Chemical oxygen demand

Using potassium dichromate

Potassium dichromate is a strong oxidizing agent under acidic conditions. Acidity is usually achieved by the addition of sulfuric acid. The reaction of potassium dichromate with organic compounds is given by:

${\displaystyle {\text{C}}_n{\text{H}}_a{\text{O}}_b{\text{N}}_c+d{\text{Cr}}_2^{}{\text{O}}_7^{2-}+(8d+c){\text{H}}^{+}⟶n{\text{CO}}_2^{}+\frac{a+8d-3c}{2}{\text{H}}_2^{}\text{O}+c{\text{NH}}_4^{+}+2d{\text{Cr}}^{3+}}$

where ${\displaystyle d=2n/3+a/6-b/3-c/2}$. Most commonly, a 0.25 N solution of potassium dichromate is used for COD determination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate is preferred.

In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all redox reactions, one reagent is oxidized and the other is reduced), forming Cr³⁺. The amount of Cr³⁺ is determined after oxidization is complete and is used as an indirect measure of the organic contents of the water sample.

Measurement of excess

An excess amount of potassium dichromate (or any oxidizing agent) must be present for all organic matter to be completely oxidized. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr³⁺ can be accurately determined. To do so, the excess potassium dichromate is titrated with ferrous ammonium sulfate (FAS) until all of the excess oxidizing agent has been reduced to Cr³⁺. Typically, the oxidation-reduction indicator ferroin is added during this titration step. Once all the excess dichromate has been reduced, the ferroin indicator changes from blue-green to a reddish brown. The amount of ferrous ammonium sulfate added is equivalent to excess potassium dichromate added to the original sample. Note: Ferroin indicator is bright red from commercially prepared sources, but it exhibits a green hue when added to a digested sample containing potassium dichromate. During the titration, the color of the indicator changes from a green hue to a bright blue hue and then to a reddish brown upon reaching the endpoint. Ferroin indicator changes from red to pale blue when oxidized.

Preparation of ferroin indicator reagent

A solution of 1.485 g 1,10-phenanthroline monohydrate is added to a solution of 695 mg FeSO₄·7H₂O in distilled water, and the resulting red solution is diluted to 100 mL.

Calculations

The following formula is used to calculate COD:

${\displaystyle \mathrm{C}\mathrm{O}\mathrm{D}=\frac{8000(b-s)n}{\text{sample volume}}}$

where b is the volume of FAS used in the blank sample, s is the volume of FAS in the original sample, and n is the normality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/L.

The COD can also be estimated from the concentration of oxidizable compound in the sample, based on its stoichiometric reaction with oxygen to yield CO₂ (assume all C goes to CO₂), H₂O (assume all H goes to H₂O), and NH₃ (assume all N goes to NH₃), using the following formula:

COD = (C/FW)·(RMO)·32

Where

C = Concentration of oxidizable compound in the sample,

FW = Formula weight of the oxidizable compound in the sample,

RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO₂, water, and ammonia

For example, if a sample has 500 Wppm (Weight Parts per Million) of phenol:

C₆H₅OH + 7O₂ → 6CO₂ + 3H₂O

COD = (500/94)·7·16*2 = 1192 Wppm

Inorganic interference

Some samples of water contain high levels of oxidizable inorganic materials which may interfere with the determination of COD. Because of its high concentration in most wastewater, chloride is often the most serious source of interference. Its reaction with potassium dichromate follows the equation:

${\displaystyle 6\mathrm{C}{\mathrm{l}}^{-}+\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+14{\mathrm{H}}^{+}\to 3\mathrm{C}{\mathrm{l}}_2+2\mathrm{C}{\mathrm{r}}^{3+}+7{\mathrm{H}}_2\mathrm{O}}$

To eliminate chloride interference, mercuric sulfate can be added to the sample prior to the addition of other reagents.

The following table lists several other inorganic substances that may cause interference. It also lists chemicals that may be used to eliminate such interference and the compounds formed when the inorganic molecule is eliminated.

Government regulation

Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in wastewater before it can be returned to the environment. For example, in Switzerland, a maximum oxygen demand between 200 and 1000 mg/L must be reached before wastewater or industrial water can be returned to the environment .

History

For many years, the strong oxidizing agent potassium permanganate (KMnO₄) was used for measuring chemical oxygen demand. Measurements were called oxygen consumed from permanganate rather than organic substances' oxygen demand. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and in many cases, biochemical oxygen demand (BOD) measurements were often much greater than results from COD measurements. This indicated that potassium permanganate could not effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing agent for determining COD.

Since then, other oxidizing agents such as ceric sulphate, potassium iodate, and potassium dichromate have been used to determine COD. Of these, potassium dichromate (K₂Cr₂O₇) is the most effective: it is relatively cheap, easy to purify, and can nearly completely oxidize almost all organic compounds.

In these methods, a fixed volume with a known excess amount of the oxidant is added to a sample of the solution being analyzed. After a refluxing digestion step, the initial concentration of organic substances in the sample is calculated from a titrimetric or spectrophotometric determination of the oxidant remaining in the sample. As with all colorimetric methods, blanks are used to control for contamination by outside material.