AET Buyers’ Guide 2005

GAS Detection

Using Electrochemical Sensors FOR TOXIC GAS MEASUREMENT

How Electrochemical Sensors Detect GasSubstance-specific electrochemical sensors are available for many of the most common toxicgases including hydrogen sulphide, carbon monoxide, sulphur dioxide, chlorine, chlorine dioxide,ammonia, phosphine, ethylene oxide, nitrogen dioxide, ozone and others. "EC" sensors arecompact, require very little power, exhibit excellent linearity and repeatability, and generally havea long life span. The detection technique is very straightforward in concept. Gas that enters thesensor undergoes an electrochemical reaction that causes a change in the electrical output of thesensor. The difference in the electrical output is proportional to the amount of gas present. ECsensors are usually designed to minimise the effects of interfering contaminants, making thereadings as specific as possible for the gas being measured.

Figure 1 illustrates the major components included in a typical electrochemical sensor. Thegas enters the sensor through an external diffusion barrier that is porous to gas but nonporous toliquid. Many sensor designs include a capillary diffusion barrier that limits and controls the amountof gas that enters the sensor. The sensing electrode is designed to catalyse a specific detectionreaction. Depending on the sensor, the substance being measured is either oxidised or reduced at

the surface of the sensing electrode. This reaction causes the potential of the sensing electrodeto rise or fall relative to that of the counter electrode. Current collector wires or filaments connectthe electrodes with the external pins of the sensor. The instrument supplies power to the sensor,and interprets the output of the sensor by readings obtained through the external pins.

Electrochemical sensors are stable, long lasting, require very little power and are capable ofresolution (depending on the sensor and contaminant) to ± 0.1 PPM or even lower.Electrochemical sensors are normally usable over a wide range of temperatures, in some casesfrom - 40 to 50 °C (- 40 to 120 °F). However, the uncorrected sensor output may be stronglyinfluenced by changes in temperature. For this reason instruments generally include temperaturecompensating software and/or hardware for the EC sensors installed.The simplest sensor designs use a two-electrode system. In two-electrode designs, the potentialof the sensing electrode is compared directly to that of the counter electrode. In three electrodedesigns, what actually is measured is the difference between the sensing electrode and referenceelectrode. Since the reference electrode is shielded from any reaction, it maintains a constantpotential. This provides a true point of comparison. The change in potential of the sensingelectrode is due solely to the concentration of gas. The current generated by the sensor isproportional to the amount of gas present. The amount of current generated per ppm (parts-per-million) of gas is constant over a wide concentration range. This consistency in output over a widerange explains the exceptional linearity of three-electrode electrochemical sensors.

Why H2S Sensors Don’t Wear Out Even When Exposed to HighConcentrations of GasChemical equations can be a little daunting, but working through a typical detection reaction iswell worth the effort. The oxidation of H2S in an electrochemical sensor provides a good exampleof the detection mechanism used in a non-consuming electrochemical sensor design:

H2S is oxidised at the sensing electrode:

H2S + 4H2O � H2SO4 + 8 H+ + 8 e-

The counter electrode balances out the reaction at the sensing electrode by reducing oxygenfrom the air to water:

2O2 + 8 H+ + 8 e- � 4H2O

Each molecule of H2S that is oxidised at the sensing electrode produces a currentflow of eight electrons. The amount of current produced is a function of thenumber of H2S molecules that are oxidised at the sensing electrode. Forevery 1.0 ppm of H2S in the atmosphere beingmonitored, the sensor shows a raw electricaloutput of 0.7 mµA (micro amps). Thisrelationship is linear over a very wide rangesuch that 10 ppm produces 7.0 mµA, 100ppm produces 70.0 mµA and so on. Theworking efficiency of the sensingelectrode is very high. This means thesensor is usually easily able to oxidise

Electrochemical sensors are one of the most common types of sensors used in portable gas detectors. Multi-sensor confined space

monitors generally contain an oxygen sensor, a flammable/combustible sensor and one to three additional electrochemical sensors for

specific toxic gases. Single-sensor instruments equipped with electrochemical toxic sensors are also extremely popular for use in

situations where a single toxic hazard is present. In spite of the very large number of electrochemical toxic sensors in use, there is still

a lot of misinformation and misunderstanding when it comes to the performance characteristics and limitations of this very important

type of sensor.

Author Details

Robert E. HendersonVice President, Business DevelopmentBW Technologies, 2840 - 2 Avenue S. E.Calgary, AB, Canada T2A 7X9Tel: (403) 248-9226 Fax: (403) 273-3708Website: www.gasmonitors.com E-mail: bhenderson@bwtnet.com

Figure 2: Electrochemical sensor equipped personal gas detectorscan last up to two years withoutrequiring battery replacement or

calibration adjustment

Figure 1: Major Components of a Typical Electrochemical Sensor

AET Buyers’ Guide 2005

the incoming H2S as fast as it reaches the sensing electrode. If the concentration of incoming gasexceeds the ability of the sensing electrode to oxidise the gas, the sensor becomes saturated, inwhich case the output reaches a maximum value and can't rise any higher. However, as soon asthe concentration of gas in the atmosphere drops below this critical concentration, the sensorrapidly recovers with no damage done to the sensor.

The sulphuric acid produced in the reaction simply accumulates in the sulphuric acidelectrolyte. Water from the electrolyte is used, but is regenerated during the course of thereaction. The only materials consumed during the detection reaction are the molecules ofhydrogen sulphide, power from the battery of the instrument and oxygen. As long as the sensoris located in an atmosphere containing even trace amounts of oxygen, the sensor will be able toreplenish itself directly from the atmosphere. This is the reason that non-consumingelectrochemical sensors have such long life spans. The lifespan of the sensor is not affected byexposure to the contaminant that it measures. No part of the sensor is consumed during thedetection reaction. You can expose the sensor to H2S calibration gas every single day withoutshortening or affecting the lifespan of the sensor.

Similar non-consuming reactions are used for the detection of a variety of other toxic gasesincluding carbon monoxide, sulphur dioxide, chlorine, chlorine dioxide, nitrogen dioxide, ozone,phosphine and most other gases detected by means of electrochemical sensors, (more on theexceptions later). Because the electrolyte contains a certain amount of dissolved oxygen, for shortperiods, non-consuming sensors can detect the contaminant they are designed to measure evenin the absence of oxygen. This is fortuitous since many reactive gases (such as chlorine) havevery short shelf lives when packaged in calibration mixtures that include oxygen. Gas mixturesused to calibrate sensors for highly reactive gases, such as chlorine, frequently contain nooxygen. For example, a typical calibration gas mixture used to calibrate a chlorine sensor mightcontain 5 ppm of chlorine in nitrogen. The chlorine sensor has no trouble operating in an oxygenfree atmosphere for the duration of the calibration procedure.

Certain environmental conditions may limit use of this type of sensor. For instance, a non-consuming electrochemical sensor would not be usable for long-term monitoring for H2S in anenvironment containing zero percent oxygen. Once all of the oxygen available in the electrolyteis consumed, the sensor will lose the ability to respond to hydrogen sulphide. When re-exposedto an oxygen-containing atmosphere, however, the sensor will regain its ability to detect H2S.Another problem is prolonged exposure to extremely dry conditions. Water in the sensorelectrolyte is not consumed, but is necessary for the detection reaction to proceed. If sufficientmoisture is lost from the electrolyte through evaporation, the sensor may not be able to detectgas. Because the sensor electrolyte contains moisture to begin with, short-term exposure to verydry conditions does not generally cause damage or interfere with the proper operation of thesensor. The sensor is normally able to replenish any water lost through evaporation as long as theatmosphere contains even trace concentrations of water. However, when operated for prolongedperiods in excessively dry conditions (such as in a stream of chemically dried air) sensors caneventually dry out to the point that the damage may not be recoverable.

Special Types of Electrochemical SensorsA bias voltage is sometimes applied to sensors used to detect less electrochemically active gasessuch as hydrogen chloride, ethylene oxide (ETO) and nitric oxide. The bias voltage helps to drivethe detection reaction. Biased sensors may take a significant amount of time - up to 24 hours ormore in some cases - to stabilise completely when first installed in an instrument, or if the sourceof power used to maintain the biasing voltage is interrupted.

Several other gases (such as ammonia and hydrogen cyanide) are detectable by lessstraightforward reactions that consume parts of the sensor. In the case of a hydrogen cyanidesensor, the sensor includes a gold sensing electrode. The gold in the electrode is consumedduring the detection reaction. Once all of the available gold is consumed, the sensor will need tobe replaced.

In the case of the ammonia sensor, it is the electrolyte that is consumed. The lifespan of the ammonia sensor is directly related to its exposure to NH3. An ammonia sensor that has a lifespan of one year when continuously exposed to 2 ppm of ammonia would last only 6months when exposed to 4 ppm, or three months when exposed to 8 ppm, etc. This type of sensorshould be used only when the normal ambient background concentration of ammonia issufficiently low to allow a reasonable operational life. For example, this type of sensor should not be used at a poultry farm or nitrate fertiliser plant where ambient concentrations of ammoniamay be as high as 20 to 30 ppm. In this environment the life span of the sensor could be a matterof weeks.

Effects of Interfering GasesOne of the chief limitations of electrochemical sensors is the effect of interfering gases - the onesthat you are not trying to measure with the sensor - on the sensor readings. Substance-specificsensors are ideally supposed to respond only to the gases they are supposed to measure. Thehigher the specificity of the sensor, the less likely the sensor will be affected by other gases. Thecomposition of the electrodes and type of electrolyte, as well as the use of selective filters for theremoval of interfering gases are all ways to increase the specificity of the sensor.

For instance, on the inside, a CO sensor is very similar to a sensor used to measure H2S. Thetrick is to keep the H2S from reaching the CO sensing electrode. Most substance-specific COsensors include an internal activated carbon filter designed to remove the H2S and other acid gasinterferents before they reach the sensing electrode. Thus, the reading of the sensor is not affectedby the presence of H2S in the atmosphere being monitored.

While inclusion of a filter is frequently able to increase specificity, removal of a filter may beused to broaden response to a wider variety of gases. For instance, carbon monoxide sensors thatdo not include a filter are sometimes marketed as "dual purpose" sensors for the simultaneousdetection of both CO and H2S. This type of sensor responds to both CO and H2S, but cannot tellthem apart. The sensor produces a single signal, which is up to the instrument user to interpret.

Even though care has been taken to reduce cross-sensitivity in substance-specific designs,interferences still exist. In some cases, the interfering effect is positive and results in readings thatare higher than actual. In other cases, the interference is negative and produces readings that arelower than actual. It's important to understand clearly the effects of potential interferents on theoutput of the sensors installed. Users should consult the owner's manual or contact themanufacturer of the instrument they will be using to verify the correct values to use when makingdecisions based on interfering contaminants.

"COSH" Type CO / H2S Sensors are Really Two Sensors in a Single HousingOne of the most popular electrochemical sensors is the four-electrode "COSH" type design. Thistype of sensor essentially packs two separate sensors for the measurement of CO and H2S into asingle housing. The sensor contains two separate sensing electrodes, one for CO and one for H2S.Each sensing electrode provides an independent, substance-specific signal, and can beindividually calibrated. In order to increase specificity, the sensor is internally configured so thatincoming gas passes by the H2S electrode first. Hydrogen sulphide, which would otherwise havean interfering effect on the CO sensing electrode, is removed via the electrochemical detectionreaction at the H2S electrode, and is not present in the gas that finally reaches the CO sensingelectrode. Thus, the sensor is able to differentiate between CO and H2S, with minimal interferencebetween the two contaminants on the sensor outputs.

Electrochemical sensors are among the most dependable, stable and reliable type of gasdetecting sensors available. But no sensor can detect gas unless it is used. The only way of beingsure that toxic contaminants are not present in dangerous concentrations is to look for them withan atmospheric monitor designed for their detection. Understanding your instrument is important;using your instrument is critical.

About the author:Robert Henderson is Vice President, Business Development for BW Technologies. Mr. Henderson

has been a member of the American Industrial Hygiene Association since 1992. He is Vice Chair

of the AIHA Gas and Vapour Detection Systems Technical Committee. He is also a current

member and past chair of the AIHA Confined Spaces Committee. He is also a past chair of the

Instrument Products Group of the Industrial Safety Equipment Association.

Figure 4: Compact dual-channel “COSH” type sensorshelp make it possible for a multi-sensor instrument forO2, combustible gas, CO and H2S to be small enoughto wear on a shirt pocket

Figure 3: Compact electrochemical sensors are available for a wide variety of common toxic gases