Chemical oxygen demand

Lore & Background

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 acidic conditions. The amount of oxygen required to oxidize an organic compound to carbon dioxide, ammonia, and water is given by a specific chemical equation, though this expression does not include the oxygen demand caused by nitrification. Dichromate, the oxidizing agent for COD determination, does not oxidize ammonia into nitrate, so nitrification is not included in the standard COD test.

Potassium dichromate is a strong oxidizing agent under acidic conditions, with acidity usually achieved by the addition of sulfuric acid. In the process of oxidizing organic substances, potassium dichromate 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.

An excess amount of potassium dichromate must be present for complete oxidation. Once oxidation is complete, the excess potassium dichromate is titrated with ferrous ammonium sulfate (FAS) until all excess oxidizing agent has been reduced to Cr3+. The oxidation-reduction indicator ferroin is added during this titration step, changing from blue-green to reddish brown at the endpoint.

Reader's Guide

Chemical oxygen demand is significant as a metric for water quality, particularly in quantifying the amount of oxidizable pollutants in surface water and wastewater. It is useful for determining the effect an effluent will have on the receiving body, similar to biochemical oxygen demand (BOD). The test relies on the complete oxidation of organic compounds using a strong oxidizing agent under acidic conditions, with potassium dichromate being the standard choice. The method involves measuring the amount of Cr3+ produced as an indirect measure of organic content, with excess dichromate titrated using ferrous ammonium sulfate and ferroin indicator. The International Organization for Standardization described a standard method in ISO 6060, though this standard was withdrawn in 2024. The test does not include nitrification, as dichromate does not oxidize ammonia into nitrate. For samples with COD below 50 mg/L, a lower concentration of potassium dichromate is preferred.

Did You Know?

• The standard COD test does not include oxygen demand from nitrification.
• Potassium dichromate is reduced to Cr3+ during the oxidation of organic substances.
• Ferroin indicator changes from blue-green to reddish brown at the titration endpoint.

The Universal Solvent and the Chemistry of Life

Water, with the formula H2O, is a polar inorganic compound whose strong intermolecular hydrogen bonding gives it a remarkable capacity to interact with other molecules. Because of this polarity relative to its tiny molecular size, it dissolves more substances than any other liquid on Earth, earning the title "universal solvent." Yet it is notably poor at dissolving nonpolar materials, a limitation that shapes the chemistry of natural waters. In every known living organism, water serves as the internal fluid and solvent, making it indispensable to life even though it contributes no caloric energy and is not classified as an organic micronutrient. Natural water virtually always carries a mixture of dissolved minerals and organic compounds, and obtaining truly chemically pure water requires deliberate, special procedures. This constant state of carrying dissolved substances—both mineral and organic—makes water the medium through which countless chemical interactions unfold, from the cells of a fish to the depths of an ocean.

Three States and the Eternal Cycle

Because Earth's surface temperature and pressure sit close to water's triple point, the compound H2O is unique among common substances in existing simultaneously as a solid, a liquid, and a gas under ordinary terrestrial conditions. Ice, liquid water, and steam or water vapor all coexist across the planet. Roughly 71 percent of Earth's surface is covered by water, and the vast majority—about 96.5 percent of the total volume—resides in seas and oceans. Smaller fractions appear as groundwater (1.7 percent), in the glaciers and ice caps of Antarctica and Greenland (another 1.7 percent), and as vapor, clouds, and precipitation in the atmosphere (a mere 0.001 percent). These reservoirs are never static. Water continuously circulates through a cycle of evaporation, transpiration, condensation, precipitation, and runoff, typically ending its journey back in the sea. Rain falls, fog forms as aerosols, clouds hold suspended droplets of liquid and ice, and finely divided crystalline ice drifts down as snow.

The Density Anomaly That Shields Life

Most substances grow denser as they cool, but liquid water defies this expectation within a critical temperature range. At one atmosphere of pressure, water reaches its peak density of approximately 999.972 kilograms per cubic meter at 3.98 degrees Celsius. Below that temperature, down to the freezing point of 0 degrees, it actually expands and becomes less dense. When it finally solidifies into ice, the volume increases by roughly nine percent, dropping the density to about 917 kilograms per cubic meter. This expansion is powerful enough to burst pipes and split rocks. In a lake or ocean, the coldest liquid water at four degrees sinks to the bottom while ice forms and floats on the surface. That floating ice acts as an insulating blanket, keeping the water beneath from freezing solid. Without this protective layer, most aquatic organisms living in temperate lakes would die through the winter. The same anomalous behavior also plays a key role in thermohaline circulation, the process that distributes heat throughout the planet's oceans.

Economic Backbone and Industrial Workhorse

Water's economic footprint is enormous and multifaceted. Around seventy percent of all fresh water drawn by humans is devoted to agriculture, underscoring its centrality to food production. Fishing in both saltwater and freshwater bodies remains a major source of sustenance worldwide, supplying roughly 6.5 percent of global protein intake. Beyond food, much of the long-distance trade in commodities—oil, natural gas, manufactured goods—moves by boat across seas, rivers, lakes, and canals. In industry and in homes, large quantities of water, ice, and steam are employed for heating and cooling. Because water dissolves a wide range of both mineral and organic substances, it is indispensable in industrial processing, cooking, and washing. Its role extends into recreation as well: swimming, pleasure boating, boat racing, surfing, sport fishing, diving, ice skating, snowboarding, and skiing all depend on water in its liquid, solid, or gaseous form. The word itself traces back through Old English wæter to a Proto-Indo-European root meaning "water" or "wet," a lineage shared with Greek hydor, Russian voda, Irish uisce, and Albanian ujë.