COD (Chemical Oxygen Demand) Analysis

Chemical oxygen demand, also known as COD, is the amount of oxygen consumed when oxidizable substances (such as organic matter, nitrite, ferrous salts, sulfides, etc.) in water are oxidized by a chemical oxidant (such as potassium permanganate). It is calculated based on the amount of remaining oxidant. Similar to biochemical oxygen demand (BOD), it's a key indicator of water pollution. COD is measured in ppm or mg/L; lower values indicate less severe water pollution.

Water contains various reducible substances, including organic matter, nitrites, sulfides, and ferrous salts, with organic matter being the primary component. Therefore, COD is often used as an indicator of the amount of organic matter in water. Higher COD values indicate more severe organic pollution. COD values vary depending on the reducible substances in the water sample and the measurement method. The most common methods are the acidic potassium permanganate oxidation method and the potassium dichromate oxidation method. The potassium permanganate (KMnO4) method has a lower oxidation rate but is simpler. It can be used to determine the relative values of organic matter content in water samples and for clean surface and groundwater samples. The potassium dichromate (K2Cr2O7) method has a high oxidation rate and good reproducibility, suitable for determining the total amount of organic matter in wastewater monitoring.

Organic matter poses significant risks to industrial water systems. Water containing large amounts of organic matter will contaminate ion exchange resins when passing through desalination systems, especially anion exchange resins, reducing their exchange capacity. Approximately 50% of organic matter can be removed during pretreatment (coagulation, clarification, and filtration), but it cannot be removed in desalination systems and is often introduced into boilers through makeup water, lowering the boiler water pH. Sometimes, organic matter can also enter steam systems and condensate, lowering the pH and causing system corrosion. High organic matter content in circulating water systems promotes microbial growth. Therefore, lower COD is always preferred for desalination, boiler water, or circulating water systems, although there are no uniform limit indicators. In circulating cooling water systems, when COD (KMnO4 method) > 5 mg/L, the water quality begins to deteriorate.

COD Determination Methods

Potassium Dichromate Standard Method (also known as the reflux method)

(a) Principle

A certain amount of potassium dichromate and the catalyst silver sulfate are added to the water sample. In a strongly acidic medium, the mixture is heated under reflux for a certain time. Part of the potassium dichromate is reduced by oxidizable substances in the water sample. The remaining potassium dichromate is titrated with ammonium ferrous sulfate, and the COD value is calculated based on the amount of potassium dichromate consumed. COD Value.

Since this standard was established in 1989, there are many shortcomings when measured by current standards:

1. It is time-consuming; each sample requires a 2-hour reflux.

2. The reflux equipment is bulky, making batch determination difficult.

3. The analysis cost is high, especially for silver sulfate.

4. During the determination process, the waste of reflux water is astonishing.

5. Toxic mercury salts can cause secondary pollution.

6. Large reagent consumption and high consumable costs.

7. The testing process is complex and not easy to promote.

(b) Equipment

1. 250mL all-glass reflux apparatus

2. Heating device (electric furnace)

3. 25mL or 50mL acid burette, Erlenmeyer flask, pipette, volumetric flask, etc.

(c) Reagents

1. Potassium dichromate standard solution (c1/6K2Cr2O7 = 0.2500 mol/L)

2. Ferroin indicator solution

3. Ammonium ferrous sulfate standard solution [c(NH4)2Fe(SO4)2·6H2O ≈ 0.1 mol/L] (standardize before use)

4. Sulfuric acid-silver sulfate solution

(d) Determination Steps

Ammonium ferrous sulfate standardization: Accurately pipette 10.00mL of potassium dichromate standard solution into a 500mL Erlenmeyer flask, dilute with water to about 110mL, slowly add 30mL of concentrated sulfuric acid, and shake well. After cooling, add 3 drops of ferroin indicator solution (about 0.15mL), and titrate with ammonium ferrous sulfate solution until the solution color changes from yellow to blue-green to reddish-brown, which is the endpoint.

(e) Determination

Take 20mL of water sample (if necessary, take less and add water to 20mL or dilute before taking), add 10mL of potassium dichromate, insert the reflux apparatus, then add 30mL of sulfuric acid-silver sulfate solution, heat and reflux for 2h, cool, and rinse the condenser wall with 90.00mL of water, remove the Erlenmeyer flask.

After the solution cools again, add 3 drops of ferroin indicator solution, titrate with ammonium ferrous sulfate standard solution until the solution color changes from yellow to blue-green to reddish-brown, which is the endpoint. Record the volume of ammonium ferrous sulfate standard solution used.

Simultaneously with the determination of the water sample, take 20.00mL of redistilled water and perform a blank experiment using the same procedure. Record the volume of ammonium ferrous sulfate standard solution used in the blank titration.

(f) Calculation

CODCr (O2, mg/L) = [8 × 1000 (V0 - V1) · C] / V

(g) Precautions

1. Up to 40mg of chloride ions can be complexed using 0.4g of mercuric sulfate. For a 20.00mL water sample, this allows for complexing up to 2000mg/L of chloride ions. If the chloride concentration is lower, less mercuric sulfate can be added, maintaining a mercuric sulfate:chloride ion ratio of 10:1 (W/W). A small amount of mercuric chloride precipitate does not affect the determination.

2. This method determines COD in the range of 50-500mg/L. For water samples with a chemical oxygen demand of less than 50mg/L, a 0.0250mol/L potassium dichromate standard solution should be used. A 0.01mol/L ammonium ferrous sulfate standard solution should be used for back titration. Water samples with COD greater than 500mg/L should be diluted before measurement.

3. After heating and refluxing the water sample, the remaining amount of potassium dichromate in the solution should ideally be 1/5 to 4/5 of the initial amount.

4. When using potassium hydrogen phthalate standard solution to check reagent quality and operational techniques, since the theoretical CODCr of 1g of potassium hydrogen phthalate is 1.176g, dissolve 0.4251g of potassium hydrogen phthalate (HOOCC6H4COOK) in redistilled water, transfer to a 1000mL volumetric flask, and dilute to the mark with redistilled water to make a 500mg/L CODcr standard solution. Prepare fresh when needed.

5. The CODCr determination results should be retained to four significant figures.

6. The ammonium ferrous sulfate standard titrant solution should be standardized for each experiment, especially paying attention to its concentration changes at higher temperatures. (Alternatively, 10.0ml of potassium dichromate standard solution can be added to the blank after titration and titrated with ammonium ferrous sulfate to the endpoint.)

7. Water samples should be fresh and tested as soon as possible.

Rapid Digestion Spectrophotometry

(a) Principle

A known amount of potassium dichromate solution is added to the sample, and in a strong sulfuric acid medium, with silver sulfate as a catalyst, after high-temperature digestion, the photometric method is used to determine COD value.

Because this method has a short determination time, low secondary pollution, small reagent volume, and low cost, most laboratories currently use this method. However, this method has a higher instrument cost and lower operating cost, suitable for long-term testing COD unit usage.

(b) Equipment

Foreign equipment development started earlier, but the price is very high, and the determination time is longer, the reagent price is generally unaffordable for users, and the accuracy is not very high. This is because the monitoring standards of foreign instruments are different from those in China, mainly because the foreign water treatment level and management system are different from those in China;

The rapid digestion spectrophotometric method is mainly based on the common methods of domestic instruments, and catalyzes rapid determination The COD method is the standard for this method, which was invented in the early 1980s. After more than 30 years of application, it has become an environmental protection industry standard. Domestic 5B-type instruments have been widely used in scientific research and official monitoring. Domestic instruments have been widely used due to their price advantage and timely after-sales service.

(3) Determination Steps

Take 2.5ml sample --- add reagents --- digest for 10 minutes --- cool for 2 minutes --- pour into cuvette --- the instrument display directly shows the sample COD concentration.

(4) Precautions

1. High-chloride water samples should use high-chloride reagents.

2. The waste liquid is about 10ml, but it is highly acidic and should be collected and treated centrally.

3. Ensure that the light-transmitting surface of the cuvette is clean.