Chemical Oxygen Demand

Test Method Technical Information

TECHNICAL SPECIFICATION

Tests For: Assessing effluent and wastewater quality prior to discharge
Reagent Chemistry Used: Dichromate
Basis of Test Method: Standard Method 5220-D, ISO15705:2005, BS 6068-8.80:2002

*The Method Detection Limit (MDL) is defined as the minimum measured concentration of a substance that can be reported with 99% confidence to be different from the method blank results.

**The Limit of Quantification (LOQ) is the smallest quantity that can be detected with reasonable certainty for a given analytical procedure.

Testing for Chemical Oxygen Demand

Chemical oxygen demand is a vital test for assessing the quality of effluents and waste waters prior to discharge. The Chemical Oxygen Demand (COD) test predicts the oxygen requirement of the effluent and is used for monitoring and control of discharges, and for assessing treatment plant performance.

The impact of an effluent or wastewater discharge on the receiving water is predicted by its oxygen demand. This is because the removal of oxygen from the natural water reduces its ability to sustain aquatic life. The COD test is therefore performed as routine in laboratories of water utilities and industrial companies.

Reagent Chemistry

The Palintest COD method conforms to the sealed tube reflux version of the method detailed in Standard Method 5220-D. Over the range of the test, the colour changes from orange to green. The results are expressed as milligrams of oxygen consumed per litre of sample.

Interferences

Effluents and wastewater samples may contain undissolved or particulate material. Such samples may be homogenised in a blender prior to the test to improve accuracy and reproducibility.

Chloride is the main potential interference in the COD test. High chloride levels may result in an apparent high COD result. The Palintest COD tests will not be significantly affected by chloride levels up to 1000 mg/L. Samples containing above this level should be diluted to reduce the concentration to 1000 mg/L or below and the test carried out on the diluted sample.

If sample dilution is not possible then it may be necessary to suppress chloride interference. The method most used in standard analytical methods is the addition of mercuric sulphate to the reagent system. The table below shows different Palintest COD mercury tests available. COD/20000/M or 2M are suitable for use with seawater samples.

Best Practice Advice for Testing

• COD test reagents are light-sensitive. Store tubes in the original contained and keep the box closed when not in use. Store in cool, dry conditions.
• The Palintest COD test is a simplified laboratory procedure and should be carried out in accordance with good laboratory working practice. The reagent tubes contain 84% sulfuric acid and must be handled with care.
• Reagent tubes should not be opened whilst hot as pressure build-up may cause acid spillage. Do not open tubes during or after sample digestion.
• In this test a reagent blank is used instead of the usual water blank referred to in the general photometer operating instructions. The reagent blank is prepared by adding deionised or distilled water to the reagent tube and then digesting the tube in the same manner as for the water sample.
• The used COD Tubetests tubes contain strong sulphuric acid and other chemical reagents, and care must therefore be exercised in their disposal. The tube contents should be disposed of in accordance with Local Authority requirements.
• The tubes must not be re-used as they are designed for single use only.

EPA, Definition and Procedure for the Determination of the Method Detection Limit, Revision 2, Dec 2016.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”).