Conductivity Meter
Conductivity Meter
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Tips On Conductivity Measurements
Listed below are some basic tips and other facts regarding conductivity measurement in the lab. Most conductivity work is done using potassium chloride, not sodium chloride. Sodium Chloride is preferred in natural salt water/brine sample testing. Conductivity solutions of less than 100 umhos are usually not stable for over 3-6 months with respect to a 1% tolerance which is common by manufacturers.
This is primarily due to absorption of atmospheric CO2. This results in an increase of the ions dissolved in the solution, thereby increasing the conductivity value. Of course, the lower the conductivity value, the higher the proportion of the added conductivity contributed by the contaminant CO2. CO2 absorption into conductivity standards higher than 5000 or 10,000 micromhos has a minimal effect under NORMAL laboratory conditions. "Minimal" means the conductivity which absorbed CO2 contributes to the conductivity standard solution will normally be much less than the 1% error tolerance stated on the label. Above a conductivity standard of about 50,000 micromhos the effect is insignificant, and if you get a suspicion that something is very wrong with your standard, you should look toward other sources of contamination. Do not store conductivity standard solutions in the refrigerator as this will obviously just attract more CO2 to the solution standard!
Always store platinum probes submerged in clean DI water. You will get quicker, more stable conductivity readings when calibrating. Opt for the lower frequency settings on your meter if given a choice and select the desired conductivity range. Pre rinse beakers and probes with the conductivity solution after a DI water rinse just before calibration. Choose high density polyethylene for containers, and avoid soft plastics which contain phthalate plasticizers such as DEP or DEHP Take readings and calibrate at 25C whenever possible. Temperature compensating sensors/thermistors can go bad with little warning on some meters!
As an alternative to certified external conductivity standard solutions, you can make your own as a second source using good lab procedures. Material must be dried reagent or better grade, water must be ASTM Type I, vessels must be class A calibration type, and scales recently calibrated. High purity water will absorb CO2 very rapidly to the order of up to 0.5 or 0.6 micromhos in a 4 hour period. If you are making a very low conductivity standard you must, after generating the ultrapure water, dissolve, mix, package, and cap as soon as possible.
About the Author
Joseph E. Papa, Sr. Chemist
-Chemical Testing Solutions
For Research and Industry-
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