Ionic Concentrations

Overview:

An ion is a charged atom or molecule in which the number of electrons does not equal the number of protons. Ionic compounds are compounds where two or more ions are held together by electrical attraction. One of the ions has a positive charge (cation) and the other has a negative charge (anion). Ionic compounds in an aqueous solution dissociate from their mutual attractions and distribute free ions throughout the solution. This ionic solution can typically conduct an electrical current and is referred to as an electrolyte. Ions can be separated from the solution by deposition on an electrically charged electrode, such as an Ion-Selective Electrode (ISE).

ISE are one of the most frequently used potentiometric sensors in laboratory analysis as well as in industry, process control, physiological measurements and environmental monitoring. The pH electrode is the most well-known and simplest form of ISE.

pH is a measure of the hydrogen ion concentration, [H⁺], and is calculated using the following formula: pH = -log10[H⁺]. The pH scale measures how acidic or basic a substance is and ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic.

Method:

An Ion-Selective Electrode (ISE) produces a potential that is proportional to the concentration of an analyte. Making measurements with an ISE is therefore a form of potentiometry, where potential is measured under the conditions of no current flow. ISEs consist of the ion-selective membrane, an internal reference electrode, an external reference electrode, and a voltmeter. The most common ISE is the pH electrode, which contains a thin glass membrane that responds to the hydronium ion [H⁺] concentration in a solution. Commercial ISEs often combine the two electrodes into one unit that are then attached to a pH meter.

pH can also be measured using either pH indicators (like phenolpthalein) in the form of solution or pH strips which are useful when all you need is 0.2-0.5 pH unit accuracy. However, when you require higher precision then potentiometric methods are recommended.

The ADInstruments pH Amp or any pH meter capable of providing an analog output can be connected to a PowerLab system for continuous real-time pH or ionic concentration monitoring.

Electrodes and Meters

Two-Electrode pH Measurements
pH can be measured using two electrodes, an indicator (H⁺ - selective) 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 Electrodes
The indicator and reference electrodes may also be combined into one unit to form a combination electrode.

pH Meters and Temperature Compensation
A 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:

• Dissolved O2
• CO2 and NH3
• Conductivity

Software:

The LabChart Advantage:

(may require additional Modules and Extensions)

• Units Conversion for easy calibration into appropriate units such as pH, μM, etc
• Multipoint Calibration LabChart Extension for Windows and Macintosh for multiple point (up to 12 points) calibration of non-linear pH or ion-selective electrodes.
• 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

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
This 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:

• MLA060 Redox Electrode
• MLA042 pH Electrode
• MI-405 Miniature Glass Electrode for pH
• MI-409 Miniature Reference Electrode 

Transducers and Accessories

• 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. 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

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