User:Rameez Usman

Electrochemical Sensors 1 Introduction: Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor (a metal or a semiconductor) and an ionic conductor (the electrolyte), and which involve electron transfer between the electrode and the electrolyte or species in solution. If a chemical reaction is driven by an external applied voltage, as in electrolysis, or if a voltage is created by a chemical reaction as in a battery, it is an electrochemical reaction. In contrast, chemical reactions where electrons are transferred between molecules are called oxidation/reduction (redox) reactions. In general, electrochemistry deals with situations where oxidation and reduction reactions are separated in space or time, connected by an external electric circuit 2 History; In the late 18th century the Italian physician and anatomist Luigi Galvani marked the birth of electrochemistry by establishing a bridge between chemical reactions and electricity on his essay "De Viribus Electricitatis in Motu Musculari Commentarius" (Latin for Commentary on the Effect of Electricity on Muscular Motion) in 1791 In 1800, William Nicholson and Johann Wilhelm Ritter succeeded     in decomposing water into hydrogen and oxygen by electrolysis. By the 1810s     William  Hyde  Wollaston made   improvements to  the galvanic cell. Sir Humphry Davy's work with electrolysis led to the conclusion  that   the production n of electricity  in simple electrolytic  cells  resulted from chemical action and  that chemical combination occurred between  substances of opposite  charge. In 1836, John Daniell invented a primary cell in which hydrogen was eliminated in  the generation o f the electricity. 3 Electrochemical sensors : A useful definition for a chemical sensor is ‘‘a small device that as the result of a chemical interaction or process between the analytic gas and the sensor device, transforms chemical or biochemical information of a quantitative or qualitative type into an analytically useful digital signal. Portable electrochemical sensor methods include instruments employing this technology in the determination of oxygen and several toxic gases in the field, using battery-supplied power. They range in size from those small enough to fit into a shirt pocket and weighing less than one pound (0.45 kg) to larger units that weigh as much as six pounds (2.7 kg). Sensors are practical devices and, as such, activities are both fundamental and applied. Also, understanding sensor devices requires some knowledge of a variety of academic areas. This leads to a very interdisciplinary field populated by physicists, chemists, engineers, biologists and biochemists, materials scientists, electrochemists, and others. The interdisciplinary nature of sensor research, combined with the ability of the Society to transcend singular disciplines and bring scientists and engineers together to work on complex goals like sensor systems will insure a containing role for ECS in the development of physical and chemical/biochemical sensors. One finds sensor symposia at all ECS meetings these days, as well as the meetings of other groups including Patton, FACSS, ACS, AICHE, IEEE, and the MRS in Europe, Japan, and the USA. The impact of advances in electrochemical sensors on all three continents is substantial, and detection has been recognized as a key target for technology development in the new USA Homeland Security initiative. Of course there are many other sensors that could be included in our brief discussion. Apologies are extended to any of our colleagues who may not see coverage for their favorite chemical or physical sensor. A consequence of the rapid expansion of the field has been the inability to cover all of it, even superficially, in a short article. Additional information on sensors can be found in books32,33 and recent reviews.34-36 Finally, excitement in the world of sensors comes from their ability to provide immediate feedback on the world around us just like our own five senses of taste, sight, hearing, touch, and smell. Also, sensors include the most up to date science and technology and new sensors are emerging made from bimolecular, nanostructures, and nano devices. Single molecule detection is at hand. Sensors are marching toward the day that they can smell out diseases, see danger, cook our food, spot terrorists, and help catch fugitives, improve environmental pollution control, and enable clean and efficient climate controls for human safety and comfort in our cars, workplaces, and homes. All in all, the world should be a better place because of the advances in sensors and there is no better place to promote sensor science and technology

Some Schmetic Diagrams Of Electrochemical sensors principle

4 Principle: The basis for all electrochemical sensors is the use of a porous membrane (normally PTFE) or capillary system which allows the gas to diffuse into the cell containing the liquid or gel electrolyte and the electrodes (Figure 1). The exact configuration will vary with manufacturers and between different toxic gases. When the gas comes into contact with the electrolyte, a change in electrochemical potential between the electrodes is produced. Associated electronic circuitry then will measure, amplify, and control this electronic signal. Because the reaction is proportional to the concentration (partial pressure) of gas present, the signal is easily translated into parts per million, percent, or ppm-hrs, and read on the readout meter or stored in microprocessor circuits for later readout 5 Types: ’ 1 Potentiometric Sensors When a redox reaction, Ox + Ze = Red, takes place at an electrode surface in an electrochemical cell, a potential may develop at the electrode-electrolyte interface. This potential may then be used to quantify the activity (on concentration) of the species involved in the reaction forming the fundamental of potentiometric sensors.for a potentiometric sensor to reach equilibrium conditions in order to obtain a meaningful reading can be quite long. These considerations are essential in the design and selection of potentiometric sensors for biomedical applications . 2 Amperometric sensors, that are also based on the current-potential relationship of the electrochemical cell, can be considered a subclass of voltammetric sensors. In amperometric sensors, a fixed potential is applied to the electrochemical cell, and a corresponding current, due to a reduction or oxidation reaction, is then obtained. This current can be used to quantify the species involved in the reaction. The key consideration of an amperometric sensor is that it operates at a fixed potential. However, a voltammetric

3 Voltammetric Sensors The current-potential relationship of an electrochemical cell provides the basis for voltammetric sensors

4 Electrochemical gas sensors They are gas detectors that measure the concentration of a target gas by oxidizing or reducing the target gas at an electrode and measuring the resulting current

6 Typical Models:

Zirconia sensor

A planar zirconia sensor (schematic picture) The zirconium dioxide, or zirconia, lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the atmosphere. An output voltage of 0.2 V (200 mV) DC represents a "lean mixture" of fuel and oxygen, where the amount of oxygen entering the cylinder is sufficient to fully oxidize the carbon monoxide (CO), produced in burning the air and fuel, into carbon dioxide (CO2). An output voltage of 0.8 V (800 mV) DC represents a "rich mixture", one which is high in unburned fuel and low in remaining oxygen. The ideal set point is approximately 0.45 V (450 mV) DC. This is where the quantities of air and fuel are in the optimum ratio, which is ~0.5% lean of the stoichiometric point, such that the exhaust output contains minimal carbon monoxide. The voltage produced by the sensor is nonlinear with respect to oxygen concentration. The sensor is most sensitive near the stoichiometric point and less sensitive when either very lean or very rich.

.Wideband zirconia sensor A planar wideband zirconia sensor (schematic picture) A variation on the zirconia sensor, called the "wideband" sensor, was introduced by Robert Bosch in 1994, and has been used on a lot of cars[3] in order to meet the ever-increasing demands for better fuel economy, lower emissions and better engine performance at the same time. It is based on a planar zirconia element, but also incorporates an electrochemical gas pump. An electronic circuit containing a feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, so that the pump current directly indicates the oxygen content of the exhaust gas. This sensor eliminates the lean-rich cycling inherent in narrow-band sensors, allowing the control unit to adjust the fuel delivery and ignition timing of the engine much more rapidly. In the automotive industry this sensor is also called a UEGO (for Universal Exhaust Gas Oxygen) sensor. UEGO sensors are also commonly used in aftermarket dyno tuning and high-performance driver air-fuel display equipment. The wideband zirconia sensor is used in stratified fuel injection systems, and can now also be used in diesel engines to satisfy the upcoming EURO and ULEV emission limits. Wideband sensors have three elements: •	Ion oxygen pump •	Narrowband zirconia sensor •	Heating element The wiring diagram for the wideband sensor typically has six wires: •	resistive heating element (two wires) •	sensor •	pump •	calibration resistor •	common Titania sensor A less common type of narrow-band lambda sensor has a ceramic element made of titanium dioxide (titania). This type does not generate its own voltage, but changes its electrical resistance in response to the oxygen concentration. The resistance of the titania is a function of the oxygen partial pressure and the temperature. Therefore, some sensors are used with a gas temperature sensor to compensate for the resistance change due to temperature. The resistance value at any temperature is about 1/1000 the change in oxygen concentration. Luckily, at lambda = 1, there is a large change of oxygen, so the resistance change is typically 1000 times between rich and lean, depending on the temperature. In automotive applications the titania sensor, unlike the zirconia sensor, does not require a reference sample of atmospheric air to operate properly. This makes the sensor assembly easier to design against water contamination. While most automotive sensors are submersible, zirconia-based sensors require a very small supply of reference air from the atmosphere. In theory, the sensor wire harness and connector are sealed. NO-B1 Nitric Oxide sensor 1) sensitivity：400-660nA/ppm 2) Response time：< 20s         3) size：Φ32.3*16.5 4) Linearity： -20~ -25ppm          5) Over gas limit：1200ppm 6) Resolution：0.15ppm           7) Storage period：6months   10) Temperature 8) Operating life：2years         8) range：-30～50℃ 11)  Humidity range：15～90％RH

CROSS H2S sensitivity % measured gas @ 20ppm H2S < 60 SENSITIVITY NO2 sensitivity % measured gas @ 10ppm NO2 < 5 Cl2 sensitivity % measured gas @ 10ppm Cl2 < 5 SO2 sensitivity % measured gas @ 20ppm SO2 < 4 H2 sensitivity % measured gas @ 400ppm H2 < 0.1 CO sensitivity % measured gas @ 400ppm CO < 0.1 NH3 sensitivity % measured gas @ 20ppm NH3 < 0.1 CO2 sensitivity % measured gas @ 5% Vol CO2 <0.1 KEY SPECIFICATIONS Bias voltage mV (working electrode potential is above ground) +300 Temperature range °C -30 to 50 Pressure range kPa 80 to 120 Humidity range % rh continuous 15 to 90 Storage period months @ 3 to 20°C (stored in sealed pot) 6 Load resistor Ω (recommended) 10 to 47 Oxygen sensor (O2) 0 - 20 mg/l, IP 65 •	                              The remarkable features of this oxygen sensor are its extremely long service life and the low maintenance costs due to its long-term adjustment and cleaning intervals. The adjustment can be done in air or in saturated water without any calibration tables. The replacement of the galvanic cell is simplified by the design of the sensor. The membrane is very durable and is almost un break-able (100 µm membrane thickness). Newly developed sensor electronics enable the oxygen sensor to work without an additional storage battery. The sensor can be modified to customer's needs regarding measuring range and accuracy. The response time is less than 2 minutes. The sensor can be immersed directly in water courses or tanks and does not need regular maintenance. The sensor is also available in combination with a temperature sensor.

Carbon monoxide (CO) sensor with low hydrogen Alpha sense Carbon Monoxide sensors operate using proven fuel cell technology. All sensors have an active chemical filter to remove NO x and SO2/H2S. The CO-AX provides low hydrogen cross sensitivity in a three-electrode format, which can also be retro-fitted into standard carbon monoxide measuring instruments. The A series CO sensors show excellent long-term stability and resistance to environmental extremes, proven by years of validation history. A: 20mm diameter, the industry standard size for portable gas detectors B: 32mm diameter package, the best choice for fixed site applications D: miniature, with proven long-term performance, for the next generation of gas detectors Carbon monoxide (CO) sensor TGS 5042 •	                                Figaro's carbon monoxide sensor TGS5042 is a battery operable electrochemical sensor which offers several advantages over traditional electrochemical sensors: its electrolyte is environmentally friendly, it poses no risk of electrolyte leakage, it does not consume active materials or its electrodes during operation, and it has lower sensitivity to interference gases. With long life, good long term stability, and high accuracy, this sensor is an ideal choice for CO detectors with digital display. OEM customers will find individual sensor data printed in bar code form on each sensor, allowing individual sensor tracking and enabling users to skip costly gas calibration. Utilizes a standard AA battery-sized package.

Features:
 * Battery operable
 * High repeatability/ selectivity to carbon monoxide
 * Reduced influence by various interference gases
 * Long life
 * UL recognized component
 * simple calibration
 * no sensor aging necessary
 * meets UL2034 and EN50291
 * meets RoHS requirements

Applications:
 * Residential and commercial CO detectors
 * CO monitors for industrial applications
 * Ventilation control for indoor parking garages and attic
 * Recreational vehicle CO detectors
 * Marine boat CO detectors
 * Fire detection

7 Applications: Electrochemical sensors are used extensively in many biomedical applications including blood Chemistry sensors Many practical enzymatic sensors, including glucose and lactate Sensors, also employ electrochemical sensors as sensing elements. Electrochemically based biomedical sensors have found in vivo and in vitro applications. In soil respiration studies oxygen sensors can be used in conjunction with carbon dioxide sensors to help improve the characterization of soil respiration. Typically, soil oxygen sensors use a galvanic cell to produce a current flow that is proportional to the oxygen concentration being measured. These sensors are buried at various depths to monitor oxygen depletion over time, which is then used to predict soil respiration rates. Generally, these soil sensors are equipped with a built-in heater to prevent condensation from forming on the permeable membrane, as relative humidity can reach 100% in soil. In marine biology or limnology oxygen measurements are usually done in order to measure respiration of a community or an organism. They have also been used to measure primary production of algae. The widest application for electrochemical sensors has been as Alarm dosimeter systems rather than as continuous monitors. Because of the low Power requirements and small size, the electrochemical sensor is ideally suited for use in combination monitors, that is, those that are able to monitor two or more substances at once. Many combination monitors are available, including in one package the sensors for oxygen deficiency, combustible gas, and toxic gas. Electrochemical sensors may be located several meters away from the electronics /readout unit in order to facilitate remote or pre-entry monitoring. Because of the low power requirements of these devices, it is possible for them to be used in lightweight, personal monitor/alarm devices. Electrochemical sensors for oxygen deficiency, H2S, HCN, and others have been designed into monitor/dosimeter/alarm packages that are small enough to fit into a shirt pocket, that weigh less than one pound (0.45 kg) and that operate continuously for as long as four months without changing the replaceable battery.

A three-wire oxygen sensor suitable for use in a Volvo 240 or similar vehicle automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. Tampering with or modifying the signal that the oxygen sensor sends to the engine computer can be detrimental to emissions control and can even damage the vehicle.), .8 Limitations:

The environmental conditions (temperature, relative humidity, barometric pressure) of the monitor at the time of calibration should be as near as possible to those that will be encountered during use. Temperature: One Of these three, temperature is most important.Electrochemical sensors are also quite sensitive to temperature and, therefore, the sensors are typically internally temperature-compensated. However, it is better to keep the sample temperature as stable as possible. In general, when the temperature is above 25°C, the sensor will read higher; when it is below 25°C, it will read lower. The temperature effect is typically 0.5% to 1.0% per degree centigrade, depending on normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles (50,000 to 80,000 km). Heated sensor lifetime is typically 100,000 miles (160,000 km). Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element Even with the temperature compensating circuitry employed in most sensors, some time is required for equilibrium to be reached. If it is not possible to calibrate at the working temperature, the user must allow sufficient time for field equilibration of temperature. Most manufacturers of electrochemical sensors specify the lower temperature limits, usually 32 to 50 F (0 to 10 C), and upper limits, typically 120 to 140 F (50 to 60 C). Pressure: Electrochemical sensors are minimally affected by pressure changes. However, it is important to keep the entire sensor within the same pressure since differential pressure within the sensor can cause sensor damage. Changes in barometric pressure are usually less significant than temperature changes and so are of less concern to the user. Oxygen monitors with pressure compensating circuitry should be employed whenever pressures differing by 5 kPa (0.05 atmosphere) or more from the calibration pressure [1,4] are Encountered. Life Time: Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty. Contamination of fuel: Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealing’s and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray. Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response. Contamination from the lead: Some sensors have an air inlet to the sensor in the lead, so contamination from the lead caused by water or oil leaks can be sucked into the sensor and cause failure Symptoms of a failing oxygen sensor include: •	Sensor Light on dash indicates problem •	Increased tailpipe emissions •	Increased fuel consumption •	Hesitation on acceleration •	Stalling •	Rough idling 9 Conclusion: In this Presentation I discussed the electrochemistry .some history about it. Then by starting the introduction of electrochemical sensors, which are used wildly now a days in home, offices and in industry at a large scale .I explained its definition of it. After it I explained some types of electrochemical sensors,. Then I concluded some typical examples and model of electrochemical sensors these sensors have a lot of applications. So, I have given many common and healthy uses of these sensors. As every thing have its advantages as well as its remedies. So I concluded the common limitation as well. At the end, I have references, from where I have collect the data to complete my assignment.. All principles applications & limitation are almost common for all sensors.