CE 420/520 Environmental Engineering Chemistry
Chemical Oxidation - Chemical Oxygen Demand (COD)
Laboratory Safety and Hazardous Waste Handling
Chemicals used in this laboratory are carcinogens and corrosives and hazardous wastes are generated. Observe the
following precautions:
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Wear safety glasses or goggles at all times when in the laboratory
Wear a laboratory coat
Wear the disposable gloves (provided) when setting up or titrating COD
Put all waste from the COD test in the waste jug (green tag) provided for COD waste. This includes
titrated solutions as well as any waste from flushing the dichromate or digestion acid dispensers.
Do not dump anything from the COD experiment down the drain.
Purpose
To demonstrate the application of oxidation-reduction reactions in the determination of the oxygen demand of
organic matter in water
Principles
The chemical oxygen demand (COD) of a compound is measured in terms of the amount of potassium dichromate
(K2Cr2O7) reduced by the sample during 2 hour of reflux in a medium of boiling 50% H2SO4 and in the presence of
a Ag2SO4 catalyst. Essentially complete oxidation to CO2, H2O, NH4+, H3PO4, SO42- of the organic compound is
obtained under the conditions of the reaction in the presence of the catalyst. Straight chain carboxylic acids are not
well oxidized in the absence of Ag2SO4 catalyst and complete oxidation is not obtained even in the presence of it.
The stoichiometry of the oxidation reduction reaction is as follows:
heat
CnHaOb + c Cr2O72- + 8c H+
=== > n CO2 + (a+8c)/2 H2O + 2c Cr3+
where
c = 2/3 n + a/6 - b/3
The oxidation is accomplished through reduction of dichromate to chromic acid ions:
6e- + 14H+ + Cr2O72- == > 2Cr3+ + 7H2O
Eo = 1.33 volts
When Ag2SO4 is used as a catalyst, chloride ion is precipitated as AgCl (s) and is subject to variable and
unpredictable oxidation. When Ag2SO4 is not used, chloride is oxidized to Cl2:
Cr2O72- + 6 Cl- + 14 H+ == > 2Cr3+ + 3Cl2 + 7H2O
Chloride is largely removed from the sphere of activity by complexing as HgCl2o if mercuric salt is present.
Autooxidation of dichromate and organic impurities in the reaction are accounted for by running a blank on distilled
water.
After digestion, the residual dichromate concentration is determined by titrating the dichromate with a reducing
agent Fe2+. Oxidation of Fe2+ proceeds as follows:
Fe2+ == > Fe3+ + e-
Eo = - 0.77 Volts
The endpoint of the ferrous titration of dichromate is determined colorimetrical using Ferroin indicator. Ferroin
contains 1, 10 phenanthroline which forms a red colored complex with all ferrous ions. At the endpoint after the
dichromate (orange) has been reduced to Cr3+ (green) free Fe2+ ions complex with the Ferroin indicator to form a
reddish brown color.
Experimental Procedure
The following samples will be used:
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Wastewater
Glucose solution (300 mg/L)
Sodium acetate (750 mg/L)
Sodium acetate (750 mg/L) - sulfuric acid without Ag2SO4
Distilled water (blank)
There are 2 titration stations for COD. COD requires 2 hr digestion in the oven. Set up the tubes, digest them in the
oven for 2 hrs, and remove to cool. The tubes will probably not be cool enough to titrate during this 3 hr laboratory
period. The laboratory will be open for your use for you to finish COD .
COD (Standard Methods for Examination of Water and Wastewater, 20th ed., 1998, 5220 C. Closed Reflux,
Titrimetric Method). The range of this test is up to 400 mg/L COD. Include a dilution if you think it will be greater
than 400 mg/L.
Set up
(Suggestion: prepare duplicate samples in case you over-titrate) Label the tubes with the marker provided. Pipet 5
ml of sample (or diluted sample) into digestion tubes. Using the dispensing bottles, add 3.00 ml of standard
potassium dichromate digestion solution, followed by 7.00 ml of sulfuric acid reagent. Dispense the reagents down
the inner wall of the tube by tipping the tube as you dispense. Cap the tube firmly, and immediately and invert 5
times to mix. Place in a rack in a glass tray (2 workstations must share one rack and tray) and digest in the 150°
oven for 2 hr. Remove and allow to cool. {Tray is hot. Protect your hands.)
Titration
Standardize the 0.10M ferrous ammonium sulfate titrant (FAS): Dispense 6.00 ml of dichromate reagent (2 pumps)
into a small flask; add 10 ml of distilled water and cool to room temperature. Add 1 or 2 drops of ferroin indicator
and titrate with FAS. The color will progress from yellow to green, and the endpoint is a sudden change from green
to brown. Use the average of 2 standardizations. Dump the waste in the waste bottle.
M FAS = (mL dichromate / mL FAS) * 0.1000
Put a small stir bar in each tube to be titrated. Put the tube in a small conical flask (for convenient support). Hold
the tube upright and adjust the stirring motor. Titrate to the green-to-brown endpoint as above. Dump the waste in
the waste bottle using the filter funnel to recover the stir bar.
COD, mg O2/L = [(mL FAS, blank)-(mL FAS, sample)][M FAS][8000]/mL sample
Cleanup
Remove the filter funnel from the waste jug and wash with tap water followed by distilled water. Put the cap on the
waste jug. Wash tubes and caps with tap water followed by distilled water. Place tubes upside down in a rack to
dry. Dry caps upside down on a paper towel. Use acetone (squeeze bottle) and a Kim Wipe to remove your
identification markings from the tubes.
Questions
1.
Report the COD of the municipal wastewater? What are the compounds in the wastewater that can be
oxidized?
2.
What are the theoretical CODs for glucose and sodium acetate?
3.
What percents of theoretical oxidation of glucose, sodium acetate (two samples) is obtained in the COD
test? Explain any differences between actual and theoretical.
4.
What is the effect of the silver catalyst on the oxidation of sodium acetate?