What are the guidelines for measuring Conductivity in low Ionic Strength samples?

Document ID

Document ID TE12907

Published Date

Published Date 05/13/2021
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Question
What are the guidelines for measuring Conductivity in low Ionic Strength samples?
Summary
Recommendations and considerations to make when considering measuring Conductivity, Resistivity, or Total Dissolved Solids (TDS) in samples that are considered Low Ionic Strength (LIS).
Answer

Introduction:

Low Ionic Strength (LIS) is a general term used to described samples with low Electrolytic Conductance (EC), referred to generally as Conductivity. It can be difficult to define if a sample is LIS, but generally samples that have an EC of 100 µS/cm or lower are considered LIS. Common examples are purified water sources such as distilled, demineralized, or reverse osmosis waters. But other waters can be considered LIS even if they do not go through one of these specific purification systems.

The lower the expected EC of the sample, the more difficult it can be to get accurate measurements for EC or EC related parameters (such as Resistivity, Salinity, or TDS). Error sources that have a relatively low percent error on higher ionic strength samples have a much larger percent error on LIS samples. This is because many of these error sources will have a positive offset in the reading which will be a larger percent error when the relative measurement is lower.

There are guidelines and recommended steps to take to reduce these error sources as much as possible. But the error can not be completely removed, only reduced. The importance of these steps, and the process of implementing the steps themselves cause measurement accuracy on LIS samples to be much more technique sensitive than on higher ionic strength samples.
 

Equipment:

The recommended Lab probe for measuring LIS samples is the Intellical™ CDC401 Laboratory 4-Poles Graphite Conductivity Cell (Product # CDC40101). 4 Pole sensors have better sensitivity at a wider range than sensors with fewer poles, and the graphite cells allow for less frequent calibration. This probe is compatible with the HQ1140, HQ2xxx, and HQ4xxx meters.
 

Atmospheric contamination:

When exposed to air, atmospheric Carbon Dioxide will go into solution forming Carbonic Acid. The Carbonic Acid in the solution will increase the EC of the sample. Because of this it's recommended that large sample volumes are tested while going through a flow chamber that is isolated from the air. This can be done with an LIS chamber (Product # 5189900). It's also important to work quickly.
 

Calibration:

It's recommended to perform a single point calibration for all conductivity calibrations regardless of range, and the CDC401 requires a one point calibration. It would be recommended to use a standard that is closest to the measurement range ( Can a multi-point calibration be performed using the Intellical™ CDC401 4-Poles Graphite Conductivity Cell? ) The CDC401 has a list of standards built into the probe that the user can choose from. Each of the standards in this list will use a standard specific non-linear temperature correction curve that is used during calibration. The standard that would be recommended for calibrating a CDC401 for use in LIS samples is Certified Conductivity Standard Solution, 25 µS/cm, NaCl 25, 250 mL (Product # S51M013). ( Can a custom standard be used to calibrate the IntelliCAL™ CDC401 using an HQd meter? )

Because of atmospheric contamination, it is recommended that this standard be used within hours after opening, making it one time use. It's also recommended that the calibration is done using a LIS chamber (Product # 5189900).
 

Temperature Correction:

By default the CDC401 uses an non-linear temperature correction curve based on NaCl standards. A linear temperature correction based on NaCl standards would be ~2 %/°C. ( What's the difference between temperature compensation and temperature correction? (pH verses Conductivity temperature adjustments) )

According to the theory and practice document for conductivity, Ultra Pure water has a temperature vs EC relationship that is 5.2 %/°C. ( Where can additional information on conductivity theory be found? page 23) So if the default temperature correction for sample measurement is used when measuring an LIS sample, there can be as high as 3% error per degree Celsius from the reference temperature (25 °C).

This is why it's extremely important that a sample specific linear temperature correction factor is calculated and used for sample measurement when testing LIS samples. Instructions for this can be found in this article: How can a sample specific linear temperature correction factor be calculated for Conductivity?


Rinsing and technique:

Skin is often covered in salt. Anything that is touched will have a salt residue from the skin. If and when this salt comes in contact with the sample, the conductivity of the sample will increase as the salt is dissolved into solution. This is why it's recommended to wear gloves while testing LIS samples, and even then anything that will come in contact with the sample needs to be meticulously rinsed. This includes the LIS chamber, stir bars, and the electrode itself as well as anything else the sample may come in contact with such as sample collection containers.

All rinse water will have some amount of conductivity regardless of the purity of the rinse water. Often LIS samples can be higher purity than the rinse water that is used. So any residual rinse water from rinsing can and will also increase the conductivity of the sample. This is why it's important to rinse multilpe times, blot dry all things that were rinsed after each rinse, and then also do multiple sample rinses before analysis. (Blotting dry is not required or recommended after sample rinses.)

Air drying is not as effective as blotting dry because any dissolved solids that would have contaminated the sample will still be left as a residue one the rinse water evaporates.

The testing environment as well as the experience and skill of the analyst will have an impact on the accuracy of results when testing LIS samples.

 

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