CARDINAL ENVIRONMENTAL LABS, LLC.
Standard Operating Procedure
Procedure No.: 1110 Revision Date: 04/2007
Subject: Colorimetric COD Method (410.4)
Approved By: Title: Date Revised:
Colorimetric
Method COD (410.4)
Scope and Application
This method covers the determination of COD in surface waters; and domestic and industrial wastes.
Matrix
Aqueous.
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 10 ppm, using low COD vials (0-150 mg/L COD).
Definitions
COD: chemical oxygen demand
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
- Volumetric Eppandorf – adjustable from 0.25 ml to 1 ml.
- Spectrophotometer
Reagents
-COD digestion reagent vials (HACH)
Low range 0-150 mg/L : commercially available.
-COD digestion reagent vials (HACH)
High range 0-1500 mg/L : commercially available.
-COD standard 1000 ppm : commercially available.
-COD standard 100 ppm : dilute 1:10 the 1000 ppm standard
Procedure
1.0 Determine appropriate COD vials (high/low range) and dilutions required.
1.1 Prepare each sample or dilution for digestion by pipetting, using an Eppendorf pipet, a series of dilutions to individual COD vials (usually 1 and 2 mls of sample for low range, and 0.25, 0.5, 1 and 2 mls for high range).
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 thoroughly 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 colorimetric determination.
2.0 Sample Analysis
2.1 Read absorbances for standards and samples against a reagent blank at 420 nm for low vials and 610 nm for high vials on the spectrophotometer.
2.1.1 Adjust wavelength of spectrophotometer by keying 4,2,0 followed by the “second function” key followed by the “go tol” key for low vials.
2.1.2 Adjust wavelength of spectrophotometer by keying 6,1,0 followed by the “second function” key followed by the “go tol” key for high vials.
2.1.3 Place the COD vial adapter into the cell holder with the solid piece to your left, and the roller on the right.
2.1.4 Clean the outside of all COD vials by wiping with a damp towel, followed by a dry one. This will remove finger prints or other marks.
2.2 Place the blank vial into the adapter.
2.2.1 Zero the spectrophotometer to the blank by keying “second function” key followed by the “zero” key.
2.2.2. Remove blank and insert standards and/or samples to be analyzed, read absorbances, and record in log book.
3.0 Standard Curve
3.1 For the low COD vial standard curve pipet the following volumes of COD standard , then dilute to 2 mls with ultra pure water.
mls of ppm standard concentration/ppm 2 mls per sample
0.10 mls – 100 ppm std 5 ppm
0.20 mls – 100 ppm std 10 ppm
1.00 mls – 100 ppm std 50 pm
1.50 mls – 100 ppm std 75 ppm
2.0 mls – 100 ppm std 100 ppm
0.25 mls – 100 ppm std 125 ppm
0.30 – 100 ppm std 150 ppm
3.2 Digest each standard as in steps 1.2 – 1.5.
3.3 Read each standard against the reagent blank as in steps 2.1 – 2.2.2.
3.3.1 Absorbance data is input into excel file codlocu.xls to generate a linear coefficient. A printed copy of the current LC is found in the active LC file.
3.4 For the high COD vial standard curve pipet the following volumes of COD standard, then dilute to 2 mls with ultra pure water.
mls of ppm standard concentration/ppm 2 mls per sample
0.10 mls – 1000 ppm std 50 ppm
0.20 mls – 1000 ppm std 100 ppm
0.50 mls – 1000 ppm std 250 pm
1.00 mls – 1000 ppm std 500 ppm
1.50 mls – 1000 ppm std 750 ppm
2.00 mls – 1000 ppm std 1000 ppm
3.5 Digest each standard as in steps 1.2 – 1.5.
3.6 Read each standard against the reagent blank as in steps 2.0 – 2.2.2.
3.6.1 Absorbance data is input into excel file codhicu.xls to generate linear coefficient. A printed copy of the current LC is found in the active LC file.
4.0 Calculations
4.1 Use COD vial with 2 mls sample for calculation. If the 2 ml sample vial is over high range of curve, use the vial with the largest sample volume in range of curve.
4.2 (Absorbance x LC) x 2 = ppm COD
mls sample used
Pollution Preventuion/Waste Disposal
Dump contents of COD vials into a proper acid waste container. Rinse vials thoroughly with water before throwing away the vials.
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 similar 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).
Method Performance
See QAPP section 5.2.
Aqueous Precision = 20%, Aqueous Accuracy = 90-110%
References
Hach Water Analysis Handbook
3rd Edition, 1997 Method 8000
pp. 942 – 944 and pp. 947 – 949
National Environmental Methods Index. April 1994. Method 335.2. 31 March 2004. <http://www.nemi.gov>
Procedure 1110
Revised 04/2007