Conductivity Conductivity is a measure of the ability of water to pass an electrical current –Affected by presence of +ve and –ve charged ions –Organics have low / negligable conductivity Conductivity is useful as a general measure of water quality –Each water body tends to have a relatively constant range of conductivity –Used as a baseline for comparison with regular conductivity measurements.
Conductivity Cell Resistance of liquid between electrodes converted according to: K = L / AR Where K = conductivity, L = distance (cm), A = surface area (cm 2 ), R = resistance (ohms = siemens S -1 ) Units are μS cm -1 Strongly temperature dependent (calibration and unknown must be at same temperature)
Relationship between Conductivity and Salt Content (TDS) Not simple! Ions of same charge have same conductivity Natural waters contain ions of differing charge! For water of similar composition use a conversion Total salt concentration = A x conductivity (mg l -1 ) where A is a constant in range K is % TDS
Question Why isn’t total salt content measured gravimetrically? Unfeasible to evaporate large quantities of water.
Typical Values (Cont.) Water BodyConductivity (μS cm -1 ) TDS (mg L -1 ) Lake Superior9763 Atlantic ocean Great Salt Lake Source:
Question Calculate the ‘A’ value for the above examples. Lake Superior = 0.65 Atlantic Ocean = 0.81 Salt Lake = 1.46
Controls Natural –Geology e.g. limestone leads to higher K due to dissolution of Ca 2+ –Watershed/lake ratio –Atmospheric (higher near ocean) –Evaporation –MO decomposition of organic matter Man-made –Wastewater input –Agricultural run-off –Run-off from roads e.g. salting
Micro Siemens cm -1 1 µS cm -1 = 1 µmho cm -1 mho = 1/R reflecting the inverse relationship between resistance and conductivity - the higher the resistance of the water, the lower its conductivity
Specific Conductivity Electrical current flow increases with increasing temperature In meters with temperature compensation values are automatically corrected to a standard value of 25°C Technically referred to as specific electrical conductivity
Procedure 1.Prepare the conductivity meter for use according to the manufacturer's directions. 2.Use a conductivity standard solution (usually potassium chloride or sodium chloride) to calibrate the meter for the range that you will be measuring. 3.Rinse the probe with distilled or deionized water. 4.Select the appropriate range beginning with the highest range and working down. Read the conductivity of the water sample. If the reading is in the lower 10 percent of the range, switch to the next lower range. If the conductivity of the sample exceeds the range of the instrument, you may dilute the sample. Be sure to perform the dilution according to the manufacturer's directions because the dilution might not have a simple linear relationship to the conductivity. 5.Rinse the probe with distilled or deionized water and repeat step 4 until finished. Samples that are sent to a lab for conductivity analysis must be tested within 28 days of collection. Keep the samples on ice or refrigerated
Question Biologists who study fish populations stun the animals using electroshocks to the water. Why is it a problem for them when the water is too soft? Difficult to shock the fish when the electric charge doesn’t flow readily.
Text Books Rump, H.H. (2000) Laboratory Manual for the Examination of Water, Waste Water and Soil. Wiley-VCH. Nollet, L.M. and Nollet, M.L. (2000) Handbook of Water Analysis. Marcel Dekker. Keith, L.H. and Keith, K.H. (1996) Compilation of EPA's Sampling and Analysis Methods. CRC Press. Van der Leeden, F., Troise, F.L., and Todd, D.K. (1991) The Water Encyclopedia. Lewis Publishers. Kegley, S.E. and Andrews, J. (1998) The Chemistry of Water. University Science Books. Narayanan, P. (2003) Analysis of environmental pollutants : principles and quantitative methods. Taylor & Francis. Reeve, R.N. (2002) Introduction to environmental analysis. Wiley. Clesceri, L.S., Greenberg, A.E., and Eaton, A.D., eds. (1998) Standard Methods for the Examination of Water and Wastewater, 20th Edition. Published by American Public Health Association, American Water Works Association and Water Environment Federation.