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5220 CHEMICAL OXYGEN DEMAND (COD)*
* Approved by Standard Methods Committee, 1997.
5220 B. Open Reflux Method
1. General Discussion
a. Principle: Most types of organic matter are oxidized by a boiling mixture of chromic and sulfuric acids. A sample is refluxed in strongly acid solution with a known excess of potassium dichromate (K2Cr2O7). After digestion, the remaining unreduced K2Cr2O7 is titrated with ferrous ammonium sulfate to determine the amount of K2Cr2O7 consumed and the oxidizable matter is calculated in terms of oxygen equivalent. Keep ratios of reagent weights, volumes, and strengths constant when sample volumes other than 50 mL are used. The standard 2-h reflux time may be reduced if it has been shown that a shorter period yields the same results. Some samples with very low COD or with highly heterogeneous solids content may need to be analyzed in replicate to yield the most reliable data. Results are further enhanced by reacting a maximum quantity of dichromate, provided that some residual dichromate remains.
a. Reflux apparatus, consisting of 500- or 250-mL erlenmeyer flasks with ground-glass 24/40 neck and 300-mm jacket Liebig, West, or equivalent condenser with 24/40 ground-glass joint, and a hot plate having sufficient power to produce at least 1.4 W/cm2 of heating surface, or equivalent.
c. Pipets, Class A and wide-bore.
a. Standard potassium dichromate solution, 0.04167M: Dissolve 12.259 g K2Cr2O7, primary standard grade, previously dried at 150°C for 2 h, in distilled water and dilute to 1000 mL. This reagent undergoes a six-electron reduction reaction; the equivalent concentration is 6X 0.04167M or 0.2500N.
b. Sulfuric acid reagent: Add Ag2SO4, reagent or technical grade, crystals or powder, to conc H2SO4 at the rate of 5.5 g Ag2SO4 /kg H2 SO4. Let stand 1 to 2 d to dissolve. Mix.
c. Ferroin indicator solution: Dissolve 1.485 g 1,10-phenanthroline monohydrate and 695 mg FeSO4·7H2O in distilled water and dilute to 100 mL. This indicator solution may be purchased already prepared.*
d. Standard ferrous ammonium sulfate (FAS) titrant, approximately 0.25M: Dissolve 98 g Fe(NH4)2(SO4)2·6H2O in distilled water. Add 20 mL conc H2SO4, cool, and dilute to 1000 mL. Standardize this solution daily against standard K2Cr2O7 solution as follows:
Dilute 25.00 mL standard K2Cr2O7 to about 100 mL. Add 30 mL conc H2SO4 and cool. Titrate with FAS titrant using 0.10 to 0.15 mL (2 to 3 drops) ferroin indicator.
Molarity of FAS solution
Volume 0.04167M K2Cr2O7
solution titrated, mL
Volume FAS used in titration, mL
e. Mercuric sulfate, HgSO4, crystals or powder.
f. Sulfamic acid: Required only if the interference of nitrites is to be eliminated (see 5220A.2 above).
g. Potassium hydrogen phthalate (KHP) standard, HOOCC6H4COOK: Lightly crush and then dry KHP to constant weight at 110°C. Dissolve 425 mg in distilled water and dilute to 1000 mL. KHP has a theoretical COD1 of 1.176 mg O2/mg and this solution has a theoretical COD of 500 µg O2/ mL. This solution is stable when refrigerated, but not indefinitely. Be alert to development of visible biological growth. If practical, prepare and transfer solution under sterile conditions. Weekly preparation usually is satisfactory.
a. Treatment of samples with COD of >50 mg O2/L: Blend sample if necessary and pipet 50.00 mL into a 500-mL refluxing flask. For samples with a COD of >900 mg O2/L, use a smaller portion diluted to 50.00 mL. Add 1 g HgSO4, several glass beads, and very slowly add 5.0 mL sulfuric acid reagent, with mixing to dissolve HgSO4. Cool while mixing to avoid possible loss of volatile materials. Add 25.00 mL 0.04167M K2Cr2O7 solution and mix. Attach flask to condenser and turn on cooling water. Add remaining sulfuric acid reagent (70 mL) through open end of condenser. Continue swirling and mixing while adding sulfuric acid reagent. CAUTION: Mix reflux mixture thoroughly before applying heat to prevent local heating of flask bottom and a possible blowout of flask contents.
Cover open end of condenser with a small beaker to prevent foreign material from entering refluxing mixture and reflux for 2 h. Cool and wash down condenser with distilled water. Disconnect reflux condenser and dilute mixture to about twice its volume with distilled water. Cool to room temperature and titrate excess K2Cr2O7 with FAS, using 0.10 to 0.15 mL (2 to 3 drops) ferroin indicator. Although the quantity of ferroin indicator is not critical, use the same volume for all titrations. Take as the end point of the titration the first sharp color change from blue-green to reddish brown that persists for 1 min or longer. Duplicate determinations should agree within 5% of their average. Samples with suspended solids or components that are slow to oxidize may require additional determinations. The blue-green may reappear. In the same manner, reflux and titrate a blank containing the reagents and a volume of distilled water equal to that of sample.
b. Alternate procedure for low-COD samples: Follow procedure of ¶ 4a, with two exceptions: (i) use standard 0.004167M K2Cr2O7, and (ii) titrate with standardized 0.025M FAS. Exercise extreme care with this procedure because even a trace of organic matter on the glassware or from the atmosphere may cause gross errors. If a further increase in sensitivity is required, concentrate a larger volume of sample before digesting under reflux as follows: Add all reagents to a sample larger than 50 mL and reduce total volume to 150 mL by boiling in the refluxing flask open to the atmosphere without the condenser attached. Compute amount of HgSO4 to be added (before concentration) on the basis of a weight ratio of 10:1, HgSO4:Cl¯, using the amount of Cl¯ present in the original volume of sample. Carry a blank reagent through the same procedure. This technique has the advantage of concentrating the sample without significant losses of easily digested volatile materials. Hard-to-digest volatile materials such as volatile acids are lost, but an improvement is gained over ordinary evaporative concentration methods. Duplicate determinations are not expected to be as precise as in 5220B.4a.
c. Determination of standard solution: Evaluate the technique and quality of reagents by conducting the test on a standard potassium hydrogen phthalate solution.
(A – B)X M X 8000
COD as mg O2/L =
A = mL FAS used for blank,
B = mL FAS used for sample,
M = molarity of FAS, and
8000 = milliequivalent weight of oxygenX 1000 mL/L.
6. Precision and Bias
A set of synthetic samples containing potassium hydrogen phthalate and NaCl was tested by 74 laboratories. At a COD of 200 mg O2/L in the absence of chloride, the standard deviation was ± 13 mg/L (coefficient of variation, 6.5%). At COD of 160 mg O2/L and 100 mg Cl¯/L, the standard deviation was ± 14 mg/L (coefficient of variation, 10.8%).
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MOORE, W.A., R.C. KRONER & C.C. RUCHHOFT. 1949. Dichromate reflux method for determination of oxygen consumed. Anal. Chem. 21:953.
MEDALIA, A.I. 1951. Test for traces of organic matter in water. Anal. Chem. 23:1318.
MOORE, W.A., F.J. LUDZACK & C.C. RUCHHOFT. 1951. Determination of oxygen-consumed values of organic wastes. Anal. Chem. 23:1297.
DOBBS, R.A. & R.T. WILLIAMS. 1963. Elimination of chloride interference in the chemical oxygen demand test. Anal. Chem. 35:1064.
©Standard Methods for the Examination of Water and Wastewater. 20th Ed. American Public Health Association, American Water Works Association, Water Environment Federation.