A potentiometric sensor is a type of chemical sensor used in a wide array of industrial processes to determine the volumetric presence of a compound based on the detection of ionic atoms or molecules in the compound that carry an electrical charge. The sensor doesn't require a flow of current, but merely that the compound itself, whether liquid or gas, passes between the electrodes of the sensor device. One of the most common potentiometric sensor units manufactured as of 2011 is the oxygen sensor incorporated into automobiles, but there are many other uses for the technology. The basic function of a potentiometer that is at the heart of a potentiometric sensor is that a known voltage on a reference electrode is used as a comparison to a voltage that changes on a working electrode. The voltage difference occurs when a solid electrolyte compound between the two electrodes obtains an electrical charge as a liquid or gas in the form of an ionic conductor passes by it. The level of charge is used to determine the quantity of ions present, and this value can then be compared against expected mass values and activate controls to maintain a desired equilibrium state. Oxygen sensors, also known as lambda probes, have been included in automobiles manufactured in the United States since 1980 as a routinely-used potentiometric sensor, and are present to ensure that the vehicles meet pollution control standards in certain states like California, and to optimize the performance of engines for fuel efficiency. They work by measuring the amount of ionized oxygen that is present in the exhaust stream for an automobile while the engine is running. Microprocessors in the vehicle then take this value and compare it to optimal values for exhaust gasses. Where there is a variation in the output, the computer control can adjust the vehicle's air/fuel mixture and ignition timing to ensure that there is more complete combustion of fuel and reduced pollution. All types of vehicles, whether powered by diesel fuel, gasoline, ethanol, or combinations of such fuels, make use of these sensors, and more modern versions of the devices measure a range of different exhaust gasses aside from just oxygen. Other uses for a potentiometric sensor include trace gas analysis in analytical chemistry down to parts per trillion and to determine the polar quantity of solvents such as water, alcohol, and other chemicals used in the creation of precise industrial solution formulas. Research as of 2011 has also led to the use of the potentiometric sensor in pharmaceutical analysis. They are used to detect lead and surfactants in waste water streams from treatment plants, and for many other reasons in the agricultural and medical fields, as well as for environmental pollution controls in a range of industries. Ion membrane potentiometric sensor designs used to detect heavy metals, such as lead or in determining the pH level of soil samples, can yield results in 15 seconds or less and be
repeatedly used for up to four months without degrading.