Conductivity and Water Quality

Conductivity is a measurement of the ability of an aqueous solution to carry an electrical current. An ion is an atom of an element that has gained or lost an electron which will create a negative or positive state. For example, sodium chloride (table salt) consists of sodium ions (Na+) and chloride ions (Cl-) held together in a crystal. In water it breaks apart into an aqueous solution of sodium and chloride ions. This solution will conduct an electrical current. An equation which shows this is:

Na (atom) + Cl (atom) Na+ Cl - (ionic crystal)

Na+Cl - ( in a water solution) = Na+ (ion) + Cl - (ion)

There are several factors that determine the degree to which water will carry an electrical current. These include:

1) the concentration or number of ions;

2) mobility of the ion;

3) oxidation state (valence) and;

4) temperature of the water.

Resistance, which is an electrical measurement expressed in ohms, is the opposite of conductivity. Conductivity is then expressed in reciprocal ohms. A more convenient unit of measurement in the chemical analysis of water is micromhos. The specific conductance or conductivity measurement is related to ionic strength and does not tell us what specific ions are present.

Methodology: The specific conductance of a sample is measured by a self-contained conductivity electrode.

Environmental Impact: Conductivity is a measurement used to determine a number of applications related to water quality. These are as follows:

1) determining mineralization: this is commonly called total dissolved solids. Total dissolved solids information is used to determine the overall ionic effect in a water source. Certain physiological effects on plants and animals are often affected by the number of available ions in the water.

2) noting variation or changes in natural water and wastewaters quickly;

3) estimating the sample size necessary for other chemical analyses; and

4) determining amounts of chemical reagents or treatment chemicals to be added to a water sample.

Elevated dissolved solids can cause "mineral tastes" in drinking water. Corrosion or encrustation of metallic surfaces by waters high in dissolved solids causes problems with industrial equipment and boilers as well as domestic plumbing, hot water heaters, toilet flushing mechanisms, faucets, and washing machines and dishwashers.

Indirect effects of excess dissolved solids are primarily the elimination of desirable food plants and habitat-forming plant species. Agricultural uses of water for livestock watering are limited by excessive dissolved solids and high dissolved solids can be a problem in water used for irrigation.

Criteria: Water quality criteria have been established only for the mainstem of the Ohio River. The limit is 800 micromhos/cm or 500 mg/L total dissolved solids.

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