Transcript Field Methods of Monitoring Atmospheric Systems

Field Methods of Monitoring
Aquatic Systems
Unit 7 – Conductivity
Copyright © 2006 by DBS
Conductivity
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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
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Resistance of liquid between
electrodes converted according to:
K = L / AR
Where K = conductivity, L =
distance (cm), A = surface area
(cm2), R = resistance (ohms =
siemens S-1)
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Units are μS cm-1
Strongly temperature dependent
(calibration and unknown must be
at same temperature)
Relationship between Conductivity and
Salt Content (TDS)
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Not simple!
Ions of same charge have same conductivity
Natural waters contain ions of differing charge!
For water of similar composition use a conversion
K is
110-180%
TDS
Total salt concentration = A x conductivity (mg l-1)
where A is a constant in range 0.55-0.90
Question
Why isn’t total salt content measured gravimetrically?
Unfeasible to evaporate large quantities of water.
Typical Values
Type
DI
Conductivity
(μS cm-1)
<1
Rainwater
20 - 40
Surface waters
30 - 400
Wastewater
300 - 1000
Typical Values (Cont.)
Water Body
Conductivity
(μS cm-1)
TDS
(mg L-1)
Lake Superior
97
63
Atlantic ocean
43000
35000
Great Salt Lake
158000
230000
Source: http://lakeaccess.org
Question
Calculate the ‘A’ value for the above examples.
Lake Superior = 0.65
Atlantic Ocean = 0.81
Salt Lake = 1.46
Controls
• Natural
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Geology e.g. limestone leads to higher K due to dissolution of Ca2+
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
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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
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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.