Conductivity

Overview:

Conduction or electrical conduction is the mechanism by which charge flows or electrically charged particles move through a substance (electrical conductor). This movement of charge constitutes an electric current which is controlled by the chemical properties of the substance, temperature and other environmental factors.

The standard unit of conductance is the siemens (abbreviated S), formerly known as the mho. Conductance is inversely related to resistance. If R is the resistance of a component or device (in ohms), then the conductance G (in siemens) is given by:

G = 1/R

Therefore, conductivity (electrical conductivity or specific conductivity) is a measure of the ability of a substance to conduct an electric current. In solutions, current is carried by free moving ions both positive and negative. A conductive solution is known as an electrolyte and includes solutions of ionic salts or of compounds that ionize in solution (e.g. acids and bases).

The measurement of conductivity has many applications in Chemistry, Biology and Environmental Science. Salty water conducts electricity more readily than purer water. Therefore, electrical conductivity is routinely used to measure salinity in waterways such as rivers, streams, lakes, etc.

Method:

Conductivity is a measure of the ability of a solution to pass an electrical current. Conductivity is measured with a probe and a meter. Voltage is applied between two electrodes in a probe immersed in the sample solution. The drop in voltage caused by the resistance of the solution is used to calculate the conductivity per centimetre. The meter typically converts the probe measurement to microSiemens per centimetre (µS/cm) and displays the result for the user. ADInstruments offer the Conductivity Pod for use with a PowerLab system for continuous real-time measurement of a solutions electrical conductance. Suitable conductivity meters with an analog output can also interface with PowerLab.

Note: You will also require a conductivity calibration solution (e.g. potassium chloride (KCl)) to calibrate the meter to the range you will be measuring and deionised/distilled water to thoroughly rinse the probe between readings.

Conductivity is also affected by temperature e.g. the warmer the solution, the higher the conductivity. For this reason, conductivity is typically reported as conductivity at 25°C. When comparing salinities of different samples, it is important to standardise the reading to 25°C (i.e. you must measure the solution temperature at the same time as the electrical conductivity). Increase the electrical conductivity reading by 2% per degree for samples with temperatures below 25°C, and decrease it by 2% per degree for samples above 25°C (Reference: Waterwatch Australia).

Software:

The LabChart Advantage:

(may require additional Modules and Extensions)

• Units Conversion for easy calibration into appropriate units such as µS, S/m, etc
• Multipoint Calibration LabChart Extension for Windows and Macintosh for multiple point (up to 12 points) calibration of non-linear 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

ML307 Conductivity Pod
The Conductivity Pod is used with conductivity cells for monitoring solution conductance. The Pod measures the alternating current (being positively and negatively charged) across a suitable conductivity cell by applying an AC potential across the pair of platinum plate electrodes that comprise the cell. The output is proportional to the conductivity of the solution. The signal is scaled to read in µS or mS, depending on the range, 5 µS to 20 mS, selected.

Transducers and Accessories

The following electrodes are suitable for use with the Conductivity Pod via a BNC connector:

• MLT908 Flow-Thru Conductivity Electrode: Suitable for monitoring the conductance of continuous streams of water, urine, physiological fluids, etc. Cell volume = 0.093 mL. Attaches to tubing with an internal diameter of 3.2mm (1/8").
• MLT916 Flow-Thru Conductivity Electrode: Suitable for monitoring the conductance of continuous streams of water, urine, physiological fluids etc. Cell volume = 0.017 mL. Attaches to tubing with an internal diameter of 1.6mm (1/16").
• MLT915 Dip-In Conductivity Electrode: Suitable for measuring the conductivity of samples such as water, urine and physiological fluids. Length = 14.6cm long. Outer diameter = 3.2mm. Immersion depth = 5mm.

Inhibition of lithium dendrites by fumed silica-based composite electrolytes
X-W Zhang, W Li, S A Khan and R S Fedkiw, Journal of the Electrochemical Society, A1257-A1263, 2004

Following the heart: temperature and salinity effects on heart rate in native and invasive species of blue mussels (genus Mytilus)
Braby C.E and Somero G.N., Journal of Experimental Biology, 2554-2566, 2006

Application of gradient programs for the determination of underivatized amino acids and small peptides in reversed-phase high-performance liquid chromatography with contactless conductifity detection
Kuban P and Hauser P.C., Journal of Chromatography A, 97-104, 2006