Laboratory Conductivity Sensor

A laboratory conductivity sensor is a tool to measure the electrical conductivity of an electrolyte solution and is based on the material’s ability to conduct an electric current. It is used to measure conductivity in laboratory and field applications.

The electrolytes dissolve to give ions that conduct electricity. The higher the ion concentration, the higher the conductivity. The measuring cell of the conductivity sensor consists of at least two electrically conductive poles with the opposite charge to measure the conductance of a sample.

Conductivity is based on Ohm’s law, wherein the voltage (V) set up across a solution is proportional to the flowing current (I) and resistance (R) is a constant of proportionality. R can be calculated with the measured current flow, if a known voltage is applied. Conductance (G) is defined as the inverse of resistance and to measure the conductance of a sample, the measuring cell is required. The conductance reading depends on the geometry of the measuring cell, which is described with the cell constant (K). This is the ratio of the distance (l) and area (A) of the poles. The conductance can be transformed into the standardized conductivity by multiplying the conductance and the cell constant.

Most customers measure conductivity in a quite narrow range, for example, always the same beverage or deionized water. With a 1-point calibration, the range between 0 µS/cm and this calibration point is calibrated. It is recommended to choose a standard with higher conductivity than the expected value in the sample, for example 1413 µS/cm, when expecting 1200 µS/cm. Performing a second calibration point in this example would not remarkably change the reading because the adjacent standards 500 µS/cm and 12.88 mS/cm are both quite far away. According to Method 2510B in Standard Methods for the Examination of Water and Wastewater and ASTM D1125, a one-point calibration of the cell constant at a representative conductivity is sufficient for accurate conductivity readings.

Multi-point conductivity calibration is only valid when using the same sensor over a wide range, for example from 50 to 5000 µS/cm. In this case, a suitable set of standards will be 84 µS/cm, 1413 µS/cm, and 12.88 mS/cm.

Classical 2-pole conductivity cells consist of two plates. Normally, the plates are surrounded by an outer tube which protects them from mechanical damage and reduces the errors caused by field effects. The strength of the 2-pole conductivity cell is measuring low conductivity with high accuracy. A typical measuring range goes from 0.001 μS/cm to 1000 μS/cm. The main applications of a 2-pole cell are the conductivity measurement of pure water, highly diluted aqueous solutions, and non-aqueous solutions.

A 4-pole cell design consists of an outer pole and an inner pole. The outer poles are the current poles to which an AC is applied. They are driven in the same manner as the 2-pole sensor. The inner measuring poles are placed within the electric field of the current poles and measure the voltage by using a high-impedance amplifier. Hence, very little current flow in the inner poles where the measurement is taken. Thereby no polarization effects occur which influence the measurement. The strength of a 4-pole conductivity cell is measuring conductivity over a wide measuring range from 10 μS/cm up to 1000 mS/cm. The main applications of this sensor type are measurements in seawater, wastewater, or diluted acids or bases.

Choosing the right laboratory conductivity sensor is critical for obtaining accurate and reliable results. The right sensor is the one that fits the needs of the application best.

a. A basic requirement is that no chemical reactions occur between the sample and the sensor. For chemically reactive samples, glass and platinum are often the best choices because they have the best chemical resistance of all commonly used cell materials. For field applications and many laboratory applications, the mechanical stability of the sensor is a more critical factor. A conductivity sensor with an epoxy body and graphite electrodes is often used, as it has been shown to be highly durable and has good chemical resistance. For low-reactive aqueous solutions and organic solvents, the use of cells made of steel or titanium is often a good alternative. The choice becomes particularly important for non-aqueous, low conductivity, protein-rich, and viscous samples where routine pH sensors are possible sources of error.

b. A suitable cell constant correlates with the conductivity of the sample. The lower the expected conductivity of the sample, the smaller the cell constant of the sensor should be. To make a decision between a 2-pole cell and a 4-pole cell, the following rough-and-ready rule can be used: For low conductivity measurements, a 2-pole cell should be used. For mid to high-conductivity measurements, a 4-pole cell is preferred, especially for measurements over a wide conductivity range.
 

There are several ways to compensate for temperature.

Conductivity in an aqueous solution is highly affected by temperature (~2 %/°C). That’s why it is conventional to link every measurement with a reference temperature. 20 °C or 25 °C are the commonly used reference temperatures in the case of conductivity measurement.

Different temperature correction methods have been developed to suit different users:

• Linear: for medium and highly conductive solutions
• Non-linear: natural waters such as groundwater, surface water, drinking water, and wastewater
• Pure water: ultrapure water, deionized water, distilled water
• None: some standards such as USP <645> prohibit any temperature compensation

The impact of temperature on different ions, and even varying concentrations of the same ion can be challenging. Hence, a compensation factor, called temperature coefficient (α), must be determined for each type of sample. (This is also the case for the calibration standards. All METTLER TOLEDO meters can automatically account for this compensation using preset temperature tables.)

All user manuals provide the necessary information about the short and long-term storage of the respective sensor. Generally, lab conductivity probes should be stored dry for long-term storage.

Laboratory conductivity electrodes have no expiration date. When the sensor is used within the specified temperature limits and neither severe mechanical force nor harsh chemical conditions are applied to the sensor and its cable, it can theoretically be used forever. Nevertheless, shifts of the cell constant may occur due to deposits of fatty substances and precipitates. In most cases, rinsing with ethanol, isopropyl alcohol, or acetone can restore the sensor.

Low conductivity range sensors like InLab 741, InLab 742 and InLab Trace come with a measured cell constant on their certificate. These are certified cell constants and are determined after the manufacturing process directly at the plant with traceability according to ASTM and NIST. With a maximum uncertainty of ± 2% they are accurate enough and can be used for conductivity measurement by directly entering the cell constant value in the meter, without the need for a calibration. The certified cell constant is stated on the quality certificate, printed on the sensor cable, and stored on the ISM sensor chip.

As these sensors are particularly designed for use in low conducting media, such as pure water, ultrapure water, distilled water, and deionized water, the measuring cell is very unlikely to be affected by contamination. Hence, the cell constant can be regarded as stable. Nevertheless, regular verification of the precision with a conductivity standard (e.g., 10 µS/cm) is crucial.

All other conductivity sensors from METTLER TOLEDO have nominal cell constants printed on the certificates. These sensors have to be calibrated before use with the appropriate calibration standard solutions.

If the exact cell constant is unknown, then calibration must be performed. When the exact cell constant is known, then verification is sufficient. This is the case with sensors with a certified cell constant or sensors which have been previously calibrated.

Yes, it is possible. Organic substances also have dissociative properties. Organic compounds like benzene, alcohols, and petroleum products generally have very low conductivity.

The sensor should be rinsed after every measurement with deionized water. If the sensor has been exposed to a sample immiscible with water, it should be cleaned with a solvent miscible with water, for example, ethanol or acetone, and carefully rinsed afterward with deionized water. If there is a build-up of solids inside the measuring cell, carefully remove it with a cotton bud soaked in detergent solution, and then rinse the sensor with deionized water.

(Caution: sensors with platinized poles should never be cleaned mechanically, as this could damage the sensors).