What is conductivity?
Electrical conductivity measures the ability of water to conduct electricity, which provides a measure of the substances dissolved in the water. It is the opposite of resistance. Pure, distilled water is a poor conductor of electricity. When salts and other inorganic chemicals dissolve in water, they break down into tiny electrically charged particles called ions. Ions increase the ability of water to conduct electricity. Common ions in water that conduct electricity include sodium, chloride, calcium and magnesium. Because dissolved salts and other inorganic chemicals conduct electrical currents, conductivity increases with increasing salinity. Organic compounds, such as sugars, oils and alcohols, do not form conductive ions.
Why is conductivity important?
Aquatic animals and plants are adapted to a certain range of salinity. Beyond this range, they will be negatively affected and may die. Some animals can handle high salinity, but not low salinity, while others can handle low salinity, but not high salinity.
In addition to its direct effects on aquatic life, salinity has many other important effects on water chemistry and water density.
Electrical conductivity can be used as a general measure of water quality. Each body of water tends to have a relatively constant range of conductivity that, once determined, can be used as a baseline for comparison with conventional conductivity measurements. Significant changes in conductivity may indicate that a discharge or some other source of pollution has entered an aquatic resource. Often, anthropogenic disturbances tend to increase the amount of dissolved solids entering the water, which leads to an increase in conductivity. Water bodies with elevated conductivity may also have other indicators of impairment or alteration.
How is conductivity measured?
Salinity is most often reported in parts per thousand or the equivalent term grams per liter. For example, the average salinity of seawater is 35 ppt, which is equivalent to adding 35 grams of salt to 1 liter of water
Conductivity is reported in units called Siemens or its smaller version, milliSiemens is one thousandth of a Siemens and microSiemens is one millionth of a Siemens. Most commonly a special type of conductivity is used, called specific conductivity.
Both conductivity and salinity are measured by an electrical probe on the data logger. This probe measures how much current is passing through the water. The salinity is then calculated from that value.
Conductivity is determined by measuring how easy it is for the current to flow between two metal plates. These metal plates are called electrodes and are spaced a specific distance apart. The dissolved salt in the solution is attracted to the plate with the opposite charge. In many probes, a four-electrode cell is used. Two of the electrodes measure the current of the solution, while the other two electrodes maintain a constant current between them and are used as a reference.
The best method to determine salinity is chemical analysis of the concentration of different ions in water, such as calcium, sodium, chloride and carbonate. However, since this method is time consuming, tedious and expensive, salinity is estimated based on electrical conductivity. Because salt in water conducts electrical currents, the conductivity will be proportional to the salt concentration. Data loggers use a complex mathematical equation to estimate salinity from conductivity. This equation accounts for the temperature dependence of conductivity.