Organic matter and its measurement

Measuring the amount of organic matter

There are various methods for measuring the amount of organic matter in water. These methods include measuring the volatile fraction, measuring total solids, BOD and COD. Because measuring the volatile fraction and measuring total solids has a relatively large error, they are used more than measuring biochemical oxygen demand (BOD) and chemical oxygen demand (COD) to obtain the amount of organic matter in water.

Biochemical Oxygen Demand (BOD)

Those substances that are decomposed by bacteria are measured by this index. In this measurement, instead of determining the amount of substances that are oxidized, the amount of oxygen that aerobic decomposing microorganisms require to oxidize that amount of substance is measured. In this method, a 5-day standard is used, represented by (BOD5), and it is a measurement of the oxygen consumed by bacteria in water over 5 days at 20°C to oxidize organic matter in the water. Therefore, to measure BOD, the difference between dissolved oxygen at the time of sampling and after 5 days in the sample must be obtained.

BOD5=DO0-DO5

Chemical Oxygen Demand (COD)

With the help of this index, the amount of organic matter present in water that can be oxidized with the help of a strong oxidizer such as chromic acid or permanganate in an acidic environment can be obtained. COD is the total chemical oxygen demand consumed in the oxidation and decomposition reactions of organic matter in an acidic environment and in the presence of a strong oxidizing agent.

The advantages of this method over BOD are that it is easier to perform, does not take time, and the entire measurement operation is completed in a few hours. It can also measure both biodegradable and non-biodegradable organic matter. The main reaction is carried out with dichromate as a strong oxidizer.

Cr³⁺ + CO₂ + H₂O → Cr₂O₇^2- + H⁺ + organic matter

Usually, the COD of a sample is higher than its BOD because, as mentioned above, both biodegradable and non-biodegradable substances can participate in the COD method.

Total Organic Carbon (TOC)

Another method used to measure organic matter in water, especially for trace amounts of organic matter in water, is the TOC method. In this method, a portion of the sample is injected into a furnace at a very high temperature and burned, and the amount of CO produced is measured by an infrared device. Now, with the help of the amount of CO obtained, the amount of organic matter in the sample can be calculated. Of course, in this method, since inorganic carbon is also measured, the amount of inorganic carbon in the water must be measured before performing the above test. After the difference between inorganic carbon and carbon obtained from the TOC method, the amount of carbon related to organic materials is obtained.

Total Oxygen Demand (TOD)

Another useful method used to determine organic materials is the TOC method. In this method, a portion of the sample is converted into stable final products in a furnace in the vicinity of a platinum catalyst. Then, the oxygen present in the nitrogen carrier gas is calculated and the results are adjusted to the COD method.