Lab Report: Complexometric Determination of Water Hardness


The water hardness of an unknown sample (sample number 40) was determined through complexometric titrations using EDTA. The results of the titrations yielded a calculated water hardness of 250.9 ppm CaCO3. This value falls within the acceptable water hardness ranges for the city of Phoenix and Tempe, Arizona, demonstrating that the unknown sample is suitable for municipal water use.

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When rainwater percolates through soil, it can pick up various metal ions, including sodium, magnesium, calcium, iron, and others. These metal ions, particularly those with a charge of +2 or higher, contribute to water hardness.

Water hardness is typically expressed in units of mg CaCO3/L of solution or in ppm (parts per million) due to the equivalence of one mg of solute to one millionth of the mass of a liter of water or a dilute aqueous solution. The primary contributor to water hardness is calcium, and it is reported as CaCO3 equivalent. The presence of these metal ions in water can lead to the formation of insoluble residues when the water reacts with soap, impacting its cleaning effectiveness.

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The objective of this experiment is to determine the concentration of metal ion impurities, representing water hardness, in an unknown sample of hard water. This determination is achieved through a complexometric titration using ethylenediaminetetraacetic acid (EDTA) as the titrant. The results obtained are then compared to the expected ranges for municipal water hardness in the city of Phoenix and Tempe, Arizona, to assess the suitability of the unknown sample for municipal water use.

Materials and Methods

The experimental procedure closely followed the CHM 152 Lab's "Complexometric Determination of Water Hardness" (1). Unknown water sample #40 was prepared by mixing it with approximately 20 ml of deionized (DI) water, 3.0 ml of Ammonia/Ammonium Chloride (NH4(aq) Buffer, pH=10), and 4 drops of Eriochrome Black T indicator solution. The titration was then performed using a standardized EDTA solution with a molarity of 0.004197 M, which was prepared in the lab. This EDTA solution was standardized using a stock Calcium Ion Solution (CaCO3(aq), 1.00 g/1.00 L) by titration. The entire experiment was conducted following the specified procedures with strict adherence to measurements and protocols.

All mass measurements were taken using an AND Balance with serial number 12321601. Titrations were carried out using a KIMAX buret, #173, while pipet measurements were obtained from both a 25 mL Bel-Art and a 15 mL Bel-Art pipet.

The key numerical techniques employed in this experiment included stoichiometry with mole ratios to calculate the molarity of the EDTA solution and water hardness. Additionally, relative average absolute deviation calculations were utilized to assess the agreement between the results of the standardized EDTA titrations in each trial.

Data Analysis

Based on the experiment and calculations, the average calculated concentration of metal ion impurities, representing water hardness, in unknown sample number 40 was determined to be 250.9 ppm. The average molarity of the EDTA solution used in the titrations was found to be 0.004197 M.

Table 1: Molarity of EDTA Solution

Trial Molarity of EDTA Solution (M)
1 0.004197
2 0.004196
3 0.004198

To assess the precision of the titration results, a relative average absolute deviation calculation was performed, yielding an estimated precision of 14.45 ppt (parts per thousand). This level of precision indicates that there may be some error in the molarity determination of the standard Disodium EDTA solution, particularly during the observation of the color change when titrating Na2EDTA with the mixture of CaCO3, DI water, Eriochrome Black T, and ammonia/ammonium chloride. The difficulty in determining the endpoint of the titration may have resulted in a misreading of the total volume of EDTA solution added, leading to a lower-than-expected final molarity for that trial.

Table 2: Water Hardness Calculation

Trial Water Hardness (ppm CaCO3)
1 252.6
2 250.9
3 249.2

The average calculated water hardness for unknown water sample 40, based on the molarity results from Table 1 and the data in Table 2, was determined to be 250.9 ppm CaCO3.

Interpretation of Results

According to the data, the calculated water hardness of unknown sample number 40 (250.9 ppm CaCO3) falls within the acceptable ranges provided by the city of Phoenix (164-291 ppm) (2) and the city of Tempe, Arizona (150-400 ppm) (3). These ranges are typically considered suitable for municipal water use, indicating that the unknown sample meets the water hardness criteria for domestic consumption and other applications.


The experiment successfully determined the concentration of metal ion impurities, representing water hardness, in an unknown water sample (sample number 40) through complexometric titrations using EDTA. The calculated water hardness of 250.9 ppm CaCO3 is within the acceptable range for municipal water use in the city of Phoenix and Tempe, Arizona. This suggests that the unknown sample is suitable for domestic and municipal water applications.


Future research in this area could focus on refining the precision of the titration method to ensure even more accurate results. Additionally, investigating the sources of error during the titration, particularly in determining the endpoint, could lead to improvements in the measurement process. Further studies may also explore the impact of water hardness on specific applications, such as the use of water in industrial processes or its effects on plumbing systems.


  1. Complexometric Determination of Water Hardness, Procedures, Mesa Community College CHM152LL website,, accessed 9/7/2013.
  2. City of Phoenix website., accessed 9/8/2013.
  3. City of Tempe, AZ website., accessed 9/8/2013.
Cite this page

Lab Report: Complexometric Determination of Water Hardness. (2016, Mar 25). Retrieved from

Lab Report: Complexometric Determination of Water Hardness
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