Chemical Sensors

What are chemical sensors?

Dave Davis | Kent State University

Chemical sensors for the aquatic environment, in general, produce concentration data for specific chemical species within a lake or other body of water. Typically, separate sensors are used to measure different chemical parameters; however, these sensors are often combined into multi-functional sensor platforms, such as sondes.

Sensor technology often incorporates materials and circuits that only respond to specific chemical species. Therefore, while a large number of chemical sensors exist for study of aquatic ecosystems, these sensors can differ greatly from each other in the technologies that they incorporate. One of the most common types of technology for chemical sensors are ion selective electrodes (ISEs).

What do chemical sensors measure?

Several important chemical parameters that are commonly measured include:

• pH
• salinity/conductivity
• dissolved oxygen/carbon dioxide
• nutrients

pH Sensors

pH is a measure of the acidity of a body of water or other solution. Mathematically, pH is defined as the negative logarithm of the concentration of acidic hydrogen ions (also referred to as protons, designated H⁺). Through this mathematical calculation, a pH scale is obtained, with a pH of 1 being highly acidic, 7 being neutral, and 14 being the most basic level on the scale. In some extreme cases, however, pH values can occur outside this scale¹. It is important to note that pH is a log scale, meaning a pH change of 1 number (e.g. from 6 to 5) means the sample is 10 times more acidic.

For the casual observer, many methods exist for the measurement of pH, ranging from indicator papers and solutions to complex electronic probes. When working in the field, lake managers and scientists use electronic probes on sensor platforms to measure water pH. These probes measure pH values indirectly, as it is not possible to “count” the number of ions within a given sample of water. pH and other ion sensors use a technology called ion selective electrodes (ISEs). pH is measured via the conductivity of the water surrounding the probe. The sensor houses a small glass bulb containing an electrode that sends a low electrical current through the water¹. The voltage is then measured, and this value is used to determine the pH based on the calibration of the probe. pH sensors drift quickly because the glass electrode does not produce a long-term reproducible signal over time. Therefore, pH sensors require frequent calibration. For high precision laboratory measurements, this means as often as every day or every hour; for field applications, it is often advisable to calibrate at least once a month.

Water pH is an important environmental issue for several reasons. Organisms living within an aquatic environment depend on a stable, specific pH range to carry out their normal functions. pH deviating from this range can cause a large amount of stress on a lake ecosystem and can be harmful to organisms by reducing survival, growth, and reproduction. Lake pH also provides a means of studying various types of environmental pollution, including acid precipitation, acidic water runoff, and soil leaching of chemical fertilizers².

Salinity/Conductivity Sensors

Salinity refers to the concentration of salts dissolved in a given sample of water. The term salt in this case refers to any material that consists of an ionic bond between a metallic cation and a nonmetallic anion. This can include common table salt (sodium chloride), but also includes other salts, such as potassium chloride and sodium bicarbonate¹. Salinity is generally reported as the concentration of the salt, either in parts per thousand or as a percent salinity.

Salinity is measured in a manner similar to pH. Salty solutions contain more ions — charged particles — than less salty solutions. Thus, salty water will conduct electricity better than fresh water. Salinity sensors contain an electrode that measures the conductivity of the water, and this conductivity value corresponds to the salinity when compared to a series of standards. With these methods, bodies of water can be identified as freshwater or saltwater. Salinity values can also be monitored to watch how it impacts other parameters and how they are impacted in turn.

Most conductivity meters give readings in micro Siemens per cm (µS/cm). Freshwater lakes are usually less than 100 µS/cm conductivity. Some slightly salty water is around 1,800 µS/cm. Very brackish water could be around 27,000 µS/cm, and oceans have a conductivity of around 54,000 µS/cm³.

Salinity is an important parameter for water quality. It is a major determining factor in whether a water source is fit for human consumption as well as determining what types of organisms can live in an aquatic environment. Many organisms require a specific balance of salt both inside and outside the cell. If this osmotic balance is interrupted, water will tend to travel from the less concentrated source to the more concentrated in an attempt to reestablish this balance. In extreme cases, this can cause the cell to fill with water and burst (lyse) if the salt concentration within the cell is much higher than outside, or it can cause the cell to shrink and die if the concentration of salt is much higher outside the cell, as much of the water will leave the cells in an attempt to dilute the salt outside the solution⁴. However, not all organisms require the same balance, as some have evolved to live in fresh water, and other organisms are adapted to live in marine ecosystems. Some organisms, such as extremophile bacteria, are even adapted to survive in very saline water in which most other organisms could not survive. Salinity can vary depending on many factors, such as rainfall, river discharge, and runoff from saline sources⁵.

Dissolved Oxygen/CO₂ Sensors

Like solids, gasses can also dissolve in lake water. However, unlike dissolved solids, the concentration of gasses in water can change rapidly over time due to factors such as temperature, consumption and production, and atmospheric exchange of gasses. Because of this, dissolved oxygen and dissolved carbon dioxide are often continuously measured at short intervals — as frequently as every 5-10 minutes. Changes in dissolved oxygen and dissolved carbon dioxide are related to physical, chemical, and biological processes that occur underwater. These gasses are both major inputs and outputs in metabolism of aquatic and terrestrial organisms⁵. Dissolved gasses are commonly measured and reported as a percent saturation, with higher saturations corresponding to a larger amount of gas dissolved in a specific volume of water at a specific temperature. Often, gasses become less soluble at higher temperatures.

Dissolved oxygen and carbon dioxide sensors are available in several varieties. Traditionally, dissolved oxygen sensors have been electrode sensors that contain a membrane through which oxygen passes and is then reduced, creating an electrical current that can be measured and converted to an oxygen measurement through calibration⁶. These sensors have limitations because they consume oxygen while taking a measurement, meaning that if left still in one place, the dissolved oxygen reading would drop over time. Additionally, they are prone to sensor drift, meaning they have to be frequently calibrated. However, new optically-based oxygen sensors have recently been developed. Optical sensors consist of a membrane that is fluorescent when exposed to light. This membrane reacts with oxygen, reducing the fluorescence⁷. This fluorescence is then detected, and the signal is converted into the dissolved gas concentration. Carbon dioxide sensors function in much the same way oxygen sensors do and are available in both conductivity and optical models.

Dissolved oxygen and carbon dioxide are especially important measures of an aquatic environment’s productivity. These two gasses are involved in the metabolism of both heterotrophs and autotrophs. Heterotrophs are organisms that require outside sources of organic carbon to survive, while autotrophs are organisms that can synthesize organic carbon from inorganic sources. The activity of these organisms can be observed indirectly in aquatic environments through measurement of dissolved oxygen and carbon dioxide, as heterotrophs intake oxygen and output carbon dioxide through respiration, while autotrophs intake carbon dioxide and output oxygen⁴. Therefore, if the water contains a relatively higher amount of oxygen than carbon dioxide, the autotrophs within the environment dominate the net production; conversely, if carbon dioxide is higher, heterotrophs dominate production⁶.

Nutrient Sensors

Nutrients are chemicals whose intake is necessary to sustain life of specific organisms. Nutrients are classified into two categories: macronutrients and micronutrients. Macronutrients are nutrients organisms need to consume in large quantities. These include certain minerals, amino acids, sugars, and vitamins, composed of simpler chemical elements, such as carbon, hydrogen, nitrogen, phosphorus, and sulfur⁸. Micronutrients include nutrients that are only needed in trace quantities, generally nonessential vitamins, amino acids, and also metals necessary in the formation of proteins necessary for metabolism.

Various sensor technologies exist for different types of nutrients; however there are few easy-to-use nutrient sensors for long term, in-lake applications. Development of specific methodologies for the detection of individual nutrients and classes of nutrients represent an important field of study, and new technologies are being applied and tested, such as liquid crystal (LC)-based sensors that measure the change in optical properties of a liquid crystal layer in response to the introduction of other organic chemicals. These sensors include a chemical binder layer that will bind targeted molecules, which makes the sensors highly selective to their intended analyte. The binding of these molecules then in turn cause a change in LC orientation, which changes their optical properties⁹.

Sources:

1. Moore, J.W., Stanitski, C.L., Jurs, P.C. 2005. Chemistry: The Molecular Science, 2^nd ed. New York: Thomson.
2. Nakahara, O. et. al. 2010. Soil and stream water acidification in a forested catchment in central Japan. Biogeochemistry, 97, 141-158.
3. Fresh, Brackish or Saline Water for Hydraulic Fracs – What are the Options? Environmental Protection Agency. (n.d.). Retrieved online at: https://www.epa.gov/sites/default/files/documents/02_Godsey_-_Source_Options_508.pdf
4. Campbell, N.A, Reese, J.B. 2004. Biology, 7^th ed. New York: Pearson.
5. Somura, H., et. al. 2009. Impact of climate change on the Hii River basin and salinity in Lake Shinji: a case study using the SWAT model and a regression curve. Hydrological Processes, 23, 1887-1900.
6. Wetzel, Robert G. 2001. Limnology, 3^rd ed. San Diego: Elsevier.
7. Wikipedia. Oxygen Sensor. 2010. Retrieved online at: http://en.wikipedia.org/wiki/Oxygen_sensor
8. Nelson, D.L., Cox, M.M. 2004. Principles of Biochemistry 4^th ed. New York: Freeman.
9. Sridharamurthy, S.S., et. al. 2008. A microstructure for the detection of vapor-phase analytes based on orientational transitions of liquid crystals. Smart Materials and Structures, 17(12), 1-4.