Conductivity-Resistivity-TDS-Salinity

What is Conductivity, Resistivity, TDS (Total Dissolved Solids), Salinity, and Their Relationship?

11/15/2017
by info we

Water has the ability to conduct electricity due to the presence of charged ions in solution. Ions are atoms of molecules that have a net electrical charge, and they include cations (positively charged ions) and anions (negatively charged ions). The most abundant charged ions in natural water typically include the cations sodium (Na+), potassium (K+), calcium (Ca+2) and magnesium (Mg+2) and the anions chloride (Cl-), sulfate (SO4-2), nitrate (NO3-) and bicarbonate (HCO3-). Many other ions can be also found in water, including organic ions and other inorganic ions.

These ions carry electrical charge and can move through water, which allows water to conduct an electrical current. The measure of the ability of water to carry electrical current is called its electrical conductivity. Higher concentrations of ions in water increase its ability to conduct electricity and thus its conductivity. Distilled water, on the other hand, has a very low concentration of ions and a low conductivity.

Technical note: Sometimes electrical conductivity is referred to as specific conductance.

The opposite of conductivity is resistivity. Resistivity is the ability of a material (such as water) to resist the flow of electricity. Resistivity is the reciprocal of conductivity, such that

Resistivity = 1/Conductivity

From this relationship, we can see that water with a high conductivity has a low resistivity, and vice versa. For example, distilled water will have a high resistivity and a low conductivity.

The typical unit for reporting conductivity is microsiemens per cm (µS/cm). This unit is also sometimes written as micromhos per cm (µmho/cm), where 1 µS/cm equals 1 µmho/cm. Potable water typically has conductivity values ranging from 50 to 1500 µmho/cm[1]. At higher conductivities, the water starts to become too salty to drink.

Technical note: Notice that “mho” is the reverse spelling of “ohm,” the common unit for electrical resistance.

Because conductivity varies slightly with temperature, conductivity values are usually reported as temperature-compensated values that represent what the conductivity would be at 25°C. This makes it easier to compare conductivity values for samples with different temperatures.

How is conductivity related to total dissolved solids (TDS)?

Total dissolved solids (TDS) refers to the total amount of dissolved material present in water. TDS is usually reported in milligrams per liter (mg/L) or ppm (parts per million). This means that, if one liter of water with a TDS of 500 mg/L was completely evaporated, 500 mg of solid residue would be left behind. Usually, the dissolved solids include mostly dissolved mineral ions such as sodium, chloride, and the other ions mentioned above. TDS can also include other inorganic ions, dissolved organic material, and non-ionic matter such as dissolved silica. Although a relatively small amount of the TDS includes non-ionic matter that does not carry electrical charge, waters with higher values of TDS generally have higher values of conductivity.

Because of this, a measurement of conductivity (which is quick and easy) can be used to estimate TDS (which is more expensive and time consuming to measure directly). However, the relationship between conductivity and TDS varies with the chemistry of the water because ions differ in their ability to transmit electrical charge through water. Some ions carry electrical charges faster than others because of factors such as the size and mass of the ions and how they interact with water molecules.

The general equation for estimating TDS from conductivity is as follows:

TDS (mg/L) = k · EC (µS/cm)

where EC is electrical conductivity, and k is the conversion factor, which is related to the chemical composition of the water.

For typical natural waters such as stream and lake water, the value of the conversion factor is usually between 0.6 and 0.7, and a value of 0.64 is considered to be typical. For a solution containing mostly sodium and chloride ions, values of 0.49 to 0.56 are typical, depending on the concentration of salt.

For a precise estimate of TDS from conductivity, the chemistry of the solution should be considered in the selection of the conversion factor. If the composition of the solution is known, then the true TDS of a representative sample of water can be calculated by taking the sum of the measured concentrations. Alternatively, the true TDS value of a representative sample can be directly measured. The correct value of the conversion factor can then be calculated based on the true TDS and the measured conductivity.

If the correct value of the conversion factor cannot be calculated, then a typical or default value of the conversion factor (such as 0.64) will result in a TDS estimate that is at least in the right ballpark.

How is conductivity related to salinity?

Salinity refers to the salt content of water. Because most dissolved solids typically consist of inorganic ions, which are the components of salts, the concepts of salinity and TDS are very similar. In fact, the two concepts are sometimes considered to be synonymous. However, salinity is often expressed in terms of mass of salt per mass of water. For example, ocean water typically has about 35 grams of salt in one kilogram of water, so its salinity can be expressed as 35/1000 or 0.035. This can also be expressed as 3.5% or 35 parts per thousand (ppt).

Salinity is often used to describe seawater and brackish water, but it can also be used to describe fresh water and brines. Because the proportions of the most important ions in seawater are nearly constant, oceanographers can use very precise formulas to estimate salinity from electrical conductivity and temperature[1].

In cases where salinity is measured in mg/L (for example, for lake water, swimming pools, or irrigation water), salinity can be estimated from electrical conductivity using the same formula presented for TDS in the previous section.



References

[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.

Applications of Conductivity Measurement

11/15/2017
by info we

Testing the electrical conductivity of water provides much practical information about a solution. Not only is the conductivity measurement itself useful, but it can also be used to estimate the total dissolved solids (TDS) or salinity of water. Because conductivity measurements are simple, and fast, they are highly suitable for routine testing and long-term monitoring. Some examples of applications of conductivity measurement are described below.

Natural Waters, Aquaculture and Environmental Applications

In natural waters, conductivity is mainly used to estimate the concentrations of dissolved salts in the water, which in can provide insights into processes affecting the water. In river waters, for example, the conductivity (and TDS) of water may increase in the summer when evapotranspiration is high and decrease when the water is diluted by snowmelt or heavy rains. In coastal areas, the conductivity of water may change with mixing with salt water, and the conductivity of water may rise when it becomes contaminated with road salt in areas with cool climates.

For water resources, the conductivity may indicate whether or not the water is too saline to be drinkable or useable for irrigation or industrial use.

In places where there is potential for water to become polluted, the water may be monitored for changes in conductivity that could indicate contamination from a spill or leak. In ecosystems and aquaculture, aquatic plants and animals have certain ranges of salinity that they can tolerate. Because of this, the conductivity of water bodies such as ponds may be monitored to warn if the salinity is in danger of falling outside of the tolerable range for certain fish species, for example.

Water Treatment and Industrial Applications

Water treatment may be used to make water safe to drink or suitable for industrial use. In many industrial applications, scale (precipitation of mineral deposits) or corrosion may be a concern. Because conductivity can be used to estimate the dissolved mineral content of water, it may be used to monitor demineralization processes used to prevent scale or remineralization processes used to prevent corrosion. Conductivity may also be used to monitor the effectiveness of desalinization, which is another water treatment process that removes salts to make water drinkable or useable for industrial processes.

In other industrial applications, conductivity measurements may be used to detect leaks (such as in heat exchangers), where the leaking water may have a higher conductivity. Conductivity may also be used to monitor the effectiveness of rinsing procedures, where a low conductivity of water in contact with the rinsed object indicates an effective rinse. In special circumstances, such as in ammonia solutions, conductivity can even be used to measure pH with more precision than a typical pH meter due to the strong relationship between conductivity and pH.

Agricultural and Hydroponics Applications

For irrigation, the salinity of water is an important factor. If the salinity is too high, salts will accumulate in soil as the water evaporates, which may degrade soil quality and inhibit plant growth. Water with a conductivity of less than 700 uS/cm is acceptable for unrestricted irrigation use, and the use of water with conductivity values greater than 3000 uS/cm should be severely restricted[1].

Conductivity can also be used to monitor nutrient concentrations in liquid fertilizers. A quick check of the conductivity of liquid fertilizers can guard against mistakes such as improper mixing or malfunctioning injectors, protecting crops from wasteful over-fertilization or inadequate fertilizer application.

Similar to fertilizer application, conductivity is used in hydroponics to monitor the concentrations of nutrient solutions. If the conductivity gets too high, indicating a nutrient concentration at toxic levels, plants may be harmed or die. Low conductivities can indicate inadequate nutrient supply. Conductivity monitoring can be used as part of automated nutrient supply systems. In addition to monitoring nutrient supply, conductivity measurements can be used to make sure that salt concentrations are in the range tolerated by the plant.

Conclusion

Conductivity measurements are simple and fast, making them very practical for making routine assessments of the salt concentrations of water. Whether assessing the concentrations of salts, contaminants or nutrients, measuring conductivity can reduce the need for more expensive or time-consuming tests. There are many factors that affect conductivity, such as the concentrations and types of dissolved salts present in the water, so knowledge of the chemistry of the system in question is often necessary for interpreting conductivity measurements.



References

[1] Abrol, I. P., Yadav, J. S. P., & Massoud, F. I. (1988). Salt-affected soils and their management (No. 39). Food & Agriculture Org.

Testing Conductivity

11/15/2017
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With an appropriate instrument, electrical conductivity (EC) measurements are relatively fast and simple. EC measures the ability of water to conduct an electric current, which in turn depends on the concentrations of ions in the solution. Because of this, EC provides useful information about the solution and can be used to estimate its total dissolved solids (TDS).

The conductivity of water is measured using a probe that is inserted into the water. Using the electrodes in the probe and the electronics in the instrument, the instrument is able to measure the conductivity and report a temperature-compensated conductivity value (units of µS/cm are most typical). To ensure an accurate result, the instrument is usually calibrated with one of more standards prior to the measurement.

Calibration of the instrument

To make accurate measurements, a conductivity instrument is usually calibrated using potassium chloride (KCl) solutions of known concentration. Typically, a standard composed of 0.01 M KCl is used, which has a conductivity of 1412 µS/cm at 25°C [1], but a standard that has a conductivity similar to the solutions being analyzed is ideal. For greater accuracy over a wide range of conductivity values, up to 3-5 standards of different KCl concentrations can be used to calibrate the instrument.

Factors affecting conductivity

There are three main factors that affect the conductivity of a solution: the concentrations of ions, the type of ions, and the temperature of the solution.

1) The concentration of dissolved ions. An electrolyte consists of dissolved ions (such as Na+ and Cl-) that carry electrical charges and can move through water. As each ion is able to carry an electrical charge, water with more ions present is able to conduct a greater amount of current. This is the most important of the three main factors.

2) The types of ions in solution. Different ions have different abilities to transmit charge. Inorganic ions such as Na +, K+, Mg+2, Ca+2, HCO 3-, Cl- and SO4-2, tend to conduct electricity well, although each ion has a different ability to conduct electricity. This depends on factors such as the charge of the ion, its size, and its tendency to interact with water molecules. Heavier ions tend to move slower, but small ions can often attract water molecules more strongly, resulting in a slow-moving hydrated ion. For example, the lightweight ion Li + actually moves electricity only about half as well as the heavier K+ ion[2] because of its stronger interaction with water molecules.

Organic substances tend to make poorer electrolytes than inorganic substances largely because they have a relatively weak tendency to dissociate into ions. For example, acetic acid is a weak acid with a tendency to stay in its uncharged CH 3COOH0 form rather than separate into the hydrogen (H +) and acetate (CH3COO-) ions. Because many organic substances are weak acids, the conductivities of solutions containing them will tend to rise as pH increases. This is because organic acids tend to become converted to their ionic forms as the solution becomes more basic.

3) Temperature. This is a relatively small, but significant, effect. Because ions can move faster in warmer water, the conductivity of water increases with rising temperature. Conductivity will increase by approximately 1.9% for each 1°C increase in temperature [1] (or a little more than 1% for each 1°F difference), which makes it necessary to compensate for temperature so that different conductivity measurements can be compared.

Temperature compensation

To make it easier to compare results for samples tested at different temperatures, conductivity measurements are usually reported as temperature-compensated values. This means that the value reported is what the conductivity would be if the temperature was 25°C. For example, the actual conductivity of a solution tested at 20°C will be lower than the reported temperature-compensated value. Temperature compensation is usually done automatically with a built-in thermistor in the conductivity probe. If the conductivity readings are not temperature compensated, especially when the temperature is far away from 25˚C, the results would not be dependable.

Can conductivity be determined without using a conductivity instrument?

As described above, the conductivity of water depends on the type and amounts of charged ions in solution. If the chemical composition of a solution is known, and if the ions present are limited to well-characterized inorganic ions such as Na +, K+, Mg+2, Ca+2, HCO3-, Cl - and SO4-2 or some organic ions, the conductivity of the solution can be calculated based on the conductance properties of each ion. This is most easily accomplished using specialized chemical software such as PHREEQC [3]. However, it is usually simpler and more direct to measure the conductivity with an instrument.


Suggested Apera Instruments Conductivity Meters

Apera Instruments Basic Pocket Conductivity Meter: EC20 Value Pocket Conductivity Tester Kit

Apera Instruments Premium Pocket Conductivity Meter: EC60 Premium EC/TDS/Salinity Pocket Tester Kit

Apera Instruments Premium Pocket Multi-parameter Meter: PC60 Premium Multiparameter (pH/EC/TDS/Salinity/Temp.) Pocket Tester Kit

Apera Instruments Lab-grade Conductivity Meter: EC700 Benchtop Conductivity Meter Kit


References

[1] American Public Health Association (APHA) (2005) Standard methods for examination of water and wastewater, 21st edn. APHA, AWWA, WPCF, Washington.

[2] Haynes, W. M. (2009). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton: CRC Press.

[3] Parkhurst, D.L., and Appelo, C.A.J. (2013), Description of input and examples for PHREEQC version 3--A computer program for speciation, batch- reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p.