Image credit: Mettler-Toledo Process Analytics | ABB Measurement
Dissolved CO2 instruments measure the amount of dissolved carbon dioxide within a liquid sample.
Measurement Principles and Applications
Carbon dioxide, along with oxygen, is one of principal gases dissolved in water sources. In fact, CO2 is roughly 200 times more soluble than oxygen and can therefore enter liquids much more easily. The surfaces of large bodies of water, such as oceans, are constantly and rapidly exchanging carbon dioxide molecules with the surrounding atmosphere.
Dissolved CO2 in bodies of water is an integral part of the carbon cycle which allows for sustainable life on Earth. Carbon dioxide naturally diffuses into water, and much of the carbon from these molecules remains in a marine "carbon sink." Between aquatic biomass, oceanic stores, and marine sedimentary rocks, the marine carbon sink stores over 100 quadrillion (1017) metric tons of carbon, which is the most of any sink on Earth.
The carbon cycle; CO2 which diffuses into bodies of water is measured as dissolved CO2.
Image credit: Miami University of Ohio
Like oxygen, carbon dioxide is vital to sustainable aquatic ecosystems. Aquatic plants consume carbon dioxide and water in order to form carbohydrate fuel. When plants expel oxygen as a waste product of photosynthesis, they in turn create a sustainable environment for aquatic life such as fish and other organisms.
Difficulties in Measurement and Methods
Dissolved carbon dioxide is considerably more difficult to measure than dissolved oxygen due to the former gas's tendency to dissolve both physically and chemically. When dissolved in water, certain carbon dioxide molecules react with hydrogen and oxygen to form hydrogen carbonate (HCO3-). These new chemical solutions may interfere with dissolved CO2 measurements.
Dissolved CO2 is commonly measured via chemical analysis using titration, but this method is capable of registering erroneously-high levels of carbon dioxide. This is due to the fact that additional solutions within the sample—including phosphates, silicates, and ammonium—may interfere with titration; this problem is especially prevalent when testing aquaculture samples.
Most modern dissolved CO2 instruments are designed to test for the specific gas pressure of dissolved carbon dioxide. This method is considerably more accurate and rules out carbon dioxide which has formed carbonates. It is particularly helpful when testing water quality in environmental applications, as physically-dissolved CO2 is the type which primarily affects fish health.
Applications and Product Types
Considering the information above, it becomes clear that dissolved carbon dioxide is vital to the sustainability of marine ecosystems; for this reason, dissolved CO2 meters are extensively used in water quality testing applications.
Another common application is quality control within beverage, chemical, and pharmaceutical manufacturing industries. Dissolved CO2 meters are frequently used to check CO2 levels in carbonated beverages and to monitor levels during fermentation of beer and similar products. Food and beverage sensors tend to be in-line process types, while those used in biotechnology and pharaceutical applications are typically sterile probe-type devices.
(left to right) An in-line beverage industry sensor; a benchtop probe meter; a handheld meter for field measurements.
Image credit: Mettler-Toledo | Indiamart
Within the brewing and biotechnology industries in particular, dissolved CO2 meters can be used to measure a number of different characteristics about fermentation reactions. Low CO2 concentration can result in poor yield, and the formation of products and by-products is largely dependent upon carbon dioxide levels within fermenting materials. Carbon dioxide measurement has a number of other applications within biotechnology, including those listed below.
Potential for micro-organism growth; certain CO2 concentrations can inhibit and/or promote metabolism and growth.
Tracking micro-organism growth; continuous measurement yields important data about growth rates.
Controlling anaerobic fermentation.
Dissolved CO2 instruments include any device used to measure dissolved carbon dioxide, including meters, sensors, and probes. Meters are typically digital devices which attach to a probe and process an output reading based on an analog or digital signal from the probe or sensor.
Like pH instruments and dissolved oxygen meters, dissolved CO2 measurement instruments often measure other water quality parameters as well, including conductivity, temperature, pH, dissolved oxygen, and total dissolved solids (TDS). These instruments are typically fitted with interchangeable probes, although they may be able to measure two different parameters simultaneously with the same probe.
Dissolved CO2 instruments may be used and maintained according to various standards. A common water quality standard which addresses dissolved carbon dioxide is ASTM D513 (standard test method for total and dissolved carbon dioxide in water). Other water quality measurement standards can be found at the IHS Standards Store.
BioWorld - Water analysis: dissolved carbon dioxide
Related Products & Services
Conductivity and Resistivity Meters
Conductivity meters, dissolved solids meters, and resistivity meters are analytical instruments that measure the conductivity, dissolved solids, and/or resistivity of a liquid sample.
Dissolved Oxygen Meters
Dissolved oxygen meters are analytical instruments that are used to measure the amount of oxygen dissolved in a liquid sample.
Ion Specific Electrode Meters
Ion specific electrode meters are millivolt meters that interface with ion selective electrodes (ISEs). These meters take the potential generated by the electrode and convert it into units of concentration.
Oil in Water Monitors
Oil in water monitors are used to detect the presence of hydrocarbons in water.
pH instruments are used to measure or monitor potential of hydrogen (pH) in a solution.
Turbidity instruments measure the average volume of light scattering over a defined angular range. Both particle size and concentration of suspended solids as well as dissolved solids can affect the reading.
Water Quality Testing Instruments
Water quality testing instruments are used to test water for chemical and biological agents, and to measure variables such as clarity and rate of movement.