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Test kits for water analysis
Test kits for water analysis

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3.4 Chemical oxygen demand

Chemical oxygen demand (COD) is a measure of the overall level of organic pollution in water. It is the oxygen required to chemically oxidise all the carbon containing material present in a body of water. If organic matter decomposes it will use up dissolved oxygen, thus threatening living organisms.

The COD test, which is widely used for environmental and effluent water samples, starts with a digestion step which involves heating the sample with acidified potassium dichromate (a strong oxidising) agent. The reaction is catalysed by silver sulfate.

COD is then calculated by determining the concentration of unconsumed dichromate either photometrically or titrimetrically. Potassium dichromate is generally regarded as the most effective oxidising agent for the job, and will react with most organic compounds, exceptions being pyridine (and related compounds) and volatile hydrocarbons.

The oxidation of a generic water soluble organic compound by dichromate may be represented as:

(CaHbOcNd)(aq) + x Cr2O72 (aq) + H+(aq) = CO2(g) + H2O(l) + NH4+ (aq) + Cr3+(aq)
Equation label: (12)

where a, b, c and d will depend on the formula of the organic species concerned. From this a theoretical value for the number of moles, x, of dichromate required may be calculated from:

x = 2a/3 + b/6 + c/3 + d/2
Equation label: (13)

So, during digestion:

  • carbon is converted to carbon dioxide
  • hydrogen is converted to water
  • nitrogen is converted to aqueous ammonium ions.

Equation 12 presents us with two options to quantify COD photometrically: Cr2O72 ions are orange and Cr3+ ions are blue/green, so either can be measured photometrically.

The most common interference in this measurement is due to chloride ions; these are oxidised as shown in Equation 14.

6Cl (aq) + Cr2O72 (aq) + 14H+(aq) = 3Cl2 (g) + 2Cr3+ (aq) + 7H2O(l)
Equation label: (14)

To suppress the interference from chloride ions, mercury (II) sulfate is added to the reagent mixture, which reacts to form mercury (II) chloride.

Hg2+ (aq) + 2Cl(aq) = HgCl2 (aq)
Equation label: (15)

On a practical level, portable apparatus is commercially available for COD measurement. Typically this contains a block heater, which allows for multiple digestions to be carried out simultaneously. This is a major advance on the older macro-digestion methods which used standard laboratory glassware, condensers, hotplates and associated paraphernalia - clearly not ideal for on-site analysis. In fact, this, coupled with the advantage of reduced waste (less mercury to get rid of for a start) and smaller sample volumes, has meant that COD test kits are favoured in many large analytical laboratories too.

Activity 2  The COD test: health and safety considerations

Timing: Time allowed: 1.5 hours

So far in this course, we've alluded to a number of health and safety issues arising from the use of toxic reagents in the field. The COD test is a case in point as it involves the use of potassium dichromate and mercury.

With reference to their respective Materials Safety Data Sheets, describe what precautions and special handling procedures you would use when disposing of used reagents or in the event of accidental spillage during or after the COD test.