Conductivity Standards

METTLER TOLEDO offers certified standard solutions with a conductivity of 10 μS/cm and lower. These standards are only intended for checking the reading of low conductivity measurements because they are easily affected by any mishandling.

10 μS/cm conductivity standard is highly vulnerable and is directly affected by contamination or influence of CO₂ when in contact with air. Due to this high likelihood of a quick value change, it is intended only for measurement verification and not calibration.

The 84 μS/cm standard should be used for the best accuracy of low-conductivity calibrations. The consistent linearity of METTLER TOLEDO measuring systems between 84 μS/cm and low conductivity values provides much better accuracy than is typically obtained by using standard solutions with a conductivity below 84 μS/cm.

Since low-conductivity sensors are factory-calibrated, we recommend performing a daily verification rather than a full calibration. If the sensor does not measure correctly (due to a defect or damage on the poles), a cleaning and possibly new calibration can be performed.

While standards such as the 1.3 µS/cm standard are strictly single-use, others like the 10 µS/cm standard and 84 µS/cm standard (if handled properly) can be used within their specified shelf life, but CO₂ absorption may alter them rapidly with time. These standards have no buffering effect, unlike pH buffers, so you must avoid leaving such bottles open.

Equipment

Measurements of samples with conductivity values lower than 10 μS/cm need special equipment and procedures. First, a two-pole cell featuring a low cell constant (0.01– 0.1 cm-1) is needed. InLab™ Trace and InLab 741/742 are specially designed for these requirements.

Measurement arrangement

CO₂ from the atmosphere greatly influences the conductivity measurement at this low level. To get reliable conductivity results below 10 μS/cm, the follow­ing two approaches are recommended:

• Inert gas:

The sample is protected by an inert gaseous layer of nitrogen or argon to prevent contamination from the atmosphere. This allows the use of normal dip cells to measure samples in the range of 1 to 10 μS/cm.

• Flow cell:

A closed system, like a flow cell, eliminates the influence of CO₂. This is the best practice to measure samples with a conductivity value lower than 10 μS/cm. However, samples with higher con­ductivities may also be measured in this manner. For non-pressurized samples, flow cells can be used with a peristaltic or a membrane pump, given that they are completely sealed systems. With other types of pumps, the sample or standard comes into contact with the pump, which can lead to contamination and erroneous readings. For sample and standards, the flow must not introduce any air bubbles.

Contamination from one sample to another or from a calibration standard to a sample can cause incorrect readings. Rinsing the flow cell, tubes, and sensor between each mea­surement helps to prevent this problem. The following steps are recommended:

1. Rinse thoroughly with deionized water (high flow rate recommended)
2. Rinse with sample or standard
3. Measure sample or calibrate with standard
4. Start at point 1 again

To decrease the volume of sample or standard solution that is wasted, the rinsing step 2. can also be performed with deionized water instead.