Ions are atoms or molecules that have lost or gained one or more valence electrons, therefore making it positively or negatively charged. Ions are still denoted in the same way as electrically neutral atoms or molecules with an additional superscript indicating the sign of the net electrical charge and the number of electrons lost or gained.
The presence of specific ions in mixture can be detected by chemical sensors, which usually converts the chemical information into an electrical or optical signal. A type of chemical sensor that measures a potential under no current flow is called a potentiometric sensor. The potentials measured by these sensors may be used to determine the analytical concentration of an analyte (a substance or chemical constituent that is determined in an analytical procedure) in a gas or solution. An ion-selective membrane is the key component of all potentiometric ion sensors. It establishes the preference with which the sensor responds to the analyte in the presence of various interfering ions from the sample.
The ion-selective electrodes (ISE) are one of the most frequently used potentiometric sensors during laboratory analysis as well as in industry, process control, physiological measurements and environmental monitoring. ISE are used in a wide variety of applications for determining the concentrations of specific ions in aqueous solutions. The pH electrode is the most well-known and simplest form of ISE.
Electrodes & MetersTwo-Electrode pH MeasurementsThe pH can be measured using two electrodes, an indicator electrode and a reference electrode. When both electrodes are immersed in a solution, a potential (voltage) difference develops between them. Ideally, the potential of the indicator electrode should change with varying pH while the potential of the reference electrode should remain constant.
Combination ElectrodesThe indicator and reference electrodes may also be joined into a single physical entity to form what is known as a "combination electrode".
pH Meters and Temperature CompensationA pH meter is used to measure the potential difference (in mV) between the electrodes and converts it to display pH. For reliable and consistent results, the influence of temperature variations must be controlled. The use of a temperature sensor provides both temperature correction and temperature display.
Related areas of interest include the following application pages:
LabChart
LabChart software (for Windows and Macintosh) combines the familiar simplicity of a traditional strip chart recorder with the powerful analysis features of a digital acquisition system. LabChart software and a PowerLab data acquisition unit provide data integrity, easy selection of hardware settings, powerful online and offline analysis, procedure automation, seamless extraction of experimental data and flexible display options. Acquisition and analysis capabilities can be further increased with LabChart Extensions and LabChart Modules. LabChart Modules are available as part of LabChart Pro and LabChart Extensions are free for download from the website for existing LabChart users.
Most pH electrodes are connected directly to a special pH meter that performs the calibration automatically. However, with PowerLab and LabChart, you can also use the:
- Units conversion feature in LabChart for a standard 2-point calibration, which is sufficient for most electrodes in pH units or (for ISE's) log of the concentration.
- Multipoint Calibration LabChart Extension for Windows and Macintosh for multiple point (up to 12 points) calibration of non-linear pH or ion selective electrodes.
- pH Measurement LabChart Extension for Windows and Macintosh for single or 2-point calibration of pH (with manual or automatic temperature compensation) or ion selective electrodes and it reports percentage Nernstian response.
GLP and 21 CFR Part 11For 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.
Signal ConditionersML165 pH AmpThis amplifier is a dual front-end suitable for pH, ion selective, and temperature measurements in solution recordings. It is supplied with a RTD Temperature Probe. It can be used with:
Biosensors - 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 - This electrode is designed for use with any pH or ion selective electrodes and used with the ML165 pH Amp. It has an internal Ag-AgCl internal reference electrode with KCl filling solution.
Third-Party ElectrodesThird-party electrodes and meters may be connected to a PowerLab data acquisition system, provided that they have a ± 10 V analog output range. Such electrodes may include:
- Ion Selective Electrodes and Microelectrodes
- pH Field Effect Transistor (pHFET) Electrodes
- Ion Selective Field Effect Transistor (ISFET) Electrodes
- Fiber Optic Probes