The Chemistry of Natural Waters and their Hardness

Lab Report

Robert Derosky

November 12, 2015

Chemistry 111: Experimental Chemistry

Section 107

Group Members

José Del Río Pantoja

Erika Del Pilar

Robert Donohue

TA

Ray Zhu

 

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Introduction

Water hardness is something that affects everyone from families at their homes to the

agency that overlooks the pipes that transport water across the city. Water hardness is simply the

amount of magnesium and calcium dissolved in a sample of water1. Water hardness is an

important topic in the world because it affects everyone, not only those working in the water

industry. Calcium and magnesium scale buildup can cause little problems like affecting the flow

of water from a faucet, or large problems, like clogging water pipes completely, causing a

drainage problem, or even flooding. These buildups do not only affect households, as large

companies also must maintain water pipes and faucets plagued by buildup. Even fish are affected

by water hardness. The same water hardness that affects humans can affect the creation of

columnaris disease, which can damage a fish’s gills, costing the United States Aquaculture over

$40 million per year2.

Typically, water hardness is measured in milligrams per liter (mg/L). Water that has 0 to

17 mg/L is considered soft water, 18 to 60 mg/L is considered slightly hard water, 61 to 120

mg/L is considered moderately hard water, 121 to 180 mg/L is considered hard water, and 181

mg/L and greater is considered very hard water11. A common conversion is 1 mg/L to 1 ppm

(parts per million), so all of the previously listed classifications are equal in size in parts per

million as in mg/L3.

There are two main processes used to determine the hardness value of parts per million.

First, the EDTA titration method uses ethylenediaminetetracetic acid (EDTA) to determine the

total divalent cation concentration. Using a sample of water, EBT indicator fluid, and a

NH3/NH4Cl/MgEDTA buffer, all of an equal size of one drop, and then by adding a serially

titrated amount of EDTA solution, the concentration of divalent cations can be calculated4.

 

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Another method to determine water hardness, atomic absorption spectrophotometry (AA), uses

an atomic absorption spectrophotometer to determine the absorption values of either calcium or

magnesium. The machine uses a high temperature flame to burn the aerosol of a water sample,

while shining a light, with a specific wavelength, through the flame. The light that remains after

being passed through the flame is measured, and a reading is given based on the amount of light

absorbed by the ions being burned. This reading is then compared to readings of known

concentration solutions to determine the hardness concentration. Due to the accuracy of the

machine being used, the user is able to determine the concentration of Ca2+ and Mg2+ separately.

In order to find total water hardness (CaCO3), the concentrations of calcium and magnesium are

added together3. Both EDTA and AA methods are used because of less focused results obtained

from EDTA for low concentrations of magnesium and calcium5.

Experiment Number 10 from the PSU Chemtrek4 is an experiment that involves

becoming acquainted with the various methods for determining water hardness, and using these

methods on water samples students obtained. This project is about the procedure I used, the

results myself and my group members obtained, and how these results correlate to my

hypotheses. In our group of four, we obtained four different samples of water to be tested. All of

these samples were found in State College, Pennsylvania, on the Pennsylvania State University

campus. José Del Río Pantoja obtained his water sample from the pond outside of Osmond

Laboratory, Erika Del Pilar obtained her sample from a bottle of Aquafina Purified Water,

Robert Donohue obtained his sample from the tap in the bathroom of Geary Hall, and I, Robert

Derosky, obtained my sample from a water bottle fountain in Ritner Hall.

In the fountain outside of Osmond Laboratory, the water is a mixture of the initial water

used, presumably tap water, along with rain water, which is soft. The mixture is being run

 

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through a filter as it is redistributed, which will remove some degree of hardness, but not a

significant amount. According to the State College Borough Water Authority (SCBWA), the

water in the State College, PA area has between 120 and 190 ppm of hardness6. I expect this

water to be moderately hard. I expect the Aquafina bottled water to have the lowest level of

hardness of all of our samples, because bottled water is filtered extensively so that the water is

pure for providing a better taste. I expect the sample of tap water from Geary Hall to have a

moderate amount of hardness. The SCBWA states that the water in this area has between 120

and 190 ppm of hardness, however I do not notice extensive amounts of scale buildup around the

fixtures, which is typically present around hard waters. For my sample, I used many physical

observations of the water and its source, as well as the information from the SCBWA. I believe

the filtering throughout the fountain would reduce the amount of hardness somewhat. Also, I

visually inspected the source and water itself. I did not see any visible particles in or tints of the

water, and I did not notice extensive white scaling on the fixture. Taking into account all of the

information I collected, before I completed the experiment, my hypothesis was that my sample of

water would be slightly hard.

Procedure

As previously mentioned, the PSU Chemtrek4 lab manual was explicitly used for the

water sample hardness experiment; every step was followed exactly as published. Section A of

the experiment involves testing the water samples in an atomic absorption spectrophotometer.

Due to a high-expected level of hardness, I was directed to dilute my water sample. To do so, I

mixed 25mL of my water sample with 25mL of distilled water. This is considered using a 1:1

dilution factor. Two separate small samples of the diluted samples were used for AA

spectroscopy. Due to the spectrophotometer required to be calibrated for calcium and magnesium

 

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separately, one sample was used to test for calcium and another for magnesium. An absorption

value was recorded for each. In section B, a drop of my undiluted water sample, distilled water,

and 1.00 x 10-3 M Ca2+, were each placed on a piece of aluminum foil and then placed on a hot

plate in order to evaporate the water. The leftover material is the total dissolved solids that were

in the water. Section C focuses on the acquaintance with EDTA titration. First, we are shown the

effect of NH3/NH4Cl/MgEDTA buffer solution on different solutions. In the second part of

section C, I mixed a titrated serially amount of EDTA solution combined with a drop of EBT

indicator, NH3/NH4Cl/MgEDTA buffer, and a drop of Ca2+ in each well, of a 12 well strip. Then,

I tested the accuracy of EDTA titration, solving for the concentration of Ca2+ and comparing it to

the concentration given on the bottle. Also, the concentration of a drop of Ca2+ and Mg2+

combined was found using the same procedure. This is important, because it provides insight

into how accurate EDTA titration will be when used with water samples.

In section D, we used EDTA titrations to determine the hardness of our water samples.

Using the same solutions listed above with our water samples instead of the Ca2+, we calculated

the total CaCO3 hardness of our water samples. Section E is focused on a method to reduce the

hardness of water. After mixing cation exchange resin with our water sample, I used the resulting

water to test the hardness using EDTA titration like before. Section F is mostly calculations to

determine the hardness value of our water samples using AA. Using calibration data given in

class per AA spectrophotometer, graphs and trend lines were created and used to find the

concentration of calcium and magnesium in the water sample. Then, using conversions, the

concentration was found in ppm and then added to find the total hardness of the fountain water

sample.

 

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My partners and I had four water samples total. José Del Río Pantoja7 obtained a sample

of pond water from the pond located outside of Osmond Laboratory. This sample was diluted

with a 1:1 factor for both AA and EDTA titration. Erika Del Pilar8 obtained a sample of purified

drinking water of Aquafina Bottled Water. This sample was not diluted due to its small amount

of expected hardness. For EDTA titration, a sample of one drop of 1.00×10!! M Ca2+ was used

instead of the bottled water sample due to the very little hardness present. After calculating, the

actual water hardness value will be less than that of Ca2+. Robert Donohue9 obtained a sample of

tap water from a faucet in Geary Hall. This sample was diluted with a 1:1 factor. I, Robert

Derosky10, obtained a sample of tap water from a water bottle fountain in Ritner Hall. This

sample was diluted with a 1:1 factor. None of these samples were filtered.

Results

Table 1: Comparison of Atomic Absorption Spectroscopy (Section A)

Sample Absorption Value for Ca2+ (at 422.7 nm)

Absorption Value for Mg2+ (at 202.5 nm)

Osmond Lab Pond Water7 0.2579 0.2752 Aquafina Bottled Water8 0.0013 0.0000

Tap Water9 0.2297 0.2201 Water Bottle Fountain Water10 0.2046 0.2187

Table 2: Calibration data for AA *Data found by AA Operator White

Ca2+ Concentration (ppm) Absorbance Value (at 422.7 nm) Check Standard (ppm) 1.000 0.00574 1.04 5.00 0.04020 5.28 10.00 0.07657 10.24 25.0 0.18331 25.01 50.0 0.34689 50.53

Mg2+ Concentration (ppm) Absorbance Value (at 202.5 nm) Check Standard (ppm) 1.000 0.01315 1.28 5.00 0.06371 4.59 10.00 0.14213 9.90 25.0 0.30911 25.58 30.0 0.34672 29.62

 

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Figure 1: AA spectrophotometer calibration of calcium (Ca2+) concentration in water

Figure 2: AA spectrophotometer calibration of magnesium (Mg2+) concentration in water

Calculation 1: Ca2+ and Mg2+ concentration in water sample using AA

The equation of the trend line for Ca2+: y = 0.0069x + 0.0048

(y = absorbance value, x = Ca2+ concentration)

Absorbance value for Ca2+ of the water bottle fountain water10: y = 0.2046

0.2046 = 0.0069x + 0.0048 (solve for x)

x = 28.95 ppm Ca2+ (because of a 1:1 dilution factor, this value needs to be multiplied by 2)

28.95 × 2 = 57.90 ppm Ca2+

Mg2+ concentration in water bottle fountain water10: 35.94 ppm Mg2+

y  =  0.0069x  +  0.0048  R²  =  0.99884  

0  0.05  0.1  0.15  0.2  0.25  0.3  0.35  0.4  

0   10   20   30   40   50  

Absorbtion  Values  for  Ca

2+  (at  422.7  

nm)  

Ca2+  Concentration  (ppm)  

AA  Standards  for  Ca2+  

y  =  0.0116x  +  0.0102  R²  =  0.99357  

0  0.05  0.1  0.15  0.2  0.25  0.3  0.35  0.4  

0   5   10   15   20   25   30  Absorbtion  Values  for  Mg2

+  (at  

202.5  nm

)  

Mg2+  Concentration  (ppm)  

AA  Standards  for  Mg2+  

 

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Calculation 2: Finding the total CaCO3 water hardness using AA

57.90  ppm  Ca!!×

100.09  g  CaCO!1  mole  CaCO!40.08  g  Ca!!1  mole  Ca!!

= 144.59  ppm  CaCO!  hardness

35.94  ppm  Mg!!×

100.09  g  CaCO!1  mole  CaCO!24.31  g  Mg!!1  mole  Mg!!

= 147.97  ppm  CaCO!  hardness

To find the total water hardness, we add the hardness values of Ca2+ and Mg2+:

144.59 ppm + 147.97 ppm = 292.56 ppm CaCO3 hardness

Table 3: Comparison of TDS residues from one drop of various samples (Section B)

Sample Observation Distilled Water10 No residue

1 x 10-3 M CaCl2 (reference)10 Faint white ring Osmond Lab Pond Water7 Much heavier ring compared to reference Aquafina Bottled Water8 No residue

Tap Water9 Heavier ring compared to reference

Water Bottle Fountain Water10 A strong white ring that is more pronounced than reference

Calculation 3: Finding the % error in EDTA titration using a known solution (Section C)

We use the equation: MEDTA VEDTA = MCa2+ VCa2+ (M= molarity, V= volume)

MEDTA is given on the bottle of the EDTA solution: MEDTA = 2.00×10-4 M

(5 drops of EDTA solution is used for Ca2+ and 10 drops used for Ca2+ and Mg2+)

(2.00x10-4 M) (VEDTA) = (MCa2+) (1 drop)

(2.00x10-4 M) (5 drops) = (MCa2+) (1 drop)

MCa2+ = 1.00×10!! M Ca2+

For the second part of Section C, we used 1 drop of Ca2+ and 1 drop of Mg2+

(2.00x10-4 M) (10 drops) = (MCa2+ and Mg2+) (2 drops)

MCa2+ and Mg2+= 1.00×10!! M Ca2+ and Mg2+

Concentrations found on the bottles of solutions: 1.00×10!! M Ca2+, and 1.00×10!! M Mg2+

 

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%  error =  1.00×10!!  M  Ca! +−1.00×10!!  M  Ca!

1.00×10!!  M  Ca!×  100%   = 0%  error

0% error for Ca2+ and Mg2+ combined

Table 4: Comparison of the number of drops of EDTA solution needed to turn the mixture blue

(denoting where there is excess EDTA not used by the reaction) (Section D)

Sample Average number of Drops of EDTA (unsoftened) (VEDTA)

Average number of Drops of EDTA (resin softened)

(VEDTA) Osmond Lab Pond Water7 9.5 1 Aquafina Bottled Water8 1 1

Tap Water9 7.5 2 Water Bottle Fountain Water10 6.5 1

Calculation 4: Hardness of the water bottle fountain water10 using EDTA titration

Mwater = 1.30×10-3 M CaCO3 (using equation in Calculation 3)

To find ppm, we use the following conversion:

1.3×10!!  M  CaCO! =!.!×!"!!  !"#  !"!#!

!"× !"".!"#  !"!#!

!  !"#  !"!#!× !"""#$  !"!#!

!"  !"!#!= 130  !"

!=

130  ppm  CaCO! (Note: 1 !"!

= 1 ppm)3

130 ppm × 2 (dilution factor of 1:1) = 260 ppm CaCO! (unsoftened)

Mwater = 40 ppm CaCO3 (resin softened)

Calculation 5: Percent change in CaCO3 hardness from an unsoftened sample to a resin softened

sample (using water bottle fountain water10)

%  change =260  ppm− 40  ppm

260ppm ×100% = 84.6%  change

 

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Table 5: Comparison of water hardness values using both EDTA and AA methods

Source Hardness (unsoftened, AA)

Hardness (unsoftened, EDTA)

Hardness (resin softened, EDTA)

Osmond Lab Pond Water7 (diluted, 1:1)

371.29 ppm CaCO3 380 ppm CaCO3 40 ppm CaCO3

Aquafina Bottled Water8 (undiluted)

≈ 0 ppm CaCO3 < 20 ppm CaCO3 < 20 ppm CaCO3

Tap Water9 (diluted, 1:1) 311.74 ppm CaCO3 300 ppm CaCO3 80 ppm CaCO3 Water Bottle Fountain Water10 (diluted, 1:1)

292.56 ppm CaCO3 260 ppm CaCO3 40 ppm CaCO3

*Dilution factors are accounted for in these hardness values

 

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Discussion

During Section A, an absorbance value was obtained for calcium and magnesium. From

these values, concentrations of each divalent cation were calculated, along with the final total

water hardness. This hardness value is the total amount of hardness contributed by calcium and

magnesium, the main two contributors of water hardness1. The total hardness value, using AA on

the water bottle fountain water10, was found to be 292.56 ppm, which is considered to be very

hard water11. Both the tap water9 and Osmond Lab pond water7 are considered to be very hard

samples of water as well11. The Aquafina bottled water8, however, is considered to be soft,

because it is less than 20 ppm11.

My hypothesis for the Aquafina bottled water8 was correct, as very little hardness is

present. For the other samples, my hypotheses were incorrect. For the Osmond Lab pond water7,

the tap water9, and the water bottle fountain water10, the actual hardness values were much

higher than expected. I expected these samples to be considered less than hard, but the results

show that these samples are considered very hard11. A higher regard for the filtering involved in

the water dispensaries is the main reason for the underestimation in my hypothesis. From the

results obtained, I can now say with confidence that the Osmond Lab pond, tap, and water bottle

fountain waters are filtered minimally.

The two different methods used to determine water hardness were AA and EDTA

titration. On average, there is an 18.25-ppm difference between the results from each method.

For both the tap water9 and water bottle fountain water10, the EDTA titration result was less than

that of AA, and for the pond water from Osmond Lab7, the EDTA titration result was greater

than that of AA. These results are expected, as EDTA titration results should theoretically be less

than those of AA, because a specific ending point for the reaction is unable to be found; only an

 

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estimate can be found using EDTA titration. EDTA titration measures the total divalent cation

(all ions with a 2+ charge) concentration4, while AA measures only the cations the user is

specifically interested in. According to the SCBWA, the tap water has a 0.020 ppm concentration

of Ba2+, which is a divalent cation as well6. Also, even though not mentioned on the Authority’s

webpage, iron is also possibly part of the group of divalent cations encompassed by EDTA

titration.

Accuracy and precision are major considerations in calculating hardness using EDTA

titration and AA. Accuracy is defined as the degree of conformity and correctness when

compared to a true value; precision is defined as how consistently something is exact, or the

consistency of the outcomes of the same sample12. The precision of EDTA titration of the water

bottle fountain water10 was found to be 100 ppm, meaning there was a 100 ppm difference in

results of the two duplicate procedures from the average of them. The large value of precision is

caused by different numbers of drops of EDTA being used for each duplicate analysis: for the

first, 6 drops were used, and for the second, 7 drops were used. It was also found that if there

were a one-drop difference in EDTA volume, there would be a 40 ppm difference in

concentration of CaCO3. The accuracy of EDTA is represented by the percent error in

Calculation 3, which was 0% for each solution used. This gives an estimate of the accuracy of

the total water hardness value found using EDTA titration, which, while considering the percent

error, is very high.

Using AA, the accuracy of the water hardness is very high as well. AA is also precise due

to an elimination of subjectivity of water hardness. Two absorbance values are given for calcium

and magnesium, and no other cations will be factored into the AA hardness calculation. For the

accuracy of AA, we refer to the check standard values listed in Table 2. These values show the

 

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concentration values calculated through the same procedure as in Calculation 1. The average

percent error of the concentration of Ca2+ is 2.50%; for Mg2+ it is 8.16%. Because of the

relatively low error percentages, AA is said to be accurate as well5. Also, due to the much

quicker analysis by AA, it is the preferred method for multiple analyses5.

Conclusion

After the experiment was completed, there were two main pieces of information found:

the Aquafina bottled water8 is soft, and the Osmond Lab pond water7, tap water9, and water bottle

fountain water10 are all very hard. These pieces of information prove that my hypothesis relating

to the bottled water was correct, as this water is filtered and purified extensively and is therefore

soft. However, this information also proves that my hypotheses relating to the pond, tap, and

fountain waters were on the correct path, but not to the correct extent, as these samples were

much harder than expected. I agree with my original hypothesis for the bottled water, but I

disagree with my hypotheses for the pond, tap and fountain waters.

References

1. “Water Hardness” http://water.usgs.gov/edu/hardness.html (accessed November 2015)

2. Avant, S. Water Harness Has Big Role In Fish Disease. Agricultural Research. 2015, pp

1-2.

3. “Milligrams/Liter to Parts/Million (Ppm)” http://www.unitconversion.org/concentration-

solution/milligrams-per-liter-to-parts-per-million-ppm-conversion.html (accessed

November 2015)

4. PSU Chemtrek; Keiser, J., Ed.; Hayden-McNeil: Plymouth, MI, 2015; pp. 10-1 – 10-24.

5. “Atomic Absorption Spectrophotometric and Ethylenediaminetetraacetate-Titration

Methods for Calcium and Magnesium Determinations”

 

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http://www.journalofdairyscience.org/article/S0022-0302(69)86513-6/abstract (accessed

November 2015)

6. “State College Borough Water Authority” http://www.scbwa.org (accessed November

2015)

7. Del Río Pantoja, José, Chem 111 Laboratory Notebook, fall 2015, pp. 49-58.

8. Del Pilar, Erika, Chem 111 Laboratory Notebook, fall 2015, pp. 22-26.

9. Donohue, Robert, Chem 111 Laboratory Notebook, fall 2015, pp. 27-30.

10. Derosky, Robert, Chem 111 Laboratory Notebook, fall 2015, pp. 36-40.

11. Moore, J. W.; Stanitski, C. L.; Jurs, P. C. Chemistry: The Molecular Science; 1st ed.;

Thompson Brooks/Cole: Pacific Grove, CA, 2002; p. 718.

12. “Accuracy vs. Precision” http://www.diffen.com/difference/Accuracy_vs_Precision

(accessed November 2015)