Theory of Operation of Electrochemical Sensors

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3. Page 3 o f 3 ChemDAQ Inc. • 300 Business Center Dr ive Suite 330 • Pittsburgh, PA • 15275 phone 412.787.0202 • fax 412.788.2526 filter 2 that allows the ethylene oxide to pass through unimpeded, but which reacts with and removes both the carbon monoxide and alcohols. While electrochemical sensors offer many advant ages, they are not suitable for every gas. Since the detection mechanism involves th e oxidation or reduction of the gas, electrochemical sensors are usually only suit able for gases which are electrochemically active. 3 Summary Electrochemical sensors have found widespr ead use in gas detection for occupational safety and for other applications where sma ll size, low cost and reliable operation are required. Electrochemical sensors work well for many applications and gases, but are unsuitable for others. 2 For example, the ChemDAQ ethylene oxide monito rs f or ethylene oxide employ electrochemical sensors and each ethylene oxide sensor is fitted with a ChemDAQ chemical filter (patent pending). 3 There are some exceptions to this rule. For e xample, Neotronics PLC (now part of Zellweger Analytics) sold an electrochemi cal sensor for carbon dioxide. The carbon dioxide reacted with the electrolyte to produce an electroch emically active species that wa s reduced to give the output current. D. Pletcher, J. Evans, P.R. Warburton, T.K. Gibbs, US Patent 5,071,526, Dec. 10, 1991, “Acidic Gas Sensors and Method of Using the Same”

1. Page 1 o f 3 ChemDAQ Inc. • 300 Business Center Dr ive Suite 330 • Pittsburgh, PA • 15275 phone 412.787.0202 • fax 412.788.2526 Theory of Operation of Electrochemical Sensors P. R ichard Warburton, Ph.D. Int roduction Electrochemical sensors are a widely used sensor for work place gas monito r ing. The sensors operate by oxidizing or red u cing the targ et gas at an electrode an d measuring the r esulting current. The sens ors are usu ally q uite small, and for workplace safety applications typically range from two inch es long down to about half an inch lo ng , th ough smaller ones are also available. Electrochemical sensors of fer th e adv antage of low cost, high sens itivity and often good selectivity towards the target gas. In addi tion, electrochemical senso r s offer continuous mo nitoring, fast response times and are usually relativ ely low cost compared to other technologies. Electrochemical sensors are co mmercially available for a large n umber of d ifferent gas types. In the healthcare workpl ace, electrochemical sensors ar e used for the d etection of ethylene oxide, hydrogen per oxide, oxygen, formaldehyd e, ozone and other g ases. Construction Th e sensors contain two o r th ree electrodes, occasionally fo ur, in co ntact with an electrolyte. The electrodes ar e typically fabricated by fixing a high surface area precious metal on to the porous hydrophobic me mbrane (e.g. PTFE). One of the electrodes, the wo r k in g electrode, contacts both the electrol yte and the ambient air to b e mo nitored usually via a porous membrane. The electrolyte most commonly used is a mineral acid, b ut organic electrolytes are also used for some sensors. The electrodes and housing are u sually in a plastic housing which contains an entry hole for the gas and electrical co n tacts. T heory of Operation The gas diffuses into the sensor, through the ba ck of the porous membrane to the working electrode where it is oxidized or redu ced. This electrochemical reaction results in an electric current that passes thro u g h the ex ternal circuit. In ad dition to measur ing , amplifying and performing other signal pr ocessing functions, the external circuit maintains the voltage across the senso r b etw een the work ing and c ounter electr o d es for a two electrode sensor or b etween the work ing and reference electrodes for a three electrode cell. At the counter electrode an e qual an opposite reaction o ccurs, such that if th e wo r k in g electrode is an oxidation, th en the counter electrode is a reduction. For example, in an electroch emical sens or for hydrogen peroxide, at the working electrode, the hydrogen p eroxi de is oxidized to oxygen

2. Page 2 o f 3 ChemDAQ Inc. • 300 Business Center Dr ive Suite 330 • Pittsburgh, PA • 15275 phone 412.787.0202 • fax 412.788.2526 H 2 O 2 => O 2 + 2H + + 2e and at the counter electrode there is an equal magnitude reducti on current, reduction of oxygen to water. ½ O 2 + 2H + + 2e => H 2 O The net reaction of the sensor is therefore: H 2 O 2 + ½ O 2 => 2H 2 O It is worth noting that in this reaction, ther e is no net consumption of the sensor, and thus electrochemical sensors for toxic gases of ten have long service lives (years). Diffusion Controlled Response The magnitude of the current is controlled by how much of the target gas is oxidized at the working electrode. Sensors are usually de signed so that the gas supply is limited by diffusion and thus the output from the se nsor is linearly proportional to the gas concentration. This linear output is one of the advantages of electrochemical sensors over other sensor technologies, (e.g. infrared), whose output must be linearized before they can be used. A linear output allows for more precise measurement of low concentrations and much simpler calibration (only base line and one point are needed). Diffusion control offers another advantage. Changing the diffusion barrier allows the sensor manufacturer to tailor th e sensor to a particular targ et gas concentration range. In addition, since the diffusion barr ier is primarily mechanical, 1 the calibration of electrochemical sensors tends to be more stable ove r time and so electrochemical sensor based instruments require much less maintenan ce than some other detection technologies. Cross Sensitivity For some gases such as ethylene oxide, cross sensitivity can be a problem because ethylene oxide requires a very active work ing electrode catalys t and high operating potential for its oxidation. Therefore gases which are more easily oxidized such as alcohols and carbon monoxide will also give a response, two compounds which are often found near where ethylene oxide is used for st erilization in healthcare. These false alarms can be very disruptive, and ha ve tarnished the reputation of electrochemical sensors for ethylene oxide with some users. Similar problems are found with some other sensor technologies such as metal oxide sensors that are also used to de tect ethylene oxide. These cross sensitivity problems can be eliminated though through the use of a chemical 1 In principle, the sensitivity can be calculated base d on the diffusion properties of the gas path into the sensor, though experimental errors in the measurement of the diffusion properties make the calculation is less accurate than calibrating with test gas “Amperometric Gas Sensor Response Times,” P.R. Warburton, M.P. Pagano, R. Hoover, M. Logman, and K. Crytzer, Yi.J. Warburton; Anal. Chem., 70 (5), 998 -1006, 1998.


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