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3. Conductivity/Salinity

Conductivity is expressed in /umho/cm and normally increases with increase in content of ions (solids) in natural water. Distilled water has a conductivity of about 1 /umho/cm and natural waters have conductivity of 20 to 1500 /umho/cm (Boyd, 1979). Conductivity gives a measure of mineralization and richer waters, having higher natrient concentration would have higher values of conductivity. As already indicated a measure of productivity (MEI) is a product of conductivity and depth of water.

Conductivity can be measured by using conductivity meters. Often pH conductivity and salinity - conductivity meters are available, with built-in temperature compensation.

Both the parameters can also be obtained by titration for estimation of chloride (see Practical handouts). Using suitable concentrations of silver nitrate, chloride content in fresh and salt waters can be estimated accurately. The chloride values or chlorinity can be converted to salinity values as shown below:

Salinity ppt = chlorinity ppt × 1.805 + 0.03
where chlorinity is chloride in g/litre

While salinity is the total concentration of all ionic constituents present in a water it is defined, as indicated earlier while composition of sea water was explained.

Chloride content and conductivity are so well correlated that a straight line fitted to a double-log plot of chloride concentration against conductivity can be used as a carlibration curve. Care should be taken that standard sodium chloride and a conductivity meter with built-in temperature-compensation should be used. The same curve is extrapolatable to the lower and higher ranges (sufficiently counter checked) if a multicycle double-log paper is used. At the lower level the conductivity measurement is so sensitive that it can be used to measure chloride leached out from a fish kept in distilled water.

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