CH 456: Determination of Water Hardness of Filtered and Unfiltered Water

Introduction

Water hardness is defined as the capacity of water to react with soap, and is caused by the presence of a variety of polyvalent metal ions.1 The two metals that contribute most heavily to water hardness are calcium and magnesium, and hardness is expressed in milligrams of calcium carbonate per liter of water (a measure of parts per million, ppm) by convention.1 These dissolved metallic salts most often leech into water from sedimentary rocks, runoff, and human activities such as industry and agriculture.

1 Water below 60 ppm is termed “soft” water, meaning that it lacks a large amount of these polyvalent metal ions, while concentrations of 100 ppm are common in natural water sources, such as springs.1

Founded in 19662, the Brita company manufactures water filters that are advertised to remove chlorine, lead, mercury, cadmium, benzene, asbestos, small particulates, copper, and zinc from drinking water3. The filter itself is a multi-stage filter, first removing particulates with a mesh, then employing activated carbon to reduce mercury and chlorine and an ion-exchange resin to remove copper, zinc, and cadmium.

4 Ion-exchange resins facilitate the transfer of ions from a liquid phase to a solid exchange material.5 These resins are often durable and reusable, and can be manufactured to target specific ions, allowing for efficient separations.

To test the effectiveness of the Brita filtration system at reducing water hardness, a complexometric titration was employed. Complexometric titrations rely on the formation of metal complexes for endpoint determination.6 Ethylenediaminetetraacetic acid (EDTA) is the most widely used chelating ligand titrant used in this type of chemical analysis, as it binds quantitatively to metal ions.

Get quality help now

WriterBelle

Verified

Proficient in: Calcium Carbonate

4.7 (657)

“ Really polite, and a great writer! Task done as described and better, responded to all my questions promptly too! ”

+84 relevant experts are online

Hire writer

6 In this method of titration, the indicator is a chelating agent that produces a color change when it is exchanged for the titrant at the metal center.7

The endpoint was determined qualitatively using the color-change indicators Eriochrome Black T (EBT) and Murexide; Murexide binds to calcium but does not bond to magnesium, while EBT binds to both.7 These are metallochromic indicators, meaning that they change color upon complexing with a metal ion or vice versa.8 The structures of EDTA, EBT, and Murexide can be seen in Figure 1. Indicators of this type are often heavily pH dependent6, necessitating the use of buffer solutions. Using these indicators and an EDTA titrant, the hardness of both filtered and unfiltered water can be calculated and compared.

Structures and binding modes of EDTA and EBT.

• structure of EDTA,
• binding mode of EDTA9,
• structure of EBT,
• structure of Murexide.

Results

The total hardness of the unfiltered water decreased from 85 ± 8 ppm to 51 ± 1 ppm. The unfiltered calcium and magnesium hardness were 10.3 ± 0.5 ppm and 18.2 ± 0.9 ppm, respectively, while the filtered calcium and magnesium hardness were 4 ± 1 ppm and 14 ± 4 ppm, respectively. A full tabulation of metal content, standard deviation, and %RSD can be seen in Table 1 and Table 2. Table 1. Total hardness for filtered and unfiltered water with %RSD and 90% confidence interval.

Mean total hardness (ppm)

SD (ppm) %RSD 90% Confidence

Interval (ppm)

Unfiltered 85 ±8 9.84 ±1𝑥101

Filtered 51 ±1 2.63 ±2

Table 2. Calcium and magnesium content in filtered and unfiltered water.

Ca hardness (ppm) Ca SD (ppm) Mg hardness (ppm) Mg SD (ppm)

Unfiltered 10.3 ±0.5 18.2 ±0.9

Filtered 4 ±1 12 ±4

The mean value was calculated using Equation 1 (E1), standard deviation was calculated using Equation 2 (E2), while the %RSD was calculated using Equation 3 (E3) and the 90% confidence interval was calculated using Equation 4 (E4). Standard deviation for magnesium content was calculated by using calcium %RSD and working backwards to find the magnesium standard deviation. Total hardness in terms of CaCO3 standard was calculated with Equation 5 (E5), and calcium and magnesium hardness were calculated with Equation 6 (E6) and Equation 7 (E7), respectively.

𝑥 =∑𝑥 𝑛 E1 𝑠 = √ ∑(𝑥 − 𝑥 )2 𝑛 E2%𝑅𝑆𝐷 =𝑠 𝑥 ∗ 100 E3 90% 𝐶𝐼 = 𝑡 ∗ 𝑠 √𝑛 E4

𝑇𝑜𝑡. ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 (𝐶𝑎𝐶𝑂3 𝑒𝑞𝑢𝑖𝑣. ) = 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 ∗ ( 1 𝑚𝑜𝑙 𝑖𝑜𝑛𝑠 1 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 ) ∗ 𝑀𝑊𝐶𝑎𝐶𝑂3 ∗ ( 1000 𝑚𝑔 1 𝑔 ) ∗ ( 1 𝑉𝑎𝑛𝑎𝑙𝑦𝑡𝑒 ) E5 𝐶𝑎 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 = 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 ∗ ( 1 𝑚𝑜𝑙 𝑖𝑜𝑛𝑠 1 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 ) ∗ 𝑀𝑊𝐶𝑎 ∗ ( 1000 𝑚𝑔 1 𝑔 ) ∗ ( 1 𝑉𝑎𝑛𝑎𝑙𝑦𝑡𝑒 ) E6 𝑀𝑔 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 = (𝑇𝑜𝑡. ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠 − 𝐶𝑎 ℎ𝑎𝑟𝑑𝑛𝑒𝑠𝑠) ∗ ( 1 𝑀𝑊𝐶𝑎𝐶𝑂3 ) ∗ 𝑀𝑊𝑀𝑔 E7

Discussion

It can be said with 90% confidence that the true total hardness of the unfiltered and filtered water samples was 8𝑥101 ± 1𝑥101 and 51 ± 2 ppm, respectively. The Brita filter reduced the water’s calcium content, as the measured content dropped from 10.4 ± 0.5 ppm to 4 ± 1 ppm upon filtering. The filter also reduced the magnesium content, as the measurements of magnesium content for unfiltered water was 18.2 ± 0.9 ppm and the measurement of magnesium content for filtered water was 12 ± 4 ppm. The total hardness decreased from 85 ± 8 ppm to 51 ± 1 ppm.

The Brita filter is rated to reduce calcium and overall hardness, but not magnesium, so the Brita filter’s ability to filter magnesium from the water sample is unexpected.3 The standard deviation for the total unfiltered hardness was 8.39 (9.84%), while the standard deviation for the filtered total hardness was 1.34 (2.62%). The large %RSD for the unfiltered total hardness is likely due to systematic error in measurement.

Improved experimental technique would likely eliminate this error and allow for the determination of whether or not the Brita filter reduces magnesium content in the water. This method of complexometric titration has the ability to be precise, as can be seen from the titration to determine filtered total hardness, which had a standard deviation of 1.34 ppm (2.63%). Overall, this method of titration can be done relatively quickly and reproducibly, and, in trained hands, can be precise.

Conclusion

Here, it was determined that a Brita filter reduced the total hardness of a water sample from 85 ± 8 ppm to 51 ± 1 ppm, the calcium hardness from 10.3 ± 0.5 ppm to 4 ± 1 ppm, and the magnesium hardness from 18.2 ± 0.9 ppm to 12 ± 4 ppm. These results corroborate Brita’s claims using a technique that is not time-intensive and is easily reproducible. The most significant source of error was systematic experimenter error that can be eliminated with the proper training.

References

1. Guidelines for drinking-water quality, 2nd ed.; World Health Organization: Geneva, 1996; Vol. 2.
2. BRITA history: 50 years of success. https://www.brita.co.uk/group-history (accessed Feb 19, 2019).
3. What Does Brita Filter Out? Chlorine & More. https://www.brita.com/why-brita/what-we-filter/ (accessed Feb 19, 2019).
4. Brita®. https://www.brita.com/why-brita/health/how-do-brita-filters-work (accessed Feb 19, 2019).
5. Wheaton, R. M.; Lefevre, L. J. Fundamentals of Ion Exchange. http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0032/0901b803800326ca.pdf (accessed Feb 18, 2019).
6. Harris, D. C.; Lucy, C. A. Quantitative chemical analysis, 9th ed.; W.H. Freeman & Company: New York, 2016.
7. Ueno, K. Journal of Chemical Education 1965, 42 (8), 432.
8. Chalmers, R. A.; Miller, F. I. The Analyst 1971, 96 (1139), 97.
9. EDTA Chelation: Rise of the Undead Therapy. http://cbr.ubc.ca/edta-chelation-rise-of-the-undead- therapy/ (accessed Feb 19, 2019).

Appendix

1.  Title Page
2. Lab Report
3. Pre-lab grade sheet
4. Carbon-copy notebook pages
5. Excel Spreadsheet and Calculations`
6. Example calculations

Example Calculations

• 𝑈𝑛𝑓𝐶𝑎𝐶𝑜𝑛𝑡 = 10.69 𝑝𝑝𝑚 + 9.99 𝑝𝑝𝑚 2 = 10.35 𝑝𝑝𝑚 (EX1)
• 𝑠𝑈𝑛𝑓𝐶𝑎𝐶𝑜𝑛𝑡 = √ (10.35 − 17.31)2 + (10.35 − 17.31)2 2 = 0.50 𝑝𝑝𝑚 (EX2)
• %𝑅𝑆𝐷𝑈𝑛𝑓𝐶𝑎𝐶𝑜𝑛𝑡 = ( 0.50 𝑝𝑝𝑚 10.35 𝑝𝑝𝑚 ) ∗ 100 = 4.83% (EX3)
• 90% 𝐶𝐼𝑈𝑛𝑓𝐶𝑎𝐶𝑜𝑛𝑡 = (2.92 ∗ 0.50) √2 = 0.84 𝑝𝑝𝑚 (EX4)
• 𝑀𝐸𝐷𝑇𝐴 = 6.77𝑥10−4 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 0.25 𝐿 = 0.00271 𝑀 (EX5)
• 𝑇𝑜𝑡. 𝐻𝑎𝑟𝑑𝑛𝑒𝑠𝑠 (𝐶𝑎𝐶𝑂3 𝑒𝑞) = (0.00271 𝑀 𝐸𝐷𝑇𝐴 ∗ 0.01475 𝐿 𝐸𝐷𝑇𝐴) ∗ ( 1 𝑚𝑜𝑙 𝑖𝑜𝑛𝑠 1 𝑚𝑜𝑙 𝐸𝐷𝑇𝐴 ) ∗ 100.0869 𝑔 𝐶𝑎𝐶𝑂3 1 𝑚𝑜𝑙 𝐶𝑎𝐶𝑂3 ∗ ( 1000 𝑚𝑔 1 𝑔 ) ∗ ( 1 0.005000 𝐿 ) = 80.0 𝑝𝑝𝑚 (EX6)
• UnfCaCont = Unfiltered Calcium Content