Conductivity FAQs
- What am I measuring when I measure the conductivity of my solution?
- How does temperature affect conductivity readings?
- Can conductivity be measured in aqueous solutions only?
- How are conductivity and conductance related?
- What is a cell constant and why are there different ones?
- How are conductivity and TDS related?
- How do I calibrate my meter for TDS if my dissolved solids are not the same as those in the calibration solution that is sold in the Cole-Parmer catalog?
- The standardization solution I purchased has three values listed on it. Which one should I use?
- What is the difference between micromhos and microsiemens?
- How do I clean my electrode?
- How often do platinum probes need to be replatinized?
- How should I store my conductivity electrode?
- How do I condition a probe?
- What is the difference between conductivity and salinity?
- How far can the probe be from the meter?
- Is there an expiration date on the standardizing solutions?
- Are conductivity probes interchangeable with meters?
- How and when do I need to calibrate the probe?
- How do I find the correct temperature coefficient when not working with water?
Conductivity is the measurement of the electrolytes in a solution. It is defined as the conductance in a given volume. Conductance is the ability of the solution to conduct electric current.
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The effect of temperature on conductivity readings depends on the solution being
measured. The effect is greatest in low ionic strength (low conductivity) solutions. A general
rule to follow is there will be a 2% change (increase)/degree C. This rule can be followed
for most aqueous solutions, however if you require a high degree of accuracy, you should
consult a chart for the particular solution you are measuring. Organics also have very
different temperature curves.
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No, all substances possess some conductive properties. Generally organic compounds (such as benzene, alcohols, and petroleum products) have very low conductivities, while metals have very high conductivities. Measuring the conductivity of highly flammable liquids is very risky.
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Conductance is the ability of a solution to conduct electric current, while conductivity is the conductance in a given volume (usually measured in umho/cm).
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The cell constant, K, is equal to the area normal to the current flow in centimeters squared divided by the length in centimeters between the electrodes. For solutions with low conductivities the electrodes can be placed closer together or made smaller so that the cell constant is less than one. This has the effect of raising the conductance so as to produce a value more easily interpreted by the meter. The converse also applies, in high conductivity solutions, the electrodes are placed farther apart or made larger. Different cell constants are used as range multipliers.
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Salts, minerals, and even dissolved gases contribute uniformly to the conductivity of a solution. This means that the conductivity can be used as an indicator of the amount of dissolved materials in a solution. TDS can be used fairly accurately when comparing the status of a single source, such as NaCl, but error can arise when trying to compare two different types of solutions. It is necessary to calibrate the meter using the same dissolved materials that are in the test solution.
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Making your own standard will yield the most accurate results. This is done by
formulating a mixture of salts in relative proportions that simulate the solution to be
tested, then dissolving the mixture into distilled water. This should be done according
to the following formula:
1 mg salt mixture/liter of distilled water = 1 ppm TDS,
or in other words,
X ppm TDS = X mg of salts + one liter of distilled water.
Remember that "X mg of salts" is the number of milligrams of a mixture of salt of which
proportions simulate your test solution, not X milligrams of each salt in the mixture.
An appropriate value for "X" is determined by the following rule:
Choose a ppm value for a calibrated solution as close as possible to the expected ppm values
of the test solutions. If the ppm content of the test solution is expected to vary a great
deal, it is best to choose a ppm value for the calibrated solution in the upper 1/3 of the
TDS indicator's measurement range.
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This depends on what you are measuring. If it is conductivity, use the value
with the units of micromhos or microsiemens. If it is TDS, use the one labeled ppm/NaCl
if you are measuring the TDS of a sodium chloride solution. If you are measuring natural
water, use the 442 formulation (442 is 40% sodium sulfate, 40% sodium bicarbonate, and 20%
sodium chloride). Whichever you choose, you are referencing your solution to that standard.
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There is no difference. Micromhos is more common in the U.S., while microsiemens
is more common in Europe.
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Clean cells with mild liquid detergent and/or dilute nitric acid (1% wt) by
dipping or filling the cell with solution and agitating for 2 to 3 minutes. Dilute HCl
(hydrochloric acid) or H2SO4 (sulfuric acid) may also be used. When stronger cleaning is
needed, try concentrated HCl mixed into 50% isopropanol (rubbing alcohol). Rinse the
cell several times with distilled or deionized water and recalibrate before use.
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Some platinum electrodes are coated with platinum black before calibration. This
coating is extremely important to cell operation, especially in solutions of high conductivity.
Electrodes are platinized to avoid errors due to polarization. Polarization is a condition
in which conductivity plates are screened by ions in the solution, leading to inaccurate
readings. Cells should be inspected periodically and after each cleaning. If the
platinum black coating appears to be wearing or flaking off the electrode, the cell
should be replatinized. It is recommended to replatinize the electrode approximately
every six months.
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Rinse it in tap water when you are finished using it. You can store your electrode
either wet or dry. If it is stored dry you will need to recondition the electrode before use.
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Place the probe in a standard solution or tap water and have power running to
the probe. Let the probe soak for 30 minutes to 1 hour unless otherwise specified.
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The probe is the same for conductivity and salinity, but in a salinity meter a
correction factor is applied to the reading. The correction factor takes the conductivity
reading and converts it to ppm of a specific salt. The salt varies from manufacturer to
manufacturer. Some use NaCl while others use CaCO3.
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Generally the probe can be about 200 feet from the meter. The meter sends a
small AC voltage signal to the probe.
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Unopened with an unbroken seal, the solution will last one year. Opened, but
placed in an airtight container, the solution will last six months. Opened and exposed to
air, the solution will last one day.
Note: Standards below 100 uS will degrade at a faster rate than others due to air.
The conductivity of air is approximately 120 uS; any standard lower than 120 uS will
slowly rise in conductivity until it reaches a state of equilibrium. Evaporation
drastically increases conductivity values.
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No, you cannot place one manufacturer's probe on another manufacturer's meter. The cell
constant may be different and the pin configurations are usually different. The type of thermistor used for temperature compensation is usually different also. The cell constant is not a big problem, but not matching the pin configuration and temperature compensating element used would be a problem.
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Calibrate using a standard solution in the range of the samples you are testing.
Place the probe in standard solution, condition, rinse probe in second sample of standard
solution, use a third sample of standard solution to calibrate, and then adjust the cell
constant until the specified value is displayed. Recalibrate when you change ranges, or if
readings seem to be incorrect.
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For water, the correction factor is 2% change per degree C. Check the conductivity
of the sample at 25°C, then using the same sample, find the conductivity at another
temperature to see what the percent change is. This will give you the temperature correction
factor.
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Complete Selection of Conductivity Controllers
Complete Selection of Conductivity/Resistivity/TDS Meters
Conductivity Technical Information
Conductivity Theory and Technical Tips
Conductivity Values µS/cm at 77°F (25°C)
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