Conductivity, Salinity & Total Dissolved Solids - Environmental Measurement Systems
When ions are present in water, the water is able to conduct electricity. The higher the concentration of ions in solution, the higher its conductivity will be. Conductivity depends on the concentration of ions as they are the charge carriers which are responsible for the conduction. So, the conductivity. relationship between conductivity and strong and weak electrolytes. To do this, you will add Conductivity is a measure of the concentration of ions in solution.
However, the ionic composition should be considered if calculating total dissolved solids. If a project allows for it, the TDS constant should be determined for each specific site based on known ionic constituents in the water 6. Why is Conductivity Important? Factors that affect water volume like heavy rain or evaporation affect conductivity.
Runoff or flooding over soils that are high in salts or minerals can cause a spike in conductivity despite the increase in water flow. Conductivity, in particular specific conductance, is one of the most useful and commonly measured water quality parameters 3. In addition to being the basis of most salinity and total dissolved solids calculations, conductivity is an early indicator of change in a water system.
Most bodies of water maintain a fairly constant conductivity that can be used as a baseline of comparison to future measurements 1. Significant change, whether it is due to natural flooding, evaporation or man-made pollution can be very detrimental to water quality. Seawater cannot hold as much dissolved oxygen as freshwater due to its high salinity.
Conductivity and salinity have a strong correlation 3. As conductivity is easier to measure, it is used in algorithms estimating salinity and TDS, both of which affect water quality and aquatic life.
Salinity is important in particular as it affects dissolved oxygen solubility 3. The higher the salinity level, the lower the dissolved oxygen concentration. This means that, on average, seawater has a lower dissolved oxygen concentration than freshwater sources.
Aquatic Organism Tolerance Euryhaline including anadromous and catadromous species have the widest salinity tolerance range as they travel between both saltwater and freshwater. Most aquatic organisms can only tolerate a specific salinity range The physiological adaption of each species is determined by the salinity of its surrounding environment. Most species of fish are stenohaline, or exclusively freshwater or exclusively saltwater However, there are a few organisms that can adapt to a range of salinities.
These euryhaline organisms can be anadromous, catadromous or true euryhaline. Anadromous organisms live in saltwater but spawn in freshwater. Catadromous species are the opposite — they live in freshwater and migrate to saltwater to spawn True euryhaline species can be found in saltwater or freshwater at any point in their life cycle Estuarine organisms are true euryhaline.
Euryhaline species live in or travel through estuaries, where saline zonation is evident. Salinity levels in an estuary can vary from freshwater to seawater over a short distance While euryhaline species can comfortably travel across these zones, stenohaline organisms cannot and will only be found at one end of the estuary or the other. Species such as sea stars and sea cucumbers cannot tolerate low salinity levels, and while coastal, will not be found within many estuaries Some aquatic organisms can even be sensitive to the ionic composition of the water.
An influx of a specific salt can negatively affect a species, regardless of whether the salinity levels remain within an acceptable range Most aquatic organisms prefer either freshwater or saltwater.Conductivity and Concentration
Few species traverse between salinity gradients, and fewer still tolerate daily salinity fluctuations. Salinity tolerances depend on the osmotic processes within an organism.
Fish and other aquatic life that live in fresh water low-conductivity are hyperosmotic Thus these organisms maintain higher internal ionic concentrations than the surrounding water On the other side of the spectrum, saltwater high-conductivity organisms are hypoosmotic and maintain a lower internal ionic concentration than seawater. Euryhaline organisms are able to adapt their bodies to the changing salt levels.
Each group of organisms has adapted to the ionic concentrations of their respective environments, and will absorb or excrete salts as needed Altering the conductivity of the environment by increasing or decreasing salt levels will negatively affect the metabolic abilities of the organisms. Even altering the type of ion such as potassium for sodium can be detrimental to aquatic life if their biological processes cannot deal with the different ion Conductivity Change can Indicate Pollution Oil or hydrocarbons can reduce the conductivity of water.
Lamiot via Wikimedia Commons A sudden increase or decrease in conductivity in a body of water can indicate pollution.
Agricultural runoff or a sewage leak will increase conductivity due to the additional chloride, phosphate and nitrate ions 1. An oil spill or addition of other organic compounds would decrease conductivity as these elements do not break down into ions In both cases, the additional dissolved solids will have a negative impact on water quality. Salinity affects water density. The higher the dissolved salt concentration, the higher the density of water 4. The increase in density with salt levels is one of the driving forces behind ocean circulation When sea ice forms near the polar regions, it does not include the salt ions.
Instead, the water molecules freeze, forcing the salt into pockets of briny water This brine eventually drains out of the ice, leaving behind an air pocket and increasing the salinity of the water surrounding the ice.
As this saline water is denser than the surrounding water, it sinks, creating a convection pattern that can influence ocean circulation for hundreds of kilometers Conductivity and salinity vary greatly between different bodies of water. Most freshwater streams and lakes have low salinity and conductivity values.
The oceans have a high conductivity and salinity due to the high number of the dissolved salts present. Freshwater Conductivity Sources Many different sources can contribute to the total dissolved solids level in water. In streams and rivers, normal conductivity levels come from the surrounding geology 1. Clay soils will contribute to conductivity, while granite bedrock will not 1. The minerals in clay will ionize as they dissolve, while granite remains inert. Likewise, groundwater inflows will contribute to the conductivity of the stream or river depending on the geology that the groundwater flows through.
Groundwater that is heavily ionized from dissolved minerals will increase the conductivity of the water into which it flows. Saltwater Conductivity Sources Most of the salt in the ocean comes from runoff, sediment and tectonic activity Rain contains carbonic acid, which can contribute to rock erosion.
As rain flows over rocks and soil, the minerals and salts are broken down into ions and are carried along, eventually reaching the ocean Hydrothermal vents along the bottom of the ocean also contribute dissolved minerals As hot water seeps out of the vents, it releases minerals with it.
Submarine volcanoes can spew dissolved minerals and carbon dioxide into the ocean The dissolved carbon dioxide can become carbonic acid which can erode rocks on the surrounding seafloor and add to the salinity. As water evaporates off the surface of the ocean, the salts from these sources are left behind to accumulate over millions of years Discharges such as pollution can also contribute to salinity and TDS, as wastewater effluent increases salt ions and an oil spill increases total dissolved solids 1.
When does Conductivity Fluctuate? Water flow and water level changes can also contribute to conductivity through their impact on salinity. Water temperature can cause conductivity levels to fluctuate daily. In addition to its direct effect on conductivity, temperature also influences water density, which leads to stratification.
Stratified water can have different conductivity values at different depths. Water flow, whether it is from a spring, groundwater, rain, confluence or other sources can affect the salinity and conductivity of water.
Likewise, reductions in flow from dams or river diversions can also alter conductivity levels Water level changes, such as tidal stages and evaporation will cause salinity and conductivity levels to fluctuate as well. Conductivity and Temperature Conductivity is temperature dependent. When water temperature increases, so will conductivity 3. Temperature affects conductivity by increasing ionic mobility as well as the solubility of many salts and minerals This can be seen in diurnal variations as a body of water warms up due to sunlight, and conductivity increases and then cools down at night decreasing conductivity.
This standardized reporting method is called specific conductance 1. Seasonal variations in conductivity, while affected by average temperatures, are also affected by waterflow.
In some rivers, as spring often has the highest flow volume, conductivity can be lower at that time than in the winter despite the differences in temperature In water with little to no inflow, seasonal averages are more dependent on temperature and evaporation.
Conductivity and Water Flow The effect of water flow on conductivity and salinity values is fairly basic. If the inflow is a freshwater source, it will decrease salinity and conductivity values Freshwater sources include springs, snowmelt, clear, clean streams and fresh groundwater On the other side of the spectrum, highly mineralized groundwater inflows will increase conductivity and salinity 1.
Agricultural runoff, in addition to being high in nutrients, often has a higher concentration of dissolved solids that can influence conductivity For both freshwater and mineralized water, the higher the flow volume, the more it will affect salinity and conductivity Rain itself can have a higher conductivity than pure water due to the incorporation of gases and dust particles However, heavy rainfall can decrease the conductivity of a body of water as it dilutes the current salinity concentration Flooding can increase conductivity when it washes salts and minerals from the soil into a water source.
If heavy rainfall or another major weather event contributes to flooding, the effect on conductivity depends on the water body and surrounding soil. In areas with dry and wet seasons, conductivity usually drops overall during the wet season due to the dilution of the water source Though the overall conductivity is lower for the season, there are often conductivity spikes as water initially enters a floodplain.
If a floodplain contains nutrient-rich or mineralized soil, previously dry salt ions can enter solution as it is flooded, raising the conductivity of water If coastal water floods, the opposite effect can occur.
Though turbidity will increase, the conductivity of water often decreases during a coastal flood Seawater will pick up suspended solids and nutrients from the soil, but can also deposit its salts on land, decreasing the conductivity of the water Dams and river diversions affect conductivity by reducing the natural volume of water flow to an area.
When this flow is diverted, the effect of additional freshwater lowering conductivity is minimized Areas downstream of a dam or a river diversion will have an altered conductivity value due to the lessened inflow Conductivity and Water Level As water flow fluctuates in an estuary, so will salinity levels. The conductivity of water due to water level fluctuations is often directly connected to water flow.
Conductivity and salinity fluctuations due to water level changes are most noticeable in estuaries. As tides rise, saltwater from the ocean is pushed into an estuary, raising salinity and conductivity values When the tide falls, the saltwater is pulled back toward the ocean, lowering conductivity and salinity Evaporation can cause salinity concentrations to rise. As the water level lowers, the ions present become concentrated, contributing to higher conductivity levels This is why conductivity and salinity values often increase in summer due to lower flow volume and evaporation On the other side of the scale, rain can increase water volume and level, lowering conductivity Salinity and Stratification Temperature and salinity levels alter water density, and thus contribute to water column stratification Just as a decrease in temperature increases water density, an increase in salinity will produce the same result.
Vertical stratification due to salinity. Deeper water has a greater density and higher salinity than the surface water. Stratification can be vertical through the water column seen in lakes and oceansor horizontal, as seen in some estuaries 8. These strata are separated by an boundary known as a halocline 9. The halocline divides layers of water with different salinity levels 9.
When salinity levels differ by a great amount often due to a particularly fresh or saline inflowa halocline develops A halocline often coincides with a thermocline temperature boundary and a pycnocline density boundary These clines mark the depth at which water properties such as salinity, temperature and density undergo a sharp change. Estuaries are unique in that they can have horizontal or vertical haloclines. Vertical haloclines are present when salinity levels decrease as the water moves into the estuary from the open ocean 8.
Vertical haloclines often occur when tides are strong enough to mix the water column vertically for a uniform salinity, but levels differ between the freshwater and oceanic sides of the estuary 8. Estuaries can stratify horizontally between a freshwater source and the saline ocean. Horizontal stratification is present in estuaries where tides are weak. The incoming freshwater from rivers can then float over the denser seawater and little mixing occurs Horizontal stratification also exists in the open ocean due to salinity and temperature gradients.
The salinity of an inflow can contribute to stratification. A pycnometer is a device that is used to accurately measure the density of a solution.
A pycnometer is a small metal cylinder with a close-fitting metal stopper that has a capillary hole through it. It holds a specific amount of liquid so any excess liquid is expelled through this hole. A pycnometer also has a cylindrical component that binds the cap and the body of the pycnometer together. To measure the density of a compound, the pycnometer is first filled with distilled water because its density is known and its volume can be found with the following equation: Since the volume of the pycnometer is always constant, the following equation can be obtained: Once density is known, the density-concentration graph can be plotted and by performing linear interpolation: Conductivity is a measure of the number of free moving ions in a solution.
The more concentrated a solution is, the more conducting ions the solution will have and thus the solution will have a higher conductivity.
Conductivity, Salinity & Total Dissolved Solids
Therefore, conductivity should be in a direct linear relationship with concentration. The following equation represents this relationship: Once the composition of the compound is known, the conductivity of the unknown solution can be compared to that of three other solutions of the same salt but of different concentrations.
By linear interpolation in a conductivity-concentration graph, the specific concentration of the unknown solution can be found. Solutions of different concentrations can be obtained through dilution of a standard solution that was prepared.
A set amount of mass of salt is first measured using an electronic balance and used to create a mL solution in a volumetric flask. This would act as the standard solution with the concentration calculated using the formula: A second solution of the same composition but different concentration can then be obtained using dilution. A fixed amount of the standard solution is extracted using a pipette; for higher accuracy and added to a new and clean volumetric flask before filling up the flask with distilled water to form a new diluted solution of volume mL.
The theory used in dilution is that the number of moles in the extracted volume of the standard solution is the same as new solution formed. Therefore, as a fixed amount of volume is extracted from the standard solution, a fraction of the moles of the standard is being transferred into the new solution. When the solution is made into a mL solution by adding distilled water, the number of moles in the flask is retained but a decrease in concentration is observed due to the increase in volume.
Hence, since the number of moles remains the same in both solutions but the volume increases, concentration decreases. This relationship can be shown by the following equation: The relationship between concentration and conductivity can then be found by measuring the conductivity of these solutions.
A conductivity meter is used for the accurate measurement of conductivity. Approximately 5mL of the solution would be poured out into a smaller cup and its conductivity is measured.
Since the 5mL solution is taken to represent the conductivity of the entire solution, extra care has to be taken to ensure that the solution was swirled thoroughly.
Also, the metal rods on the conductivity meter that takes the measurements have to be cleaned and dried properly. Contamination of the rod will lead to errors in the reading of conductivity while excess distilled water left on the rods after drying may dilute the solution and would defeat the purpose of the experiment. With the results obtained, a conductivity-concentration curve can be constructed and linear interpolation can be carried out to find the conductivity of the unknown solution.
This acts as the standard solution for the salt.
Conductivity Vs. Concentration | Sciencing
Repeat this two more times until there is 30mL of standard solution in the flask. This is the first dilution needed of the standard salt solution.
Now there should be three salt solutions of different concentrations. It is important to label each of the volumetric flasks. Determining Conductivity of Solutions 1 The metal rods on the conductivity meter were washed with distilled water 2 The rods were then wiped dry with Kimwipes.
Assuming the conductivity of the extracted solution to be that of the standard solution. Weighing the Solutions 1 Label, clean with RO water, dry and weigh a pycnometer 2 Fill pycnometer with water, dry with a Kimwipe the outside surface of pycnometer and weigh it.
Make sure to dry every spot where there may be any fluid bottom, treads, etc. The presence of fluids anywhere other than inside the pycnometer will influence the weight and produce error. Furthermore, it is key that the droplet at the tip of the pycnometer remains somewhat constant throughout each measurement to ensure a constant volume.
Lastly, do not put the Kimwipe directly into the hole of the pycnometer because it will absorb fluid found inside the pycnometer. Reference source not found. The table suggests that a greater concentration of salt leads to greater density of Table 4 — Densities of Compounds at solution. This falls within reason because as is 22OC shown in Table 4, the densities Sigma-Aldrich of Compound Density all the salts are well above the density of water.
Another trend observed from Error! Reference source not Na2SO4 2.