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What is a conductivity standard used for?
A conductivity standard is used for calibration or verification. A low conductivity standard requires special handling and is mostly used for verification purposes. Additionally, the low conductivity standard is directly affected by CO2 when in contact with air. Thus, they have a shorter shelf life and require protection with inert gas or the usage of a flow cell. These types of conductivity standard are only intended for verification and not calibration.
Is the 10 µS/cm conductivity standard appropriate for calibration?
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 verification of a measurement and not calibration.
We always recommend to calibrate with a 84 μS/cm conductivity standard and do check with the 10 μS/cm standard, even when measuring in very low conducting media such as pure water.
What precautions must be taken when measuring in low conductivity standards and samples?
Measurements of samples with conductivity values lower than 10 μS/cm need a special equipment and procedure. First of all 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.
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:
Calibration and Verification 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. For best accuracy of low conductivity calibrations, the 84 μS/cm standard should be used. 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 conductivity below 84 μS/cm.
Conductivity standards of 10 μS/cm and lower can only be used for one calibration or verification right after opening. 84 μS/cm standard should be used within one day after opening.