Determination of Total Hardness of Water Samples for Chemistry students PDF

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Chem 2206L Analytical Chemistry 2 LaboratorySecond Semester, SY 2020- 21 March 28, 20 21Page 1Experiment 1:Determination of Total Hardness of Water SamplesEspiritu, P., Dela Rosa, J., Ledda, Y.M., Garcia,I.E., Arellano, E., Calilung, N., Padua, J.Physical Science Department, College of Science, Pama...

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Chem 2206L Second Semester, SY 2020-21

Analytical Chemistry 2 Laboratory March 28, 2021

Experiment 1: Determination of Total Hardness of Water Samples Espiritu, P.R., Dela Rosa, J.J., Ledda, Y.M.A., Garcia, I.E.A., Arellano, E.J., Calilung, N.K., Padua, J.V. Physical Science Department, College of Science, Pamantasan ng Lungsod ng Maynila

Abstract: Hardness of Water is defined by the presence of cations, usually calcium, magnesium and other heavy metals in water. Complexometric Titration was conducted with EDTA as a titrant and use of metallochromic indicator to serves as a chemical indicator, which is commonly used as a technique for determination of a mixture of different metal ions in the solution. Thus, to be able to determine the total hardness of a water sample, a 100 mL volume of sample undergo in three trials and each trial was calculated to get the concentration of Calcium (Ca) present in the sample. Few drops of Eriochrome Black T as an indicator to form complex with Calcium and 10 M buffer solution using ammonia and ammonium chloride was added to ensure the titration must proceed in preferred pH of the solution, which is in basic solution. Blue color of solution is observed signifies the end of the titration; EDTA freed all the Calcium in the indicator. The average metal concentration of the Calcium was calculated which is expressed and measured in ppm, 58.72 mg/L, present in the sample was classified as soft, in which it has a concentration of less than 60 mg/L. Hence, the water sample taken in this experiment means it has low quantity of minerals; a poor mineral content that tends to pull some minerals where it can and even other contaminants and it is much preferred for cleaning, as it does not tend to cause soap scum or mineral stains. Keywords: Water Hardness, Complexometric Titration, EDTA, Metallochromic indicator, Minerals, Eriochrome Black T.

1. INTRODUCTION Water hardness is defined by the presence of cations, usually calcium, magnesium, and other heavy metals in water that can replace the sodium or potassium present in soaps which leads to formation/precipitation of scum that is slippery in texture. However, in natural waters, calcium and magnesium are present in concentrations that exceed the other metal ions. Thus, the concentration of calcium carbonate is used to determine the hardness of water sample as it is equivalent to the total concentration of other present cations. Determination of water hardness is helpful in industries since hard waters can clog pipes when subjected to high temperature. (Skoog, et al, 2014, p. 346) To determine water hardness, complexation titration is done using EDTA as the titrant. But before carrying out the titration, the water sample should be buffered at pH 10. The EDTA that is added combines quantitatively with the cation at a 1:1 mole ratio to form the complex. The end point occurs when essentially all of the cation has reacted with EDTA which is observable through a physical change with the help of an indicator. Usually, a magnesium-ion indicator is used since both EDTA and calcium ion are colorless. Addition of this indicator, such Eriochrome

Black T, will form complex with the magnesium ions giving a color of wine red to the water sample. When EDTA is continuously added, it will form complex with the free calcium and magnesium ions until all of it has been turned into chelates. When this happens, EDTA will displace the magnesium in the indicator complex and the wine-red color turns to blue which indicates the end point of the titration. The principle and method discussed above is necessary to understand in order to conduct this experiment that will assess water hardness of water samples.

2. METHODOLOGY To determine the total hardness of a water sample, a Complexometric titration with EDTA is performed using a burette, pipette, beaker, conical flask, 10M buffer solution using ammonia and ammonium chloride, 0.005000M EDTA, and Eriochrome Black T Indicator. The burette is rinsed out with deionized water and titrant before proceeding. Afterwards, it is filled with EDTA until the bottom of the meniscus observed at eye level is at the zero point. Next, the pipette is rinsed with the water sample. Using the same pipette to take 100.0

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Chem 2206L Second Semester, SY 2020-21

Analytical Chemistry 2 Laboratory March 28, 2021

mL water sample, the liquid is drawn until the lower meniscus seen at eye level is in line with the graduated mark. The water sample is then transferred onto a conical flask. The remaining liquid is not blown out instead the tip of the pipette is touched off the side of the flask. This procedure can be repeated to ensure that 100.0 mL of water sample is obtained. Few drops of 10 M buffer are added to the water sample. The Eriochrome Black T indicator is added as well. A wine-red solution is observed as the indicator forms a complex with Calcium. Begin titrating with EDTA while continuously swirling the flask. A blue solution must be seen once the EDTA removes all of the Calcium in the water sample indicating the endpoint.

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3. RESULTS AND DISCUSSION Trial Volume (mL)

1 2 3 Ave rage

12.00 11.50 11.70 11.73

Volume EDTA( ) of the 𝒎𝑳 Sample (mL) 0.005000 100.0 0.005000 100.0 0.005000 100.0 0.005000 100.0 𝒎𝒎𝒐𝒍

Hardness of the water (ppm) 60.054 57.552 58.553 58.720

Table 1. Amount of Calcium carbonate (ppm) in the water sample

The table above shows the result of hardness of water samples by measuring the concentration of Calcium carbonate that was conducted in three trials. Thus, each trial was titrated with the same concentration of titrant EDTA in a 100 mL volume of the sample. In trial 1, hardness of the water sample was 60.054 ppm. Followed by the second trial which is 57.552 ppm, and third trial for 58.553 ppm. Average concentration of the water sample is calculated and results in 58.720 ppm. Significance of these result is that these can categorized the water either soft or hard. Soft water contains relatively very few minerals, while hard water is abundant in calcium and magnesium minerals. Water hardness is a measurement of mineral content in a water sample, which is normally shown as parts of a million (ppm) or mg/L as calcium carbonate. Ethylenediaminetetraacetic acid (EDTA) is used as the titrant that complexes Ca2+ and Mg2+ ions for the determination of water hardness. By introducing a metallochromic indicator to the water sample, the end point of the titration can be visually identified. The change of color shows the end point where the EDTA eliminates the ion Mg2+ attached to

the indicator after complexing unbound Ca2+and Mg2+ ions.

all

of

the

Figure 1. Water hardness classification

The figure above presents the water hardness classification. A total hardness of less than 60 mg/L is considered as soft; a total hardness of 61 to 120 is considered as moderately hard; a total hardness of 121 to 180 is considered hard; and a total hardness of greater than 180 is considered at very hard. The water sample is considered as soft as shown in the results of the three trials because the overall hardness is less than 60 mg/L, which means the quantity of minerals is low. Minerals can occur naturally from rainwater, ponds, or wetlands. And water that occurs naturally is not actually drinkable because of contamination. Furthermore, due to its poor mineral content, soft water tends to pull minerals where it can and even other contaminants. The benefits of this soft water sample are that— it is soft to the touch and doesn't damage any equipment as much as hard water; because of its low mineral composition, it is much better for the hair and skin; and that there will be better washing — because the water which is rinsing away leaves without any residual material; unlike hard water which leaves minerals behind. Calculations: Trial 1: 𝑚𝑔 𝑚𝑚𝑜𝑙 𝐸𝐷𝑇𝐴) 𝐶𝑎2+ = (12 𝑚𝐿 𝐸𝐷𝑇𝐴) (0.005 𝑚𝑙 𝐿 𝑚𝑔 1 𝑚𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3 ) (100.09 ( 𝐶𝑎𝐶𝑂3 ) 1 𝑚𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 𝑚𝑚𝑜𝑙 1 𝐿 𝑠𝑎𝑚𝑝𝑙𝑒) = 𝟔𝟎. 𝟎𝟓𝟒 𝒑𝒑𝒎 ( 0.1 Trial 2: 𝑚𝑔 2+ 𝑚𝑚𝑜𝑙 𝐸𝐷𝑇𝐴) 𝐶𝑎 = (11.5 𝑚𝐿 𝐸𝐷𝑇𝐴) (0.005 𝑚𝑙 𝐿 𝑚𝑔 1 𝑚𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3 ) (100.09 ( 𝐶𝑎𝐶𝑂3) 1 𝑚𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 𝑚𝑚𝑜𝑙 1 ( 𝐿 𝑠𝑎𝑚𝑝𝑙𝑒) = 𝟓𝟕. 𝟓𝟓𝟐 𝒑𝒑𝒎 0.1 Trial 3: 𝑚𝑚𝑜𝑙 𝑚𝑔 2+ 𝐶𝑎 = (11.7 𝑚𝐿 𝐸𝐷𝑇𝐴) (0.005 𝐸𝐷𝑇𝐴) 𝐿 𝑚𝑙 𝑚𝑔 1 𝑚𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3 ) (100.09 ( 𝐶𝑎𝐶𝑂3) 1 𝑚𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 𝑚𝑚𝑜𝑙 1 𝐿 𝑠𝑎𝑚𝑝𝑙𝑒) = 𝟓𝟖. 𝟓𝟓𝟑 𝒑𝒑𝒎 ( 0.1

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Chem 2206L Second Semester, SY 2020-21

Analytical Chemistry 2 Laboratory March 28, 2021 7.

4. CONCLUSION In this experiment, we can determine water hardness through EDTA titration wherein metallochromic indicator is introduced to the sample and to visually identify the end point of the titration. In addition, an indicator is also used to indicate the presence of the Calcium and Magnesium based on its color change. Thus, metallochromic indicator forms complex with the metal ions present in the solution that is why EDTA titrant must free all the metal ions from the indicator to signifies the end point. Performing comple xometric titration requires to perform in a basic solution as much as possible that’s why additional of buffer presented in the solution. Moreover, these mineral contents are measured in ppm (mg/L) and are classified based on their amounts. Also, these metals assessed through stepwise calculations which mainly deals with stoichiometry. In this experiment, 60.054 ppm is measured in trial 1, 57.552 ppm in trial 2, and 58.553 ppm in trial 3. Having the average concentration of the Calcium in water sample of 58.720 ppm which was considered and classify as a relatively soft water.

8.

W. (2019, July 10). Hard water vs soft water – which is better? WaterCare. Retrieved from https://www.watercare.co.uk/hard-water-vs-softwater-which-is-better/ Yappert, M.C., & DuPré, D. (1997, December 12). Complexometric Titrations: Competition of Complexing Agents in the Determination of Water Hardness with EDTA. Journal of Chemical Education, Vol. 74, No. 12. Retrieved from https://www.chem.uci.edu/~unicorn/M3LC/hand outs/Week7/calmagite.pdf

5. REFERENCES 1.

2.

3.

4.

5.

6.

Adey, W., & Loveland, K. (2007). Water hardness - an overview | ScienceDirect topics. ScienceDirect. Retrieved from https://www.sciencedirect.com/topics/agricultura l-and-biological-sciences/water-hardness Committee, A. C. T. B. C. (2015, October 24). Water hardness and pH – understanding ingredients for the canadian baker. Pressbooks. Retrieved from https://opentextbc.ca/ingredients/chapter/waterhardness-and-ph/ Corrosionpedia. (2014, May 22). Total hardness. Retrieved from https://www.corrosionpedia.com/definition/1104/ total-hardness Hancock, N. (2019, August 15). Total hardness analysis for elementary operation water drop. Safe Drinking Water Foundation. Retrieved from https://www.safewater.org/operation-water-droplistings/2017/2/4/total-hardness-analysis-forelementary-operation-water-drop https://www.ccri.edu/chemistry/courses/chem_11 00/wirkkala/labs/Calcium_Analysis_by%20EDT A_Titration.pdf Skoog, D.A., West, D.M., Holler, F.J., Crouch, S.R. (2014). Calcium Analysis by EDTA Titration. Fundamentals of Analytical Chemistry, 9th ed.. Cengage. Retrieved from

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