a gas sensor areis so calledwith the use of interference filters. They are designed to eat 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. But Tthe major problem with filters is that you never know if they are saturated or polluted. SoTherefore, the performance of the gas sensor does not depend on the sensor itself but on status of the filter whichfilter, 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.level? Another important criterion how to choosein selecting a sensor technology is to check if this the sensor has interferences to gases that might occur where the sensor is to be usedmay 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 than the target gas may be acceptable. Alarms created by non- dangerous 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 higherhigher, it is not.
Instruments responding to harmless gas concentrations which respond to harmless gas concentrations, will lead to unnecessary production interruptions and will soon be ignored.

Non selectiveNon-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 COCl2ClO2, 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 of course be used tobe 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. as tThe TWA of chlorine dioxide is at 0.1 ppm., instrumentsInstruments 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 plantsplants, still chlorine is present for some applicationsstill used. The high sensitivity of such sensor technology to chlorine might create many false alarmscould, and usually does, create false alarms.
An even more dangerous aspect is the negative interference to H2S whichH2S which is very likely to occur 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”. It is specified to be – 1.5 ppm chlorine dioxide reading at + 15 ppm hydrogen sulphide concentration.
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 selectivenon-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.
Although Tthe 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.with this new sensor generation everything 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. SoThis allows the the chlorine dioxide gas can accessto 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 withppm with a detectable limit of 0.02 ppm. Even in this low rangerange, they will operate false alarm free free of false alarms even in the corrosive atmosphere of a paper mill.in the corrosive environment of a paper mill.

Calibration
Having good sensor technology on hand, now another problem has surfaced is one thing; ease of use and calibration is another.: Since ClO2 span gas is not available in gas cylinders, Hhow to calibratecan ClO2a ClO2 gas detector accurately be calibrateds? ClO2 span gas is not available in gas cylinders.

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 knowing his site best, 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. Most These commonly they use a NaClO2 / Na Cl solution or reduction of chlorate in oxalic acid. To As of the writing of this reference guide, the authors knowledge doesn’t knownone of any theseof the calibration devices can generate a lowdevices that can generate a low enough concentration to be used to calibrate a state of the art chlorine dioxide gas detector. Another issue is sStability: None of the calibrators in on the market has 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.
SoTherefore, 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 Nno 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 the a breakthrough for reliable Chlorine Dioxide gas detection.

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