1. Field Methods of Monitoring Aquatic Systems
Unit 4 – Dissolved Oxygen and Oxygen Demand
Copyright © 2006 by DBS

2. All animal life in natural waters is dependent on the presence of dissolved oxygen

3. 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 mg L-1 for survival
O2 saturation ranges from 7 mg/L (hot) to 15 mg/L cold

4. CH2O(aq) + O2(aq) → CO2(g) + H2O(aq)
Oxygen Demand
The most common substance oxidized by DO in water is organic matter (plant debris, dead animals etc.)
CH2O(aq) + O2(aq) → CO2(g) + H2O(aq)
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
0 to -2
Microbial process
Aerobic decay C, H, N, S converted into CO2, H2O, NO3-, SO42-
0 to +4

5. What is the opposite of aerobic decay?
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!

6. Redox Chemistry in Natural Waters
O2(aq) = O2(g)
KH = [O2 (aq)]
pO2
Concentration of O2 is low (10 ppm average)
At 25 °C,
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 g mol-1
= 8.7 x 10-3 g L-1 x 1000 mg = 8.7 mg L-1 = 8.7 ppm
1 g

7. Depletion of DO Temperature (inc) Pressure (dec) Salts (inc)
Organic matter (inc)

8. % 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

9. 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.

10. Oxygen Analysis Dissolved Oxygen Oxygen demand
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

11. Dissolved Oxygen 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

12. QA/QC Considerations 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 M potassium bi-iodate (812.4 mg KH(IO3)2 in 1 L H2O)

13. Source: http://www. ecy. wa
Source: plants/management/joysmanual/4oxygen.html

14. Mn2+ + 2OH- + ½ O2 → MnO2(s) + H2O
Azide-Winkler Method
O2 is fixed after sampling by reaction with Mn2+ (MnSO4) together with alkaline iodide/azide mixture
Mn2+ + 2OH- + ½ O2 → MnO2(s) + H2O
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
I2 is then titrated with thiosulfate and starch indicator
I2 + 2S2O32- → S4O I-
1:4

15. Remove sample from refrigerator ~30 mins prior to analysis
Azid-Winkler Method
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.
2. 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.
3. Add 2 mL of alkali-iodide-azide reagent in the same manner.
4. 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.
5. 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.
6. 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.
7. Add 2 mL of starch solution so a blue color forms.
8. 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-.

16. Remove sample from refrigerator ~30 mins prior to analysis
Azid-Winkler Method

17. Example Calculation
1 mL of 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 S2O32-
mols S2O32- x 1 mol O2 = mols O2
4 mols S2O32-
0.025 mol/L x 1 mL x 1 L / 1000 mL = 2.5 x 10-5 mols S2O32-
2.5 x 10-5 mols S2O32- x mol O2 = 6.25 x 10-6 mols O2
There are 6.25 x 10-6 mols of O2 in the 200 mL sample
6.25 x 10-6 mol = 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

18. QA/QC Considerations
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

19. DO Meter and Probe
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-
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
Extech Instruments model

20. DO Method Remove sample from refrigerator ~30 mins prior to analysis
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

21. Biochemical Oxygen Demand - BOD
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

22. Biochemical Oxygen Demand - BOD
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 , 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

23. 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

24. 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
Cr2O H+ + 6e- → 2Cr 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

25. Question
A 25 mL sample of river water was titrated with M Na2Cr2O7 and required 8.7 mL to reach the endpoint. What is the COD (mg O2/L)?
No. moles Cr2O72- = 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 g mol-1 = 4.2 x 10-4 g
0.42 mg / L= 17 mg/L

26. 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

27. Comparison of BOD and 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

28. 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

29. 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.