2172:
373:
828:
133:
932:
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 Cr can be accurately determined. To do so, the excess
1496:
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
949:
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
544:
505:
368:{\displaystyle {\mbox{C}}_{n}{\mbox{H}}_{a}{\mbox{O}}_{b}{\mbox{N}}_{c}+\left(n+{\frac {a}{4}}-{\frac {b}{2}}-{\frac {3}{4}}c\right){\mbox{O}}_{2}\rightarrow n{\mbox{CO}}_{2}+\left({\frac {a}{2}}-{\frac {3}{2}}c\right){\mbox{H}}_{2}{\mbox{O}}+c{\mbox{NH}}_{3}}
1459:(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.
1309:
1321:
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.
1040:
923:
reactions, one reagent is oxidized and the other is reduced), forming Cr. The amount of Cr is determined after oxidization is complete and is used as an indirect measure of the organic contents of the water sample.
823:{\displaystyle {\ce {C}}_{n}{\ce {H}}_{a}{\ce {O}}_{b}{\ce {N}}_{c}\ +\ d{\ce {Cr2O7^2-}}\ +\ (8d\ +\ c){\ce {H+ ->}}n{\ce {CO2}}\ +\ {\frac {a+8d-3c}{2}}{\ce {H2O}}\ +\ c{\ce {NH4+}}\ +2d{\ce {Cr^3+}}}
392:
910:
916:
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.
1408:
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
1155:
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
945:
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
1169:
950:
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.
517:
1695:
1060:
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
2061:
1490:
1640:
521:
1516:
984:
513:, the oxidizing agent for COD determination, does not oxidize ammonia into nitrate, so nitrification is not included in the standard COD test.
1770:
1667:
1057:
is the normality of FAS. If milliliters are used consistently for volume measurements, the result of the COD calculation is given in mg/L.
2056:
2217:
1926:
1615:
1596:
2207:
1417:
2016:
1986:
1956:
1755:
1455:
of organic substances. Potassium permanganate's effectiveness at oxidizing organic compounds varied widely, and in many cases,
2202:
1775:
500:{\displaystyle {\mbox{N}}{\mbox{H}}_{3}+2{\mbox{O}}_{2}\rightarrow {\mbox{N}}{\mbox{O}}_{3}^{-}+{\mbox{H}}_{3}{\mbox{O}}^{+}}
919:
In the process of oxidizing the organic substances found in the water sample, potassium dichromate is reduced (since in all
2006:
1830:
1534:
2197:
1931:
2071:
1838:
941:(FAS) until all of the excess oxidizing agent has been reduced to Cr. Typically, the oxidation-reduction indicator
1843:
1538:
1525:
1510:
1456:
96:
2212:
1660:
1163:
is often the most serious source of interference. Its reaction with potassium dichromate follows the equation:
946:
938:
2031:
1971:
1898:
1740:
510:
20:
1888:
1883:
1431:
913:
836:
2097:
1908:
1475:
1471:
531:
40:
1109:
RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO
534:
is a strong oxidizing agent under acidic conditions. Acidity is usually achieved by the addition of
2011:
1996:
1951:
1750:
1653:
1528: – calculated amount of oxygen required to oxidize a compound to its final oxidation products
2222:
2137:
2102:
2046:
1966:
1941:
1710:
1690:
1626:
1564:
1304:{\displaystyle \mathrm {6Cl^{-}+Cr_{2}O_{7}^{2-}+14H^{+}\rightarrow 3Cl_{2}+2Cr^{3+}+7H_{2}O} }
2176:
1961:
1858:
1705:
1611:
1592:
1412:, a maximum oxygen demand between 200 and 1000 mg/L must be reached before wastewater or
1349:
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2127:
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107:
The basis for the COD test is that nearly all organic compounds can be fully oxidized to
2152:
2132:
2122:
2117:
2107:
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2026:
1921:
1853:
1795:
1780:
1725:
1463:
959:
108:
2191:
1810:
1735:
1730:
1362:
535:
379:
88:
72:
2157:
2041:
1991:
1863:
1820:
1760:
2147:
2142:
2066:
2021:
1981:
1878:
1785:
1409:
67:. The most common application of COD is in quantifying the amount of oxidizable
16:
Measure of the amount of oxygen that can be consumed by reactions in a solution
2112:
1976:
1676:
1156:
84:
520:
describes a standard method for measuring chemical oxygen demand in ISO 6060
1946:
1893:
1745:
1438:
1435:
1117:
For example, if a sample has 500 Wppm (Weight Parts per
Million) of phenol:
934:
68:
52:
970:
O in distilled water, and the resulting red solution is diluted to 100 mL.
538:. The reaction of potassium dichromate with organic compounds is given by:
1447:) was used for measuring chemical oxygen demand. Measurements were called
1873:
1765:
1700:
1339:
1160:
1035:{\displaystyle \mathrm {COD} ={\frac {8000(b-s)n}{\text{sample volume}}}}
92:
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ISO 6060: Water quality - Determination of the chemical oxygen demand
1610:(20th ed.). Washington, DC: American Public Health Association.
1441:
120:
48:
32:
1318:
can be added to the sample prior to the addition of other reagents.
1497:
oxidant remaining in the sample. As with all colorimetric methods,
1720:
1537: – Ways to test the suitability of wastewater discusses both
1493:, and can nearly completely oxidize almost all organic compounds.
920:
80:
64:
56:
1606:
Lenore S. Clescerl; Arnold E. Greenberg; Andrew D. Eaton (1998).
1474:
have been used to determine COD. Of these, potassium dichromate (
116:
76:
44:
1649:
1645:
1106:
FW = Formula weight of the oxidizable compound in the sample,
378:
This expression does not include the oxygen demand caused by
59:). A COD test can be used to quickly quantify the amount of
1868:
1501:
are used to control for contamination by outside material.
123:
required to oxidize an organic compound to carbon dioxide,
1608:
Standard
Methods for Examination of Water & Wastewater
1587:
Clair N. Sawyer; Perry L. McCarty; Gene F. Parkin (2003).
1489:) is the most effective: it is relatively cheap, easy to
757:
703:
619:
51:
of solution, which in SI units is milligrams per liter (
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Pages displaying short descriptions of redirect targets
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monohydrate is added to a solution of 695 mg FeSO
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Pages displaying wikidata descriptions as a fallback
523:. However, this ISO standard was withdrawn in 2024.
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1513: – Oxygen needed to remove organics from water
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1034:
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499:
367:
1053:is the volume of FAS in the original sample, and
91:by providing a metric to determine the effect an
978:The following formula is used to calculate COD:
1049:is the volume of FAS used in the blank sample,
518:International Organization for Standardization
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8:
31:) is an indicative measure of the amount of
1462:Since then, other oxidizing agents such as
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954:Preparation of ferroin indicator reagent
1591:(5th ed.). New York: McGraw-Hill.
1556:
1517:Carbonaceous biochemical oxygen demand
1541:and COD as measures of water quality.
7:
1565:"General Chemistry Online: Glossary"
1314:To eliminate chloride interference,
1451:from permanganate, rather than the
1416:can be returned to the environment
2057:Ultraviolet germicidal irradiation
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1146:COD = (500/94)·7·16*2 = 1192 Wppm
2171:
2170:
1084:), using the following formula:
382:, the oxidation of ammonia into
1987:Industrial wastewater treatment
1957:Decentralized wastewater system
97:biochemical oxygen demand (BOD)
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43:. It is commonly expressed in
1:
2007:Rotating biological contactor
1535:Wastewater quality indicators
87:. COD is useful in terms of
1427:For many years, the strong
958:A solution of 1.485 g 1,10-
2241:
2072:Wastewater treatment plant
1839:Adsorbable organic halides
1627:"General Chemistry Online"
527:Using potassium dichromate
119:conditions. The amount of
2166:
1844:Biochemical oxygen demand
1526:Theoretical oxygen demand
1511:Biochemical oxygen demand
1457:biochemical oxygen demand
1072:O (assume all H goes to H
127:, and water is given by:
2218:Water quality indicators
1080:(assume all N goes to NH
1064:(assume all C goes to CO
947:ferrous ammonium sulfate
939:ferrous ammonium sulfate
933:potassium dichromate is
912:. Most commonly, a 0.25
47:of oxygen consumed over
35:that can be consumed by
2208:Environmental chemistry
2032:Sewage sludge treatment
1972:Fecal sludge management
1932:API oil–water separator
1899:Wastewater surveillance
21:environmental chemistry
1889:Total suspended solids
1884:Total dissolved solids
1849:Chemical oxygen demand
1432:potassium permanganate
1305:
1151:Inorganic interference
1036:
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2203:Chemical oceanography
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1569:antoine.frostburg.edu
1404:Government regulation
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2198:Anaerobic digestion
2012:Secondary treatment
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1751:Infiltration/Inflow
1328:Inorganic molecule
1223:
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2177:Category: Sewerage
2138:Septic drain field
2103:Infiltration basin
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2128:Reclaimed water
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2002:Reverse osmosis
1903:
1825:
1791:Reverse osmosis
1716:Boiler blowdown
1679:
1674:
1637:
1632:
1625:
1618:
1605:
1599:
1586:
1582:
1577:
1576:
1563:
1562:
1558:
1553:
1548:
1529:
1520:
1507:
1487:
1483:
1479:
1449:oxygen consumed
1446:
1429:oxidizing agent
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1406:
1370:
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1180:
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2133:Sanitary sewer
2130:
2125:
2123:Marine outfall
2120:
2118:Marine dumping
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2108:Injection well
2105:
2100:
2095:
2090:
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2082:
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2074:
2069:
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2039:
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111:with a strong
109:carbon dioxide
104:
101:
39:in a measured
15:
13:
10:
9:
6:
4:
3:
2:
2236:
2235:
2224:
2221:
2219:
2216:
2214:
2211:
2209:
2206:
2204:
2201:
2199:
2196:
2195:
2193:
2178:
2169:
2168:
2165:
2159:
2156:
2154:
2151:
2149:
2146:
2144:
2141:
2139:
2136:
2134:
2131:
2129:
2126:
2124:
2121:
2119:
2116:
2114:
2111:
2109:
2106:
2104:
2101:
2099:
2096:
2094:
2091:
2089:
2086:
2085:
2083:
2079:
2073:
2070:
2068:
2065:
2063:
2060:
2058:
2055:
2053:
2050:
2048:
2045:
2043:
2040:
2038:
2035:
2033:
2030:
2028:
2025:
2023:
2020:
2018:
2017:Sedimentation
2015:
2013:
2010:
2008:
2005:
2003:
2000:
1998:
1995:
1993:
1990:
1988:
1985:
1983:
1980:
1978:
1975:
1973:
1970:
1968:
1965:
1963:
1960:
1958:
1955:
1953:
1950:
1948:
1945:
1943:
1940:
1938:
1935:
1933:
1930:
1928:
1925:
1923:
1920:
1918:
1915:
1914:
1912:
1910:
1906:
1900:
1897:
1895:
1892:
1890:
1887:
1885:
1882:
1880:
1877:
1875:
1872:
1870:
1867:
1865:
1862:
1860:
1857:
1855:
1852:
1850:
1847:
1845:
1842:
1840:
1837:
1836:
1834:
1832:
1828:
1822:
1819:
1817:
1814:
1812:
1811:Sewage sludge
1809:
1807:
1804:
1802:
1799:
1797:
1794:
1792:
1789:
1787:
1784:
1782:
1779:
1777:
1774:
1772:
1769:
1767:
1764:
1762:
1759:
1757:
1754:
1752:
1749:
1747:
1744:
1742:
1739:
1737:
1736:Cooling water
1734:
1732:
1731:Cooling tower
1729:
1727:
1724:
1722:
1719:
1717:
1714:
1712:
1709:
1707:
1704:
1702:
1699:
1697:
1696:Ballast water
1694:
1692:
1689:
1688:
1686:
1682:
1678:
1671:
1666:
1664:
1659:
1657:
1652:
1651:
1648:
1642:
1639:
1638:
1634:
1631:
1628:
1619:
1617:0-87553-235-7
1613:
1609:
1604:
1600:
1598:0-07-248066-1
1594:
1590:
1585:
1584:
1579:
1570:
1566:
1560:
1557:
1550:
1545:
1540:
1536:
1533:
1527:
1524:
1518:
1515:
1512:
1509:
1508:
1504:
1502:
1500:
1494:
1492:
1488:
1473:
1469:
1465:
1460:
1458:
1454:
1453:oxygen demand
1450:
1443:
1440:
1437:
1433:
1430:
1422:
1420:
1418:
1415:
1411:
1403:
1396:
1393:
1391:
1388:
1387:
1383:
1380:
1378:
1375:
1374:
1366:
1364:
1363:Sulfamic acid
1361:
1359:
1356:
1355:
1351:
1348:
1346:
1343:
1341:
1338:
1337:
1333:
1330:
1327:
1326:
1323:
1319:
1317:
1292:
1284:
1281:
1276:
1273:
1262:
1259:
1254:
1243:
1235:
1227:
1224:
1219:
1216:
1211:
1201:
1190:
1185:
1174:
1166:
1165:
1164:
1162:
1158:
1150:
1145:
1144:
1120:
1119:
1118:
1108:
1105:
1102:
1099:
1098:
1097:
1092:/FW)·(RMO)·32
1091:
1087:
1086:
1085:
1058:
1056:
1052:
1048:
1028:sample volume
1023:
1017:
1014:
1011:
1005:
999:
981:
980:
979:
973:
971:
961:
953:
951:
948:
944:
940:
936:
927:
925:
922:
917:
915:
899:
895:
891:
888:
885:
881:
877:
874:
871:
867:
863:
860:
857:
853:
849:
846:
843:
840:
814:
811:
802:
799:
796:
787:
782:
773:
767:
752:
741:
737:
734:
731:
728:
725:
722:
719:
710:
698:
689:
680:
668:
662:
656:
653:
644:
635:
632:
627:
614:
605:
599:
591:
579:
567:
555:
541:
540:
539:
537:
536:sulfuric acid
533:
526:
524:
522:
519:
514:
512:
492:
478:
466:
461:
456:
432:
420:
417:
412:
389:
388:
387:
385:
381:
380:nitrification
360:
348:
345:
333:
320:
316:
311:
308:
303:
298:
295:
289:
285:
280:
268:
260:
247:
243:
238:
235:
230:
225:
222:
217:
212:
209:
204:
201:
197:
193:
188:
174:
160:
146:
130:
129:
128:
126:
122:
118:
114:
110:
102:
100:
98:
94:
90:
89:water quality
86:
82:
78:
74:
73:surface water
70:
66:
62:
58:
54:
50:
46:
42:
38:
34:
30:
26:
22:
2158:Vacuum sewer
2042:Sewer mining
1992:Ion exchange
1942:Chlorination
1864:Heavy metals
1848:
1821:Urban runoff
1761:Ion exchange
1741:Fecal sludge
1624:
1607:
1588:
1568:
1559:
1495:
1461:
1452:
1448:
1426:
1407:
1377:Ferrous iron
1320:
1313:
1154:
1116:
1100:
1095:
1089:
1059:
1054:
1050:
1046:
1044:
977:
974:Calculations
957:
931:
918:
832:
530:
515:
509:
377:
106:
28:
24:
18:
2148:Storm drain
2143:Sewage farm
2067:Vermifilter
2022:Septic tank
1982:Imhoff tank
1879:Temperature
1786:Return flow
1776:Papermaking
1410:Switzerland
2192:Categories
2113:Irrigation
1977:Filtration
1677:Wastewater
1546:References
1157:wastewater
1076:O), and NH
511:Dichromate
85:wastewater
69:pollutants
2223:Limnology
1947:Clarifier
1894:Turbidity
1746:Greywater
1551:Citations
1241:→
1220:−
1186:−
1015:−
889:−
875:−
732:−
686:⟶
636:−
462:−
438:→
304:−
266:→
231:−
218:−
71:found in
37:reactions
1874:Salinity
1766:Leachate
1701:Bathroom
1505:See also
1390:Sulfides
1352:complex
1340:Chloride
1161:chloride
935:titrated
103:Overview
93:effluent
61:organics
41:solution
1801:Septage
1580:Sources
1423:History
1358:Nitrite
1129:OH + 7O
1096:Where
1088:COD = (
943:ferroin
384:nitrate
125:ammonia
2175:
1816:Toilet
1806:Sewage
1771:Manure
1614:
1595:
1499:blanks
1491:purify
1470:, and
1045:where
833:where
794:
771:
765:
714:
708:
666:
660:
648:
642:
603:
597:
121:oxygen
117:acidic
115:under
81:rivers
75:(e.g.
49:volume
33:oxygen
23:, the
1721:Brine
1133:→ 6CO
937:with
921:redox
83:) or
77:lakes
65:water
2062:UASB
1612:ISBN
1593:ISBN
1371:gas
1137:+ 3H
1068:), H
1006:8000
516:The
79:and
45:mass
1539:BOD
966:·7H
63:in
29:COD
19:In
2194::
1869:pH
1567:.
1480:Cr
1466:,
1439:Mn
1419:.
1397:-
1394:-
1384:-
1381:-
1228:14
1159:,
808:Cr
779:NH
695:CO
611:Cr
386::
355:NH
275:CO
99:.
53:mg
1669:e
1662:t
1655:v
1629:.
1620:.
1601:.
1571:.
1486:7
1484:O
1482:2
1478:2
1476:K
1445:4
1442:O
1436:K
1434:(
1369:2
1367:N
1298:O
1293:2
1289:H
1285:7
1282:+
1277:+
1274:3
1270:r
1266:C
1263:2
1260:+
1255:2
1251:l
1247:C
1244:3
1236:+
1232:H
1225:+
1217:2
1212:7
1208:O
1202:2
1198:r
1194:C
1191:+
1182:l
1178:C
1175:6
1141:O
1139:2
1135:2
1131:2
1127:5
1125:H
1123:6
1121:C
1111:2
1101:C
1090:C
1082:3
1078:3
1074:2
1070:2
1066:2
1062:2
1055:n
1051:s
1047:b
1024:n
1021:)
1018:s
1012:b
1009:(
1000:=
996:D
993:O
990:C
968:2
964:4
914:N
900:2
896:/
892:c
886:3
882:/
878:b
872:6
868:/
864:a
861:+
858:3
854:/
850:n
847:2
844:=
841:d
815:+
812:3
803:d
800:2
797:+
788:+
783:4
774:c
768:+
761:O
753:2
749:H
742:2
738:c
735:3
729:d
726:8
723:+
720:a
711:+
699:2
690:n
681:+
677:H
672:)
669:c
663:+
657:d
654:8
651:(
645:+
633:2
628:7
624:O
615:2
606:d
600:+
592:c
587:N
580:b
575:O
568:a
563:H
556:n
551:C
493:+
487:O
479:3
473:H
467:+
457:3
451:O
443:N
433:2
427:O
421:2
418:+
413:3
407:H
399:N
361:3
349:c
346:+
341:O
334:2
328:H
321:)
317:c
312:2
309:3
299:2
296:a
290:(
286:+
281:2
269:n
261:2
255:O
248:)
244:c
239:4
236:3
226:2
223:b
213:4
210:a
205:+
202:n
198:(
194:+
189:c
183:N
175:b
169:O
161:a
155:H
147:n
141:C
57:L
55:/
27:(
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