Spencer Schrock

November 13th, 2012

Analyzing Water Hardness of Groundwater and Surface Water Sources

Chem 111- Section 105

Group Members: Rachel Thomas, Priyanka Solanki, Menaka Suri, and Wilton Smith

TA: Sha Sun

Schrock 1

Introduction:

Water hardness is a quality of water that is based upon the presence of dissolved

polyvalent cations in water. Most of the hardness for a water sample comes from Ca2+ and

Mg2+, however other polyvalent cations contribute to water hardness in lesser quantities.

This occurs because water is a broad solvent and easily dissolves many minerals it comes

in contact with, so when water percolates through soil and rock it dissolves minerals.1

Water hardness is important to analyze because it plays a direct role in both industrial and

domestic water uses. In domestic use the polyvalent cations can precipitate soap, which

creates soap curd on surfaces and necessitates the use of more soap for cleaning

processes.15 The challenges of using hard water for industrial purposes are a result of the

polyvalent cations reacting to form deposits known as scale. Scale adheres to plumbing

and other machine parts, increasing wear, clogging parts, and in general increasing

maintenance costs.2

The measurements for water hardness vary, but they all convey concentration in

one form or another (either mass per volume or mass per mass ratios). The different units

for water hardness include grains per gallon (gpg), parts per million (ppm), and

milligrams per liter (mg/L).2 In order to measure water hardness, two different methods

are used: ethylenediaminetetraacetic acid (EDTA) titration and atomic absorption

spectrophotometry (AA).

EDTA titration relies on the eriochrome black T (EBT) dye, specifically at a high

pH of about 10. The dye starts in its blue form (HD2-) and reacts with Mg2+ to form a red

chelate (MgD-), turning the water sample red. When the EDTA is added, it forms chelates

with Ca2+ (and other polyvalent ions). Once all the other polyvalent ions are chelated, the

Schrock 2

EDTA interacts with the MgD- indicator until all of the Mg2+ ions are removed from the

MgD- and the indicator returns to its initial blue form. The color change creates a visible

start and end point for the titration and allows for the concentration of polyvalent cations

to be determined form a known concentration of EDTA.3

AA consists of monochromatic light projected through the atomized water

sample. Atoms have discrete electron energy levels that and are unique for each atom, so

a different hallow cathode lamp (HCL) is needed for each element to be analyzed. When

the HCL is powered, the voltage across the electrodes excites the gaseous element in the

lamp; when an electron returns from its excited state the electron releases monochromatic

light specific to the transition and is therefore specific for the element. The

monochromatic light emitted travels through the atomized water sample. Only the

element that matches the element in the lamp (and therefore exactly matches the energy

of the emitted light) absorbs the monochromatic light. The remaining light passes through

a grating in the monochrometer so that only the desired wavelength passes through to the

photomultiplier tube (PMT). Because of the absorption of light by the matching atoms in

the sample, the concentration can be determined through the Beer-Lambert law where

Absorption = abc, a and b are both constants, and c is concentration of the ion.3

The two methods are different not only in their approach, but also in their

analysis. AA only measures the concentrations of the ions in the lamp (in this case Ca2+

and Mg2+), while the EDTA measurements are a result of all polyvalent ions in the

sample. Since Ca2+ and Mg2+ make up the majority of water’s hardness but not all, the

difference in analysis leads to a higher water hardness with the EDTA method than with

the AA.

Schrock 3

If water hardness is found to be an issue, the water can be treated to remove most

of the polyvalent ions from the water. The most common treatment is an ion-exchange

resin where polyvalent ions are exchanged for monovalent ions (for example Na+ and H+)

in the resin. The resin can then be reused when run through a brine tank, a high

concentration salt solution, so that the polyvalent ions in the resin are replaced with

monovalent ions, restoring the resin to its initial state.4 Other treatments include products

which react with the ions to form insoluble salts which can be filtered out of the water.

Five water samples were tested for this experiment, each from a different town in

Pennsylvania including: Harrisburg, Mechanicsburg, Lansdale, Yardley, and State

College. The hypothesis was that water whose source is groundwater will have a higher

hardness than water whose source is surface water. State College retrieves most of their

water from underground.5 Yardley’s source is the Delaware river which is surface water.6

Mechanicsburg also gets its water from creeks and rivers, so its water is surface water.7

Harrisburg’s source is somewhat misleading. The Harrisburg Airport from which the

sample was taken gets its water from Middletown, whose source is groundwater.8 Lastly,

Lansdale also uses a local creek as their source, which is surface water. 9 All of the towns

that retrieve their water from surface water would be expected to travel through less soil

and rock and would be softer as a result. This hypothesis meant that State College and

Harrisburg would have the highest water hardness, while Yardley, Mechanicsburg, and

Lansdale would be lower.

Procedure:

The following procedure was taken from the PSU Chemtrek.3

Schrock 4

In order to analyze the sample via AA, an aspirator is placed in the sample to

draw the liquid to the flame and vaporize it. The absorbance values were recorded. Using

calibration data for known concentrations analyzed earlier in the day, a calibration line

was constructed. From this line the concentrations of the water samples were interpolated

from the absorption values. Some samples were too concentrated to be analyzed without

dilution, therefore the State College, Lansdale and Harrisburg samples were diluted in a

1:1 ratio of sample water and distilled water.

To carry out the EDTA titration, one drop of the water sample (or diluted sample

for the towns diluted in AA) was placed in each well of a 1 x 12 well strip. One drop of

EBT and one drop of NH3/NH4Cl/MgEDTA buffer (to get a high pH) were also added to

the wells in order to set up the initial state. The 2 x 10-4M EDTA solution was serially

titrated with one drop in the first well, two in the second and so on. The first well to turn

blue was considered the end point for the titration and would indicate the volume of

EDTA used later in the calculations. The same EDTA procedure was used for the water

treated with the ion-exchange resin and the Arm and Hammer Baking Soda. To treat the

water, 20mg of baking soda was added to the 1cm of sample water (or diluted sample) in

a vial, or for the ion-exchange resin method, enough resin was added to cover the bottom

of the vial.

Schrock 5

Results:

Table 1: Absorption Calibration Data for Ca2+

Ca2+ concentration (ppm) Absorbance at 422.7 nm Check Standard (ppm)

0

1.000 0.01082 1.27

5.00 0.05310 5.09

10.00 0.10000 9.81

25.0 0.23103 24.08

50.0 0.43298 50.45

Table 2: Absorption Calibration Data for Mg2+

Mg2+ concentration (ppm) Absorbance at 202.5 nm Check Standard (ppm)

0

1.000 0.01517 1.48

5.00 0.08718 5.62

10.00 0.18376 10.53

25.0 0.39579 24.72

30.0 0.48868 29.02

Schrock 6

Graph 1: Ca2+ Calibration Line

0 10 20 30 40 50 600

0.1

0.2

0.3

0.4

0.5

0.01082

0.0531

0.1

0.23103

0.43298f(x) = 0.00854824680210685 x + 0.0100079082016554R² = 0.998684146580191

Absorbance vs. Ca2+ Concentration

Ca2+ Concentration (ppm)

Absorbance(at 422.7 nm)

Graph 2: Mg2+ Calibration Line

0 5 10 15 20 25 30 350

0.1

0.2

0.3

0.4

0.5

0.6

0.01517

0.08718

0.18376

0.39579

0.48868f(x) = 0.0159016552582452 x + 0.00831249533291845R² = 0.997150551449768

Absorbance vs. Mg2+ Concentration

Mg2+ Concentration (ppm)

Absorbance(at 202.5nm)

Schrock 7

Table 3: Absorbance Values for Water Samples

Sample Dilutions Ca2+ Abs. Mg2+ Abs.

Harrisburg10 1 dH20 : 1 sample 0.3459 0.1953

Mechanicsburg11 N/A 0.3489 0.2440

Lansdale12 1 dH20 : 1 sample 0.1547 0.2391

State College13 1 dH20 : 1 sample 0.1920 0.2323

Yardley14 N/A 0.2020 0.1741

Table 4: AA Total Hardness

Sample Hardness (ppm)

Harrisburg10 294

Mechanicsburg11 161

Lansdale12 213

State College13 223

Yardley14 99

To calculate total hardness from absorption values, the absorption value was

plugged into the y-value of the calibration line to solve for concentration. For Ca2+ this

line was , where y is the absorption and x is the concentration in ppm.

The Harrisburg Ca2+ absorbance was 0.3459, so solving for x, a concentration of 39.5

ppm. Once a concentration was interpolated it must be converted into an equivalent

CaCO3 ppm. The Ca2+ ppm must be converted to CaCO3 ppm by a ratio of molar masses:

Schrock 8

. For the Harrisburg Ca2+ this would be

. In order to do the same with Mg2+, simply

substitute the molar mass of Mg2+ (24.3 ) instead of Ca2+’s and use Mg2+’s

calibration line, for the Harrisburg sample the results fore Mg2+ was 48.4 ppm CaCO3. If

the sample was diluted the concentration was adjusted accordingly here. In the case of the

Harrisburg sample, the total ppm would be multiplied by a factor of two since the sample

analyzed was half as concentrated, which yields a hardness of 294ppm.

Table 5: EDTA Titration End Points

Sample Untreated (Well #) Baking Soda Treated (Well #) Ion-Exchange Resin Treatment (Well #)

Harrisburg10 9^ 5^ 2^

Mechanicsburg11 9.5 7 1

Lansdale12 6^ 4^ 0*^

State College13 6^ 5^ 1^

Yardley14 6 4 0*

*All wells were blue in serial titration. ^Samples were diluted in a 1:1 ratio.

Schrock 9

Table 6: EDTA Molarities Found by Serial Titration

Sample Untreated (M) Baking Soda Treated (M) Ion-Exchange Resin Treatment (M)

Harrisburg10 3.6 x 10-3 2.0 x 10-3 8.0 x 10-4

Mechanicsburg11 1.9 x 10-3 1.4 x 10-3 2.0 x 10-4

Lansdale12 2.4 x 10-3 1.6 x 10-3 0*

State College13 2.4 x 10-3 2.0 x 10-3 4.0 x 10-4

Yardley14 1.2 x 10-3 8.0 x 10-4 0*

*All wells were blue in serial titration.

Molarities were found using the equation , where M is

molarity and V is volume. Since the concentration of EDTA is known (2.00 x 10-4 M) and

the volumes are known, the equation can be solved for Msample. In the untreated Harrisburg

sample, the titration end point was Well 9, which means VEDTA is 9 drops and Vsample is 1

drop. So the molarity of the diluted sample is .

Considering the dilution was a factor of two, the actual concentration is twice that.

Schrock 10

Table 7: EDTA Total Hardness for Untreated and Treated Water Samples

Sample Untreated Hardness (ppm) Baking Soda Treated Hardness (ppm)

Ion-Exchange Resin Treatment Hardness

(ppm)

Harrisburg10 360 200 80

Mechanicsburg11 190 140 20

Lansdale12 240 160 0

State College13 240 200 40

Yardley14 120 80 0

To achieve ppm CaCO3, molarity is converted into mg/L. Since the density of

dilute aqueous solutions is close to a density of 1g/mL, mg/L is equivalent to ppm.3

For the untreated Harrisburg sample whose

molarity is 3.6 x 10-3 M, the calculation would be:

Discussion:

The hardest water sample came from Harrisburg, which had an AA hardness of

294 ppm and an EDTA value of 360ppm. Although State College and Lansdale shared

equal EDTA values of 240ppm, State College has a higher AA hardness (223ppm vs.

213ppm) and is the second hardest water sample with Lansdale following close behind.

Mechanicsburg ranked 4th in terms of AA and EDTA hardness with values of 161ppm

and 190ppm, respectively. Yardley had the least hard water at 99ppm for AA and

120ppm for EDTA titration. These results partially support the hypothesis. The towns

Schrock 11

with a groundwater water supply had the higher water hardness values and those with

surface water were lower, which correlates with the hypothesis, however the proximity of

Lansdale’s hardness to that of State College’s hardness shows that the correlation is not

perfect.

It is important to compare the results to other sources however. Yardley’s water

authority published quality information indicates a range of hardness from 60ppm-

230ppm.16 While the hardness range is not specific, both the AA and EDTA hardness

values fall within the reported range. Mechanicsburg’s water authority also reported a

range of water hardness, this hardness was between 60-284ppm.17 Again, the observed

hardness values fell between the ranges provided. The State College Borough Water

Authority reports a typical hardness of 120-190ppm.5 Unlike previous samples, this was

below both the EDTA and AA observed in the lab, which were above 200ppm. While

published values for Harrisburg and Lansdale could not be found, the data for the other

towns were generally in agreement with the observations in the lab.

Other trends emerged as well, such as differences between EDTA and AA

methods. The AA values, as seen above, were always lower than the EDTA hardness

values, a phenomenon discussed in the introduction. EDTA values were also less precise.

The precision for EDTA was 1 drop of 2.00 x 10-4 M EDTA solution, which equates to a

20ppm shift. Compared to AA whose standards are almost exact with check standards

(50ppm vs. 50.45ppm for Ca2+), EDTA is much less accurate. Due to this precision, the

EDTA are a probable source of error, in some cases up to 100% in the case of treated

water whose calculated hardness was 20ppm with a precision of 20ppm. AA analysis is

not impervious to error however; ambient light could affect absorbance values (although

Schrock 12

a grating prevents most of ambient light). In addition the concentrations were interpolated

from a trendline. Small differences in absorption values can translate into differences in

concentrations as a result.

Conclusions

In closing, towns with groundwater sources have high water hardness than towns

with surface water supplies. While this supports the hypothesis, the difference between

groundwater and surface water hardness vary and the correlation is not as simple as

source. For example while Lansdale and Yardley are both surface water they differ by

over 100ppm or a two-fold difference. The range in reported hardness values reported by

water authorities does not add to the certainty either and indicate variance within a water

supply as well. As a result all that is left is general support for the proposed hypothesis,

but more data (ideally more precise than the EDTA in this lab experiment) is needed to

reach any meaningful conclusions.

References:

1. “Explanation of Water Hardness” www.fcwa.org/Water/hardness.htm (Accessed

Nov 2012)

2. “Hardness” water.mecc.edu/exam_prep/hardness.html (Accessed Nov 2012)

3. PSU Chemtrek

4. “Water Quality Association” wqa.org/sitelogic.cfm?ID=207 (Accessed Nov 2012)

5. “State College Borough Water Authority” www.scbwa.org/pages/faq (Accessed

Nov 2012)

Schrock 13

6. “Pennsylvania American Water Company - Yardley Intake”

elibrary.dep.state.pa.us/dsweb/Get/Document-59599/RS1090074001%20Yardley.

pdf (Accessed Nov 2012)

7. “2011 Annual Water Quality Report” amwater.com/files/PA_7210029_CCR.pdf

(Accessed Nov 2012)

8. “Middletown Borough Services” middletownborough.com/services_utility.php

(Accessed Nov 2012)

9. “About Your Water” northpennwater.org/ourwater/aboutyourwater (Accessed

Nov 2012)

10. Schrock, Spencer, Chem 111 Laboratory Notebook, pp 25-28.

11. Smith, Wilton, Chem 111 Laboratory Notebook, pp 35-41.

12. Suri, Menaka, Chem 111 Laboratory Notebook, pp 37-44.

13. Solanki, Priyanka, Chem 111 Laboratory Notebook, pp 36-42.

14. Thomas, Rachel, Chem 111Laboratory Notebook, pp 30-34.

15. Nagarajan, M.; Paine H. Water hardness control by detergent builders. Journal of

the American Oil Chemists' Society. 1984, 1475-1478.

16. “Typical Water Quality Information” amwater.com/files/WQ%20Sheet%20for

%20Yardley%20Final%2001242012.pdf (Accessed Nov 2012)

17. “Typical Water Quality Information”

amwater.com/files/Mechanicsburg_TWQ.pdf (Accessed Nov 2012)

Schrock 14