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The analysis of the COD value in waste and surface waters is one of the most expressing to determine the degree of pollution of the water.

COD stands for chemical oxygen demand and reflects the total quantity of oxidisable components, whether it is carbon (C) and hydrogen (H) from hydrocarbons, nitrogen (N) from proteins, or sulphur (S) and phosphorus (P) from detergents.

Unlike the TOC value (total organic carbon), which only expresses the carbon coming from pollutants, the COD value expresses everything.

The principle of COD analysis is the depletion of an oxidiser given to the sample and being consumed by the pollutants, whereas the TOC analysis is based on the generation of CO2 caused by the oxidation of the organic components, usually being detected by an infra-red detector.

There are several oxidisers, all strong enough to oxidise the various components in an aqueous sample.

However, not too many are stable enough to suit a quantitative chemical analysis, as most of the oxidisers not only oxidise the pollutant but also themselves, which obviously would lead to false results in the analysis.

The most stable oxidiser is a solution of potassium dichromate (K2Cr2O7) mixed with sulphuric acid.

Hence the reason this oxidiser has been chosen to base the normalised method on (DIN 38409-44, EPA 410.4).

This is the true COD test and is used in all wastewater testing.

Another very suitable oxidiser is a solution of potassium permanganate (KMnO4).

This is often used in the potable water industry and with water authorities to determine COD.

The permanganate test is also suitable for use in wastewater and has the advantage that it doesn’t release a harmful waste (Cr3+ and Cr2O7), although the quantities are minute.

To execute the tests, a small amount of sample is mixed with an excess of oxidiser solution and boiled for a certain time.

According to the normalised test, this time should be two hours, but can be set shorter when the sample allows.

For example, waste waters with a predictable quickly oxidisable component (such as sugar) can be set to a short oxidation time (such as 15 minutes).

Sometimes it is recommended a catalyst (Ag+) is added to oxidise aliphatic and aromatic hydrocarbons.

In the case chloride is present in the sample (which will get oxidised to chlorine and is not considered as COD) it should be immobilised with mercury or it should be measured and deducted in the final COD result calculation.

The latter is preferred since the release of mercury is not wanted; (there are also simple ways to retrieve mercury by an absorption cartridge, which can be disposed of in an environmentally acceptable way).

This oxidation step in the determination is also called the digestion.

The next step in the determination is the detection.

The analysis of the remainder of the oxidiser is what performs the detection (either the dichromate or the permanganate).

There are two methods of analysing this remainder: titration or colorimetric, of which the first is the most accurate.

The colorimetric method requires less hardware and is attractive from an investment point of view.

In the case of dichromate titration, the digested solution is transferred to a titration vessel and titrated with a solution of ferrous (Fe2+) against a platinum electrode.

From a precision pump (burette), the ferrous solution is slowly added to the digested solution.

This causes the ferrous to be oxidised instantaneously by the remainder of the dichromate, until the potential shift at the electrode tells when all dichromate has been taken out by the ferrous.

The COD value may be calculated then from the difference of the originally added quantity of dichromate and the remainder, which is determined by the quantity of the added ferrous till titration endpoint.

For the dichromate colorimetric method, the digested solution is transferred to a colorimetric cell in which the density of the green colour of the chromium ion (Cr3+) is being measured (the dichromate gets reduced to chromium in the oxidation process).

The measurement is performed against a previously stored calibration, which is a series of chromium solutions with a different concentration.

In the case of permanganate titration, the digested solution is transferred to a titration vessel and titrated with a solution of oxalic acid against a platinum electrode.

For permanganate colorimetric, the digested solution is transferred to a colorimetric cell in which the optical density of the purple colour of the permanganate ion (MnO72-) is being measured.

Another strong oxidiser is the so-called OH radicals.

These radicals can be generated by either ozone or by an electrode (from a suitable reagent).

The ozone is preferred as an oxidiser as it can be generated from air.

A major drawback is that the OH radicals have a lifespan of a few minutes only, which means only the easily oxidised components are measured.

The ‘nasties’ are only partly oxidised.

This means the method is only suitable in cases where a single polluting component varies in concentration.

If a wide array of pollutants is present in the sample, this method shows inconsistent results, showing no correlation with the normalised method.

Here it is possible to observe the trend of (mainly hydrocarbon) pollutants in the water sample, and it is also possible to use the approved spectro-photometric method DIN 38404.

This method measures all components with an absorption ability at 254nm.

This has drawbacks similar to the previously mentioned method, but analysers based on it are not expensive to buy or to maintain.

Any of the mentioned methodologies, except the one using the OH radicals, can be applied with the ADI Process Analyzer 2040, a versatile on-line analyser.

The 2040 features the ability to incorporate two more analyses such as ammonia, nitrate or phosphate and an unlimited number of physical parameters such as optical density (uv, vis, turbidity), pH, conductivity, oxygen and ORP.

Applikon Analytical

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