C.O.D. (Chemical Oxygen Demand)

The Chemical Oxygen Demand (C.O.D.) test measures the oxygen equivalent consumed by organic matter in a sample during strong chemical oxidation. The strong chemical oxidation conditions are provided by the reagents used in the analysis.

Several methods have been described to calculate C.O.D. but the most broadly used and officially recognized (ISO 6060, NFT 90101, UNE 77004) for water analysis, is the one using potassium dichromate as oxidizing substance.

Sulphuric acid is added to yield a strong acid medium. Several reagents are added during the set up of the analysis, to drive the oxidation reaction to completion and also to remove any possible interferences. Specifically, these reagents are mercuric sulphate and silver sulphate. Mercuric sulphate is added to remove complex chloride ions present in the sample. Without the mercuric sulphate the chloride ions would form chlorine compounds in the strong acid media used in the procedure. These chlorine compounds would oxidize the organic matter in the sample, resulting in a COD value lower than the actual value.

Silver sulphate is added as a catalyst for the oxidation of short, straight chain organics and alcohols.

Excess of dichromate is titrated with ammonium iron sulphate standard solution by using ferroin as indicator. To prepare ferroin solution, a mixture of o-phenantroline and Iron (II) sulfate is dissolved in water.

We provide all separate reagents to perform C.O.D. In addition, we offer several mixtures of reagents and volumetric solutions, ready to be used in the Standard C.O.D. Methods. This will save time in your laboratory.

REAGENTS USED IN COD ANALYSIS

DESCRIPTION	CAT.NO.
Iron ammonium sulfate 6 hydrate, reagent grade	HI0316
Iron (II) sulfate 7 hydrate, reagent grade	HI0351
Mercury (II) sulfate, reagent grade	ME0227
o-Phenantroline 1 hydrate, reagent grade	FE0100
Potassium dichromate 0,04M with 80g/l mercury (II) sulfate	PO0234
Potassium dichromate 0,25N, volumetric solution	PO0232
Potassium dichromate, r.g. ACS ISO	PO0220
Silver sulfate, sol. 10 g/l in sulphuric acid	PL0072
Silver sulfate, reagent grade	PL0071
Sulfuric acid 96%, reagent grade	AC2069