COD measurement is a water-quality analysis used to evaluate the amount of organic matter and reducing substances contained in water based on the amount of oxidizing agent consumed.
COD is called Chemical Oxygen Demand and is an important indicator for considering how polluted water is, particularly how much chemically oxidizable material it contains.
It is commonly used in water-quality evaluation of river water, lake water, wastewater, domestic wastewater, industrial wastewater, and other types of water.
In a discussion of COD measurement, it is not sufficient simply to write that “the titration volume was large” or that “the COD was high.”
It is necessary to explain what consumed the oxidizing agent, how oxidant consumption is related to the amount of organic matter, what can be considered about water quality when COD is high or low, and where measurement errors arise.
This article clearly explains, as examples of discussions that can be used in laboratory reports on COD measurement, the relationship between oxidant consumption and the amount of organic matter, how to interpret COD values, blank correction, incomplete oxidation reactions, titration endpoints, effects of coexisting substances, sources of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of COD measurement results obtained in environmental chemistry experiments, analytical chemistry experiments, and basic chemistry experiments at universities and similar institutions.
For the actual COD measurement method, type of oxidizing agent, heating conditions, titration procedures, calculation formula, units, handling of effluent standards and environmental standards, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is COD Measurement?
- Main Items to Include in the Results
- Reference Experimental Values and Calculation Examples for COD Measurement
- Reference Experimental Conditions
- Concept of COD Measurement
- Blank and Sample Titration Results
- Example Calculation of the Difference in Titration Volume
- Example Calculation of COD
- Example Calculation Including the Dilution Factor
- Effect of Using an Incorrect Dilution Factor
- Comparison of COD by Water Sample
- Example Comparison With BOD
- Changes in COD Depending on Heating Time
- Effect of Chloride Ions
- Effects of Turbidity and Suspended Matter
- Example of Checking Reproducibility
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- Relationship Between Oxidant Consumption and the Amount of Organic Matter
- Discussion When the COD Value Is High
- Discussion When the COD Value Is Low
- Relationship Between COD and BOD
- Concept of the Potassium Permanganate Method
- Importance of Blank Correction
- Relationship Between Titration Volume and COD Value
- Discussion When the Oxidation Reaction Is Incomplete
- Effects of Reducing Inorganic Substances
- Effect of Chloride Ions
- Effects of Sampling and Storage
- Effects of Water Turbidity and Color
- Effect of Dilution
- When the COD Measurement Can Be Considered Good
- Example Discussion When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing COD Measurement
- Summary
What Is COD Measurement?
COD is an abbreviation for Chemical Oxygen Demand.
It is a value representing, in terms of oxygen, how much oxidizing power is required when organic matter and reducing substances in water are oxidized by an oxidizing agent.
In general, the higher the COD, the more readily oxidizable substances are considered to be present in the water.
In COD measurement, oxidizing agents such as potassium permanganate or potassium dichromate are used to oxidize organic matter and other substances in water.
The COD, converted to an amount of oxygen, is then determined from the amount of oxidizing agent consumed.
In other words, COD does not directly measure the amount of substances in the water itself, but indirectly evaluates them from how much oxidizing agent is consumed.
Example Discussion:
In COD measurement, organic matter and reducing substances in water are oxidized with an oxidizing agent, and the chemical oxygen demand is determined from the amount of oxidizing agent consumed.
Because a COD value was obtained in this experiment, it was possible to evaluate how much readily oxidizable material was contained in the sample water.
COD is useful as an indicator of organic pollution, but it should be noted that not only organic matter but also reducing inorganic substances may affect the value.
Main Items to Include in the Results
In the results of COD measurement, organize the type of sample water, sampling location, sampling date and time, measurement method, type of oxidizing agent, heating conditions, titration volume, blank value, dilution factor, calculated COD value, and other information.
Because values in water-quality analysis may change depending on sampling and storage conditions, handling of the sample is also important information.
Main Items to Include in the Results
- Type of sample water
- Sampling location
- Sampling date and time
- Weather at the time of sampling
- Water temperature
- Color and turbidity of the sample
- COD measurement method
- Oxidizing agent used
- Heating temperature
- Heating time
- Reagent used for titration
- Titration volume
- Blank value
- Dilution factor
- Calculated COD value
- Comparison with other samples
- Sources of error and points for improvement
Example of How to Write the Results:
The COD was determined by reacting the sample water with an oxidizing agent and titrating the remaining oxidizing agent.
In the sample water, the amount of oxidizing agent consumed was greater than in the blank, and a COD value was calculated.
From this result, the sample water was considered to contain organic matter or reducing substances oxidized by the oxidizing agent.
Reference Experimental Values and Calculation Examples for COD Measurement
Here, the process of measuring chemical oxygen demand (COD) and evaluating the amount of organic matter in water from oxidant consumption is organized using reference experimental values.
COD is a water-quality indicator expressed as the amount of oxygen required to chemically oxidize organic matter and reducing substances in water.
In general, water with a high COD contains large amounts of readily oxidizable substances and may have advanced water pollution.
While BOD represents oxygen consumption by microorganisms, COD represents chemical oxygen consumption by an oxidizing agent.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Measurement target | Tap water, river water, pond water, domestic wastewater, food-factory wastewater |
| Measurement method | COD measurement by the potassium permanganate method |
| Sample volume | 100.0 mL |
| Oxidizing agent | 0.00500 mol/L KMnO4 standard solution |
| Reaction conditions | Heated reaction under acidic conditions |
| Heating time | 30 min |
| Titrant | 0.0125 mol/L sodium oxalate standard solution |
| Evaluation items | Oxidant consumption, COD, dilution factor, degree of pollution, sources of error |
Concept of COD Measurement
In COD measurement, a fixed amount of oxidizing agent is added to the sample water to oxidize organic matter and reducing substances in the water.
The amount of oxidizing agent remaining afterward is determined by titration, and the amount of oxidizing agent consumed by the sample is determined from the difference from the blank.
The greater the amount of oxidizing agent consumed, the more readily oxidizable substances are considered to be present in the water.
The value obtained by converting that consumption to an amount of oxygen is COD.
Blank and Sample Titration Results
Reference examples are shown in which the remaining oxidizing agent in the blank and each water sample was titrated with a sodium oxalate standard solution.
Here, the smaller the sample titration volume is compared with the blank titration volume, the more oxidizing agent is considered to have been consumed in the sample.
| Sample | Water Sample | Pretreatment | Blank Titration Volume | Sample Titration Volume | Difference in Titration Volume | COD | How to Interpret Water Quality |
|---|---|---|---|---|---|---|---|
| A | Tap water | None | 10.20 mL | 10.05 mL | 0.15 mL | 0.47 mg/L | Very low |
| B | River water | Filtration | 10.20 mL | 9.55 mL | 0.65 mL | 2.03 mg/L | Relatively good |
| C | Pond water | Filtration | 10.20 mL | 8.80 mL | 1.40 mL | 4.38 mg/L | Contains somewhat more organic matter |
| D | Domestic wastewater | Tenfold dilution | 10.20 mL | 7.10 mL | 3.10 mL | 96.9 mg/L | High |
| E | Food-factory wastewater | 50-fold dilution | 10.20 mL | 6.00 mL | 4.20 mL | 656 mg/L | Very high |
Example Calculation of the Difference in Titration Volume
In COD calculations, the difference between the blank titration volume and the sample titration volume is determined.
The larger the difference, the more oxidizing agent is considered to have been consumed in the sample.
Difference in titration volume = Blank titration volume − Sample titration volume
For river water, the blank titration volume is 10.20 mL and the sample titration volume is 9.55 mL.
Difference in titration volume = 10.20 − 9.55 = 0.65 mL
The amount of oxidizing agent corresponding to this 0.65 mL is considered to have been consumed by organic matter and reducing substances in the river water.
Example Calculation of COD
In this reference example, under the conditions of a 100.0 mL sample and the 0.00500 mol/L KMnO4 method, a difference in titration volume of 1.00 mL is treated as corresponding to a COD of 3.13 mg/L.
COD (mg/L) = Difference in titration volume (mL) × 3.13 × Dilution factor
For river water, the difference in titration volume is 0.65 mL and the dilution factor is 1.
COD = 0.65 × 3.13 × 1 = 2.03 mg/L
Therefore, in this reference example, the COD of the river water is determined to be 2.03 mg/L.
Example Calculation Including the Dilution Factor
For samples with high COD, such as domestic wastewater, direct measurement may cause the oxidizing agent to become insufficient or the reaction range to be exceeded, so the sample is diluted before measurement.
When domestic wastewater is diluted tenfold and measured, the difference in titration volume is 3.10 mL.
COD in diluted solution = 3.10 × 3.13 = 9.70 mg/L
Because the original sample was diluted tenfold, the COD in the original sample is multiplied by 10.
COD in original sample = 9.70 × 10 = 96.9 mg/L
Therefore, in this reference example, the COD of the domestic wastewater is determined to be 96.9 mg/L.
Effect of Using an Incorrect Dilution Factor
When a high-concentration sample is diluted before measurement, forgetting to include the dilution factor in the calculation causes the COD to be greatly underestimated.
| Calculation Condition | Calculated COD | Problem |
|---|---|---|
| Tenfold dilution correctly considered | 96.9 mg/L | Correct calculation |
| Dilution factor omitted | 9.70 mg/L | Underestimated to one-tenth |
| Calculated as a fivefold dilution | 48.5 mg/L | Underestimated to half |
| Calculated as a 50-fold dilution | 485 mg/L | Overestimated |
For diluted samples, it is important to confirm the collected volume, final volume, and aliquot volume and correctly calculate the concentration of the original sample.
Comparison of COD by Water Sample
COD can be used as an indicator for comparing the amounts of organic matter and reducing substances in water.
Here, the results of measuring multiple water samples under the same conditions are organized.
| Water Sample | COD | Appearance | Direction of Discussion |
|---|---|---|---|
| Tap water | 0.47 mg/L | Clear | Contains few readily oxidizable substances |
| River water | 2.03 mg/L | Almost clear | Relatively good |
| Pond water | 4.38 mg/L | Slightly green | Effects of algae and organic matter |
| Domestic wastewater | 96.9 mg/L | Turbid | Contains much organic matter originating from daily life |
| Food-factory wastewater | 656 mg/L | Strongly turbid | Contains very large amounts of food-derived organic matter |
In this reference example, COD is low in tap water and river water and high in domestic wastewater and food-factory wastewater.
Samples with high COD may have contained large amounts of readily oxidizable substances such as sugars, proteins, and fats.
Example Comparison With BOD
COD and BOD are both indicators of water pollution, but their meanings differ.
COD reflects substances that are chemically oxidized, while BOD reflects organic matter that can be decomposed by microorganisms.
| Water Sample | COD | BOD | BOD/COD Ratio | How to Interpret the Result |
|---|---|---|---|---|
| River water | 2.03 mg/L | 1.6 mg/L | 0.79 | Contains many readily biodegradable components |
| Pond water | 4.38 mg/L | 3.4 mg/L | 0.78 | Organic matter is relatively readily decomposed |
| Domestic wastewater | 96.9 mg/L | 31 mg/L | 0.32 | Contains many chemically oxidizable components as well |
| Food-factory wastewater | 656 mg/L | 235 mg/L | 0.36 | Organic-matter concentration is very high |
When the BOD/COD ratio is high, a large amount of organic matter readily decomposed by microorganisms is considered to be present.
On the other hand, when COD is high but BOD is relatively low, components that are chemically oxidized but not readily decomposed by microorganisms may be present.
Changes in COD Depending on Heating Time
In COD measurement, the sample and oxidizing agent are heated for a fixed period to allow the reaction to proceed.
If the heating time is short, oxidation may be insufficient and COD may be underestimated.
| Heating Time | Difference in Titration Volume | COD | How to Interpret the Result |
|---|---|---|---|
| 5 min | 0.90 mL | 2.82 mg/L | Insufficient oxidation |
| 15 min | 1.20 mL | 3.76 mg/L | Reaction progresses |
| 30 min | 1.40 mL | 4.38 mg/L | Standard conditions |
| 60 min | 1.46 mL | 4.57 mg/L | Change is small |
If the heating time is short, the oxidizing agent may not react sufficiently and the COD may be measured as low.
When comparing samples, it is important to keep the heating time consistent.
Effect of Chloride Ions
In COD measurement, inorganic reducing components such as chloride ions may consume the oxidizing agent and cause COD to be overestimated.
The effects of chloride ions require particular attention in seawater and wastewater containing large amounts of salt.
| Sample Condition | Cl− Concentration | Measured COD | Corrected COD | Direction of Discussion |
|---|---|---|---|---|
| River water | 10 mg/L | 2.03 mg/L | 2.00 mg/L | Effect is small |
| Brackish water | 1800 mg/L | 18.5 mg/L | 9.2 mg/L | Effect of chloride is large |
| Wastewater containing salt | 3500 mg/L | 42.0 mg/L | 24.5 mg/L | Correction or another method is necessary |
In samples containing large amounts of chloride ions, oxidant consumption by substances other than organic matter increases and COD may be overestimated.
For samples containing salt, treatment to suppress the effects of chloride ions or another measurement method should be considered.
Effects of Turbidity and Suspended Matter
If a sample contains suspended matter, COD may change depending on whether the sample is filtered.
Without filtration, not only dissolved components but also suspended organic matter is oxidized.
| Sample Treatment | Difference in Titration Volume | COD | How to Interpret the Result |
|---|---|---|---|
| Pond water without filtration | 1.85 mL | 5.79 mg/L | COD including suspended matter |
| Pond water after filtration | 1.40 mL | 4.38 mg/L | COD mainly from dissolved components |
| Supernatant after settling | 1.25 mL | 3.91 mg/L | Effect of settleable matter is reduced |
Because COD was higher without filtration, suspended matter and algae in the pond water were also considered to have consumed the oxidizing agent.
Example of Checking Reproducibility
A reference example is shown in which the same river water was measured three times to check variation in the difference in titration volume and COD.
| Trial | Blank Titration Volume | Sample Titration Volume | Difference in Titration Volume | COD |
|---|---|---|---|---|
| 1st | 10.20 mL | 9.56 mL | 0.64 mL | 2.00 mg/L |
| 2nd | 10.18 mL | 9.51 mL | 0.67 mL | 2.10 mg/L |
| 3rd | 10.22 mL | 9.58 mL | 0.64 mL | 2.00 mg/L |
| Average | – | – | 0.65 mL | 2.03 mg/L |
The three COD values ranged from 2.00 to 2.10 mg/L and agreed relatively well.
Highly reproducible measurements can be obtained by standardizing titration-volume readings, heating time, and reagent concentrations.
Example of How to Write the Results
The COD of the water samples was measured by the potassium permanganate method.
For river water, the blank titration volume was 10.20 mL and the sample titration volume was 9.55 mL, giving a difference in titration volume of 0.65 mL.
Under these conditions, a difference in titration volume of 1.00 mL corresponds to a COD of 3.13 mg/L, so the COD of the river water was determined to be 2.03 mg/L.
When the samples were compared, the COD was 0.47 mg/L in tap water, 2.03 mg/L in river water, 4.38 mg/L in pond water, 96.9 mg/L in domestic wastewater, and 656 mg/L in food-factory wastewater.
In domestic wastewater and food-factory wastewater, the COD was considered high because large amounts of readily oxidizable organic matter and reducing substances were present.
In pond water, the COD was 5.79 mg/L without filtration and 4.38 mg/L after filtration.
Because the value was higher without filtration, particulate organic matter such as suspended matter and algae was also considered to have consumed the oxidizing agent.
Points for Connecting the Results to the Discussion
In a discussion of COD measurement, it is important not only to calculate COD from oxidant consumption but also to explain it in relation to the amount of organic matter, differences from BOD, chloride ions, heating conditions, and the effects of suspended matter.
- Has the amount of oxidizing agent consumed been determined from the difference between the blank titration volume and the sample titration volume?
- Can the difference in titration volume be converted to COD?
- If the sample was diluted before measurement, has the dilution factor been correctly considered?
- Can it be explained that water with high COD contains large amounts of readily oxidizable organic matter and reducing substances?
- Can the difference between BOD and COD be explained as the difference between microbial decomposition and chemical oxidation?
- Can the possibility that a short heating time causes COD to be underestimated be discussed?
- Can it be explained that inorganic reducing components such as chloride ions may cause COD to be overestimated?
- Can it be discussed that the components being evaluated change between dissolved components and suspended components depending on whether filtration is performed?
- Can reagent concentration, heating temperature, titration endpoint, and burette reading be explained as sources of error?
Example Discussion
In this experiment, the COD of water samples was measured using the potassium permanganate method.
COD is an indicator representing the amount of oxygen required to chemically oxidize organic matter and reducing substances in water.
In river water, the difference in titration volume was 0.65 mL and the COD was determined to be 2.03 mg/L.
This value was higher than that of tap water, suggesting that the river water contained a certain amount of readily oxidizable organic matter and reducing substances.
COD was very high in domestic wastewater and food-factory wastewater.
Domestic wastewater may contain soap, food residues, organic matter originating from excreta, and similar substances, while food-factory wastewater may contain large amounts of food-derived components such as sugars, proteins, and fats.
These components were considered to have consumed large amounts of oxidizing agent, resulting in high COD.
When BOD and COD were compared, the BOD/COD ratio was relatively high in river water and pond water, suggesting that a large proportion of the organic matter may have been readily biodegradable.
On the other hand, domestic wastewater and food-factory wastewater had high COD values and relatively low BOD/COD ratios.
This was considered to be because they also contained components that were chemically oxidized but not sufficiently decomposed during five days of microbial decomposition.
In the comparison of heating times, COD was low after 5 minutes of heating and increased after 30 minutes.
This was because the oxidation reaction did not proceed sufficiently when the heating time was short, resulting in a smaller amount of oxidizing agent consumed.
When comparing COD values, the heating time and heating temperature must be kept constant.
Possible sources of error include chloride ions, suspended matter, reagent concentration, titration endpoint, and burette readings.
Particularly in samples containing large amounts of chloride ions, components other than organic matter may also consume the oxidizing agent and cause COD to be overestimated.
In addition, because COD was higher in unfiltered pond water, particulate organic matter such as suspended matter and algae was also found to affect COD.
Summary
COD is a water-quality indicator obtained by converting the amount of oxidizing agent consumed when organic matter and reducing substances in water are chemically oxidized into an amount of oxygen.
Oxidant consumption is determined from the difference between the blank and sample titration volumes and expressed as COD.
In this reference example, COD was low in tap water and river water and high in domestic wastewater and food-factory wastewater.
In a report, it is useful to discuss the difference in titration volume, COD calculation, dilution factor, organic pollution, differences from BOD, chloride ions, heating conditions, and errors caused by suspended matter in relation to one another.
Relationship Between Oxidant Consumption and the Amount of Organic Matter
In COD measurement, organic matter and reducing substances in water consume the oxidizing agent.
The more readily oxidizable substances the sample water contains, the more oxidizing agent is consumed.
Therefore, the greater the oxidant consumption, the higher the COD value and the greater the possibility of organic pollution.
However, organic matter is not the only substance that consumes oxidizing agents.
Reducing inorganic substances such as nitrite ions, iron(II) ions, and sulfide ions may also consume the oxidizing agent.
Therefore, COD must be considered not as “the amount of organic matter itself” but as “an indicator of the amount of chemically oxidizable substances.”
Example Discussion:
Because oxidant consumption was large in the sample water, a large amount of readily oxidizable organic matter or reducing substances was considered to be present in the water.
Because oxidant consumption is reflected in the COD value, water with high COD can be judged to have a large chemical pollution load.
However, because oxidizing agents are also consumed by reducing inorganic substances other than organic matter, the COD value cannot simply be interpreted as the amount of organic matter itself.
Discussion When the COD Value Is High
When the COD value is high, the water may contain large amounts of readily oxidizable organic matter and reducing substances.
Domestic wastewater, food-factory wastewater, agricultural drainage, decomposed fallen leaves and algae, oils, detergent components, and other substances may cause COD to increase.
The greater the amount of organic matter in the water, the more oxidizing agent is consumed.
Water with high COD may have advanced water pollution.
However, the type of pollutant cannot be determined from the COD value alone.
Combining the result with BOD, electrical conductivity, pH, nitrogen and phosphorus concentrations, and other results makes it possible to discuss the cause of pollution in greater detail.
Example Discussion:
Because the COD value was high, the sample water was considered to contain large amounts of organic matter or reducing substances oxidized by the oxidizing agent.
COD tends to increase when domestic wastewater, decomposed plant fragments, or food-derived organic matter enters the water.
However, because COD does not directly indicate the type of substance, the cause of pollution must be determined together with other water-quality parameters such as BOD, electrical conductivity, and pH.
Discussion When the COD Value Is Low
When the COD value is low, the water is considered to contain small amounts of chemically readily oxidizable organic matter and reducing substances.
COD values tend to be low in tap water, clean river water, and treated wastewater.
This provides one indication that organic pollution is low.
However, low COD does not mean that the water is necessarily free of water-quality problems.
Inorganic ions, heavy metals, microbial contamination, nutrients, and other substances that are difficult to evaluate by COD may be present.
COD is only an indicator of the amount of chemically oxidizable substances.
Example Discussion:
Because the COD value was low, the sample water was considered to contain small amounts of readily oxidizable organic matter and reducing substances.
This result indicates that pollution by organic matter was relatively low.
However, because the presence of dissolved ions, heavy metals, and microbial contamination cannot be judged even when COD is low, the result must be evaluated together with other water-quality analyses.
Relationship Between COD and BOD
COD and BOD are both indicators used to evaluate organic pollution in water, but they measure different things.
COD is evaluated by chemical oxidation using an oxidizing agent, while BOD evaluates the amount of oxygen consumed during microbial decomposition of organic matter.
Therefore, COD may be high while BOD is low.
When both COD and BOD are high, a large amount of readily biodegradable organic matter may be present.
When COD is high and BOD is low, large amounts of refractory organic matter or reducing inorganic substances may be present.
Therefore, comparing COD with BOD makes it easier to discuss the properties of organic matter in the water.
| Result | Possible Meaning | Point for Discussion |
|---|---|---|
| High COD / High BOD | Large amount of readily biodegradable organic matter | Consider domestic wastewater and food-derived organic matter |
| High COD / Low BOD | Possibility of refractory substances or reducing substances | Consider substances that are chemically oxidized but difficult for microorganisms to decompose |
| Low COD / Low BOD | Possibility of little organic pollution | Possibility of clean or treated water |
| Low COD / High BOD | Measurement conditions or sample condition must be checked | Review insufficient oxidation in COD measurement or BOD measurement conditions |
Example Discussion:
When both COD and BOD are high, the sample water may contain large amounts of organic matter readily decomposed by microorganisms.
On the other hand, if BOD is low despite high COD, refractory organic matter or reducing inorganic substances may have affected the COD.
Therefore, rather than judging COD alone, comparing it with BOD makes it possible to discuss the properties of organic matter in greater detail.
Concept of the Potassium Permanganate Method
In student experiments, COD may be measured using potassium permanganate as the oxidizing agent.
Permanganate ions are strong oxidizing agents and oxidize organic matter and reducing substances in water.
COD is calculated by titrating the oxidizing agent remaining after the reaction or determining the amount of oxidizing agent consumed.
In the potassium permanganate method, the progress of oxidation changes depending on reaction temperature, heating time, acidic or alkaline conditions, and the effects of coexisting substances.
Therefore, it is important to keep the experimental conditions constant.
If the conditions differ, the COD value may change even for the same sample.
Example Discussion:
In the potassium permanganate method, permanganate ions oxidize organic matter and reducing substances in water, and COD is determined from the amount consumed.
The greater the oxidant consumption, the more readily oxidizable substances are considered to be present in the water.
However, because the progress of the oxidation reaction is affected by heating time, temperature, and reaction conditions, the measurement conditions must be kept constant.
Importance of Blank Correction
In COD measurement, it is important to measure a blank that does not contain the sample.
Blank measurement is performed to correct for the amount of oxidizing agent consumed by the reagents themselves and by the experimental procedure.
To correctly determine oxidant consumption by the sample, the difference from the blank must be considered.
If blank correction is inappropriate, the COD value is calculated as too high or too low.
If the blank value is large, possible causes include deterioration of reagents, contamination of equipment, impurities in pure water, and operational errors.
The blank is not merely a formal measurement but serves as the reference for COD calculations.
Example Discussion:
In COD measurement, blank correction is necessary to accurately determine oxidant consumption caused by the sample.
The blank value includes oxidant consumption caused by reagents and the experimental procedure.
If blank correction is insufficient, systematic error occurs in the COD value, so it is important to correctly handle the difference between the sample measurement and blank value.
Relationship Between Titration Volume and COD Value
When titration is performed in COD measurement, the titration volume is used to calculate how much oxidizing agent remains or how much has been consumed.
When the titration volume changes, the calculated COD value also changes.
Therefore, accurate determination of the titration endpoint is important.
Even a slight error in reading the titration volume may become a relatively large error when the sample volume is small or the COD is low.
Reading of the burette scale, bubbles, dropping speed, and judgment of the endpoint color are sources of error.
Example Discussion:
Because the COD value is calculated from the titration volume, errors in reading the titration volume directly affect the COD value.
If titration continues beyond the endpoint, the calculation of oxidant consumption shifts and the COD may be overestimated or underestimated.
Particularly for low-COD samples, small errors in titration volume become relatively large, so the titrant must be added carefully near the endpoint.
Discussion When the Oxidation Reaction Is Incomplete
In COD measurement, substances in water are not necessarily completely oxidized by the oxidizing agent.
The ease of oxidation differs depending on the type of organic matter, and refractory organic matter may not be sufficiently oxidized under the measurement conditions.
In such cases, the COD value may be lower than would be expected from the actual amount of organic matter present.
The oxidation reaction may become incomplete when the heating time is short, the reaction temperature is low, the amount of oxidizing agent is insufficient, or the sample contains many substances that are difficult to oxidize.
When comparing COD values, it is important that the measurement conditions are the same.
Example Discussion:
One possible reason the COD value was lower than expected is that the organic matter in the sample was not sufficiently oxidized under the measurement conditions.
Refractory organic matter is difficult to completely oxidize with an oxidizing agent, and oxidant consumption may not sufficiently reflect the actual amount of organic matter.
In addition, if the heating time or reaction temperature was insufficient, the COD value may also have been underestimated.
Effects of Reducing Inorganic Substances
COD is used as an indicator of organic matter, but reducing inorganic substances that consume oxidizing agents also affect COD.
For example, nitrite ions, iron(II) ions, sulfide ions, and thiosulfate ions may be oxidized by the oxidizing agent.
If these substances are present, the COD value may become high even when there is not much organic matter.
Therefore, even when the COD value is high, organic matter is not necessarily the only cause.
In industrial wastewater, mine drainage, and water containing reducing components, the effects of inorganic substances must be considered.
It is important to evaluate the result together with other analytical parameters.
Example Discussion:
The high COD value may have been affected not only by organic matter but also by reducing inorganic substances.
Nitrite ions, iron(II) ions, and sulfide ions consume oxidizing agents and may increase the COD value.
Therefore, rather than linking the COD value only to the amount of organic matter, it is necessary to interpret it while considering the origin of the sample water and other analytical results.
Effect of Chloride Ions
In COD measurement, chloride ions may affect the oxidizing agent or measurement reaction.
Particularly in seawater and wastewater containing large amounts of salt, the effect of chloride ions may not be negligible.
Depending on the measurement method, chloride ions may be oxidized and consume the oxidizing agent, causing COD to be overestimated.
When measuring samples containing large amounts of salt, the correction or interference-removal method specified in the laboratory manual should be checked.
Chloride ions may not be a major problem in tap water and river water, but care is required for seawater and industrial wastewater.
Example Discussion:
If the sample water contains large amounts of chloride ions, they may affect COD measurement and cause oxidant consumption to be greater than would be expected from the actual amount of organic matter.
Particularly in seawater and wastewater containing salt, interference from chloride ions must be considered.
Therefore, for samples with high salinity, correction or interference removal appropriate for the measurement method is desirable.
Effects of Sampling and Storage
The properties of water samples continue to change after sampling.
COD values may change because of microbial decomposition, redox reactions, precipitate formation, loss of volatile components, adsorption onto containers, and other processes.
Particularly for water containing organic matter, the measured value may change as storage time becomes longer.
To accurately measure COD, it is desirable to perform the measurement as soon as possible after sampling.
Samples should be stored in a cool, dark place or preserved when necessary.
In addition, if the sampling container is dirty, organic matter may contaminate the sample and increase the COD value.
Example Discussion:
One possible source of error in the COD value is a change in water quality during storage after sampling.
Organic matter in the water sample may be decomposed by microorganisms or adsorb onto the container, causing the COD value to change with storage time.
In addition, if the sampling container is contaminated with organic matter, COD may be measured as too high, so it is important to use a clean container and perform the measurement promptly after sampling.
Effects of Water Turbidity and Color
When water is turbid, suspended matter may contain organic matter.
In such water, COD may become high.
Fragments of fallen leaves, soil particles, algae, microorganisms, suspended matter originating from wastewater, and other substances may affect COD.
In addition, if the sample water is colored, it may become difficult to judge the color at the titration endpoint.
In colored samples, misjudging the endpoint may cause an error in the titration volume and shift the COD value.
Pretreatment or another method may be considered when necessary.
Example Discussion:
If turbidity was observed in the sample water, organic matter contained in the suspended matter may have consumed the oxidizing agent and increased the COD value.
In addition, in colored samples, the endpoint color change becomes difficult to judge and errors may occur in the titration volume.
Therefore, in COD measurement, it is necessary to record the appearance of the sample and consider the effects of turbidity and color on the measurement.
Effect of Dilution
Samples with high COD may be diluted so that they fall within the measurement range.
If the dilution is not performed correctly, large errors occur in the COD value.
Main causes include calculation errors in the dilution factor, operational errors with volumetric flasks and pipettes, and insufficient mixing of the sample.
COD values measured after dilution must be converted back to the concentration of the original sample.
If the dilution factor is not applied, the COD value is greatly underestimated.
Dilution procedures are particularly important for high-COD samples.
Example Discussion:
When a diluted sample is used, handling of the dilution factor greatly affects the COD value.
If the dilution factor is not correctly reflected in the calculation, the COD of the original sample is underestimated.
In addition, if the sample is not sufficiently mixed during dilution, the portion used for measurement may not be representative and variation may occur in the COD value.
When the COD Measurement Can Be Considered Good
COD measurement can be considered to have produced good results when the difference between the blank and sample is clear, the titration endpoint is easy to determine, variation among repeated measurements is small, and the COD values do not contradict the sampling location or appearance of the sample.
For example, if COD is high in a sample considered to be affected by domestic wastewater and low in clean water, the results are easier to consider valid.
However, because COD values change depending on the measurement conditions and type of oxidizing agent, values obtained by different methods cannot simply be compared.
The basic approach is to compare samples measured by the same method under the same conditions.
Example Discussion:
In this experiment, the COD value of the sample water was clearly higher than the blank, and no large variation was observed among repeated measurements.
In addition, the magnitude of the COD values did not contradict the environment of the sampling locations or the appearance of the samples.
From these results, the COD measurements in this experiment were considered to approximately reflect the amounts of readily oxidizable substances in the sample water.
Example Discussion When the Experiment Did Not Go Well
When COD measurement does not go well, possible causes can be considered from results such as variation in titration values, difficulty identifying the endpoint, a large blank value, COD higher or lower than expected, or trends among multiple samples that cannot be explained.
The causes can be divided into nonuniformity of the sample, changes during storage, incomplete oxidation reactions, titration procedures, reagent concentration, and blank correction.
Example Discussion:
In this experiment, variation was observed in the COD values for the same sample.
Possible causes include suspended matter in the sample not being uniformly dispersed, differences in endpoint determination, and heating conditions not being completely constant.
In addition, if blank correction was inappropriate, systematic error may have occurred in the COD values of all samples.
How to Write Points for Improvement
In a discussion of COD measurement, including not only sources of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sampling and storage, reaction procedures, titration procedures, and calculation and analysis.
Improvements to Sampling and Storage
- Use a clean sampling container
- Measure as soon as possible after sampling
- Store in a cool, dark place when necessary
- Mix the sample thoroughly before measurement
- Record the sampling location, time, and weather
- Record turbidity, color, and odor
Improvements to Reaction Procedures
- Keep the heating temperature constant
- Accurately follow the heating time
- Add the oxidizing agent accurately
- Accurately measure the sample volume
- Always perform blank measurement
- Check the effects of interfering substances
Improvements to Titration and Analysis
- Read the burette scale accurately
- Add the titrant carefully one drop at a time near the endpoint
- Remove bubbles before titration
- Perform blank correction correctly
- Reflect the dilution factor in the calculation
- Perform multiple measurements and calculate the average value
- Compare with other indicators such as BOD and electrical conductivity
Example of How to Write Points for Improvement:
To improve the accuracy of COD measurement, the sample must be measured as soon as possible after sampling and thoroughly mixed before measurement.
In addition, it is important to keep the heating time and temperature constant and accurately perform blank measurement.
During titration, the titrant should be added carefully near the endpoint, and the dilution factor and blank correction should be correctly reflected in the calculation to obtain a more reliable COD value.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of COD measurement, simply writing that “COD was high” or that “the oxidizing agent was consumed” results in a superficial discussion.
A good discussion relates oxidant consumption, organic matter and reducing substances in the water, measurement conditions, and sources of error.
| Superficial Discussion | Good Discussion |
|---|---|
| COD was high. | Because COD was high, the sample water was considered to contain large amounts of organic matter or reducing substances oxidized by the oxidizing agent. Possible causes include domestic wastewater and decomposed plant fragments. |
| A large amount of oxidizing agent was consumed. | The greater the oxidant consumption, the more readily oxidizable substances are present in the water. However, reducing inorganic substances other than organic matter may also consume the oxidizing agent. |
| COD was low. | Because COD was low, the sample water was considered to contain few chemically readily oxidizable substances and the degree of organic pollution was relatively low. However, overall water quality cannot be judged from COD alone. |
| The values varied. | The variation in COD values may have resulted from nonuniformity of the sample, differences in heating conditions, errors in determining the titration endpoint, and insufficient blank correction. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of COD measurement.
Adjust the necessary parts according to your own experimental results.
- COD is an indicator reflecting the amount of readily oxidizable organic matter and reducing substances in water.
- The greater the oxidant consumption, the higher the COD value becomes.
- Because the COD value was high, organic pollution may have been present in the sample water.
- COD is not the amount of organic matter itself but an indicator of the amount of chemically oxidizable substances.
- If reducing inorganic substances are present, the COD value may be overestimated relative to the amount of organic matter.
- If the oxidation reaction is incomplete, the COD value may be underestimated.
- Blank correction is important for removing oxidant consumption caused by reagents and procedures.
- Errors in determining the titration endpoint directly affect the calculated COD value.
- COD values must be discussed together with other water-quality indicators such as BOD, electrical conductivity, and pH.
- Storage conditions after sampling and sample homogeneity also affect COD values.
Points to Check When Discussing COD Measurement
Checking the following points before writing the report makes the discussion easier to write.
- Is it explained what COD represents?
- Is the relationship between oxidant consumption and COD described?
- Is it understood that COD is not the amount of organic matter itself?
- Are the effects of reducing inorganic substances considered?
- Is the meaning of blank correction explained?
- Are titration-volume and endpoint-determination errors considered?
- Are the effects of heating temperature and heating time considered?
- Are changes caused by sampling and storage considered?
- Are the effects of turbidity and coloration considered?
- Is the dilution factor handled correctly?
- Are the results compared with other indicators such as BOD and electrical conductivity?
- Do the points for improvement correspond to the sources of error?
Summary
COD measurement is a method used to evaluate water quality by determining how much oxidizing agent is consumed by organic matter and reducing substances in water and converting that amount to an oxygen equivalent.
The greater the oxidant consumption, the higher the COD value and the more readily oxidizable substances are considered to be present in the water.
Therefore, COD is important as an indicator of organic pollution.
However, COD is not a value that directly represents the amount of organic matter itself.
Reducing inorganic substances may also consume the oxidizing agent, while organic matter that is difficult to oxidize may not be sufficiently reflected.
In addition, COD values change depending on blank correction, titration endpoint, heating conditions, sample storage, and dilution procedures.
In a report, rather than simply writing that “COD was high or low,” organize and discuss the relationship between oxidant consumption and substances in the water, the water-quality meaning of the COD value, comparison with other indicators such as BOD, sources of error, and points for improvement.
COD measurement is an effective method for evaluating water quality, but it is important to judge the results in combination with multiple water-quality indicators rather than using COD alone.
