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Complexometric Determination of Water Hardness

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Complexometric Determination of Water Hardness

By: Daniel Morgan and Melinda Allred

Abstract

        The hardness of both a known and an unknown sample of water were determined using a titration method with a cheatling agent and Calcium Carbonate. The calculated average hardness of the unknown sample was 262.029 ppm.

Introduction

        Water hardness is defined as “the amount of dissolved calcium and magnesium in the water. Hard water is high in dissolved minerals, both calcium and magnesium.” (https://water.usgs.gov/edu/hardness.html). Research shows that there can be health implications due to water hardness. “Several epidemiological investigations have demonstrated the relation between risk for cardiovascular disease, growth retardation, reproductive failure, and other health problems and hardness of drinking water or its content of magnesium and calcium.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775162/ Knowing the water hardness level is important to avoid potential health risks. There have been ranges established by different government agencies that determine what the safe levels of water hardness in a municipality’s water supply are.

        A way to determine the hardness of a water sample is through a titration process with a chelatling agent. The purpose of the chelatling agent is to attract the metal ions present in the water in order to be measured. “Because metal ions and hydrogen ions behave like Lewis acids in aqueous solutions, a comparison of the behavior of these ions offers a logical and reasonable approach to the investigation of the properties of metal ions in titration processes.” (pubs.acs.org/doi/abs/10.1021/ac60095a005) Using a standardized solution, in this case Na2 EDTA, through titration, chemists can determine an experimental value for metal ions, in this case CaCO3, present in any unknown water sample.

Procedure

Prepare approximately 500mL of .004 M Na2 EDTA solution by adding .734 g Na2 EDTA to 500 mL deionized water. Using an Erlenmeyer flask, add 10 mL of standardized calcium ion stock (1.0 g CaCO3 /L solution).  Add 30 mL deionized water to the Erlenmeyer flask. Stir solution using a magnetic stirrer and stir bar. Add 3 mL ammonia as a chloride buffer. Stir for 30 seconds. Add 4 drops of Eriochrome Black T indicator solution. Stir an additional 30 seconds. Using a titration flask, titrate the Na2EDTA into the Erlenmeyer flask. Note the starting volume on the titration flask of the Na2EDTA. Watch for a color change from bright pink, to violet, and finally to a blue color. The blue color is the end point of the titration. Note the end volume of the Na2EDTA in the titration flask. Repeat the titration process twice more and record both starting and ending volumes of Na2EDTA for each trial.

Take unknown water sample # 73 add 25.0 mL to the Erlenmeyer flask. Add 20mL of deionized water to the Erlenmeyer flask. Stir solution using a magnetic stirrer and stir bar. Add 3 mL ammonia as a chloride buffer. Stir for 30 seconds. Add 4 drops of Eriochrome Black T indicator solution. Stir an additional 30 seconds. Using a titration flask, titrate the Na2EDTA into the Erlenmeyer flask. Note the starting volume on the titration flask of the Na2EDTA. Watch for a color change from bright pink, to violet, and finally to a blue color. The blue color is the end point of the titration. Note the end volume of the Na2EDTA in the titration flask. Repeat the titration process twice more and record both starting and ending volumes of Na2EDTA for each trial.

Results and Discussion

*Data Collection from Standardization (Trials 1-3)

Calculating Molarity of Na2 EDTA from solution:

Standardizing the disodium EDTA Solution

Na2 EDTA

DI H2O

NH4Cl

Na2 EDTA [M]

Mean Molarity

Trial 1

24.8 mL

29.9 mL

3.1 mL

0.004028

0.004447

Trial 2

21.8 mL

30.0 mL

3.0 mL

        0.004580

Trial 3

21.1 mL

30.0 mL

3.0 mL

0.004734

Trial 1:

[pic 1]

Trial 2:

[pic 2]

Trial 3:

[pic 3]

Calculating the mean molarity:

[pic 4]

[pic 5]

 [Na2 EDTA]mean ≈ .004447 (mean molarity)

Calculating Relative Precision:

Performing these calculations will represent the reproducibility for this set of experiments and the degree of accuracy on each trial. The first calculation is the relative average absolute deviation which will be reported in parts per thousand (ppt) with the expectation of achieving five parts per thousand in this experiment. In this calculation, the absolute value is required for each trial’s deviation from the mean:

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