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.)

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 +4

0 to -2Aerobic decay C, H, N, S converted into CO2, H2O, NO3

-, SO42-

Microbial process

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,

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

1 g

O2(aq) = O2(g)

KH = [O2 (aq)]

pO2

Depletion of DO

• 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

• 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

Two methods

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

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 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

• 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- → S4O6

2- +2 I-

1:4

Azid-Winkler Method1. 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.

9. Calculate the DO (mmol O2 and ppm) using the 1:4 mole ration of O2 to S2O32-.

Remove sample from refrigerator ~30 mins prior to analysis

Azid-Winkler Method

Remove sample from refrigerator ~30 mins prior to analysis

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 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

4 mols S2O32-

There 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

• 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

• 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

• 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

O2 + 4H+ + 4e- → H2O2Pb(s) → 2Pb2+

(aq) + 4e-

DO Method

• 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 elementCalibrated by comparing DO concentrations (5-10% samples) measured by the probe to Azide-Winkler method and then correct all samples for any measurement error

Remove sample from refrigerator ~30 mins prior to analysis

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

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 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 oxidationBack 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

http://www.bioscienceinc.com

Accu-TEST Micro-COD System

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

• 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.