CO2 & NH3

Overview:

Carbon dioxide (CO₂) is one of the most abundant gasses in the atmosphere. CO₂ is a molecule composed of two oxygen atoms covalently bonded to a single carbon atom. CO₂ plays an important part in vital plant and animal process, such as photosynthesis and respiration. It is also generated as a by-product of the combustion of fossil fuels and many other chemical processes. Dissolved CO₂ is the amount of carbon dioxide dissolved in a liquid sample such as water and common applications include:

• Regulation of pCO₂ in Aquatic Environments
• Aquatic Respirometry
• Aquatic Photosynthesis
• Fermentation
• Algal and bacterial culture

Ammonia (NH₃) is normally encountered as a colorless gas with a characteristic sharp, penetrating odor. It is a very toxic, reactive, and corrosive gas and is widely used in fertilizers, plastics, and explosives. NH₃ is also directly or indirectly a building block for the synthesis of many pharmaceuticals. Typical NH₃ monitoring applications include:

• Waste water treatment
• Potable water treatment
• River and stream water quality
• Refrigeration coolers (using ammonia-water absorption) in dairies, breweries and beverage manufacture

Method:

Measurements of dissolved CO₂ are typically carried out using a submerged probe that is covered by a thin organic membrane. When the probe is submerged in the liquid sample, CO₂ diffuses through the membrane at a rate proportional to the partial pressure. Increasing the partial pressure increases the diffusion amount. Both temperature and pressure affect the diffusion rate measured by dissolved CO₂ instruments. Accuracy and diffusion range are typically measured in parts per thousand (ppt) or parts per million (ppm). Dissolved CO₂ instruments which can provide analog voltage outputs can be used with a PowerLab system to record a simple (usually linear) function of the measurement.

Note: For CO₂ measurements in air, refer to the Respiratory Gas Analysis application page.

CO₂ can change the pH of water therefore pH Electrodes can also be used to measure dissolved CO₂. Electrode measurements are simpler and faster than other analytical techniques. The pH electrodes work by allowing CO₂ to diffuse across a permeable plastic membrane into the inner compartment of the electrode where they reversibly react with a buffer solution and in doing so alter the pH. The pH electrodes are typically connected to a pH meter or can be used with the ADInstruments pH Amp, which can also be used if a separate reference electrode is necessary.

Ion-selective ammonia (NH3) electrodes consist of an outer membrane which allows NH₃ to diffuse through it. The change in the pH value of the inner electrolyte solution is monitored by a combined glass electrode.

Transcutaneous Carbon Dioxide Tension (PCO₂)

Transcutaneous measurements of oxygen and carbon dioxide are based on the principle that a heating element in the electrode elevates the temperature of the underlying tissues. This increases the capillary blood flow and the partial pressure of oxygen and carbon dioxide, and makes the skin permeable to gas diffusion.Transcutaneous monitoring of
carbon dioxide tension (PCO₂) has been shown to be more reliable than the transcutaneous measurement of oxygen due to the greater diffusion capacity of carbon dioxide through the skin.

Software:

The LabChart Advantage:

(may require additional Modules and Extensions)

• Units Conversion for easy calibration into appropriate units such as ppm, mg/L, µM, etc
• The Multipoint Calibration Extension performs linear and non-linear calibration on transducers, sensors, pH electrodes and other equipment
• Comments can be added in real time, viewed or moved at a later stage
• Fast data extraction, analysis and export (e.g. csv. or txt.) to other applications using Timed Add to Data Pad and Multiple Add to Data Pad
• The Event Manager Extension can be used to monitor an incoming dissolved CO₂ or NH₃ signal and perform a user defined action such as adding a comment, playing a sound/alarm, flashing a light, etc when the CO₂ or NH₃ signal rises/falls above/below a set threshold level.

LabChart

LabChart software (for Windows and Macintosh) together with a PowerLab data acquisition system offers up to 32 channels of real-time data acquisition, data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Additional acquisition and analysis functionality is provided with the use of specialized LabChart Extensions and LabChart Modules. Modules are available as part of LabChart Pro while Extensions are free for download from the website for existing LabChart users.

Multipoint Calibration

The Multipoint Calibration Extension (Win and Mac) extends Units Conversion in LabChart to allow you to perform linear and non-linear corrections on any input using up to 12 standard points. The Extension may be used online, while data is recording, or offline with previously recorded data. The calibration points should span the highest and lowest signals that you expect to encounter in your experiment. Various fitting functions are provided by the Multipoint Calibration Extension. The functions (except Point to Point) are fitted by least squares to your calibration points. Note: Some calibration functions (especially polynomials) can work poorly if subsequently used with data outside the calibration region.

GLP and 21 CFR Part 11

For those researchers working within a laboratory requiring GLP and 21 CFR Part 11 compliance the GLP Client and GLP Server are available for use with LabChart (Windows only) and PowerLab data acquisition systems. For more information, visit the Good Laboratory Practice application page or contact your nearest ADInstruments representative.

Hardware:

PowerLab Data Acquisition Systems

The PowerLab is a high-performance data acquisition unit capable of recording at speeds of up to 400,000 samples per second continuously to disk (aggregate). PowerLab units are compatible with instruments, signal conditioners and transducers supplied by ADInstruments, as well as many other third-party companies. In addition to standard single-ended BNC inputs, 4 differential Pod ports are also available for direct connection of Pod signal conditioners and appropriate transducers. Research PowerLab units include:

• PL3504 PowerLab 4/35 - 4 Channels
• PL3508 PowerLab 8/35 – 8 Channels
• PL3516 PowerLab 16/35 – 16 Channels

Signal Conditioners

ML165 pH Amp
A dual front-end suitable for pH, ion-selective, and temperature measurements in solution recordings. The unit is capable of measuring pH in the range 0-14, electrode voltages to 2.0 V and temperature in the range of 0 to 100 °C. It is supplied with a RTD Temperature Probe.

Transducers and Accessories

The ML165 pH Amp can be used with:

• MLA060 Redox Electrode: The combination Redox Electrode is suitable for operation from 0 to 60°C. The double junction is easily cleaned, making it suitable for biological samples.
• MLA042 pH Electrode: The combination pH Electrode operates from 0 to 60°C and for a pH of 0 to 14. The double junction is easily cleaned and therefore the electrode is suitable for biological samples, in particular those of low ionic strength.
• MI-405 Miniature Glass Electrode for pH: This miniature electrode has many applications in general purpose analyses and in microchemical studies. It requires the use of an external reference electrode such as the MI-409 Miniature Reference Electrode.
• MI-409 Miniature Reference Electrode: Designed for use with any pH or ion-selective electrodes. It has an internal Ag-AgCl internal reference electrode with KCl filling solution.

Third-party Equipment

Third-party electrodes and meters may be connected to a PowerLab data acquisition system, provided that they have a ± 10 V analog output range. Electrodes and meters made by Microelectrodes Inc, Diamond General and Thermo Orion may be suitable.

Transcutaneous carbon dioxide (TCpCO₂) can be monitored by using skin electrodes. An approved electrically isolated meter must be used for human use. These meters can sometimes also monitor transcutaneous oxygen levels. Manufacturers of these types of meters include Radiometer and SensorMedics.

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