Discussion Examples for Nitrate and Nitrite Ion Determination | Water Quality Indicators and Colorimetric Reactions

What Is Nitrate and Nitrite Ion Determination?

Nitrate and nitrite ion determination is an analysis used to measure the concentrations of NO3- and NO2- contained in water.
These are forms of nitrogen compounds and may be present in water because of fertilizers, domestic wastewater, livestock wastewater, industrial wastewater, runoff from soil, decomposition reactions by microorganisms, and other factors.
In water-quality analysis, they are related to the evaluation of nitrogen pollution and eutrophication.

Nitrate ions and nitrite ions are colorless, so they are often measured by spectrophotometry using a colorimetric reaction.
Nitrite ions are made to produce reddish-purple or similar colored substances through diazotization and coupling reactions.
For nitrate ions, a method may be used in which they are first reduced to nitrite ions and then subjected to the same colorimetric reaction.

Main Items to Include in the Results

In the results of nitrate and nitrite ion determination, organize the type of water sample, sampling location, measurement method, color-forming reagent, presence or absence of a reduction procedure, measurement wavelength, concentration of the standard solutions, absorbance, calibration curve, concentration of the unknown sample, and other information.
In spectrophotometry, the linearity of the calibration curve, blank correction, and control of color-development time are related to the reliability of the results.

Main Items to Include in the Results

• Type of water sample
• Sampling location
• Sampling date and time
• Weather at the time of sampling
• Color and turbidity of the sample
• Whether the measurement target is NO3- or NO2-
• Measurement method
• Color-forming reagent
• Presence or absence of a reduction procedure
• Measurement wavelength
• Concentration of the standard solutions
• Absorbance of the standard solutions
• Equation of the calibration curve
• Correlation coefficient
• Absorbance of the unknown sample
• Concentration of the unknown sample
• Dilution factor
• Blank value
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Nitrate and Nitrite Ion Determination

Here, the process of determining nitrate ions (NO3−) and nitrite ions (NO2−) in water samples using colorimetric reactions and absorbance measurements is organized using reference experimental values.

Nitrite ions react with substances such as sulfanilamide and undergo diazotization, then generate a reddish-purple dye through a coupling reaction.
By measuring the intensity of this color as absorbance, the nitrite ion concentration can be determined.
Because nitrate ions do not readily undergo the same colorimetric reaction directly, a method in which they are reduced to nitrite ions before measurement is used.

Reference Experimental Conditions

Concept of the Colorimetric Reaction

Nitrite ions react with aromatic amines under acidic conditions and undergo diazotization, then react with a coupling reagent to produce a reddish-purple azo dye.
The darker the color, the higher the nitrite ion concentration and the greater the absorbance.

Calibration Curve for Nitrite Ion Standard Solutions

An example is shown in which nitrite ion standard solutions of known concentration were subjected to color development and their absorbance was measured.
The blank absorbance is assumed to be 0.015.

In this reference example, the relationship between blank-corrected absorbance and NO2− concentration is treated as the following calibration curve.

Corrected absorbance = 3.50 × NO2− concentration (mg/L)

Therefore, the concentration can be determined using the following equation.

NO2− concentration (mg/L) = Corrected absorbance ÷ 3.50

Example Calculation of Blank Correction

Because the absorbance of a sample includes absorbance originating from reagents and the cell, blank correction is performed.

Corrected absorbance = Sample absorbance − Blank absorbance

If the measured absorbance of a river-water sample is 0.155 and the blank absorbance is 0.015,

Corrected absorbance = 0.155 − 0.015 = 0.140

This corrected absorbance is substituted into the calibration curve to determine the nitrite ion concentration.

Example Calculation of Nitrite Ion Concentration

If the corrected absorbance is 0.140 and the calibration curve is “Corrected absorbance = 3.50 × concentration,” the nitrite ion concentration is calculated as follows.

NO2− concentration = 0.140 ÷ 3.50 = 0.040 mg/L

Therefore, in this reference example, the nitrite ion concentration in the river water is determined to be 0.040 mg/L.

Nitrite Ion Measurement Results by Water Sample

Reference examples are shown for directly developing nitrite ions in each water sample without performing reduction treatment.

Concept of Nitrate Ion Measurement

When nitrate ions are measured, a reduction column or reducing agent is used to convert NO3− to NO2−, after which the same colorimetric reaction is performed.
The value measured after reduction is the total of the NO2− originally present and the NO2− converted from NO3−.

NO3−-derived concentration = Total concentration after reduction − Directly measured NO2− concentration

Example of Organizing Nitrate and Nitrite Ions Together

A reference example is shown in which nitrite ions are measured before reduction and the total of nitrate and nitrite ions is measured after reduction.
Here, the reduction efficiency is assumed to be 100%.

Example Calculation of Nitrate-Ion-Derived Concentration

In river water, the directly measured NO2− concentration is 0.040 mg/L, and the total concentration after reduction is 2.85 mg/L.

NO3−-derived concentration = 2.85 − 0.040 = 2.810 mg/L

From this result, nitrate ions are considered to be present in larger amounts than nitrite ions in the river water.

Example of Correction Considering Reduction Efficiency

In the operation of reducing nitrate ions to nitrite ions, reduction may not proceed completely.
If the reduction efficiency is 90%, the measured NO3−-derived concentration must be corrected.

Corrected NO3− concentration = Measured NO3−-derived concentration ÷ Reduction efficiency

If the measured NO3−-derived concentration in agricultural drainage is 12.48 mg/L and the reduction efficiency is 90%,

Corrected NO3− concentration = 12.48 ÷ 0.90 = 13.87 mg/L

If reduction efficiency is not considered, the nitrate ion concentration may be underestimated.

Example of Dilution of a High-Concentration Sample

In high-concentration samples such as fertilizer leachate, absorbance may exceed the range of the calibration curve.
In such cases, the sample is diluted before measurement, and the original concentration is obtained by multiplying the final result by the dilution factor.

Measuring after dilution so that the value falls within the range of the calibration curve produces a more reliable result.
The measurement result after dilution must always be multiplied by the dilution factor to restore the original concentration.

Changes in Nitrite Ions Due to Storage Conditions

Nitrite ions may change readily because of oxidation and microbial activity.
A reference example of changes caused by storage conditions after sample collection is shown below.

As time passes after collection, the ratio of nitrite ions to nitrate ions may change.
For accurate comparison, it is important to measure as soon as possible after collection or standardize the storage conditions.

Effects of Coexisting Components and Turbidity

In colorimetric methods, the color and turbidity of the sample, reducing substances, metal ions, and other components may affect absorbance.

Example of How to Write the Results

When a calibration curve was prepared using nitrite ion standard solutions, a relationship of “Corrected absorbance = 3.50 × concentration” was obtained between the blank-corrected absorbance and NO2− concentration.
For the river-water sample, the measured absorbance was 0.155 and the blank absorbance was 0.015, so the corrected absorbance was 0.140 and the NO2− concentration was determined to be 0.040 mg/L.

The total concentration after reduction treatment was 1.20 mg/L in tap water, 2.85 mg/L in river water, 4.20 mg/L in pond water, 12.6 mg/L in agricultural drainage, and 38.5 mg/L in fertilizer leachate.
When the directly measured NO2− concentration was subtracted, the nitrate-ion-derived concentration accounted for most of the total in many samples.

In agricultural drainage, the NO3−-derived concentration was 12.48 mg/L, which was higher than in tap water and river water.
This may have occurred because nitrogen components in fertilizer were washed out by rainwater or irrigation water and increased the nitrate ion concentration in the water.

Points for Connecting the Results to the Discussion

In a discussion of nitrate and nitrite ion determination, it is important not only to determine concentrations from absorbance but also to explain the colorimetric reaction, reduction operation, sample-storage conditions, nitrogen cycle, and relationship with water pollution.

• Has blank correction been performed and the concentration determined from the calibration curve?
• Can the difference between direct color development for nitrite ions and color development after reduction for nitrate ions be explained?
• Can NO3− be determined by subtracting the directly measured NO2− concentration from the total concentration after reduction?
• Can the possibility of underestimating the nitrate ion concentration when reduction efficiency is not 100% be discussed?
• Can the possibility that NO3− concentration increases because of fertilizers and domestic wastewater be explained?
• Can it be explained that sample turbidity and coloration affect absorbance?
• Can color-development time, reagent volume, measurement wavelength, and cell contamination be discussed as sources of error?
• Can the possibility that NO2− and NO3− values change depending on storage conditions after sampling be explained?

Example Discussion

In this experiment, nitrite ions and nitrate ions in water samples were determined using a colorimetric reaction and absorbance measurement.
By using the calibration curve obtained from nitrite ion standard solutions, the NO2− concentration could be determined from the blank-corrected absorbance of the sample.
In river water, the NO2− concentration was 0.040 mg/L, which was higher than that in tap water.

For nitrate ions, NO3− was converted to NO2− by reduction treatment and then measured using the same colorimetric reaction.
The NO3−-derived concentration was determined by subtracting the directly measured nitrite ion concentration from the total concentration after reduction.
In many samples, the nitrate-ion-derived concentration was greater than the nitrite-ion concentration, and most of the inorganic nitrogen in the water was considered to be present as nitrate ions.

Agricultural drainage and fertilizer leachate showed high NO3−-derived concentrations.
This was considered to be because nitrogen components contained in fertilizers dissolved into the water as nitrate ions.
Because nitrate ions are highly soluble in water and are not readily retained in soil, they can readily move into rivers and groundwater through rainfall and irrigation.
Therefore, NO3− concentration is an important indicator when considering water quality around agricultural land.

Possible sources of error include sample turbidity and coloration, insufficient color-development time, deviations in reagent volume, cell contamination, and reduced reduction efficiency.
In particular, if the reduction column or reducing agent does not function sufficiently, nitrate ions are not completely converted to nitrite ions and the NO3− concentration may be underestimated.
In addition, because nitrate and nitrite ion concentrations may change when samples are stored at room temperature or under light exposure after sampling, measurements should be performed as soon as possible after collection or storage conditions should be standardized.

Summary

In nitrate and nitrite ion determination, the colorimetric reaction of nitrite ions is used and the concentration is determined by substituting absorbance into the calibration curve.
Nitrate ions are reduced and measured as nitrite ions, and the amount is obtained by subtracting the directly measured nitrite-ion amount from the total value after reduction.

In this reference example, nitrate-ion-derived concentrations were high in agricultural drainage and fertilizer leachate.
In a report, it is useful to discuss the calibration curve, blank correction, reduction operation, differences between nitrate and nitrite, storage conditions, and errors caused by turbidity and coloration in relation to one another.

Meaning as Water-Quality Indicators

Nitrate ions and nitrite ions are used as indicators for evaluating nitrogen pollution in water.
Nitrogen is a nutrient element necessary for organisms, but if excessive amounts are present in water, they may promote the growth of algae and phytoplankton and lead to eutrophication.
Therefore, measuring nitrogen concentration is important for evaluating environmental waters.

Nitrate ions are a relatively stable form of nitrogen produced by oxidation of ammoniacal nitrogen.
Nitrite ions are intermediate forms generated during nitrification and denitrification and may not remain at high concentrations for long periods.
When nitrite ions are detected, nitrogen transformation may be progressing in the water.

The Nitrogen Cycle and Nitrate and Nitrite

Nitrogen in aquatic environments exists in forms such as ammoniacal nitrogen, nitrite nitrogen, nitrate nitrogen, and organic nitrogen.
When organic matter is decomposed, ammonia is produced, and under aerobic conditions microorganisms oxidize it to nitrite and then to nitrate.
This process is called nitrification.

On the other hand, under low-oxygen conditions, denitrification may proceed, in which nitrate ions are reduced to nitrogen gas and other products.
The concentrations of nitrate ions and nitrite ions change depending on these microbial reactions, oxygen conditions, the amount of organic matter, and inflow load.
Considering the results together with DO and BOD makes it easier to discuss nitrogen transformations occurring in the water.

Relationship With Eutrophication

Nitrate ions and nitrite ions serve as nutrients for algae and aquatic plants.
If excessive amounts of nitrogen and phosphorus are supplied to water, algal growth becomes more likely and eutrophication may progress.
As eutrophication progresses, algal blooms, reduced water transparency, unpleasant odors, and decreases in dissolved oxygen may occur.

However, eutrophication is not determined by nitrogen alone.
Phosphate ion concentration, water temperature, light, flow velocity, water-retention time, and types of algae are also involved.
Therefore, nitrate and nitrite ion concentrations should be treated as one important factor for considering eutrophication.

What Is a Colorimetric Reaction?

Nitrate ions and nitrite ions are colorless ions, so they are difficult to measure directly by spectrophotometry.
Therefore, the target component is reacted with reagents to generate a colored substance.
This reaction is called a colorimetric reaction.
The darker the color of the colored substance, the higher the concentration of the target component is considered to be.

In nitrite ion determination, colorimetric reactions using sulfanilic acid, naphthylethylenediamine, or similar reagents may be used.
Nitrite ions undergo a diazotization reaction and then produce a reddish-purple azo dye through a coupling reaction.
The concentration is determined by measuring the absorbance of this dye.

Colorimetric Reaction of Nitrite Ions

Nitrite ions react with aromatic amines under acidic conditions to form diazo compounds and then react with coupling reagents to produce colored azo dyes.
The nitrite ion concentration is determined by measuring the absorbance of this colored substance.
The intensity of the color corresponds to the amount of nitrite ions.

In the colorimetric reaction, pH, reaction time, reagent concentration, and temperature are important.
If the color-development time is too short, sufficient color is not produced and absorbance becomes low.
Conversely, if the solution is left for too long, the color may change, so standard solutions and unknown samples must be measured under the same conditions.

Nitrate Ion Determination and Reduction Operation

Compared with nitrite ions, nitrate ions may be difficult to use directly in colorimetric reactions.
Therefore, one method is to first reduce nitrate ions to nitrite ions and then use the colorimetric reaction for nitrite ions.
In this case, the efficiency of the reduction operation greatly affects the measured value.

If reduction is incomplete, the nitrate ion concentration is underestimated.
Conversely, if nitrite ions are originally present in the sample, it is necessary to distinguish between nitrite derived from nitrate and nitrite originally present.
When nitrate ions and nitrite ions are measured separately, the experimental procedure must be carefully checked.

Principle of Spectrophotometry

Spectrophotometry is a method for determining concentration by using the property that colored substances in solution absorb light of specific wavelengths.
The higher the concentration of the colored substance, the greater the amount of light absorbed and the larger the absorbance.
Using this relationship, the concentration of an unknown sample is determined from a calibration curve prepared using standard solutions.

Under constant conditions, the relationship between absorbance and concentration follows the Lambert-Beer law.
However, if the concentration is too high, if the sample is turbid, if the cell is dirty, or if color development is incomplete, absorbance may not correctly reflect concentration.

Absorbance A = εlc

Discussion of the Calibration Curve

A calibration curve represents the relationship between concentration and absorbance obtained by measuring the absorbance of standard solutions of known concentration.
In nitrate and nitrite ion determination, the linearity of the calibration curve is important.
The closer the calibration curve is to a straight line, the more reliable the determination of concentration from absorbance becomes.

If a point deviates from the calibration curve, possible causes include errors in preparing the standard solution, pipetting errors, differences in color-development time, cell contamination, and absorbance-measurement errors.
If the absorbance of an unknown sample is outside the calibration-curve range, it is desirable to dilute the sample and measure it again.

Discussion When Nitrate Ion Concentration Is High

When nitrate ion concentration is high, a large amount of nitrogen nutrients is considered to be present in the water.
Possible causes include fertilizer runoff from agricultural land, domestic wastewater, livestock wastewater, nitrogen infiltration into groundwater, and nitrification of ammoniacal nitrogen.
Nitrate ions are relatively soluble in water and readily move through groundwater and river water.

In water bodies with high nitrate ion concentrations, the presence of other nutrients such as phosphate ions may promote algal growth.
Therefore, nitrate ion concentration is useful for evaluating eutrophication and anthropogenic pollution.
However, to identify the cause of a high concentration, the sampling location and surrounding environment must be considered.

Discussion When Nitrate Ion Concentration Is Low

When nitrate ion concentration is low, the inflow of nitrogen into the water may be small, or nitrate ions may have been taken up by algae or aquatic plants.
In addition, under low-oxygen conditions, denitrification may proceed and nitrate ions may be converted to nitrogen gas and other products, lowering the concentration.

A low nitrate ion concentration may indicate a small nitrogen load, but it does not necessarily mean that the water quality is good.
If nitrogen exists as ammoniacal nitrogen or organic nitrogen, nitrate ions alone cannot be used to evaluate the entire nitrogen load.
Comparing the result with total nitrogen and ammoniacal nitrogen allows a more detailed discussion.

Discussion When Nitrite Ion Concentration Is High

When nitrite ion concentration is high, an intermediate stage of nitrification or denitrification may be progressing in the water.
Nitrite ions appear as intermediate products while ammonia is oxidized to nitrate or while nitrate is reduced.
Because they do not normally accumulate stably for long periods, their detection may indicate active nitrogen transformation in the water.

Possible causes of high nitrite ion concentration include wastewater inflow, incomplete microbial reactions, changes in oxygen conditions, water in the middle of nitrification, and water in the middle of denitrification.
Considering DO and BOD together makes it easier to discuss aerobic or anaerobic conditions.

Discussion When Nitrite Ion Concentration Is Low

When nitrite ion concentration is low, little nitrite may be present in the water, or even if it is produced, it may quickly be converted to nitrate or nitrogen gas.
In water where nitrification has progressed sufficiently, nitrite exists only temporarily as an intermediate product and nitrate ions may ultimately become predominant.

A low nitrite ion concentration does not necessarily mean that there is no nitrogen pollution.
Large amounts of nitrate ions, ammoniacal nitrogen, or organic nitrogen may still be present.
Therefore, nitrite ions must be considered together with other forms of nitrogen.

Discussion of Differences Among Sampling Locations

Nitrate and nitrite ion concentrations vary greatly depending on the sampling location.
Around agricultural land, nitrate ions originating from fertilizers readily run off, while nitrogen compounds may increase at locations affected by domestic wastewater or livestock wastewater.
Concentrations may also change between upstream and downstream sections of rivers because of human activities.

In groundwater, nitrate ions may accumulate as water infiltrates through soil.
On the other hand, in stagnant waters such as ponds and lakes, uptake by algae, denitrification in bottom sediments, and rerelease may affect concentrations.
Relating the surrounding environment at the sampling location to the results makes the discussion more persuasive.

Effect of Color-Development Time

In spectrophotometry using a colorimetric reaction, it is important to keep the color-development time constant.
If the color-development time is too short, color development does not proceed sufficiently and absorbance becomes low.
As a result, the concentration may be underestimated.

On the other hand, if the sample is left for a long period after color development, the color may change or the colored substance may become unstable.
If the color-development time differs between standard solutions and unknown samples, accurate comparison with the calibration curve cannot be made.
It is important to measure all samples at the same timing.

Discussion of Measurement Wavelength

In spectrophotometry, it is important to measure at a wavelength at which the colored substance strongly absorbs light.
By selecting an appropriate wavelength, differences in absorbance caused by concentration differences can be detected more clearly.
If the measurement wavelength is inappropriate, absorbance becomes small and the sensitivity of the calibration curve decreases.

In addition, if another component in the sample absorbs light near the same wavelength, its absorption overlaps with that of the target component and causes an error.
The wavelength specified in the laboratory manual should be used, and blank correction or pretreatment should be performed when necessary.

Importance of Blank Correction

Blank correction is performed to subtract absorbance originating from reagents, solvents, cells, pure water, and other sources.
If the color-forming reagent itself has slight absorbance or the cell is dirty, failing to perform blank correction causes the absorbance to be estimated as higher than that originating from the target component.

Because nitrate and nitrite ions are often measured at low concentrations, the effect of the blank value tends to become large.
If the blank value is high, contamination of reagents, impurities in pure water, cell contamination, and operational errors should be checked.

Effects of Sample Turbidity and Coloration

If a water sample is turbid, light scattering may cause absorbance to be measured as higher.
In addition, if the sample itself is colored, absorption from substances other than the colored reaction product is added and may cause the concentration to be overestimated.
Sample transparency is important in spectrophotometry.

For turbid samples, pretreatment such as filtration or centrifugation may be performed.
However, pretreatment may remove nitrogen components adsorbed on particles.
It is necessary to clarify whether the measurement target is the dissolved component or a component closer to the total amount when discussing the results.

Effects of Coexisting Substances

Water samples contain many components other than nitrate ions and nitrite ions.
If coexisting substances interfere with the colorimetric reaction or absorb at the same wavelength, the measured value may deviate.
Reducing substances, oxidizing substances, metal ions, organic matter, salinity, and other components may have an effect.

Particularly in complex samples such as wastewater and agricultural drainage, the effects of coexisting substances must be considered.
More accurate results can be obtained by applying the interference-removal methods and pretreatment specified in the laboratory manual.

Units of Nitrate Nitrogen and Nitrite Nitrogen

In water-quality analysis, nitrate and nitrite ion concentrations may be expressed as NO3- and NO2-, or converted to the amount of nitrogen and expressed as NO3-N and NO2-N.
Because the numerical value changes depending on which expression is used, it is important to clearly state the units.

For example, nitrate ion concentration and nitrate-nitrogen concentration do not have the same meaning.
When comparing report results with environmental standards or other data, the units and conversion method must be checked.
Confusing the units can lead to major errors in the discussion.

Effects of Sample Storage

Nitrate and nitrite ion concentrations may change during storage after water sampling.
Through microbial activity, ammonia may change into nitrite or nitrate, and nitrite may be further oxidized to nitrate.
Depending on oxygen conditions, denitrification may also proceed.

Nitrite ions are intermediate products and may readily change in concentration during storage.
Therefore, measurements should be performed as soon as possible after sampling, and cold and dark storage or preservation procedures should be used when necessary.
Storage conditions are related to the reliability of the results.

Sources of Error in Nitrate and Nitrite Ion Determination

Sources of error in nitrate and nitrite ion determination include errors in preparing standard solutions, deviations in the amount of color-forming reagent added, differences in color-development time, incorrect measurement-wavelength settings, dirty cells, bubbles, insufficient blank correction, sample turbidity and coloration, coexisting substances, and incomplete reduction operations.
In spectrophotometry, all operations that affect absorbance also affect the concentration calculation.

In addition, if the absorbance of an unknown sample is outside the range of the calibration curve, concentration estimation requires extrapolation and becomes less reliable.
High-concentration samples should be diluted before measurement, while low-concentration samples require attention to the effects of the blank and cell contamination.
It is also important to perform multiple measurements and check variation.

When the Results Can Be Considered Good

Nitrate and nitrite ion determination results can be considered good when the calibration curve shows linearity, the blank value is small, the absorbance of the unknown sample falls within the range of the calibration curve, and the color-development conditions are consistent between the standard solutions and unknown samples.
Small variation among repeated measurements is also important.

If the obtained concentrations do not contradict the surrounding environment of the sampling location, the validity of the results increases.
For example, if nitrate ions are high at a location affected by agricultural land or domestic wastewater, this can be explained in relation to environmental factors.
On the other hand, if nitrite ions are high, an intermediate stage of nitrogen transformation or the influence of wastewater inflow should be considered.

Example Discussion When the Experiment Did Not Go Well

When quantitative analysis does not go well, possible causes can be considered from results such as a calibration curve that is not linear, variation in absorbance, a large blank value, an unknown sample outside the calibration-curve range, weak color development, strong sample turbidity, or unclear distinction between nitrate and nitrite.
Organizing the causes according to standard solutions, color-development procedures, reduction procedures, measuring instruments, and sample pretreatment makes the discussion easier.

How to Write Points for Improvement

In a discussion of nitrate and nitrite ion determination, including not only sources of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into preparation of standard solutions, color-development procedures, reduction procedures, absorbance measurement, sample storage, and analysis.

Improvements to Standard Solutions and Reagents

• Prepare the standard solutions accurately
• Use volumetric flasks and volumetric pipettes correctly
• Add the color-forming reagents accurately
• Avoid deterioration and contamination of reagents
• Always measure a blank

Improvements to Color-Development and Reduction Procedures

• Use the same color-development time
• Keep the color-development temperature as constant as possible
• Keep nitrate-ion reduction conditions constant
• Standardize the procedure to prevent incomplete reduction
• Use the same standing time after color development

Improvements to Measurement and Analysis

• Measure at the specified wavelength
• Keep the cells clean
• Wipe off water droplets and fingerprints from the cell surfaces
• Avoid introducing bubbles
• Dilute unknown samples so that their absorbance falls within the calibration-curve range
• Check the units of nitrate nitrogen and nitrite nitrogen
• Measure as soon as possible after sampling
• Discuss the results together with the surrounding environment and other water-quality parameters

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of nitrate and nitrite ion determination, simply writing that “the absorbance was high” or that “the concentration was determined” results in a superficial discussion.
A good discussion relates the colorimetric reaction, calibration curve, nitrogen cycle, water-quality indicators, eutrophication, and sources of error.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of nitrate and nitrite ion determination.
Adjust the necessary parts according to your own experimental results.

• Nitrate ions and nitrite ions are water-quality indicators used to evaluate nitrogen components in water.
• If nitrate ion concentration is high, inflow of nitrogen originating from fertilizers or domestic wastewater can be considered.
• Nitrite ions are intermediate products of nitrification and denitrification and provide a clue to nitrogen transformations in water.
• Nitrogen components are nutrients for algae and therefore are related to eutrophication.
• A colorimetric reaction converts colorless ions into colored substances, allowing them to be determined by spectrophotometry.
• The higher the absorbance, the higher the concentration of the colored substance and the higher the concentration of the target ion is considered to be.
• Because the calibration curve showed linearity, a proportional relationship between concentration and absorbance was considered to hold within the measurement range.
• If reduction is incomplete during nitrate ion measurement, the concentration may be underestimated.
• Sample turbidity and coloration may affect absorbance measurement.
• When comparing results, nitrate ion concentration and nitrate-nitrogen concentration must be distinguished by unit.

Points to Check When Discussing Nitrate and Nitrite Ion Determination

Checking the following points before writing the report makes the discussion easier to write.

• Is it clear whether the measurement target is nitrate ions or nitrite ions?
• Is their meaning as water-quality indicators explained?
• Is the relationship with the nitrogen cycle described?
• Is the relationship with eutrophication considered?
• Is the meaning of the colorimetric reaction explained?
• Is the effect of the reduction operation considered in nitrate ion measurement?
• Is the linearity of the calibration curve checked?
• Is the absorbance of the unknown sample confirmed to be within the calibration-curve range?
• Is blank correction taken into account?
• Are the effects of sample turbidity and coloration considered?
• Are unit conversions handled correctly?
• Do the points for improvement correspond to the sources of error?

Summary

Determination of nitrate and nitrite ions is an analysis used to measure nitrogen components in water and evaluate the possibility of water pollution and eutrophication.
Nitrate ions are a relatively stable form of nitrogen and may increase because of fertilizer components, domestic wastewater, and progression of nitrification.
Nitrite ions are intermediate products of nitrification and denitrification and provide a clue for considering nitrogen-conversion reactions in water.

In spectrophotometry using a colorimetric reaction, colorless nitrate and nitrite ions are measured as colored substances and their concentrations are determined from a calibration curve.
The linearity of the calibration curve, color-development time, measurement wavelength, blank correction, cell condition, sample turbidity, and coexisting substances greatly affect the reliability of the results.
When nitrate ions are measured after reduction to nitrite ions, reduction efficiency is also important.

In a report, rather than simply writing that “the concentration was high or low,” organize and discuss their meaning as water-quality indicators, the nitrogen cycle, eutrophication, the surrounding environment of the sampling location, the principles of colorimetric reactions and spectrophotometry, sources of error, and points for improvement.
Nitrate ions and nitrite ions are important water-quality indicators, but to comprehensively evaluate nitrogen pollution and eutrophication, it is important to assess them together with results for phosphate ions, DO, COD, BOD, pH, and other parameters.