Discussion Examples for Chloride Ion Measurement | Precipitation Titration and Comparison of Seawater and Tap Water

What Is Chloride Ion Measurement?

Chloride ion measurement is an analysis used to determine the concentration of Cl- contained in water.
Chloride ions are produced when sodium chloride, NaCl, dissolves in water and may also be present because of seawater intrusion, domestic wastewater, industrial wastewater, deicing agents, salts in groundwater, and other sources.
In water-quality analysis, chloride ions are treated as an indicator of the effects of salinity and anthropogenic pollution.

Chloride ions are relatively stable and are not easily decomposed in water.
Therefore, differences in concentration may readily reflect the influence of the water source and surrounding environment.
By comparing seawater, brackish water, tap water, and river water, the origin of the water and the effects of salinity can be discussed from differences in chloride ion concentration.

Main Items to Include in the Results

In the results of chloride ion measurement, organize the type of sample water, sampling location, sample amount, concentration of the silver nitrate standard solution, titration volume, indicator, endpoint color change, dilution factor, chloride ion concentration, and other information.
In samples with high chloride ion concentrations, such as seawater, handling the dilution factor is particularly important.

Main Items to Include in the Results

• Type of sample water
• Sampling location
• Sampling date and time
• Sample-water volume
• Dilution factor
• Concentration of the silver nitrate standard solution
• Silver nitrate titration volume
• Type of indicator
• Color change at the endpoint
• Blank value
• Chloride ion concentration
• NaCl-equivalent value
• Average value from multiple measurements
• Comparison of seawater, tap water, river water, and other samples
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Chloride Ion Measurement by Precipitation Titration

Here, chloride ions contained in tap water, river water, seawater, and other samples are measured by precipitation titration, and the process of determining Cl- concentration from the AgNO3 titration volume is organized using reference experimental values.

Chloride ions react with silver ions to form sparingly soluble silver chloride.
Using this property, the amount of chloride ions in water can be determined from the consumption of a silver nitrate standard solution.
In samples with low chloride ion concentrations, such as tap water, the titration volume is small, while samples with high concentrations, such as seawater, must be diluted.

Reference Experimental Conditions

Reaction in Precipitation Titration

Chloride ions and silver ions react in a 1:1 ratio as follows to form a white precipitate of silver chloride.

Ag+ + Cl- → AgCl

After almost all chloride ions have precipitated, excess Ag+ reacts with chromate ions to form reddish-brown Ag2CrO4.
The point at which this reddish-brown color remains slightly is taken as the endpoint.

Confirmation Titration Using a Standard Salt Solution

First, an example is shown in which a chloride ion standard solution of known concentration is titrated to confirm the silver nitrate standard solution and endpoint determination.
It is assumed that 10.00 mL of the standard solution contains 3.545 mg of Cl-.

Because the titration volumes for the standard solution were approximately 10.00 mL and agreed closely, the concentration of the AgNO3 standard solution and the endpoint determination were considered generally appropriate under these conditions.

Titration Results for Different Water Samples

Reference examples are shown in which each water sample was titrated with 0.0100 mol/L AgNO3 standard solution.
Tap water and river water were titrated directly using 50.00 mL of sample, while seawater was diluted 100-fold before 10.00 mL was titrated.

Example Calculation of the Amount of Cl-

Ag+ and Cl- react in a 1:1 ratio.
Therefore, the amount of AgNO3 consumed corresponds to the amount of Cl- in the titrated sample.

Amount of Cl- = AgNO3 concentration × AgNO3 titration volume

For tap water, the AgNO3 concentration is 0.0100 mol/L and the titration volume is 0.82 mL = 0.00082 L.

Amount of Cl- = 0.0100 mol/L × 0.00082 L = 8.20 × 10−6 mol

This value is the amount of Cl- contained in the 50.00 mL of tap water used for titration.

Example Calculation of Cl- Concentration

The mass of Cl- is determined by multiplying the amount of substance by the molar mass of Cl-, 35.45 g/mol.

Mass of Cl- = 8.20 × 10−6 mol × 35.45 g/mol = 0.000291 g

0.000291 g = 0.291 mg

Because this is the amount of Cl- in 50.00 mL of tap water, it is converted to the amount per 1 L.

Cl- concentration = 0.291 mg ÷ 0.05000 L = 5.82 mg/L

Therefore, in this reference example, the Cl- concentration in tap water is determined to be 5.82 mg/L.

Example Calculation of NaCl-Equivalent Concentration

When chloride ions are assumed to originate from table salt, the Cl- concentration can be converted to an NaCl-equivalent concentration.
Because NaCl and Cl- correspond in a 1:1 ratio, the molar-mass ratio is used.

NaCl-equivalent concentration = Cl- concentration × 58.44 ÷ 35.45

For tap water, the Cl- concentration is 5.82 mg/L.

NaCl-equivalent concentration = 5.82 × 58.44 ÷ 35.45 = 9.60 mg/L

This value is the concentration obtained when all Cl- is assumed to originate from NaCl.

Example Calculation Including the Dilution Factor for Seawater

Because seawater contains a high concentration of chloride ions, titrating it directly would require an excessively large amount of AgNO3.
Therefore, here an example is shown in which seawater is diluted 100-fold and 10.00 mL of the diluted solution is titrated.

If the AgNO3 titration volume for 10.00 mL of the diluted seawater is 5.45 mL, the amount of Cl- in the diluted solution is calculated as follows.

Amount of Cl- = 0.0100 mol/L × 0.00545 L = 5.45 × 10−5 mol

The mass of Cl- is:

5.45 × 10−5 mol × 35.45 g/mol = 0.00193 g = 1.93 mg

Because this is the amount of Cl- in 10.00 mL of the 100-fold diluted solution, the concentration in the diluted solution is calculated as follows.

Cl- concentration in the diluted solution = 1.93 mg ÷ 0.01000 L = 193 mg/L

Because the original seawater was diluted 100-fold, the Cl- concentration in the original sample is multiplied by 100.

Cl- concentration in the original seawater = 193 mg/L × 100 = 19300 mg/L

Therefore, in this reference example, the Cl- concentration in seawater is determined to be 19300 mg/L.

Effect of Incorrectly Handling the Dilution Factor

For samples that are diluted before measurement, such as seawater, forgetting to include the dilution factor in the calculation causes the concentration to be greatly underestimated.

For high-concentration samples, it is necessary to check the collected volume, final volume, and aliquot volume and correctly convert the result back to the original sample.

Comparison of Errors Caused by Endpoint Judgment

In precipitation titration, judging the reddish-brown endpoint too early causes underestimation, while titrating until the color becomes dark causes overestimation.
Differences in endpoint judgment are compared below using tap water as an example.

Particularly for low-concentration samples such as tap water, even a difference of approximately 0.1 mL in reading or endpoint determination greatly affects the concentration calculation.

Comparison When the Sample Volume Is Increased

In low-concentration samples, the titration volume is small and relative error tends to become large.
In such cases, increasing the sample volume used for titration increases the titration volume and makes it easier to read.

The calculated concentration is the same, but increasing the sample volume increases the titration volume and can reduce the relative effect of burette-reading error.

Effect of pH Conditions

In the Mohr method using potassium chromate as the indicator, pH conditions affect the endpoint.
Under strongly acidic conditions, the state of chromate ions changes, while under strongly alkaline conditions silver ions may undergo other reactions.

If the pH is inappropriate, the endpoint may become unclear and the titration volume may shift.
If the pH of the sample is extreme, the measurement conditions must be checked.

Effects of Coexisting Components

In precipitation titration, if components other than chloride ions that react with Ag+ are present, the Cl- concentration may be overestimated.

In tap water and river water, these effects may be small, but in seawater and special wastewater the effects of coexisting ions must be considered.

Example of Checking Reproducibility

The same sample is measured multiple times to check how closely the titration volumes and calculated values agree.
Here, an example is shown in which 50.00 mL of river water was titrated three times.

The three Cl- concentrations were within the range of 9.50 to 9.64 mg/L and agreed relatively well.
However, because the titration volume is small for low-concentration samples, care is required regarding burette-reading error.

Example of How to Write the Results

When 50.00 mL of tap water was titrated with 0.0100 mol/L AgNO3 standard solution, the titration volume was 0.82 mL.
Because Ag+ and Cl- react in a 1:1 ratio, the amount of Cl- in the sample was determined to be 8.20 × 10−6 mol.
Converting this to mass gave 0.291 mg, and the Cl- concentration was 5.82 mg/L.

The Cl- concentration was 9.57 mg/L in river water and 26.9 mg/L in well water, both higher than in tap water.
On the other hand, in seawater diluted 100-fold, the Cl- concentration was 19300 mg/L after the dilution factor was taken into account.
Because seawater has a very high chloride ion concentration, sufficient dilution is required before titration.

When the Cl- concentration was converted to NaCl, the value was 9.60 mg/L for tap water and 31800 mg/L for seawater.
The NaCl-equivalent concentration is a guide obtained by assuming that all chloride ions originate from table salt, although other salts may actually be present.

Points for Connecting the Results to the Discussion

In a discussion of chloride ion measurement, it is important to relate not only the titration calculations but also the type of sample, dilution factor, endpoint determination, and effects of coexisting components.

• Has the 1:1 reaction between Ag+ and Cl- been correctly reflected in the calculation?
• Can the amount of Cl- be determined from the AgNO3 concentration and titration volume?
• Can the Cl- concentration be expressed in mg/L using the sample volume?
• For high-concentration samples such as seawater, has the dilution factor been correctly considered?
• Can the difference between Cl- concentration and NaCl-equivalent concentration be explained?
• Can it be explained that titrating until a dark reddish-brown color appears causes the concentration to be overestimated?
• Can it be discussed that in low-concentration samples the titration volume is small and reading errors have a greater effect?
• Can pH conditions, turbidity, coloration, and coexisting components such as Br- and I- be explained as sources of error?

Example Discussion

In this experiment, chloride ions in water samples were determined by precipitation titration using a silver nitrate standard solution.
The titration volume for 50.00 mL of tap water was 0.82 mL, and the Cl- concentration was determined to be 5.82 mg/L.
The values were 9.57 mg/L for river water and 26.9 mg/L for well water, showing differences in chloride ion concentration depending on the type of sample.

For seawater, the titration volume of 10.00 mL of a sample diluted 100-fold was 5.45 mL.
The Cl- concentration in the diluted solution was 193 mg/L, and when the 100-fold dilution factor was taken into account, the Cl- concentration in the original seawater was 19300 mg/L.
Seawater has a much higher chloride ion concentration than tap water and river water, so forgetting to include the dilution factor would cause a large error.

In precipitation titration, Ag+ and Cl- react in a 1:1 ratio to form a white precipitate of AgCl.
After almost all chloride ions have reacted, excess Ag+ reacts with chromate ions to form reddish-brown Ag2CrO4.
Therefore, the endpoint must be taken as the point at which a slight reddish-brown color remains within the white precipitate.

Possible sources of error include deviations in endpoint determination, burette-reading errors, dilution procedures, pH conditions, and effects of coexisting components.
Particularly for low-concentration samples such as tap water, the titration volume may be less than 1 mL, so even a difference of approximately 0.1 mL can greatly affect the calculated value.
It is important to increase the sample volume, perform multiple measurements and take the average, and standardize endpoint determination.

In addition, if ions such as Br- and I- that form precipitates with silver ions coexist, they also consume AgNO3 and may cause the Cl- concentration to be overestimated.
Because seawater and special environmental waters contain many coexisting ions, the results of precipitation titration should be treated as chloride ion measurements while also considering the effects of coexisting components.

Summary

In precipitation titration of chloride ions, the 1:1 reaction between Ag+ and Cl- is used to determine the amount of Cl- from the titration volume of the AgNO3 standard solution.
The Cl- concentration can then be expressed in mg/L by dividing by the sample volume.

In this reference example, tap water and river water contained from several mg/L to several tens of mg/L, while seawater contained 19300 mg/L after the dilution factor was taken into account.
In a report, it is useful to relate the titration volume, reaction ratio, Cl- concentration, NaCl conversion, dilution factor, endpoint determination, and errors caused by coexisting components.

What Is Precipitation Titration?

Precipitation titration is a method in which a sparingly soluble precipitate is formed by a titration reaction and the amount of the target component is determined from the reaction amount.
In chloride ion measurement, Ag+ and Cl- react to form a white precipitate of silver chloride, AgCl.
Because this reaction proceeds in a 1:1 ratio, the amount of chloride ions can be determined from the amount of silver nitrate consumed.

In precipitation titration, the point at which the target ion has almost completely precipitated is judged as the endpoint.
However, because precipitate is present near the endpoint, the solution tends to become turbid and the color change may be overlooked.
The accuracy of endpoint determination greatly affects the result.

Ag+ + Cl- → AgCl↓

Concept of the Mohr Method

The Mohr method is a representative precipitation-titration method in which chloride ions are titrated with silver nitrate.
Potassium chromate is added as an indicator, and after Cl- has finished precipitating as AgCl, excess Ag+ reacts with CrO42- to form a reddish-brown precipitate of Ag2CrO4.
The point at which this reddish-brown color appears is taken as the endpoint.

In the Mohr method, it is important that AgCl precipitate forms first and silver chromate forms after the endpoint.
If the pH is inappropriate, the state of chromate ions and precipitate formation may be affected and the endpoint may shift.
Therefore, checking the pH conditions and endpoint color is important.

2Ag+ + CrO42- → Ag2CrO4↓

Relationship Between Silver Nitrate Titration Volume and Chloride Ion Concentration

The larger the silver nitrate titration volume, the greater the amount of chloride ions considered to be present in the sample water.
Because Ag+ and Cl- react in a 1:1 ratio, the amount of Cl- can be determined from the concentration and titration volume of the silver nitrate standard solution.
This relationship is the basis of chloride ion determination.

In samples with high chloride ion concentrations, such as seawater, the silver nitrate titration volume becomes very large, so the sample is often diluted before measurement.
When the sample is diluted, the measured concentration must be multiplied by the dilution factor to return it to the concentration of the original sample.

Comparison of Seawater and Tap Water

Seawater contains large amounts of salts, including sodium chloride, and therefore has a very high chloride ion concentration.
For this reason, when seawater is titrated with silver nitrate, the consumption of silver nitrate is greater than for tap water or river water.
In experiments, seawater is often diluted before measurement.

Tap water also contains chloride ions, but the concentration is generally much lower than in seawater.
Chloride ions in tap water may be affected by the geology of the water source, water-treatment processes, disinfection, piping, and the surrounding environment.
Comparing seawater and tap water provides an easy way to discuss differences in salinity.

Comparison With River Water and Groundwater

The chloride ion concentration of river water is generally lower than that of seawater and may be similar to tap water or show somewhat variable values.
However, the concentration may increase if it is affected by domestic wastewater, industrial wastewater, road runoff, deicing agents, or upstream intrusion of seawater.

In groundwater, the chloride ion concentration changes when salts in geological formations dissolve into the water.
In coastal areas, groundwater may have a high chloride ion concentration because of the influence of seawater.
Relating the measured values to the environment at the sampling location makes the meaning of the results easier to understand.

Discussion When Chloride Ion Concentration Is High

When chloride ion concentration is high, possible influences include seawater intrusion, saline wastewater, domestic wastewater, industrial wastewater, deicing agents, and salts originating from geological formations.
High chloride ion concentration is a natural result in seawater and brackish water.
On the other hand, if a high value is obtained in freshwater, anthropogenic inflow or geological effects must be considered.

Water with a high chloride ion concentration may present problems for drinking, agricultural use, and industrial use.
It is also related to metal corrosion and salt damage.
However, because there may be more than one cause of high chloride ion concentration, it should be evaluated together with electrical conductivity, sodium-ion concentration, and information about the sampling location.

Discussion When Chloride Ion Concentration Is Low

When chloride ion concentration is low, the effects of seawater and wastewater containing salts are considered to be small.
Rainwater, mountain river water, and some tap water may show relatively low values.
In samples with strong freshwater characteristics, the silver nitrate titration volume is also small.

However, in low-concentration samples, the titration volume becomes small, so burette-reading errors and endpoint-determination errors become relatively large.
For low-concentration samples, measurement accuracy and blank correction must be carefully checked.

Effect of Dilution

In samples with high chloride ion concentrations, such as seawater and concentrated salt solutions, titration without dilution consumes a large amount of silver nitrate.
Therefore, such samples may be diluted to a concentration that is easier to measure before titration.
Dilution makes it possible to bring the titration volume into an appropriate range.

When a diluted sample is measured, the dilution factor must be correctly reflected in the calculation.
Forgetting to apply the dilution factor causes the chloride ion concentration in the original sample to be greatly underestimated.
Errors in the dilution operation itself also affect the result.

Effect of pH Conditions

In the Mohr method, pH conditions affect endpoint determination.
Under strongly acidic conditions, the state of chromate ions changes and the reddish-brown endpoint precipitate may become difficult to form.
On the other hand, if the solution is too alkaline, silver ions may react as hydroxides or oxides.

Therefore, in the Mohr method, titration under appropriate near-neutral pH conditions is important.
If the pH conditions deviate, the formation of AgCl precipitate and endpoint Ag2CrO4 may be affected, and the titration volume may become inaccurate.

Discussion of Endpoint Determination

In the Mohr method, after the white AgCl precipitate has formed, excess Ag+ reacts with chromate ions to produce a reddish-brown Ag2CrO4 precipitate.
The point at which this reddish-brown color remains visibly is taken as the endpoint.
However, when a large amount of white precipitate is present in the solution, the reddish-brown change may be difficult to see.

If the endpoint is missed and titration continues too far, the silver nitrate titration volume becomes too large and the chloride ion concentration is overestimated.
Conversely, stopping before sufficient reddish-brown color appears may cause the concentration to be underestimated.
Near the endpoint, it is important to add the titrant one drop at a time and observe while mixing thoroughly.

Error Caused by Adsorption on the Precipitate

In precipitation titration, the AgCl precipitate that forms may adsorb ions onto its surface.
If Ag+, Cl-, or indicator ions are adsorbed on the precipitate surface, endpoint determination and titration volume may be affected.
In samples producing large amounts of precipitate, the effect of adsorption must also be considered.

Thorough stirring during titration makes the reaction more uniform and can reduce localized excess addition and unevenness at the precipitate surface.
Because endpoint determination in precipitation titration is more difficult than in titrations of transparent solutions, careful operation is important.

Effects of Coexisting Ions

In chloride ion measurement, if ions other than Cl- that form precipitates with Ag+ are present, extra silver nitrate may be consumed.
For example, halide ions such as Br- and I- form sparingly soluble precipitates with silver ions.
Therefore, in samples containing these ions, the chloride ion concentration may be overestimated.

In ordinary tap water and freshwater, the effect may be small, but in seawater and special wastewater the effects of coexisting ions must be considered.
Depending on the type of sample, it is necessary to confirm whether the Mohr method is appropriate.

Concentration Error of the Silver Nitrate Standard Solution

Chloride ion concentration is calculated from the concentration and titration volume of the silver nitrate standard solution.
Therefore, if the concentration of the silver nitrate standard solution is inaccurate, systematic errors occur in all measured values.
Because silver nitrate solution may be affected by light, care is also required regarding storage conditions.

When a standard solution is prepared, it is standardized as necessary.
If the concentration is assumed to be higher than its actual value in the calculation, the chloride ion concentration will also deviate.
The reliability of the standard solution is directly related to the reliability of the titration results.

Calculation of Chloride Ion Concentration

Chloride ion concentration is determined from the concentration of the silver nitrate standard solution, titration volume, and sample-water volume.
Because Ag+ and Cl- react in a 1:1 ratio, the amount of AgNO3 corresponds to the amount of Cl-.
When necessary, the result is expressed in mg/L or as an NaCl-equivalent value.

When a sample such as seawater is diluted before measurement, the dilution factor is reflected in the calculated value.
Mistakes in unit conversion or in handling the sample-water volume and titration-volume units can greatly shift the result, so it is important to clearly show the calculation process in the report.

Sources of Error in Chloride Ion Measurement

Sources of error in chloride ion measurement include endpoint-determination errors, concentration errors in the silver nitrate standard solution, burette-reading errors, sample-aliquoting errors, mistakes in the dilution factor, inappropriate pH conditions, effects of coexisting ions, adsorption on the precipitate, and contamination of equipment.
Because the endpoint may be difficult to observe in precipitation titration, careful operation is important.

Particularly for high-concentration samples such as seawater, errors in the dilution procedure greatly affect the result.
For low-concentration samples such as tap water, the titration volume is small, so an error of one drop or an error in reading the scale becomes relatively large.
It is important to consider sources of error according to the concentration of the sample.

When the Results Can Be Considered Good

Chloride ion measurement can be considered to have produced good results when the endpoint color change is clear, repeated titration volumes agree closely, and concentration differences appropriate for the types of samples are obtained.
For example, a result showing a high concentration in seawater and a low concentration in tap water can be considered consistent with the properties of the samples.

It is also important that the dilution factor and calculation units are correct and that the concentration of the silver nitrate standard solution is reliable.
If electrical-conductivity results are available, finding high conductivity in samples with high chloride ion concentrations can further support the validity of the results.

Example Discussion When the Experiment Did Not Go Well

When chloride ion measurement does not go well, possible causes are considered from results such as an unclear endpoint, variation in titration values, too small a difference between seawater and tap water, concentrations that are higher or lower than expected, or difficulty seeing the color change because of a large amount of precipitate.
Organizing the causes according to the standard solution, endpoint determination, dilution, sample aliquoting, coexisting ions, and pH conditions makes the discussion easier.

How to Write Points for Improvement

In a discussion of chloride ion 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 sample preparation, titration procedures, endpoint determination, and calculation and analysis.

Improvements to Sample Preparation

• Accurately aliquot the sample water
• Appropriately dilute high-concentration samples such as seawater
• Record the dilution factor
• Mix the sample thoroughly before measurement
• Thoroughly clean the equipment
• Record the sampling location and sampling time

Improvements to Titration Procedures

• Confirm the concentration of the silver nitrate standard solution
• Remove air bubbles from the burette
• Read the scale at eye level
• Add the titrant one drop at a time near the endpoint
• Stir thoroughly during titration
• Perform multiple titrations and calculate the average value

Improvements to Endpoint Determination and Analysis

• Confirm the reddish-brown endpoint color in the laboratory manual
• Maintain appropriate pH conditions
• Perform blank correction
• Consider the effects of coexisting ions
• Reflect the dilution factor in the calculation
• Distinguish between Cl- concentration and NaCl-equivalent values
• Compare with other water-quality indicators such as electrical conductivity

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of chloride ion measurement, simply writing that “seawater contained more” or “a precipitate formed” results in a superficial discussion.
A good discussion relates AgCl precipitation, silver nitrate titration volume, endpoint determination, differences in salinity between seawater and tap water, 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 chloride ion measurement.
Adjust the necessary parts according to your own experimental results.

• Chloride ions serve as an indicator for evaluating the effects of salinity and seawater intrusion in water.
• Ag+ and Cl- react in a 1:1 ratio to form a sparingly soluble AgCl precipitate.
• The larger the silver nitrate titration volume, the higher the chloride ion concentration is considered to be in the sample water.
• Because seawater contains large amounts of NaCl, its chloride ion concentration is higher than that of tap water.
• In the Mohr method, the point at which excess Ag+ reacts with chromate ions to form a reddish-brown precipitate is taken as the endpoint.
• Titrating beyond the endpoint may cause the chloride ion concentration to be overestimated.
• For high-concentration samples such as seawater, the dilution factor must be correctly reflected in the calculation.
• In low-concentration samples, an error of one drop or a scale-reading error becomes relatively large.
• Coexisting halide ions may consume silver nitrate and cause the chloride ion concentration to be overestimated.
• Evaluating the results together with electrical conductivity makes it easier to discuss the amount of salts in water.

Points to Check When Discussing Chloride Ion Measurement

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

• Is it explained what chloride ions indicate as a water-quality parameter?
• Is the precipitation reaction between Ag+ and Cl- explained?
• Is the relationship between silver nitrate titration volume and Cl- concentration described?
• Is the endpoint color of the Mohr method explained?
• Is the difference in concentration between seawater and tap water explained with reasons?
• Is the dilution factor handled correctly?
• Are the effects of pH conditions considered?
• Are errors in endpoint determination considered?
• Are adsorption on the precipitate and the effects of coexisting ions considered?
• Are errors in the standard-solution concentration and burette reading considered?
• Are the results compared with other water-quality parameters such as electrical conductivity?
• Do the points for improvement correspond to the sources of error?

Summary

Chloride ion measurement is an analysis used to determine the concentration of Cl- in water by utilizing the reaction in which Ag+ and Cl- form an AgCl precipitate.
In the Mohr method, chromate ions are used as an indicator, and the point at which excess Ag+ forms a reddish-brown Ag2CrO4 precipitate is taken as the endpoint.
The larger the silver nitrate titration volume, the higher the chloride ion concentration in the sample water is considered to be.

Because seawater contains large amounts of salts such as NaCl, it has a high chloride ion concentration and requires a larger silver nitrate titration volume than tap water or river water.
In contrast, tap water and freshwater have relatively low chloride ion concentrations.
However, even freshwater may show high concentrations because of domestic wastewater, deicing agents, industrial wastewater, seawater intrusion, or salts originating from geological formations.

In a report, rather than simply writing that “seawater contained more,” organize and discuss the principle of precipitation titration, AgCl formation, the endpoint of the Mohr method, the relationship between titration volume and concentration, dilution factor, endpoint determination, coexisting ions, and sources of error.
Chloride ion measurement is an important analysis for considering the salinity of water and the characteristics of its source.