CHEMICAL OXYGEN DEMAND (COD) Chemical oxygen demand (COD) is used as a measure of oxygen requirement of a sample that is susceptible to oxidation by strong chemical oxidant. The dichromate reflux method is preferred over procedures using other oxidants (eg potassium permanganate) because of its superior oxidizing ability, applicability to a wide variety of samples and ease of manipulation. Oxidation of most organic compounds is 95100% of the theoretical value.
Dichromate Reflux Technique Standard Method. Equipmentt Required Equipmen 1. 500-millilitre (ml) Erlenmeyer flask with standard (24/40) tapered glass joints 2. Friedrichs reflux condensers (12-inch) with standard (24/40) tapered glass
joints 3. Electric hot plate or six-unit heating shelf 4. Volumetric pipettes (10, 25, and 50ml capacity) 5. Burette, 50 ml - 0.1 ml accuracy 6. Burette stand and clamp 7. Analytical balance, accuracy 0.001gram (g) 8. Spatula 9. Volumetric flasks (1,000ml capacity) 10. Boiling beads, glass 11. Magnetic stirrer and stirring bars
Chemicals Required 1. Potassium dichromate (K2Cr 2O7) 0.25N 2. Sulphuric acid (H2SO4) / silver sulphate (Ag2SO4) solution 3. Mercuric sulphate (HgSO4) crystals 4. Ferrous ammonium sulphate (FAS) [Fe(NH 4)2(SO4)2], approximately 0.01N 5. Ferroin indicator (1, 10-phenanthroline and ferrous ammonium sulphate)
Caution: In carrying out the following procedures, use proper safety measures, including protective clothing, eye protection, and a fume hood. Reagents containing heavy metals (HgSO4 and Ag2SO4) should be disposed of as toxic wastes.
Chemical Preparation 1. Dissolve 12.259g of oven-dried (primary standard grade dried at 103oC to a
constant weight) potassium dichromate in distilled water and dilute to 1 litre volume in a volumetric flask. 2. Add 22g of reagent grade silver sulphate to a 4kg bottle of concentrated
sulphuric acid (H2SO4) and mix until the silver sulphate goes into solution. 3. Use 1 g of mercuric sulphate (HgSO4) to complex 100 mg chloride (2,000
mg/l).
4. Dissolve 1.485g of 1,10-phenanthroline monohydrate and 0.695g of ferrous
ammonium sulphate heptahydrate in distilled water and dilute to approximately 100ml. (Alternatively, this indicator may be purchased as Ferroin Indicator from most scientific suppliers.) 5. Dissolve 39g reagent grade ferrous ammonium sulphate hexahydrate in
distilled water. Add 20ml of concentrated sulphuric acid (H2SO4). Cool and dilute to exactly 1 litre in a volumetric flask using distilled water. The ferrous ammonium sulfate (FAS) titrant must be standardized daily by the following procedure: Dilute 10ml of standard potassium dichromate (K2Cr 2O7) solution to 100ml with distilled water. Slowly add 30ml of concentrated sulphuric acid and cool to room temperature. Titrate with ferrous ammonium sulphate titrant, using 2 to 3 drops (0.10 to 0.15 ml) of Ferroin indicator. Normality of FAS = (ml K2Cr 2O7)(0.25) ml FAS required The deterioration of FAS can be decreased if it is stored in a dark bottle.
Procedure 1. Place a 50ml sample or an aliquot diluted to 50ml in a 500ml refluxing flask.
The blank is prepared using 50ml of distilled water. This is a precise measurement and a 50ml volumetric pipette should be used. Refer to COD Range and Sample Size below for dilution. 2. Add 5 to 7 glass boiling beads. 3. Add 1g of mercuric sulphate (HgSO 4), 5ml of concentrated sulphuric acid /
silver sulphate solution, and mix until the HgSO4 is in solution. The function of the mercuric sulphate is to bind bi nd or complex chlorides. One gram may not be required if the chloride concentration is low. (Caution: Always add acid slowly down the side of the flask while mixing to avoid overheating. It may be necessary to use gloves because of the heat generated.) 4. Accurately add 25ml of 0.25 N potassium dichromate (K2Cr 2O7) and mix. 5. Add while mixing, an additional 70ml of concentrated sulphuric acid-silver
sulphate solution. 6. After thorough mixing, attach the flask to the reflux condenser, apply heat,
and reflux for 2 hours. Refluxing time can be decreased depending on the ease of oxidation of organic materials. This time may be determined by refluxing for periods from 15 minutes to 2 hours and comparing the results. r esults. 7. A reagent blank containing 50ml of distilled water treated with the same
reagent as the sample should be refluxed with wi th each set of samples. 8. Cool the apparatus to room temperature after the refluxing period. Wash
down the interior of the condenser and flask twice with approximately 25ml portions of distilled water. 9. Remove flask from the condenser and dilute to a final volume of
approximately 350ml with distilled water. 10. Add 4 to 5 drops of Ferroin indicator and a magnetic stirring bar.
11. Place flask on a magnetic stirrer and rapidly titrate with 0.1 N ferrous
ammonium sulphate to the first red-brown endpoint. Caution: Use care in titration. The endpoint is very sharp and may be reached rapidly. Formula to determine COD: COD (mg/l) = (a-b)(N) x 8,000 / sample size (ml) Where: a = ml Fe(NH4)2(SO4)2 used for blank b = ml Fe(NH4)2(SO4)2 used for sample N = normality of FAS titrant (Fe(NH ( Fe(NH4)2(SO4)2) ml sample = the actual volume of sample used before dilution
Sources of Error 1. The largest error is caused by using a nonhomogeneous sample. Every effort
should be made to blend and mix the sample so that solids are never excluded from any aliquot. 2. Always use the largest sample practical and use the largest glassware that is
in keeping with good laboratory practice. 3. Use volumetric flasks and volumetric pipettes with a large bore. 4. The K2Cr 2O7 oxidizing agent must be precisely measured. Use a volumetric
pipette and use the same one each time if possible. 5. When titrating, be certain that the burette is clean and free of air bubbles. 6. Always read the bottom of the meniscus and position the meniscus at eye
level.
COD Range and Sample Size COD Range (mg/l) Volume of Sample (ml)
50800
1001500
2403700
4807500
120018800
24003700
40000375000
50
25
10
5
2
1
0.1
All samples high in solids should be blended for 2 minutes at high speed and stirred when an aliquot is taken. Sample volumes less than 25ml should not be pipetted directly, but serially diluted and then a portion of the diluent removed: 1. 500ml of sample diluted to 1,000 ml = 0.5 ml sample/ml of diluent, .: 50 ml of
diluent = 25 ml of sample. 2. 100 ml of sample diluted to 1,000 ml = 0.1 ml sample/ml diluent, .: 50 ml of
diluent = 5 ml of sample.
Elimination of Interference One gram of mercuric sulphate (HgSO4) will complex 100mg of chloride in a 50ml sample (2,000 mg/l). For samples higher in chloride more HgSO4 should be used in the ratio of 10:1 HgSO4.
Interference from nitrites can be prevented by the addition of 10:1 ratio of sulfamic acid:nitrite. The addition of the silver sulphate (AgSO4) concentrated sulphuric acid (H2SO4) refluxing acid will aid in the oxidation of some organic nitrogen compounds, but aromatic hydrocarbons and pyridine are not oxidized to any appreciable amount.
Chemical oxygen demand From Wikipedia, the free encyclopedia
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In environmental chemistry, chemistry, the chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic of organic compounds in water . Most applications of COD determine the amount of organic of organic pollutants pollutants found in surface water (e.g. water (e.g. lakes and rivers rivers), ), making COD a useful measure of water of water quality. quality. It is expressed in milligrams per liter (mg ( mg//L), which indicates the mass of oxygen consumed per liter of solution of solution.. Older references may express the units as parts as parts per million (ppm).
Overview The basis for the COD test is that nearly all organic compounds can be fully oxidized to carbon dioxide with a strong oxidizing agent under under acidic acidic conditions. The amount of oxygen of oxygen required to oxidize an organic compound to carbon dioxide, ammonia ammonia,, and water is given by:
This expression does not include the oxygen demand c aused by the oxidation of ammonia into nitrate. The process of ammonia being converted into nitrate is referred to as nitrification. The following is the correct equation for the oxidation of ammonia into nitrate.
The second equation should be applied after the first one to include oxidation due to nitrification if the oxygen demand from nitrification must be known. Dichromate does not oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard chemical oxygen demand test. The International Organization for Standardization describes a standard method for measuring chemical oxygen demand in ISO 6060 [1] [1]..
History For many years, the strong oxidizing agent potassium agent potassium permanganate (K MnO MnO4) was used for measuring chemical oxygen demand. Measurements were called oxygen consumed from permanganate, rather than the oxygen demand of organic substances. Potassium permanaganate's effectiveness at oxidizing organic compounds varied widely, and in many cases biochemical cases biochemical oxygen demand (BOD) measurements were often much greater than results from COD measurements. This indicated that potassium permanganate was not able to effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing agent for determining COD.
Since then, other oxidizing agents such as cer ceric ic sulf sulfate ate,, potassium iodate, iodate , and and potassium potassium dichromate have been used to determine COD. Of these, potassium dichromate (K ( K 2Cr 2O7) has been shown to be the most effective: it is relatively cheap, easy to purify purify,, and is able to 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 still remaining in the sample.
Using potassium dichromate Potassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is usually achieved by the addition of sulfuric of sulfuric acid. acid.) The reaction of potassium dichromate with organic compounds is given by:
where d = 2n/3 + a/6 - b/3 - c/2 . Most commonly, a 0.25 N 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 Cr 3+. The amount of Cr 3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
Blanks Because COD measures the oxygen demand of organic compounds in a sample of water, it is important that no outside organic material be accidentally added to the sample to be measured. To control for this, a so-called blank sample is required in the determination of COD (and BOD BOD,, for that matter). A blank sample is created by adding all reagents (e.g. acid and oxidizing agent) to a volume of distilled of distilled water . COD is measured for both the water and blank samples, and the two are compared. The oxygen demand in the blank sample is subtracted from the COD for the original sample to ensure a true measurement of organic matter.
Measurement of excess For all organic matter to be completely oxidized, an excess amount of potassium dichromate (or any oxidizing agent) must be present. Once oxidation is complete, the amount of excess potassium dichromate must be measured to ensure that the amount of Cr 3+ can be determined with accuracy. 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 Cr 3+. Typically, the oxidation-reduction indicator Ferroin indicator Ferroin is added during this titration step as well. Once all the excess dichromate has been reduced, the Ferroin indicator changes from blue-green to reddish-brown. The amount of ferrous ammonium sulfate added is equivalent to the amount of excess potassium dichromate added to the original sample. and also we can determine COD by boiling the water sample and we can determine CO2 ratio by the infra-red analyzer
Preparation Ferroin Indicator reagent
A solution of 1.485 g 1,10- phenanthroline 1,10- phenanthroline monohydrate is added to a solution of 695 mg FeSO4·7H2O in water, and the resulting red solution is diluted to 100 mL.
Calculations The following formula is used to calculate COD:
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 CO 2 (assume all C goes to CO2), H2O (assume all H goes to H 2O), and NH 3 (assume all N goes to NH 3), 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 CO 2, water, and ammonia
For example, if a sample has 500 wppm of phenol: C6H5OH + 7O2 → 6CO2 + 3H2O COD = (500/94)(7)(32) = 1191 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:
Prior to the addition of other reagents, mercuric sulfate can be added to the sample to eliminate chloride interference. The following table lists a number of other inorganic substances that may cause interference. The table also lists chemicals that may be used to eliminate such interference, and the compounds formed when the inorganic molecule is eliminated. Inorganic molecule Chloride
Eliminated by Mercuric sulfate
Elimination forms Mercuric chloride complex
Nitrite
Sulfamic acid
N2 gas
Ferrous iron
-
-
Sulfides
-
-
Government regulation Many governments impose strict regulations regarding the maximum chemical oxygen demand allowed in wastewater before they can be returned to the environment. For example, in Switzerland Switzerland,, a maximum oxygen demand between 200 and 1000 mg/L must be reached before wastewater or industrial or industrial water can water can be returned to the environment [2] [2].. PREPARATION OF SOLUTIONS Stock Iron Standard Solution, 10 ppm Primary standard solid ferrous ammonium sulfate hexahydrate, (NH 4)2(SO4)2 6H2O, 392.13 g/mol, is available on the side shelves for preparation of the standard iron solution. 1. Tap a small amount of the solid ferrous ammonium sulfate onto a sheet of glassine weighing paper that has been folded in the middle. Zero your balance. Accurately weigh about 0.07 g of pure dry ferrous ammonium sulfate (to + 0.1 mg) onto a folded sheet of glassine paper or into a small, plastic weighing boat. 2. Transfer the ferrous ammonium sulfate quantitatively into a 1-L volumetric flask, carefully squirting down the weighing boat and the neck of the flask to ensure a quantitative transfer.
Add about 100-200 mL of distilled water. Dissolve the solid completely before diluting to volume 3. Pipet 2.5 mL of concentrated sulfuric acid into the flask, rinse the neck of the flask down, and mix carefully with swirling. [Be very careful when using concentrated H 2SO4; it is quite caustic.] Dilute the solution to the mark. Calculate the iron concentration of the solution in g of iron per mL (ppm) and in molar (M) units. Because this solution is used to calibrate absorbances and prepare a calibration curve, it must be prepared very carefully and accurately. The results of the entire experiment rest on preparing this solution accurately. The iron solution must be prepared daily, so there is no point in saving the solution to re-use it if you end up needing to re-do the experiment. You will need to prepare another standard solution. Iron Standard Calibration Solutions 1. Into each of five 100-mL volumetric flasks, pipet 1, 5, 10, 20, and 35 mL of the standard iron solution, respectively. Use a combination of 1-, 5-, 10-, and 25-mL volumetric pipets. The 1- and 5-mL pipets are located in the drawer marked for the experiment. 2. Pour about 50 mL of distilled water into a 6 th flask to serve as the “blank” (i.e. zero iron concentration). 3. Obtain the unknown sample from the Teaching assistants and treat it in the same manner as the standards, as indicated below. 4. Line all seven 100-mL volumetric flasks in this order: The blank, those with 1-35 mL of iron
stock standard solution added, and your unknown sample. To each flask (including the distilled water “blank” and the unknowns), pipet in order – a. 1 mL of the hydroxylamine solution, b. 10 mL of the 1,10-phenanthroline solution, and c. 8 mL of the sodium acetate solution. Note that the “blank” solution must have all the reagents in it except for any ferrous ammonium sulfate. 5. Swirl each flask to mix the contents, then carefully dilute each solution to the 100-mL mark and mix thoroughly. 6. Allow the solutions to stand for 10 minutes to fully develop the color. Mix well again. Fill each of seven clean, dry plastic cuvettes about two-thirds full with each of the seven solutions, keeping them in the same order. (If the insides of the cuvettes are wet or spotted, rinse them out twice with the appropriate solution first.) Stock Reagent Solutions The sodium acetate buffer (1.2 M), the 1,10-phenanthroline solution (1 g/L), and the hydroxylamine hydrochloride solution (100 g/L) are prepared by the Teaching Assistants and should be available for your use.
