A GUIDE TO chemical oxygen demand testing

Part 3*

The colorimetric method for COD testing is the most popular. Although colorimetry requires a spectrophotometer or photometer, it offers convenience since most manufacturers offer premixed reagents, so it is easy to run samples with the digestion chemicals and have minimal contact. Since all the analyst needs to do is digest the samples and let the instrument do the work, colorimetry is the most common method to measure COD.

The following equipment is needed:

Heating block (HI839800-02 COD test tube heater)

Both methods for COD testing require the digestion step, so a heating block for samples is crucial for ensuring accurate and repeatable results. For best results, look for a heating block that features multiple temperatures so that it can be used for other tests, such as total phosphorus. Most heating blocks also have timers, which are critical for keeping digestion times consistent over multiple runs. For added safety, look for models that have an optional safety shield that covers the heating block in case of an accident.

Colorimeter/spectrophotometer

The colorimeter or spectrophotometer

Reagents

By Ralf Christoph, GM, Hanna Instruments South Africa

is the device that will read the absorbance of the samples after digestion in order to correlate it to the COD concentration. Both of these instruments can be used to measure COD, but the two devices are a little different from one another.

Colorimeters use filters to measure light as specific wavelengths, but spectrophotometers use a device that allows for measurement across a wide spectrum. Regardless of which instrument used, look for models that feature pre-programmed methods for COD for ease of use.

Reagents

Reagents are among the most important components of the COD testing system. These chemicals are responsible for oxidising the organic material. It is possible to prepare reagents in-house, but it is easier to purchase reagents to minimise contact with hexavalent chromium and strong acids. These COD vials are premixed and ready to use.

There are several types of reagents available commercially:

EPA-compliant reagents: The vials comply with EPA method 410.4 and Standard Methods 5220D. These reagents use the formulation for this method, which contains mercury sulfate, potassium dichromate, and sulfuric acid. Choose these vials if your work requires you to report COD results to a regulatory agency requiring EPA methodologies.

ISO-compliant reagents: The vials conform to ISO 15705:2002 methods with respect to their composition. The composition of these COD vials is similar to that of EPA standards, so they also contain mercury.

Mercury-free reagents: Most COD vials contain mercury sulfate to remove chloride interferences, which would otherwise create a falsely high COD value. COD-free vials do not contain mercury, which makes them more susceptible to chloride interferences, but greatly reduces the safety and environmental risks of handling mercury. As a result, these reagents are ideal for routine analysis where no or very low chloride concentrations are expected.

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