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 umhos/cm. Drinking water is generally between 50 to 1500 umhos/cm and domestic wastewater may have conductivities above 10,000 umhos/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 umhos/cm, and measurements taken in waters sampled the GVSU vessels usually range from 110 to 600 umhos/cm.

Instructions for use of a Conductivity Meter:

1. Using specially marked beakers found in the main cabin, obtain samples of water from the water sampling devices (Van Dorn Bottles) located on the rear deck. Use the beaker marked COND T to obtain 250 mL of the top water sample from the Van Dorn bottle marked "T". Use the beaker marked COND B to obtain 250 mL of the bottom water sample from the Van Dorn bottle marked "B". Be sure to match the symbols on the beakers with the same symbol on the Van Dorn bottle (the symbol "T" for top and "B" for bottom).
2. Bring the beakers containing the water samples back to the conductivity lab station. Measure the top water sample first then measure the bottom water sample.
3. Remove the conductivity meter probe from the deionized (D.I.) water storage container. Rinse the probe with D.I. water from the plastic squeeze bottle, catching the rinse water in the large beaker labeled WASTE WATER. Blot away excess D.I. water from the probe with paper toweling before lowering the probe into the top water sample.
4. Place the conductivity meter probe in the beaker with the top water sample. Lower the probe into the water sample so that the tip is completely submerged.
5. Press the ON/OFF (I/O) key to turn on the conductivity meter. When "READY" appears, read and record the readings in uS/cm on the data sheet (COND T).
6. Remove the probe from the top water sample and position the probe over the WASTE WATER beaker. Rinse the probe with D.I. water from the plastic squeeze bottle and blot away excess D.I. water with paper toweling.
7. Repeat steps 4 through 6 with the bottom "B" sample. Record the reading in mS/cm for the bottom sample in the appropriate place (COND B) on the data sheet. Remove the probe from the water and rinse the probe with D.I. water over the WASTE WATER beaker. Place the probe in the beaker of D.I. water.
8. Rinse the sample beakers and stir bar with D.I. water, wipe dry with paper toweling and store as they were when you started.
9. For GLOBE trips repeat steps 3 through 7 two more times and calculate an average of the readings for the top sample and the readings for the bottom sample. Record the averages on the data board.

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