Cardinal Environmental Laboratories, LLC
Standard Operating Procedure
Procedure No.: 3050
Subject: (EPAMethod 410.4) Chemical Oxygen Demand (COD)
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Rev. Date Approved By: Date
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04/2007
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COD
Description
The amount of oxygen required to oxidize organic materials in waters or wastes can be
determined using the following method. The method is sensitive to 10ppm.
Precautions
Corrosives (see procedure 0020).
Sample Preservation
Samples to be analyzed for COD can be collected in either plastic or glass containers.
Samples should be preserved by adding concentrated H2SO4 to a PH <2 and stored
at 4° C. Once preserved, samples may be held for 28 days prior to analysis.
Apparatus
-Heating block
-100 ml beakers
-25 ml buret
-10 ml buret
-Volumetric pipettes, 2 ml and 10 ml
Reagents
- Ferrous Ammonium Sulfate (FAS) 0.25 N: commercially available.
- Ferroin Indicator 0.025 M: commercially available.
- Potassium Dichromate 0.0417M: commercially available.
- Potassium Hydrogen Phthalate ( KHP) (ACS grade or better): commercially available.
- Sulfuric Acid (H2SO4) (ACS grade), concentrated.
- FAS 0.025N (High range titrant):
Prepare a 1:10 dilution of the stock 0.25N FAS solution.
Prepare daily.
- FAS 0.0125N (Low range titrant):
Prepare a 1:1 dilution of the 0.025N FAS solution.
Prepare daily.
- Potassium Dichromate 0.00417M:
Prepare a 1:10 dilution of the stock Potassium Dichromate Solution.
- COD Standard (1000 ppm):
Dissolve 850 mg (dry at 120° C) Potassium Hydrogen Phthalate in
1000 ml ultrapure water. Preserve with H2SO4 to a pH <2.
Refrigerate.
- COD Standard (100 ppm):
Dilute 1:10 the 1000 ppm standard. Preserve with H2SO4 to pH <2.
Refrigerate.
- COD vials (HACH) low range 0-150: commercailly available.
- COD vials (HACH) high range 0-1500: commercially available.
Procedure
1.0 Determine appropriate COD vials (High/Low range) and dilutions required.
1.1 Prepare each sample or dilution for digestion by pipeting, using an Eppendorf pipette, a series of dilutions to individual COD vials. (usually 0.5, 1 and 2 mls of each sample)
1.2 Each sample aliquot should have a final volume of 2 mls after required dilutions are made with ultra pure water.
1.3 Close vial cap and invert vial several times to throughly mix sample and reagents (CAUTION: vials will get hot.)
1.4 Place all samples/standards/blanks in the COD heating block and heat for two hours at 120° C (Preset).
1.5 Allow vials to cool to room temperature prior to proceeding with titration.
2.0 FAS N Determination.
2.1 High range FAS N determination
2.1.1 To a 100 ml beaker, add 2 ml 0.0417 M Potassium Dichromate, 3 ml concentrated H2SO4, 3 drops Ferroin indicator and approximately 20 ml ultra pure water.
2.1.2 Titrate with 0.025N FAS using the 25 ml buret to a bright orange endpoint. Record volume used.
2.1.3 Repeat steps 2.1.1 - 2.1.2. Record average FAS titrant volume used (see step 4.0).
2.2 Low range FAS N determination.
2.2.1 To a 100 ml beaker, add 2 ml 0.00417M Potassium Dichromate, 3 ml concentrated H2SO4, 3 drops Ferroin indicator and approximately 20 ml ultra pure water.
2.2.2 Titrate with 0.0125N FAS using the 10ml buret to a bright orange endpoint. Record volume of titrant used.
2.2.3 Repeat steps 2.2.1 - 2.2.2 and record average FAS titrant volume used (see step 4.0).
3.0 Sample Analysis
3.1 Carefully transfer digested samples/standards/blanks from COD vial to a 100 ml beaker.
3.2 Add 3 drops Ferrion indicator to beaker.
3.3 Rinse vial with ultrapure water at least twice pouring rinsate into the same beaker as sample.
3.4 Titrate sample with appropriate FAS titrant (0.025N=high range, 0.0125N=low range) to a bright orange endpoint.
4.0 Calculation
High Range: (A-B) 2000 = ppm COD
C
(Sample results <100ppm need to be run on low range. See Quality Control Section)
Low Range: (A-B) 2000 X 0.1 = ppm COD
C
A= mls FAS Blank
B =mls FAS Sample
C= mls FAS used to standardize (average of 2)
Quality Control
* Each analytical batch analyzed for COD must be accompanied by a method blank consisting of 2 ml ultrapure water taken through entire digestion step.
* Each batch must also be accompanied by two standards containing 1000 and 100 mg/l COD as Potassium Hydrogen Phthalate.
* All weights, volumes, comments, etc. for each batch must be entered in notebook form for data review.
* Every 5th sample of a similiar matrix shall be analyzed in duplicate and% RPD calculated. Every 10th sample shall be analyzed as a duplicate spike and % recoveries and % RPD calculated. (see procedure 0110 for calculations).
* If a sample analyzed using high range vials calculates to <100ppm, then a second digestion using low range vials is required. The low range analysis results will be reported.
* The current acceptable limits for Q. C. parameters are listed in procedure 0120.
* Failure to meet Quality Control acceptable limits requires corrective action. (see procedure 0150).
References
Standard Methods for the Examination of Water and Wastewaters,
18th Edition 1992 Method 5220 C. pp 5-8 thru 5-9.
Hach water analysis handbook 1989
Oxygen demand, chemical pp. 461-477
Procedure 3050
Revised 04/07