What makes conductivity

Electrical conductivity in water, is defined by the ability for a fluid to pass an electrical current. For example, if you were to compare drinking water with seawater you would find that the conductivity level of seawater is much higher.

Contributing factors to this can be material or sediment in the water, such as salt. However, these can also be contaminants which can have a harmful effect on animals and human health. Conductivity is measured using a unit called siemens S. The value for conductivity is often provided in two basic formats, these are actual and specific conductivity. Actual conductivity will provide the reading of conductivity at the temperature of the measured sample. Conductance is part of conductivity, but it is not a specific measurement on its own.

Electrical conductance is dependent on the length of the conductor, just as resistance is Conductance is measured in mhos or siemens As such, the conductance of water will change with the distance specified.

But as long as the temperature and composition remains the same, the conductivity of water will not change. Salinity is an ambiguous term. As a basic definition, salinity is the total concentration of all dissolved salts in water 4. These electrolytes form ionic particles as they dissolve, each with a positive and negative charge. As such, salinity is a strong contributor to conductivity. While salinity can be measured by a complete chemical analysis, this method is difficult and time consuming Seawater cannot simply be evaporated to a dry salt mass measurement as chlorides are lost during the process More often, salinity is not measured directly, but is instead derived from the conductivity measurement 6.

This is known as practical salinity. These derivations compare the specific conductance of the sample to a salinity standard such as seawater 6. Salinity measurements based on conductivity values are unitless, but are often followed by the notation of practical salinity units psu There are many different dissolved salts that contribute to the salinity of water.

The major ions in seawater with a practical salinity of 35 are: chloride, sodium, magnesium, sulfate, calcium, potassium, bicarbonate and bromine Many of these ions are also present in freshwater sources, but in much smaller amounts 4. The ionic compositions of inland water sources are dependent on the surrounding environment.

Most lakes and rivers have alkali and alkaline earth metal salts, with calcium, magnesium, sodium, carbonates and chlorides making up a high percentage of the ionic composition 4. Freshwater usually has a higher bicarbonate ratio while seawater has greater sodium and chloride concentrations While the Practical Salinity Scale is acceptable in most situations, a new method of salinity measurement was adopted in This method, called TEOS, determines absolute salinity as opposed to the practical salinity derived from conductivity.

Absolute salinity provides an accurate and consistent representation of the thermodynamic state of the system Absolute salinity is both more accurate and more precise than practical salinity and can be used to estimate salinity not only across the ocean, but at greater depths and temperature ranges TEOS is derived from a Gibbs function, which requires more complex calculations, but offers more useful information The units used to measure salinity fluctuate based on application and reporting procedure.

Now salinity values are reported based on the unitless Practical Salinity Scale sometimes denoted in practical salinity units as psu As of , an Absolute Salinity calculation was developed, but is not used for database archives TEOS offers pre-programmed equations to calculate absolute salinity.

All three methods are based on an approximate salinity value of 35 in seawater However, there are some distinctions that must be made. Practical salinity units are dimensionless and are based on conductivity studies of potassium chloride solutions and seawater These studies were done with This north Atlantic sea water was given a set practical salinity of 35 psu The practical salinity scale is considered accurate for values between 2 and 42 psu These are the most common units used, and practical salinity remains the most common salinity value stored for data archives The historical definition of salinity was based on chloride concentration which could be determined by titration This calculation used the following equation:.

This method is only acceptable for seawater, as it is limited in estuaries, brackish and freshwater sources While salinity and chlorinity are proportional in seawater, equations based on this are not accurate in freshwater or when chlorinity ratios change It is consistent with other SI units as a true mass fraction, and it ensures that all thermodynamic relationships density, sound, speed and heat capacity remain consistent Absolute salinity also offers a greater range and more accurate values than other salinity methods when ionic composition is known Total dissolved solids TDS combine the sum of all ion particles that are smaller than 2 microns 0.

This includes all of the disassociated electrolytes that make up salinity concentrations, as well as other compounds such as dissolved organic matter. In wastewater or polluted areas, TDS can include organic solutes such as hydrocarbons and urea in addition to the salt ions While TDS measurements are derived from conductivity, some states, regions and agencies often set a TDS maximum instead of a conductivity limit for water quality Depending on the ionic properties, excessive total dissolved solids can produce toxic effects on fish and fish eggs.

Salmonids exposed to higher than average levels of CaSO4 at various life stages experienced reduced survival and reproduction rates Dissolved solids are also important to aquatic life by keeping cell density balanced In water with a very high TDS concentration, cells will shrink.

TDS can also affect water taste, and often indicates a high alkalinity or hardness TDS can be measured by gravimetry with an evaporation dish or calculated by multiplying a conductivity value by an empirical factor While TDS determination by evaporation is more time-consuming, it is useful when the composition of a water source is not known.

Deriving TDS from conductivity is quicker and suited for both field measurements and continuous monitoring When calculating total dissolved solids from a conductivity measurement, a TDS factor is used. This TDS constant is dependent on the type of solids dissolved in water, and can be changed depending on the water source.

Most conductivity meters and other measurement options will use a common, approximated constant around 0. Likewise, fresh or nearly pure water should have a lower TDS constant closer to 0. Several conductivity meters will accept a constant outside of this range, but it is recommended to reanalyze the sample by evaporation to confirm this ratio As seen in the table below, solutions with the same conductivity value, but different ionic constitutions KCl vs NaCl vs will have different total dissolved solid concentrations.

To make accurate measurements, a conductivity instrument is usually calibrated using potassium chloride KCl solutions of known concentration. Typically, a standard composed of 0. For greater accuracy over a wide range of conductivity values, up to 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.

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. Different ions have different abilities to transmit charge. 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. Organic substances tend to make poorer electrolytes than inorganic substances largely because they have a relatively weak tendency to dissociate into 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.

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. Temperature compensation. To make it easier to compare results for samples tested at different temperatures, conductivity measurements are usually reported as temperature-compensated values. Temperature compensation is usually done automatically with a built-in thermistor in the conductivity probe. 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.

However, it is usually simpler and more direct to measure the conductivity with an instrument. CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Geological Survey Techniques and Methods, book 6, chap. A43, p. 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. These things are the solutes dissolved in water. Don't worry, though—if you swallow a snowflake, it won't hurt you; it may even contain some nice minerals your body needs to stay healthy.

Water stops being an excellent insulator once it starts dissolving substances around it. Salts , such as common table salt sodium chloride NaCl is the one we know best. In chemical terms, salts are ionic compounds composed of cations positively charged ions and anions negatively charged ions.

In solution, these ions essentially cancel each other out so that the solution is electrically neutral without a net charge. Even a small amount of ions in a water solution makes it able to conduct electricity so definitely don't add salt to your "lightning-storm" bathwater. When water contains these ions it will conduct electricity, such as from a lightning bolt or a wire from the wall socket, as the electricity from the source will seek out oppositely-charged ions in the water.

Too bad if there is a human body in the way. Interestingly, if the water contains very large amounts of solutes and ions, then the water becomes such an efficient conductor of electricity that an electrical current may essentially ignore a human body in the water and stick to the better pathway to conduct itself—the masses of ions in the water.

That is why the danger of electrocution in sea water is less than it would be in bathwater. Lucky for hydrologists here at the USGS, water flowing in streams contains extensive amounts of dissolved salts. Otherwise, these two USGS hydrologists might be out of a job. Many water studies include investigating the fish that live in streams, and one way to collect fish for scientific study is to shoot an electrical current through the water to shock the fish "zap 'em and bag 'em".

Want to know more about conductivity and water?