Instructor's Manual - Conductivity
What is conductivity?
Conductivity or specific conductance is the measure of the water's ability to conduct an electric current. Conductivity depends upon the number of ions or charged particles in the water. The ease or difficulty of the flow of electrical current through liquids makes it possible to divide them into two broad categories: electrolytes and nonelectrolytes. Electricity passes easily through water that is high in electrolytes or ions, and poorly through low electrolyte materials such as pure water or many organic solvents such as alcohol or oil. The opposition to the flow of electricity is called resistance and it is measured in units called ohms. Substances with low resistance and high conductivity pass electricity easily.
How is conductivity measured?
A conductivity meter is used to measure the ability of the water sample to conduct electricity. The specific conductance is measured by passing a current between two electrodes (one centimeter apart) that are placed into a sample of water. The unit of measurement for conductivity is expressed in either microSiemens (uS/cm) or micromhos (umho/cm) which is the reciprocal of the unit of resistance, the ohm. The prefix "micro" means that it is measured in millionths of a mho. MicroSiemens and micromhos are equivalent units. Distilled water has a range of conductivity from 0.5 to 2 uS/cm. Drinking water is generally between 50 to 1500 uS/cm and domestic wastewater may have conductivities above 10,000 uS/cm. The warmer the water, the higher the conductivity with an increase of about 1.9% per Celsius degree. Conductivity is reported at standard temperature of 25.0° C.
What is the significance of conductivity?
Conductivity determinations are useful in aquatic studies because they provide an estimate of dissolved ionic matter in the water. Low values of specific conductance are characteristic of high-quality, oligotrophic (low nutrient) lake waters. High values of specific conductance are observed in eutrophic lakes where plant nutrients (fertilizer) are in greater abundance. Very high values are good indicators of possible pollution sites. For instance, industrial discharges, road salt, and failing septic tanks can raise conductivity. A sudden change in conductivity can indicate a direct discharge or other source of pollution into the water.
Conductivity readings do not provide information the specific ionic composition and concentrations. Water, itself, contains hydrogen (H+) and hydroxide ions (OH-) with relative amounts reflected in the pH readings. Chloride, phosphate, sulfate, and nitrate anions (negative ions) as well as calcium, magnesium, iron, aluminum, and sodium cations (positive ions) contribute to overall conductivity as well.
Lakes and rivers vary in conductivity based on the geology of an area. Water bodies underlain by granite have lower conductivity than those areas of clay soils. Conductivity in rivers in the United States range from 50 to 1,500 uS/cm, and measurements taken in waters sampled the GVSU vessels usually range from 110 to 600 uS/cm.
Page last modified January 10, 2013