Transcript Field Methods of Monitoring Atmospheric Systems

Field Methods of Monitoring
Aquatic Systems
Unit 4 – Dissolved Oxygen
and Oxygen Demand
Copyright © 2006 by DBS
All animal life in natural waters is dependent on the
presence of dissolved oxygen
• While it is not a water quality measurement the level
of DO is indicative of the concentration of nutrients
and organic matter in the water
• Low DO = high concentration OM
• Fish require 5 - 6 mg L-1 for survival
• O2 saturation ranges from 7 mg/L (hot) to 15 mg/L
cold
Oxygen Demand
• The most common substance oxidized by DO in water is organic
matter (plant debris, dead animals etc.)
Microbial process
0 to -2
CH2O(aq) + O2(aq) → CO2(g) + H2O(aq)
Aerobic decay
C, H, N, S
converted into
CO2, H2O, NO3-, SO42-
0 to +4
• DO is also consumed by NH3 and NH4+ in the nitrification process
• Water in streams and rivers is constantly replenished with oxygen
• Stagnant water and deep lakes can have depleted oxygen
Question
What is the opposite of aerobic decay?
Anaerobic decay: final products are CH4, NH3, and H2S
Products are toxic, smelly and flammable..avoid at all costs!
Redox Chemistry in Natural Waters
Concentration of O2 is low (10 ppm average)
At 25 °C,
O2(aq) = O2(g)
KH = [O2 (aq)]
pO2
KH = 1.3 x10-3 mol L-1 atm-1
[O2 (aq)] = (1.3 x10-3 mol L-1 atm-1 ) x 0.21 atm = 2.7 x 10-4 mol L-1
[O2 (aq)] = 2.7 x 10-4 mol L-1 x 32.00 g mol-1
= 8.7 x 10-3 g L-1 x 1000 mg = 8.7 mg L-1 = 8.7 ppm
1g
Depletion of DO
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Temperature (inc)
Pressure (dec)
Salts (inc)
Organic matter (inc)
% Saturation
Pair up the mg/l of dissolved oxygen you measured and the
temperature of the water in degrees C. Draw a straight line between
the water temperature and the mg/l of dissolved oxygen
The percent saturation is the value where the line intercepts the
saturation scale
Question
Which of the following rivers would take up oxygen more quickly?
Which would have the highest oxygen demand?
1. A fast-flowing mountain stream
2. A slow moving river in a heavily industrialized area
3. A slow moving river in unspoilt countryside
Turbulence in the mountain stream would ensure rapid uptake of
O2 and the water would be saturated with O2. Unlikely to be large
amounts of OM from vegetation or industrial effluent. O2 demand
would be low.
Slow flowing rivers take up O2 more slowly. O2 consuming
effluent and vegetation would increase O2 demand.
Oxygen Analysis
Two methods
• Dissolved Oxygen
– Direct measurement of O2 concentration. Gives an indication
of the health of the water body at a particular location and
time
– Less useful for determining the overall health as O2 level
varies dramatically
• Oxygen demand
– Measurement of the amount of material which, given time,
could deplete the O2 level
– Useful for determining the overall health of the water body
since O2 demand is unlikely to change suddenly
Dissolved Oxygen
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Titration (Winkler method) or electrode
Sample transport is a problem – agitation
introduces more O2
Use special collection bottle for BOD
– Point bottle downstream
– Gently lower into water
– Cap underwater when full
– No air bubbles
– If sampling apparatus is used
cannot be poured into bottle!
Flared mouth forms a water seal to
prevent air from being drawn into the
bottle during incubation
Shoulder radius provides an interior
shape that sweeps entrapped air out of
the stopper opening
QA/QC Considerations
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Quality Control
– Measure DO immediately after taking sample (on site if possible)
– Do not shake sample
– Do not change temperature
– Do not dilute sample
– Do not let air in while sampling or measuring
– Starch supports bacterial growth, shelf lie is 1 month unless a
preservative is added
– Sodium thiosulfate (6.205 g Na2S2O3.5H2O with 0.4 g NaOH in 1 L
H2O) must be standardized against the primary standard 0.0021 M
potassium bi-iodate (812.4 mg KH(IO3)2 in 1 L H2O)
Source: http://www.ecy.wa.gov/programs/wq/
plants/management/joysmanual/4oxygen.html
Azide-Winkler Method
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O2 is fixed after sampling by reaction with Mn2+ (MnSO4) together with
alkaline iodide/azide mixture
Mn2+ + 2OH- + ½ O2 → MnO2(s) + H2O
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I- is needed for titration, N3- prevents NO2- and Fe3+ interference
(production of excess I2 from KI)
After transport to lab sample is acidified with H2SO4
(dissolves Mn4+ floc), and Mn4+ oxidizes iodine ions:
MnO2 + 2I- + 4H+ → Mn2+ + I2 + 2H2O
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I2 is then titrated with thiosulfate and starch indicator
I2 + 2S2O32- → S4O62- +2 I-
1:4
Remove sample from refrigerator ~30 mins prior to analysis
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Fill a 300-mL glass stoppered BOD bottle with sample water. Remember – no bubbles!
(siphon, allow to overflow 3 times). Turn upside down to remove water stuck in the well.
Immediately add 2mL of manganese sulfate to the collection bottle by inserting the calibrated
pipette just below the surface of the liquid. (If the reagent is added above the sample surface,
you will introduce oxygen into the sample.) Squeeze the pipette slowly so no bubbles are
introduced via the pipette.
Add 2 mL of alkali-iodide-azide reagent in the same manner.
Stopper the bottle with care to be sure no air is introduced. Mix the sample by inverting
several times. Discard the sample and start over if any air bubles are seen. If oxygen is
present, a brownish-orange cloud of precipitate or floc will appear. When this floc has settled
to the bottom, mix the sample by turning it upside down several times and let it settle again.
Add 2 mL of concentrated sulfuric acid via a pipette held just above the surface of the
sample. Carefully stopper and invert several times to dissolve the floc. At this point, the
sample is "fixed" and can be stored for up to 8 hours if kept in a cool, dark place. As an
added precaution, squirt distilled water along the stopper, and cap the bottle with aluminum
foil and a rubber band during the storage period.
In a glass flask, titrate 200(?) mL of the sample with sodium thiosulfate to a pale straw color.
Titrate by slowly dropping titrant solution from a calibrated pipette into the flask and
continually stirring or swirling the sample water.
Add 2 mL of starch solution so a blue color forms.
Continue slowly titrating until the sample turns clear. As this experiment reaches the
endpoint, it will take only one drop of the tritrant to eliminate the blue color. Be especially
careful that each drop is fully mixed into the sample before adding the next. It is sometimes
helpful to hold the flask up to a white sheet of paper to check for absence of the blue color.
Calculate the DO (mmol O2 and ppm) using the 1:4 mole ration of O2 to S2O32-.
Azid-Winkler Method
Remove sample from refrigerator ~30 mins prior to analysis
Azid-Winkler Method
Example Calculation
1 mL of 0.025 M S2O32- is required to reach the blue starch end-point of a 200 mL sample. Calculate the
moles of O2 dissolved in the sample, and the mg/L DO.
1 mol O2 = 4 mols S2O32mols S2O32- x 1 mol O2
4 mols S2O32-
= mols O2
0.025 mol/L x 1 mL x 1 L / 1000 mL = 2.5 x 10-5 mols S2O322.5 x 10-5 mols S2O32- x mol O2 = 6.25 x 10-6 mols O2
4 mols S2O32There are 6.25 x 10-6 mols of O2 in the 200 mL sample
6.25 x 10-6 mol = 3.125 x 10-5 mol O2 / L x 32 g / mol = 1 x 10-3 g /L = 1 mg O2 / L = 1 ppm DO
0.2 L
QA/QC Considerations
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To test the method, you need to have samples with a known oxygen
concentration
– 100 % saturation solution prepared by bubbling air into distilled
water
– A zero DO solution can be made by adding excess sodium sulfite
and a trace of cobalt chloride to a sample
– In a professional lab, a calibration standard would be analyzed with
each batch of samples run
Randomly select 5 to 10 percent of the samples for duplicate laboratory
analysis
DO Meter and Probe
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Probe uses a thin O2-permeable membrane stretched over electrodes
The O2 diffusing through the membrane is reduced with contact with the cathode, flow to
anode, oxidizing it, generating a current measured by the meter
O2 + 4H+ + 4e- → H2O
2Pb(s) → 2Pb2+(aq) + 4e-
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Flow of e- from cathode to anode is
proportional to O2 concentration
passing through the membrane
The electrode requires a constant
current of water across the surface
since O2 is consumed
Less precise and less accurate than
the Winkler method, particularly at
concentrations below 1ppm
DO Method
Remove sample from refrigerator ~30 mins prior to analysis
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Calibrate the probe
Place the probe below the surface of the water
Set the meter to measure temperature and allow the
temperature reading to stabilise
Switch the meter to 'dissolved oxygen‘
For saline waters, measure electrical conductivity level or use
correction feature
Re-test water to obtain a field replicate result
NOTE: The probe needs to be gently stirred to aid water
movement across the membrane
DO probes ruined through deterioration of the membrane, trapping of air bubbles
under the membrane, and contamination of the sensing element
Calibrated by comparing DO concentrations (5-10% samples) measured by the probe
to Azide-Winkler method and then correct all samples for any measurement error
Biochemical Oxygen Demand - BOD
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DO oxidizes organics and inorganics altering their chemical and
physical states and their capacity as a nuisance to the customer
Measurement of DO is the basis for the BOD test in wastewaters
Biochemical Oxygen Demand - BOD
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The capacity of the organic and biological matter in a sample of natural
water to consume oxygen, a process usually catalyzed by bacteria, is
called BOD
Procedure: Take two samples (completely filled) measure DO of the
first and store the second at 20 °C, pH 6.5-8.5, in the dark. Measure O2
content of second bottle after 5 days. The difference is the BOD
– BOD5 corresponds to about 80% of the actual value. It is not
practical to measure the BOD for an infinite period of time
– Surface waters have a BOD ~ 0.7 mg/L – significantly lower
than the solubility of O2 in water (8.7 mg/L)
– If O2 level is 0 after 5 days it is not possible to tell what the
BOD level is. Dilute the original sample by a factor that results
in a final DO level of at least 2 mg L-1
High-Throughput Labs
• Dilution-BOD (EPA Method)
• BOD Self-Check (Hg free)
– MOs in the sample consume the
oxygen and form CO2
– absorbed by NaOH creating a
vacuum
– read directly as a measured
value in mg/l BOD
Chemical Oxygen Demand (COD)
O2 + 4H+ + 4e- → 2H2O
• Dichromate ion, Cr2O72- dissolved in sulfuric acid is a powerful
oxidizing agent. It is used as an oxidant to determine COD
Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7 H2O
• Excess dichromate is added to achieve complete oxidation
Back titration with Fe2+ gives the desired endpoint value
# moles of O2 consumed = 6/4 x (#moles Cr2O7 consumed)
Note: Cr2O72- is a powerful oxidizing agent and can oxidize species
that are not usually oxidized by O2 - hence gives an upper limit
Question
A 25 mL sample of river water was titrated with 0.0010 M Na2Cr2O7
and required 8.7 mL to reach the endpoint.
What is the COD (mg O2/L)?
No. moles Cr2O72- = 0.0010 mol L-1 x (8.7 x 10-3 L) = 8.7 x 10-6 mols
No. moles O2 = 1.5 moles Cr2O72- = 1.5 x (8.7 x 10-6 mols)
= 1.3 x 10-5 mols O2
1.3 x 10-5 mol x 32.00 g mol-1 = 4.2 x 10-4 g
0.42 mg / 0.025 L= 17 mg/L
High-Throughput Labs
• Spectrophotometric COD
determination at 620 nm using
microscale quantities of chemicals
• 2 mL aliquots heated in a tube with
premixed reagents
Accu-TEST Micro-COD
System
http://www.bioscienceinc.com
Comparison of BOD and COD
BOD
COD*
5 days
Rapid
Closely related to natural
processes
Less relationship to natural
process
Difficult to reproduce
Good reproducibility
Care has to be taken with
polluted water
Can analyze heavily polluted
water
*Affected by inorganic reducing or oxidizing agents
Question
On the basis of these comparisons suggest appropriate
applications of the two techniques
BOD – long-term monitoring of natural waters
COD – rapid analysis of polluted samples e.g. industrial effluent
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.