Discussion Examples for Water Hardness Measurement | Interpreting Ca and Mg Ions with EDTA Titration

What Is Water Hardness?

Water hardness is an indicator representing mainly the amount of calcium ions and magnesium ions contained in water.
In general, water containing large amounts of Ca2+ and Mg2+ is considered to have high hardness, while water containing small amounts has low hardness.
Hardness is often expressed as a concentration converted to CaCO3.

In water with high hardness, soap tends to lather poorly, and scale tends to form more readily upon heating.
On the other hand, water with low hardness is called soft water and tends to allow soap to lather easily.
Water hardness varies depending on geology, water source, water-treatment processes, and differences in mineral composition.

Main Items to Include in the Results

In the results of water hardness measurement, organize the type of sample water, sampling location, concentration of the EDTA standard solution, sample-water volume, titration volume, indicator, buffer solution, pH conditions, total hardness, calcium hardness, magnesium hardness, and other information.
In titration experiments, reading the titration volume and determining the endpoint greatly affect the results, so recording the operating conditions makes the discussion easier.

Main Items to Include in the Results

• Type of sample water
• Sampling location
• Sample-water volume
• Concentration of the EDTA standard solution
• EDTA titration volume
• Type of indicator
• Type of buffer solution
• pH during titration
• Color change at the endpoint
• Total hardness
• Calcium hardness
• Magnesium hardness
• CaCO3-equivalent value
• Average value from multiple measurements
• Standard deviation and variation
• Comparison with other water samples
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Water Hardness Measurement

Here, the process of determining calcium ions (Ca2+) and magnesium ions (Mg2+) in water by EDTA chelatometric titration and calculating total hardness, calcium hardness, and magnesium hardness is organized using reference experimental values.

Water hardness is a value expressing the amounts of Ca2+ and Mg2+ contained in water as a calcium carbonate (CaCO3) equivalent.
In general, the more Ca2+ and Mg2+ a water sample contains, the higher its hardness, affecting soap lathering, scale formation, and the way the water tastes.

Reference Experimental Conditions

Concept of EDTA Titration

EDTA forms chelates with Ca2+ and Mg2+ in a 1:1 ratio.
Therefore, the total amount of Ca2+ and Mg2+ in water can be determined from the amount of EDTA consumed in the titration.

EDTA Titration Results by Water Sample

Reference examples are shown in which 50.00 mL of each water sample was titrated with 0.0100 mol/L EDTA.
Total hardness was measured as the sum of Ca2+ and Mg2+, while calcium hardness was measured as Ca2+ alone.

Example Calculation of Total Hardness

EDTA reacts with Ca2+ and Mg2+ in a 1:1 ratio.
First, the amount of EDTA is determined.

Amount of EDTA = EDTA concentration × EDTA titration volume

For tap water, the EDTA concentration is 0.0100 mol/L and the total-hardness titration volume is 2.85 mL = 0.00285 L.

Amount of EDTA = 0.0100 mol/L × 0.00285 L = 2.85 × 10−5 mol

This amount is converted to a CaCO3 equivalent.
Taking the molar mass of CaCO3 as 100.1 g/mol, the CaCO3-equivalent amount in 50.00 mL of sample is as follows.

CaCO3-equivalent amount = 2.85 × 10−5 mol × 100.1 g/mol = 0.00285 g

0.00285 g = 2.85 mg

Because this is the amount in 50.00 mL of sample, it is converted to the amount per 1 L.

Total hardness = 2.85 mg ÷ 0.05000 L = 57.1 mg/L

Therefore, in this reference example, the total hardness of the tap water is determined to be 57.1 mg/L as CaCO3.

Simplified Formula for Hardness Calculation

When the sample volume is 50.00 mL and the EDTA concentration is 0.0100 mol/L, the hardness can be simplified as follows.

Hardness (mg/L as CaCO3) = EDTA titration volume (mL) × 20.0

For tap water, the total-hardness titration volume is 2.85 mL, so:

Total hardness = 2.85 × 20.0 = 57.0 mg/L

This simplified formula can be used when the sample volume and EDTA concentration are the conditions shown above.
If the conditions change, the calculation must be redone from the amount of substance.

Example Calculation of Ca Hardness and Mg Hardness

Calcium hardness is determined by titrating Ca2+ with EDTA at approximately pH 12 while suppressing the effect of Mg2+.
Magnesium hardness is obtained by subtracting calcium hardness from total hardness.

For tap water, the Ca-hardness titration volume is 2.10 mL and the total-hardness titration volume is 2.85 mL.

Ca hardness = 2.10 × 20.0 = 42.0 mg/L

Mg hardness = Total hardness − Ca hardness = 57.0 − 42.0 = 15.0 mg/L

Therefore, in this tap water, of the total hardness of 57.0 mg/L, Ca hardness is 42.0 mg/L and Mg hardness is 15.0 mg/L.

Example Conversion to Ca and Mg Concentrations

Hardness is expressed as a CaCO3 equivalent, but it can be converted to Ca2+ and Mg2+ concentrations when necessary.

Ca2+ concentration (mg/L) = Ca hardness × 40.1 ÷ 100.1

Mg2+ concentration (mg/L) = Mg hardness × 24.3 ÷ 100.1

For tap water, Ca hardness is 42.0 mg/L and Mg hardness is 15.0 mg/L.

Ca2+ concentration = 42.0 × 40.1 ÷ 100.1 = 16.8 mg/L

Mg2+ concentration = 15.0 × 24.3 ÷ 100.1 = 3.64 mg/L

In this way, hardness expressed as a CaCO3 equivalent and the actual Ca2+ and Mg2+ concentrations have different numerical values.

Example Classification of Hardness

From the hardness value, water can be roughly classified as soft water, moderately hard water, or hard water.
Classification criteria may differ depending on the source, but here they are organized as follows for reference.

According to this classification, tap water at 57.1 mg/L is soft water, well water at 156.2 mg/L is hard water, and mineral water at 228.2 mg/L can be judged to be close to very hard water.

Changes in Hardness Before and After Boiling

In water containing temporary hardness, boiling may cause calcium carbonate and other substances to precipitate, lowering the hardness.
A reference example for boiling well water is shown below.

Because the hardness decreased after boiling, the well water may have contained components related to temporary hardness, such as calcium hydrogen carbonate.

Differences in Endpoint Depending on pH and Indicator

In EDTA titration, if the pH conditions deviate, the reaction between metal ions and EDTA and the color change of the indicator become unclear, making the endpoint difficult to judge.

In EDTA titration, it is important to keep the pH constant with a buffer solution and to use the same amount of indicator.

Comparison of Errors Caused by Endpoint Judgment

With EBT indicator, the color changes from reddish purple to blue at the endpoint.
If the color change is judged too early or titration is continued until a deep blue color is obtained, the calculated hardness shifts.

Judging the endpoint color using the same criterion each time improves the reproducibility of the measured values.

Example of Checking Reproducibility

A reference example is shown in which the same tap-water sample was measured three times to check variation in titration volume and hardness.

The three hardness values ranged from 56.6 to 57.2 mg/L and agreed well.
Highly reproducible measurements can be obtained by standardizing pH adjustment, indicator amount, endpoint judgment, and burette reading.

Example of Confirmation by Standard Addition

To check whether components in the water sample interfere with EDTA titration, a known amount of Ca2+ may be added and the recovery rate determined.

If the recovery rate is low, EDTA titration may have been affected by precipitation of metal ions, turbidity, coexisting ions, insufficient pH adjustment, or other factors.

Example Calculation of Recovery Rate

Consider a case in which Ca2+ corresponding to 20.0 mg/L of hardness is added to tap water and the measured hardness becomes 76.4 mg/L.
The original hardness is 57.0 mg/L.

Recovery rate (%) = (Measured hardness after addition − Original hardness) ÷ Added hardness × 100

Recovery rate = (76.4 − 57.0) ÷ 20.0 × 100 = 97%

From this result, major interference with EDTA titration in tap water is considered small.

Example of How to Write the Results

A 50.00 mL water sample was titrated with 0.0100 mol/L EDTA standard solution, and the hardness was determined as a CaCO3 equivalent.
For tap water, the total-hardness titration volume was 2.85 mL, and the total hardness was determined to be 57.1 mg/L.
In addition, because the Ca-hardness titration volume was 2.10 mL, the Ca hardness was 42.0 mg/L and the Mg hardness was 15.0 mg/L.

When the samples were compared, the hardness was 7.0 mg/L for softened water, 57.1 mg/L for tap water, 72.1 mg/L for river water, 156.2 mg/L for well water, and 228.2 mg/L for mineral water.
Well water and mineral water were considered to have high hardness because they contained large amounts of Ca2+ and Mg2+.

When the well water was boiled, the total hardness decreased from 156.2 mg/L to 121.1 mg/L.
This was considered to be because calcium hydrogen carbonate and similar substances precipitated as calcium carbonate upon heating and were removed by filtration.

Points for Connecting the Results to the Discussion

In a discussion of water hardness measurement, it is important not only to calculate hardness from the titration volume but also to relate differences between Ca2+ and Mg2+, the origin of the water, changes caused by boiling, and endpoint errors.

• Has the 1:1 reaction between EDTA and Ca2+ and Mg2+ been correctly reflected in the calculation?
• Can the hardness as a CaCO3 equivalent be determined from the EDTA titration volume?
• Can the relationship among total hardness, Ca hardness, and Mg hardness be explained?
• Can the difference between hardness and Ca2+ and Mg2+ concentrations be explained?
• Can the reason well water and mineral water have high hardness be explained in relation to the origin of the water and contact with geological formations?
• Can the reason hardness decreases after boiling be explained in relation to temporary hardness and precipitation of calcium carbonate?
• Can the pH buffer, indicator amount, endpoint judgment, and burette reading be discussed as sources of error?
• Can the possibility that turbidity and coexisting metal ions affect EDTA titration be explained?

Example Discussion

In this experiment, Ca2+ and Mg2+ in water samples were determined by EDTA chelatometric titration and expressed as hardness on a CaCO3-equivalent basis.
The total-hardness titration volume for 50.00 mL of tap water was 2.85 mL, and the total hardness was determined to be 57.1 mg/L.
This value is classified as soft water and indicates water containing a relatively small amount of mineral components.

Comparing total hardness and Ca hardness, of the total hardness of 57.0 mg/L in the tap water, Ca hardness was 42.0 mg/L and Mg hardness was 15.0 mg/L.
From this, Ca2+ was considered to be the main hardness component in this tap water, with a certain amount of Mg2+ also present.
Because hardness is expressed as a CaCO3 equivalent, care is required because the numerical values differ from the actual Ca2+ and Mg2+ concentrations.

When the samples were compared, well water and mineral water had higher hardness than tap water.
This was considered to be because groundwater and mineral water remained in contact with limestone and minerals in geological formations for long periods and dissolved larger amounts of Ca2+ and Mg2+.
On the other hand, hardness was greatly reduced in softened water, suggesting that Ca2+ and Mg2+ had been removed.

When the well water was boiled, the total hardness decreased from 156.2 mg/L to 121.1 mg/L.
This was considered to be because calcium hydrogen carbonate in the water precipitated as calcium carbonate upon heating and was removed by filtration.
Therefore, this well water may have contained components related to temporary hardness.

Possible sources of error include insufficient pH buffer, variation in indicator amount, endpoint-color judgment, and burette-reading errors.
In EDTA titration, if the pH is inappropriate, complex formation between metal ions and EDTA and the color change of the indicator become unclear.
In addition, if titration is continued until the endpoint becomes deep blue, excess EDTA is added and the hardness may be overestimated.

Summary

In water hardness measurement, the property that EDTA forms 1:1 chelates with Ca2+ and Mg2+ is used to determine hardness as a CaCO3 equivalent from the EDTA titration volume.
Mg hardness can also be obtained by subtracting Ca hardness from total hardness.

In this reference example, tap water showed a soft-water value, while well water and mineral water showed values closer to hard water.
In a report, it is useful to discuss the titration volume, total hardness, Ca hardness, Mg hardness, CaCO3 conversion, changes caused by boiling, and errors caused by endpoint judgment and pH conditions in relation to one another.

What Is EDTA Chelatometric Titration?

EDTA chelatometric titration is a titration method that uses the property of EDTA to form stable complexes with metal ions.
EDTA reacts with Ca2+ and Mg2+ in a 1:1 ratio.
Therefore, if the concentration and titration volume of the EDTA standard solution are known, the amount of metal ions in the sample water can be determined.

In water hardness measurement, titration proceeds as EDTA captures Ca2+ and Mg2+ in the water.
At the endpoint, the metal ions that had been bound to the metal indicator are also taken by EDTA, causing the indicator to change color.
This color change is used to determine the endpoint.

M2+ + EDTA → M-EDTA complex

Relationship Between Ca and Mg Ions and Hardness

The ions mainly related to water hardness are Ca2+ and Mg2+.
These ions enter the water when minerals in rocks and soil dissolve.
For example, in areas containing large amounts of limestone, the Ca2+ concentration may become high and the hardness may increase.

Ca2+ and Mg2+ react with fatty-acid ions in soap to form insoluble salts, so soap lathers poorly in hard water.
In addition, upon heating they may precipitate as carbonates and cause scale.
Therefore, hardness is also relevant to daily life and evaluation of industrial water.

What Is Total Hardness?

Total hardness is the hardness corresponding to the combined amount of Ca2+ and Mg2+ contained in water.
In EDTA titration, both Ca2+ and Mg2+ can be titrated together by adjusting the conditions.
The value obtained under these conditions is total hardness.

Total hardness is often expressed as a CaCO3 equivalent.
By expressing it as a CaCO3 equivalent, hardness components containing calcium and magnesium can be compared using a common basis.
Total hardness is an important indicator when comparing water quality.

Calcium Hardness and Magnesium Hardness

Calcium hardness is the hardness originating from Ca2+ in water.
Magnesium hardness, on the other hand, is the hardness originating from Mg2+.
Total hardness can be considered the sum of calcium hardness and magnesium hardness.

In some experiments, total hardness is measured first, calcium hardness is measured under different conditions, and magnesium hardness is obtained from the difference.
Evaluating Ca2+ and Mg2+ separately makes it possible to discuss water-quality characteristics in greater detail.

Magnesium hardness = Total hardness − Calcium hardness

EBT Indicator and Endpoint Determination

In total-hardness measurement by EDTA titration, Eriochrome Black T, abbreviated EBT, may be used as the indicator.
EBT shows a reddish-purple color when bound to metal ions and changes to blue when all the metal ions are captured by EDTA.
This color change is used to determine the endpoint.

Near the endpoint, the color may change gradually, making overtitration and reading errors more likely.
If too much EDTA is added beyond the endpoint, the hardness may be overestimated.
Endpoint determination is an important source of error in water hardness measurement.

Importance of pH Conditions

pH conditions are extremely important in EDTA chelatometric titration.
The ease with which EDTA forms complexes with metal ions changes with pH.
In addition, the color and function of EBT as a metal indicator are also affected by pH.
Therefore, a buffer solution is added during hardness measurement to keep the pH constant.

Total-hardness measurement is generally performed under alkaline conditions.
If the pH is inappropriate, the reaction between Ca2+ or Mg2+ and EDTA may not proceed sufficiently, or the endpoint color change may become unclear.
As a result, errors occur in the measured hardness.

Discussion When Hardness Is High

When hardness is high, the water is considered to contain large amounts of Ca2+ and Mg2+.
Groundwater, well water, water that has passed through limestone areas, and mineral water may have high hardness.
The geology of the water source greatly affects hardness.

Water with high hardness has characteristics such as poor soap lathering, easy scale formation in heating equipment and pipes, and a mineral taste.
However, high hardness is not necessarily undesirable, and evaluation varies depending on personal preference and intended use as drinking water.

Discussion When Hardness Is Low

When hardness is low, the water is considered to contain small amounts of Ca2+ and Mg2+.
Rainwater, surface water, some tap water, and water flowing through granite areas may have low hardness.
Tap water in Japan is generally considered to be relatively soft.

Water with low hardness tends to allow soap to lather easily and scale to form less readily.
On the other hand, because it contains fewer mineral components, its taste as drinking water may differ from hard water.
Low hardness may also reflect the characteristics of the water source and geology.

Comparison of Tap Water, Groundwater, and Mineral Water

In hardness measurement, comparing multiple water samples makes discussion easier.
The hardness of tap water varies by region, but in Japan it tends to be relatively low.
Groundwater remains in contact with geological formations for long periods, so it may contain larger amounts of Ca2+ and Mg2+ and therefore have higher hardness.

The hardness of mineral water varies greatly depending on the source and product.
Water with high hardness requires a larger EDTA titration volume.
Relating differences among samples to differences in water source, geology, and treatment method leads to a good discussion.

Temporary Hardness and Permanent Hardness

Water hardness may be divided into temporary hardness and permanent hardness.
Temporary hardness originates mainly from calcium hydrogen carbonate and magnesium hydrogen carbonate and is hardness that readily precipitates as carbonates upon heating.
Permanent hardness originates from sulfates, chlorides, and similar salts and is difficult to remove by heating.

Many experiments determine only total hardness, but the difference between temporary and permanent hardness is also useful when considering the causes of hardness.
Scale formation caused by heating is closely related to temporary hardness.

Discussion When the EDTA Titration Volume Is Large

A large EDTA titration volume indicates that the sample water contains a large amount of metal ions that react with EDTA.
In hardness measurement, this mainly means that there are large amounts of Ca2+ and Mg2+.
Therefore, the larger the EDTA titration volume, the higher the hardness.

However, if metal ions other than Ca2+ and Mg2+ are present, they may also react with EDTA and affect the titration volume.
In ordinary water samples, Ca and Mg are the main hardness components, but in industrial wastewater and special samples, the effects of other metal ions must also be considered.

Discussion When the EDTA Titration Volume Is Small

A small EDTA titration volume indicates that the sample water contains small amounts of Ca2+ and Mg2+.
In this case, the hardness is low and the water may have characteristics close to soft water.
Rainwater, some tap water, and water from regions with few mineral components require smaller amounts of EDTA.

However, if the titration volume is extremely small, it is necessary to check whether there were problems with the EDTA concentration, sample volume, endpoint determination, indicator amount, or pH conditions.
Particularly for low-hardness samples, small reading errors in the titration volume become relatively large.

Effects of Coexisting Ions

EDTA forms complexes not only with Ca2+ and Mg2+ but also with other metal ions.
For example, if Fe3+, Cu2+, Zn2+, and similar ions coexist, they may consume EDTA and affect hardness measurement.
The effect is often small in ordinary drinking water, but caution is required for wastewater and special water samples.

If coexisting metal ions consume EDTA, a value higher than the actual hardness originating from Ca and Mg may be obtained.
Masking agents may be used when necessary to suppress the effects of interfering ions.
It is important to follow the reagents and conditions specified in the laboratory manual.

Concept of Hardness Calculation

Hardness is calculated from the concentration of the EDTA standard solution, titration volume, and sample-water volume.
Because EDTA basically reacts with Ca2+ and Mg2+ in a 1:1 ratio, the amount of hardness components can be determined from the amount of EDTA.
In many cases, the obtained value is converted to a CaCO3 equivalent.

In the calculation, unit conversion, sample-water volume, dilution factor, and EDTA concentration must be handled correctly.
Particularly in CaCO3 conversion, errors in molar mass or concentration units cause large deviations in the result.
Clearly writing the calculation process makes the report easier to understand.

Discussion When the Endpoint Is Difficult to Determine

In EDTA titration, the endpoint color change may not be clear.
Possible causes include inappropriate pH, too much or too little indicator, low metal-ion concentration, colored sample water, and excessively fast titration.
If endpoint determination is unclear, an error occurs in the titration volume.

Near the endpoint, EDTA is added one drop at a time and the color change is checked while mixing thoroughly.
If the color change is missed and titration continues too far, the hardness may be overestimated.
Conversely, stopping before the endpoint may cause the hardness to be underestimated.

Discussion When Titration Values Vary

If titration values vary among repeated measurements, possible causes include burette-reading errors, differences in endpoint determination, sample-aliquoting errors, concentration errors in the EDTA standard solution, insufficient mixing, and differences in pH conditions.
Because the titration volume is small for low-hardness water, even slight errors can greatly affect the result.

If the variation in titration values is large, not only the average value but also the cause of the variation should be discussed.
If possible, multiple values that are close to one another should be used, and values clearly considered to result from operational mistakes should be handled with reasons stated.

Sources of Error in Water Hardness Measurement

Sources of error in water hardness measurement include errors in the EDTA standard-solution concentration, burette-reading errors, endpoint-determination errors, deviations in pH conditions, indicator amount, sample-aliquoting errors, effects of coexisting ions, and changes during storage of the sample water.
In titration experiments, small differences in operation affect the measured value.

In addition, if Ca2+ or Mg2+ enters the sample because of insufficient cleaning of the equipment, the hardness may be determined as too high.
Conversely, if sample water is spilled or the aliquot volume is too small, the hardness may be determined as too low.
Sources of error are easier to organize when divided into factors that increase and decrease the titration volume.

When the Hardness Measurement Can Be Considered Good

Hardness measurement can be considered to have produced good results when the endpoint color change is clear, multiple titration values agree closely, and the magnitude of the hardness does not contradict the type and source of the sample water.
For example, results showing high hardness in mineral water and groundwater and low hardness in tap water and rainwater are easier to consider reasonable.

In addition, if the concentration of the EDTA standard solution is accurate and the pH conditions are appropriately maintained, reliable titration can be performed.
In the discussion of the results, not only the numerical values but also the reproducibility of the measurement operation should be checked.

Example Discussion When the Experiment Did Not Go Well

When hardness measurement does not go well, possible causes are considered from results such as difficulty identifying the endpoint, variation in titration values, hardness that is higher or lower than expected, or trends among samples that are difficult to explain.
Organizing the causes according to EDTA concentration, pH, indicator, endpoint determination, coexisting ions, equipment contamination, and calculation mistakes makes the discussion easier.

How to Write Points for Improvement

In a discussion of water hardness 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 operations, endpoint determination, and calculation and analysis.

Improvements to Samples and Equipment

• Thoroughly clean the equipment
• Accurately aliquot the sample water
• Mix the sample water thoroughly before measurement
• Record the sampling location and sampling time
• Compare multiple water samples when necessary

Improvements to Titration Operations

• Accurately confirm the concentration of the EDTA 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
• Mix thoroughly during titration
• Perform multiple titrations and calculate the average value

Improvements to Endpoint Determination and Analysis

• Keep the pH constant with a buffer solution
• Add an appropriate amount of indicator
• Confirm the endpoint color change in the laboratory manual
• Distinguish between total hardness and calcium hardness
• When magnesium hardness is determined from the difference, consider propagation of error
• Check unit conversions for the CaCO3 equivalent
• Consider the effects of coexisting ions

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of water hardness measurement, simply writing that “the hardness was high” or “EDTA was used” results in a superficial discussion.
A good discussion relates Ca and Mg ions, EDTA complexes, endpoint determination, pH conditions, differences in water source, 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 water hardness measurement.
Adjust the necessary parts according to your own experimental results.

• Water hardness is an indicator mainly reflecting the amounts of Ca2+ and Mg2+.
• EDTA forms stable complexes with Ca2+ and Mg2+.
• Because EDTA and metal ions react in a 1:1 ratio, the amount of hardness components can be determined from the EDTA titration volume.
• The larger the EDTA titration volume, the greater the amount of hardness components considered to be present in the sample water.
• Total hardness is the sum of the hardness originating from Ca2+ and Mg2+.
• Magnesium hardness can be determined from the difference between total hardness and calcium hardness.
• If the endpoint color change is unclear, an error may occur in the titration volume.
• If the pH conditions are inappropriate, formation of EDTA complexes and the color change of the indicator are affected.
• If coexisting metal ions are present, they may consume EDTA and cause the hardness to be overestimated.
• Differences in hardness may reflect differences in water source, geology, and mineral composition.

Points to Check When Discussing Water Hardness Measurement

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

• Is it explained what hardness represents?
• Is the relationship between Ca2+ and Mg2+ described?
• Is the reaction ratio between EDTA and metal ions understood?
• Are total hardness and calcium hardness distinguished?
• Is the method for determining magnesium hardness explained?
• Is the importance of pH conditions considered?
• Is the color change of the EBT indicator explained?
• Are errors in endpoint determination considered?
• Is variation in EDTA titration volume checked?
• Are the effects of coexisting ions considered?
• Is the relationship with the water source and geology discussed?
• Do the points for improvement correspond to the sources of error?

Summary

Water hardness measurement is an analysis used to determine the amounts of Ca2+ and Mg2+ in water by EDTA chelatometric titration.
Because EDTA forms 1:1 complexes with metal ions, the amount of hardness components can be calculated from the amount of EDTA required for titration.
Total hardness originates from the combined amount of Ca2+ and Mg2+ and is often expressed as a CaCO3 equivalent.

Water with high hardness contains large amounts of Ca and Mg ions and tends to show reduced soap lathering and increased scale formation.
Water with low hardness contains smaller amounts of these ions and has properties closer to soft water.
Differences in hardness may reflect differences in water source, geology, contact time with groundwater, and mineral composition.

In a report, rather than simply writing that “the hardness was high or low,” organize and discuss complex formation between Ca and Mg ions and EDTA, titration volume, pH conditions, indicator color change, endpoint determination, coexisting ions, and sources of error.
In water hardness measurement, it is important to understand the relationship between the principle of chelatometric titration and water-quality evaluation.