EDTA Chelatometric Titration Discussion Examples | Water Hardness Measurement and Key Points for Endpoint Determination

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What Is EDTA Chelatometric Titration?

EDTA chelatometric titration is a titration method used to determine the concentration of metal ions by utilizing the property of EDTA to form stable chelate complexes with metal ions.
Because EDTA reacts with many metal ions in a 1:1 molar ratio, it is easy to calculate the amount of metal ions from the titration volume.

In water hardness measurement, the amounts of calcium ions and magnesium ions contained in water are mainly expressed as calcium carbonate equivalents.
This makes it possible to evaluate whether the water is closer to hard water or soft water.

EDTA titration uses a complex-formation reaction between metal ions and EDTA rather than a neutralization reaction such as that used in acid-base titration.
Therefore, pH conditions and the color change of the metal indicator are important points in the discussion.

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What Is Water Hardness?

Water hardness is an index representing the amounts of calcium ions, magnesium ions, and other such ions contained in water.
In general, water containing large amounts of these metal ions is called hard water, while water containing small amounts is called soft water.

Hardness is often expressed not as the actual amounts of calcium ions and magnesium ions, but as an equivalent amount of calcium carbonate.
Therefore, in reports, the calculated result may be shown as “hardness as CaCO3.”

Hardness = the amount of calcium ions and magnesium ions in water expressed as an equivalent amount of CaCO3

Water with high hardness may have characteristics such as poor soap lathering and a tendency for scale to form in kettles and pipes.
In the experiment, the amount of metal ions in water is determined by EDTA titration and evaluated as hardness.

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Reaction Ratio Between EDTA and Metal Ions

An important point in EDTA chelatometric titration is that EDTA reacts with many metal ions in a 1:1 molar ratio.
Calcium ions and magnesium ions also form complexes with EDTA in a 1:1 ratio.

Metal ion + EDTA → metal-EDTA complex

Using this 1:1 relationship, the total amount of metal ions in the sample can be determined from the amount of EDTA used in the titration.
When measuring total water hardness, EDTA reacts with both calcium ions and magnesium ions, so the titration volume reflects the combined amount of both ions.

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Results to Examine in EDTA Titration

In the results of EDTA chelatometric titration, organize the concentration of the EDTA solution, the volume of sample water, each titration volume, the average titration volume, the calculated hardness, and the color change at the endpoint.
If the color change at the endpoint was difficult to distinguish, this also becomes an important point in the discussion.

Main Items to Include in the Results

• Volume of sample water
• Concentration of the EDTA standard solution
• Metal indicator used
• Type of buffer solution or pH condition
• Titration volume for each trial
• Average titration volume
• Color change at the endpoint
• Calculated hardness
• Variation when measurements were repeated

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Reference Experimental Values and Analysis Example for Water Hardness Measurement by EDTA Titration

Here, reference experimental values are organized for determining calcium ions, magnesium ions, and total hardness in water samples by EDTA titration.
EDTA consumption, total hardness, calcium hardness, magnesium hardness, CaCO3 conversion, indicator color changes, pH conditions, and sources of error are summarized in a form that is easy to use in reports.

EDTA forms stable complexes with metal ions as a multidentate ligand.
In water hardness measurement, Ca2+ and Mg2+ are quantified using the fact that they react with EDTA in a 1:1 ratio.
The amount of metal ions is determined from the titration volume and is commonly expressed as hardness in terms of CaCO3.

Reference Experimental Conditions

Basic Reaction of EDTA Titration

EDTA forms complexes with divalent metal ions such as Ca2+ and Mg2+ in a 1:1 ratio.

Ca2+ + EDTA4− → [Ca-EDTA]2−

Mg2+ + EDTA4− → [Mg-EDTA]2−

Example Titration Data for Total Hardness Measurement

The following are reference data obtained by titrating 50.00 mL of a water sample with 0.0100 mol/L EDTA standard solution.
In total hardness measurement, the amount of EDTA corresponding to the combined amount of Ca2+ and Mg2+ is determined.

In this example, the first trial is judged to have been slightly over-titrated in order to confirm the endpoint color change, so the average of the second to fourth trials is used as the representative value.

Example of Total Hardness Calculation

Consider the case where the EDTA concentration is 0.0100 mol/L, the average titration volume is 6.12 mL, and the sample volume is 50.00 mL.

n(EDTA) = 0.0100 mol/L × 0.00612 L = 6.12×10−5 mol

Because Ca2+ and Mg2+ react with EDTA in a 1:1 ratio, the total amount of hardness components in the sample is 6.12×10−5 mol.

Because hardness is often expressed as CaCO3 equivalent, the formula mass of CaCO3, 100.09 g/mol, is used for the calculation.

CaCO3 equivalent amount = 6.12×10−5 mol × 100.09 g/mol = 0.00613 g

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

Total hardness = 0.00613 g ÷ 0.05000 L = 0.1226 g/L = 122.6 mg/L

Therefore, the total hardness of this water sample is approximately 123 mg/L as CaCO3.

Simplified Formula for Hardness Calculation

The hardness as CaCO3 equivalent can be calculated from the EDTA concentration, titration volume, and sample volume as follows.

Hardness (mg/L as CaCO3) = CEDTA × VEDTA × 100.09 × 1000 ÷ Vsample

If volumes are handled in mL, care must be taken with the conversion factor.

Example of Calcium Hardness Measurement

This is an example in which conditions are adjusted to mainly measure Ca2+ and determine calcium hardness.
Suppose 50.00 mL of sample is titrated with 0.0100 mol/L EDTA.

If the calcium hardness is 80.4 mg/L and the total hardness is 122.6 mg/L, the magnesium hardness can be determined from the difference.

Example of Magnesium Hardness Calculation

Magnesium hardness as CaCO3 equivalent can be determined by subtracting calcium hardness from total hardness.

Magnesium hardness = total hardness − calcium hardness

Magnesium hardness = 122.6 − 80.4 = 42.2 mg/L as CaCO3

Because magnesium hardness is often determined by difference rather than by direct measurement, the errors in both total hardness and calcium hardness affect the result.

Example of Conversion to Ca2+ and Mg2+ Concentrations

Hardness expressed as CaCO3 equivalent may be converted into the actual mass concentrations of Ca2+ and Mg2+.

Because hardness is often expressed as CaCO3 equivalent, it should not be confused with the actual concentrations of Ca2+ or Mg2+.

Example of Water Hardness Classification

Hardness classifications vary depending on the standard used, but as a reference, the properties of water can be compared using values expressed as CaCO3 equivalent.

In this reference example, the total hardness of 122.6 mg/L can be treated as moderately hard to slightly hard water, close to the hard-water range.

Example of Indicator Color Change

In total hardness measurement, a metal indicator is used to determine the endpoint.
The indicator shows a color when bound to metal ions, and the color changes when EDTA removes the metal ions.

The endpoint is often judged not as a single instant when the color changes suddenly, but as the point at which the reddish-purple color disappears and a stable blue color remains, making individual differences in judgment likely.

Effect of pH Conditions

Complex formation between EDTA and metal ions is affected by pH.
If the pH is inappropriate, complex formation may become incomplete or metal hydroxide precipitates may form.

In total hardness measurement, a buffer solution is often added to keep the pH constant, and pH control greatly affects the clarity of the endpoint.

Effect of Deviation in EDTA Standard-Solution Concentration

If the actual concentration of the EDTA standard solution differs from the stated value, the hardness calculation is directly affected.

Errors can be reduced by standardizing the EDTA standard solution and using its actual concentration in the calculations.

Example of Blank-Test Correction

If trace metal ions or EDTA consumption originate from reagents, buffer solution, or indicator, correction is performed using a blank test.

Using the corrected titration volume of 6.07 mL gives a total hardness of approximately 121.5 mg/L.
Even if the blank-test amount is small, it may not be negligible for low-hardness samples.

Example Comparison of Hardness of Multiple Samples

The larger the EDTA titration volume, the greater the amounts of Ca2+ and Mg2+ in the sample and the higher the hardness is judged to be.

Effect of Passing the Endpoint

If excess EDTA is added, the calculation indicates that more metal ions were present than actually existed, resulting in overestimation of hardness.

If the endpoint color change is ambiguous, over-titration is more likely, so judging the endpoint using the same color tone improves reproducibility.

Effects of Interfering Ions

EDTA also forms complexes with metal ions other than Ca2+ and Mg2+.
Therefore, if coexisting metal ions are present, additional EDTA may be consumed and the hardness may be overestimated.

Because EDTA titration reacts broadly with metal ions, components other than the hardness components may also be titrated depending on the composition of the sample.

Points to Note for Samples With Low Hardness

In samples with low hardness, the EDTA titration volume is small, so the influence of slight reading errors and the blank-test volume becomes large.

When measuring low-hardness water, increasing the sample volume and thereby increasing the titration volume can reduce the relative error.

Example of How to Write the Results

When 50.00 mL of a water sample was titrated with 0.0100 mol/L EDTA standard solution, the average of three satisfactory titration volumes was 6.12 mL.
Because EDTA forms complexes with Ca2+ and Mg2+ in a 1:1 ratio, the amount of hardness components was determined from the amount of EDTA consumed.
The calculation gave a total hardness of 122.6 mg/L as CaCO3.

When calcium hardness was measured under different conditions, the average EDTA titration volume was 4.02 mL, and the calcium hardness was 80.4 mg/L as CaCO3.
Subtracting the calcium hardness from the total hardness gave a magnesium hardness of 42.2 mg/L as CaCO3.
Therefore, the proportion of hardness components derived from calcium is considered relatively large in this sample.

At the endpoint, the color of the metal indicator changed from reddish purple to blue.
This occurred because the metal ions were bound to the indicator before titration, but EDTA more stably incorporated the metal ions, causing the free indicator to show a different color.
Because the endpoint color change is somewhat subjective, over-titration and reading errors may cause overestimation of the hardness.

Points to Connect to the Discussion

In a discussion of EDTA titration, it is important not only to calculate hardness from the titration volume, but also to relate complex formation, pH conditions, indicator color changes, interfering ions, and errors in endpoint determination.

• Can you explain that EDTA reacts with Ca2+ and Mg2+ in a 1:1 ratio?
• Can you determine the amount of hardness components from the EDTA concentration and titration volume?
• Can you calculate hardness as CaCO3 equivalent?
• Can you distinguish total hardness, calcium hardness, and magnesium hardness?
• Have you avoided confusing hardness values with actual Ca2+ and Mg2+ concentrations?
• Can you explain the indicator color change from the difference in stability between the metal-indicator complex and the EDTA complex?
• Can you discuss how pH affects complex formation and precipitation?
• Can you explain the meaning of blank-test correction and standardization of the EDTA standard solution?
• Can you discuss the possibility that interfering ions such as Fe3+ and Cu2+ cause overestimation of hardness?
• Can you explain endpoint determination, over-titration, titration-volume reading, and relative error in low-hardness samples?

Example Discussion Text

Summary

In EDTA titration, water hardness can be determined using the fact that Ca2+ and Mg2+ form complexes with EDTA in a 1:1 ratio.
Total hardness is expressed as the combined amount of Ca2+ and Mg2+ in terms of CaCO3, and magnesium hardness can be determined from the difference between total hardness and calcium hardness.

This reference example covered total hardness, calcium hardness, magnesium hardness, conversion to CaCO3 equivalents, conversion to actual ion concentrations, indicator color changes, pH conditions, blank-test correction, interfering ions, endpoint determination, and errors in low-hardness samples.
In a report, it is useful to discuss not only the calculation from the titration volume, but also the principle of complex formation and the effects of measurement conditions on the results.

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Role of the Metal Indicator

In EDTA titration, a metal indicator may be used to determine the endpoint.
In water hardness measurement, indicators such as Eriochrome Black T may be used.

A metal indicator has different colors when bound to a metal ion and when free from the metal ion.
Before titration, the indicator binds to the metal ions and shows a particular color.
As EDTA is added, EDTA forms more stable complexes with the metal ions, causing the metal ions to move from the indicator to EDTA.
As a result, the color of the indicator changes near the endpoint.

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Key Points for Endpoint Determination

The endpoint of EDTA titration is determined from the color change of the indicator.
However, the color change may not be abrupt and intermediate colors may be visible, so care is required when determining the endpoint.

In water hardness measurement, a color change from reddish purple to blue before and after titration is commonly observed.
However, the appearance of the color is affected by the solution concentration, pH, amount of indicator, components of the sample water, and lighting conditions.

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Why pH Conditions Are Important

Complex formation between EDTA and metal ions is affected by pH.
Therefore, in EDTA titration, a buffer solution is often added and titration is performed under an appropriate pH condition.

If the pH is inappropriate, complex formation between EDTA and metal ions may not proceed sufficiently, or the color change of the indicator may become unclear.
Other reactions, such as precipitation of metal ions as hydroxides, may also occur.

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Role of the Buffer Solution

A buffer solution is added in EDTA titration to maintain a constant pH during titration.
Complex formation between metal ions and EDTA, the color change of the indicator, and the dissolved state of the metal ions are all affected by pH.

If the amount of buffer solution is insufficient or the buffer solution is not thoroughly mixed, the pH of the solution may not remain stable and endpoint determination may be affected.

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Concept of Hardness Calculation

In water hardness measurement, the total amount of calcium ions and magnesium ions in the water is determined from the EDTA titration volume.
The hardness is then expressed as an equivalent amount of calcium carbonate.

Basically, because EDTA reacts with metal ions in a 1:1 ratio, the total amount of metal ions is determined from the amount of EDTA.
However, follow the units and conversion methods specified in your class or laboratory manual.

Amount of EDTA = EDTA concentration × titration volume

Amount of EDTA = total amount of Ca2+ and Mg2+ in the sample

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Causes of Variation in Titration Volumes

In EDTA titration, the titration volumes obtained from repeated measurements may not completely agree.
Possible causes of variation include differences in endpoint determination, errors in reading the burette, differences in sample volume, differences in pH conditions, and insufficient mixing.

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Discussion When the Endpoint Is Exceeded

If too much EDTA solution is added beyond the endpoint in EDTA titration, the titration volume becomes larger than the amount actually required.
As a result, the amount of metal ions in the sample is overestimated and the hardness may also be calculated higher than the actual value.

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Discussion When the Endpoint Is Judged Too Early

Conversely, if the color change is judged too early, the titration is stopped with less EDTA than is actually required.
In this case, the amount of metal ions is underestimated and the hardness may be calculated lower than the actual value.

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Burette-Reading Error

In EDTA titration, errors in reading the burette scale are also important sources of error.
If the initial or endpoint reading is incorrect, the titration volume becomes larger or smaller than the actual value.
Because the titration volume is used directly in the hardness calculation, reading errors affect the result.

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Error Due to Sample-Water Volume

If the volume of sample water is not measured accurately, the amount of EDTA required for titration also changes.
If more sample water is collected, the amount of metal ions increases and the titration volume becomes larger.
Conversely, if less sample water is collected, the titration volume becomes smaller.

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Error Due to the Concentration of the EDTA Standard Solution

If the concentration of the EDTA standard solution is inaccurate, the calculated hardness will be incorrect even if the titration volume is accurate.
Possible causes include weighing errors during preparation of the standard solution, errors in aligning the volumetric-flask calibration mark, and insufficient mixing.

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Effects of Interfering Ions

EDTA may also form complexes with metal ions other than calcium ions and magnesium ions.
If other metal ions are present in the sample water, they may also react with EDTA and increase the titration volume.

In that case, the amount of EDTA consumed does not reflect only calcium ions and magnesium ions, so the hardness may be overestimated.
Depending on the laboratory manual, procedures may be specified to suppress the effects of interfering ions.

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Discussion When Water Hardness Is High or Low

If the determined hardness is high, the sample water is considered to contain large amounts of calcium ions and magnesium ions.
If the hardness is low, the water is considered to contain small amounts of these metal ions.

However, rather than simply stating that the hardness was high or low, relating the result to the type of sample water, sampling location, regional differences, or whether the water has been treated makes the discussion more substantial.

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Discussion When Measurements Are Repeated

When titration is repeated multiple times, similar titration volumes indicate high reproducibility.
On the other hand, if the variation is large, there may have been problems with endpoint determination or operating conditions.

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How to Write About Excluding a Titration Value

In EDTA titration, values that clearly exceeded the endpoint or differed greatly from the other measurements may be excluded.
In such cases, the reason must be written based on observed facts.

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When the Result Can Be Considered Good

A good result in EDTA chelatometric titration is indicated when multiple titration volumes are close to one another, the endpoint color change is clear, and the pH conditions are appropriately maintained.
In addition, if the determined hardness does not greatly contradict the properties or known value of the sample water, the reliability of the measurement result is considered relatively high.

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Example Discussion When the Experiment Did Not Go Well

If EDTA titration does not go well, possible causes can be considered from results such as an unclear color change, variation in titration values, or hardness that differs greatly from the expected value.
Checking endpoint determination, pH conditions, the buffer solution, EDTA standard solution, and interfering ions makes the discussion easier to write.

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How to Write Points for Improvement

In a discussion of EDTA chelatometric titration, including points for improvement as well as sources of error makes the report easier to organize.
It is important to write improvements that correspond to the actual sources of error considered.

Methods for Improving Endpoint Determination

• Add the EDTA standard solution one drop at a time near the endpoint
• Confirm the color change carefully before recording the endpoint
• Titrate while stirring the solution thoroughly
• Perform multiple titrations under the same conditions to confirm reproducibility
• Adjust the lighting conditions to make the color change easier to observe

Methods for Improving Reaction Conditions

• Add an appropriate amount of buffer solution to keep the pH constant
• Prepare or standardize the EDTA standard solution accurately
• Collect an accurate volume of sample water
• Consider the effects of interfering ions when necessary
• Carefully rinse the apparatus with the solution and read the scales accurately

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Difference Between a Superficial Discussion and a Good Discussion

In a discussion of EDTA titration, simply writing that “the color was difficult to see” or “the hardness was different” results in a superficial discussion.
A persuasive discussion can be produced by relating endpoint determination, pH conditions, and the reaction ratio between EDTA and metal ions.

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Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of EDTA chelatometric titration and water hardness measurement.
Adjust the necessary parts according to your own experimental results.

• EDTA forms complexes with calcium ions and magnesium ions in a 1:1 molar ratio.
• The amount of EDTA required for titration corresponds to the total amount of metal ions in the sample water.
• Water hardness is a value obtained by expressing the amounts of calcium ions and magnesium ions as an equivalent amount of calcium carbonate.
• The endpoint was taken as the point at which the color of the indicator changed from reddish purple to blue.
• Because endpoint determination was performed visually, individual differences may have occurred in the timing used to judge the color change.
• If the pH conditions are inappropriate, complex formation between EDTA and the metal ions and the indicator color change are affected.
• A buffer solution is added to maintain a constant pH during titration.
• If EDTA is added beyond the endpoint, the amount of metal ions may be overestimated and the hardness may be calculated too high.
• If interfering ions react with EDTA, the hardness may be estimated higher than the actual value.
• Because multiple titration volumes were close to one another, the reproducibility of endpoint determination is considered relatively high.

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Points to Check When Discussing EDTA Chelatometric Titration

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

• Can you explain the reaction ratio between EDTA and metal ions?
• Have you explained what the water hardness was converted to?
• Have you correctly recorded the color change at the endpoint?
• Was there any uncertainty in endpoint determination?
• Was there variation in the titration volumes?
• Can you explain why the buffer solution was added?
• Have you considered the effect of pH conditions on the results?
• Have you considered burette-reading errors?
• Was there any error in the amount of sample water collected?
• Was the concentration of the EDTA standard solution accurate?
• Have you considered the effects of interfering ions?
• Have you related the reason for high or low hardness to the properties of the sample water?

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Summary

EDTA chelatometric titration is an analytical method used to determine the amount of metal ions by utilizing the property of EDTA to form stable complexes with metal ions.
In water hardness measurement, the combined amount of calcium ions and magnesium ions is determined by EDTA titration and the hardness is expressed as a calcium carbonate equivalent.

In a discussion of EDTA titration, it is important to explain that EDTA and metal ions react in a 1:1 ratio, the color change of the metal indicator, pH conditions, the role of the buffer solution, and sources of error in endpoint determination.
In particular, because endpoint determination is performed visually, variation may occur in the titration volume.

In a report, do not simply write that “the hardness was determined.”
Specifically discuss how endpoint determination and pH conditions affected the results.
Considering the concentration of the EDTA standard solution, sample volume, and effects of interfering ions results in a more persuasive discussion.

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