The Chemistry of Natural Water
Lauren Kenney
Chemistry 111
November 14, 2012
TA: Tammy Badger
1
Introduction
Water hardness is important because it not only is a huge part of water
quality, but it also has a big affect on both life and industry. Since all living creatures
need water to survive, it is necessary to understand the chemistry behind natural
water. The purpose of this experiment was to test several different water samples to
determine the hardness, find out what makes the samples hard, and test a common
water softening technique. Water hardness is defined as the sum of both the calcium
and magnesium concentrations, expressed as calcium carbonate in milligrams per
liter. Water is considered hard when there are high concentrations of the divalent
cations Magnesium and Calcium and water is considered soft when there is a low
combined concentration of Calcium and Magnesuim1. Hard water causes scale
formation, large build-ups of calcium carbonate, that narrows pipes and is very
costly to remove, which causes appliances like laundry machines and dishwashers
to break easily, and when hard water is evaporated it leaves behind white residue.
Hard water also effects how well soaps work. The soap reacts with the metal ions in
hard water instead of the dirt on the surface being cleaned and produces a scum.
Water softening agents such as washing soda and lime are used to slow down the
scale build-up. They work by precipitating the Calcium and Magnesium ions as
insoluble salts which are then removed by filtration or settling2.
General guidelines for classification of waters are: 0 to 60 mg/L (milligrams
per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately
hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard3. Water
hardness depends on what the water had contact with for example rainwater flows
2
over rocks dissolving some of the elements of the rocks. Areas rich in limestone and
chalk tend to have hard water because the minerals are more soluble, while soft
water usually comes from areas with granite and other impermeable rocks5.
The four water samples for this experiment were obtained from various
lakes and creeks around Pennsylvania and Ohio including Lake Moshannon,
Loyalsock Creek, Spring Creek, and a test pit in Wilmington, Ohio. All of these
samples come from lakes and creeks which get the main supply if water from rain or
springs. While the majority of the samples will be classified as soft to moderately
hard, they will be rank ordered on which is the hardest out of all four samples. Small
streams and springs feed Lake Moshannon so this water sample should be the
second hardest out the all the samples because the springs send water from under
ground where there are many minerals and the streams flow into the lake picking
up cations on the way, and the entire body of water sits over rock and sand6.
Loyalsock Creek is mostly gets it’s water from rain, however the underlying geology
is interbedded sediment and sandstone7. Since sandstone is soluble in water, it is
likely that the flowing water of the creek will also be mildly softer than Lake
Moshannon due to the ions that it picks up from the sandstone. The water sample
from Spring Creek should be the hardest water out of the four samples because the
underlying bedrock of Nittany Valley and Penn’s Valley is limestone8. Limestone is
very soluble in water and the endless flowing of the creek dissolves the sediment
picking up metal ions. The water from Wilmington Ohio test pit should be the third
hardest sample in between Lake Moshannon and Loyalsock Creek. The geology of
Ohio is very similar to the geology in Pennsylvania where the other three samples
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are from and the sample sits in a pit for a period of time collecting dissolved cations.
Atomic Absorption Spectrophotometry (AA) and ethylenediaminetetracetic
acid (EDTA) titration were used to determine the hardness for the water samples in
this experiment. AA works by using Beer’s Law that states that absorbance and
concentration are directly related. EDTA is an inexpensive way of calculating water
hardness and works by reacting with all divalent cations and not just calcium and
magnesium. In a titration method, the EDTA complexes with Magnesium and
Calcium and the indicator eriochrome black T (EBT). The water will turn blue if it is
soft1. In AA only the specific concentration of calcium and magnesium divalent ions
are tested for. The AA method is indeed more expensive, however it is extremely
accurate. The water sample is aspirated into a flame in the device and a light beam is
directed at the flame onto a detector, which measures the amount of light absorbed
by the aspirated water. Since each element has a unique wavelength, the source
lamp is made of the element (calcium or magnesium). The amount of energy that the
wavelength absorbed by the flame is proportional to the concentration of the
element in the sample over a limited concentration range4.
It is expected that the hardness of each sample rank in this order (from
softest to hardest): Loyalsock Creek, Wilmington Ohio, Lake Moshannon, and Spring
Creek.
Procedure
A water sample was taken from each of the following locations: Spring Creek,
Lake Moshannon, Loyalsock Creek, and Wilmington, Ohio. Each group member took
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a sample and performed a series of experiments including AA and EDTA titration to
determine the hardness of the sample.
Atomic Absorption Spectroscopy was performed by shinning a
monochromatic light though the sample to measure the amount of light absorbed.
According to Beer’s Law, the amount of absorbance is directly related to the
concentration. Two calibration graphs and equations were created from data points
obtained by testing known concentrations of calcium and magnesium, recording
their absorbance, and creating a trend line for the data. Each water sample was then
put into each of the machines and the absorbance was read off the machines. The
data was applied to the specific calibration line equation and the hardness of each
sample was calculated.
EDTA titration was performed on each water sample. One drop of
NH3/NH4CL/MgEDTA, EBT indicator, and the water being tested were added to each
well of a 1x12 strip. Then a serial titration was performed and the EDTA solution
was titrated across the well strips so that the first well had 1 drop and the last well
had 12 drops. The first well that turned blue after the titration was recorded. The
number of EDTA drops in that well were used to determine the hardness of that
water sample by equations seen in the results section. A more detailed procedure
can be found in PSU Chemtrek: Small-Scale Chemistry for General Chemistry.
Written by Stephen Thompson, edited by Joseph T. Keiser 2012-2013, published by
Hayden McNeil, and located on pages 10-1 to 10-221.
5
Results
Figure 1: AA calibration graph for Ca2
Absorbance (Ca2+) vs Concentration
(ppm) an 427.7nm
0.5
y = 0.0081x + 0.0089
R² = 0.9989
Absorbance
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
Concentration
35
40
45
50
Figure 2: AA calibration graph for Mg 2+
Absorbance (Mg2+) vs Concentration
(ppm) at 202.5nm
0.5
y = 0.0142x + 0.0227
R² = 0.9961
Absorbance
0.4
0.3
0.2
0.1
0
0
5
10
15
20
Concentration
25
30
35
6
Table 1: AA hardness results for Lake Moshannon water sample
Absorbance
ppm hardness
Calcium
0.0508
12.9
Magnesium
0.0329
7.16
Total Hardness
20.06
The calibration line equation in Figure 1 (y =0.0081x + 0.0089) was used to
calculate the concentration in ppm. The Ca2+ absorbance was substituted for the y
variable to solve for x.
0.0508=0.0081x+0.0089
0.0419=0.0081x
x = 5.17 ppm Ca2+
To get ppm CaCO3, it is necessary to multiply the concentration by the molar mass of
CaCO3 divided by the molar mass of Ca2+.
CaCO3
5.17 ppm Ca2+ * ((100g CaCO3/ 1mole)/(40.0g Ca2+/ 1mole)) = 12.9 ppm
The same process was used to calculate concentration in ppm for Mg2+.
The calibration line equation in Figure 2 (y=0.0142x + 0.0227) was used to calculate
the concentration in ppm of Mg2+. The absorbance was substituted for the y variable
to solve for x.
0.0329 = 0.0142x + 0.0227
x = 0.718 ppm Mg2+
To convert that concentration to ppm of CaCO3, multiply the concentration by the
molar mass of CaCO3 divided by the molar mass of Mg2+.
CaCO3
0.718n ppm Mg2+ * ((100g CaCO3/ 1 mol)/(24.3g Mg2+/ 1mol)) = 2.96 ppm
Table 2: AA hardness results for all water samples
Absorbance units
ppm hardness
.0508
.0329
12.9
2.96
15.86
.0417
.0139
10.1
0
10.1
Lake Moshannon
Ca2+
Mg2+
Total Hardness:
Loyalsock
Creek10
Ca2+
Mg2+
Total Hardness:
Spring Creek11
7
Ca2+
Mg2+
.3263
.2957
195.9*
158.2*
354.1
.0435
.0181
10.7
0
10.7
Total Hardness:
Wilmington Ohio12
Ca2+
Mg2+
Total Hardness:
* Concentration multiplied by 2 due to dilution
Table 3: EDTA Titration results for Lake Moshannon water sample
Drops of EDTA
ppm hardness
Water sample
2
40
Softened sample
1
20
To convert the drops of EDTA to ppm hardness we first must figure out the
concentration of the divalent cations using the equation (MEDTA VEDTA = MCations
VCations)1
2*10-4 * 2 drops = MCations * 1 drop
4*10-4 = MCations
To covert the concentration of CaCO3 to hardness we must multiply by CaCO3 molar
mass and then convert it into mg CaCO3:
(4*10-4 mol CaCO3/ liter) * (100 g CaCO3/ mol CaCO3) *
(1000mg CaCO3/ 1g CaCO3) = 40 mg CaCO3/ liter
40 mg CaCO3/L =40 ppm hardness because the density of water is 1000g/L and
1ppm = 1 mg CaCO3/1000g water.
To convert the ppm into grains/gallon, multiply the hardness by the conversion
factor: (1 grain CaCO3 / gallon) is equivalent to 17.1 ppm
40ppm *(1 grain per gallon /17.1ppm) = 12 grains per gallon
Table 4: EDTA hardness for all water samples
Drops EDTA
Lake Moshannon:
Water Sample
2
Loyalsock Creek10:
Water Sample
6
11
Spring Creek :
Water Sample
2
12
Wilmington Ohio :
Water Sample
11
* Concentration multiplied by 2 due to dilution
ppm hardness/grains per
gallon
40/2.34
120/7.02
80*/4.68
220/12.9
8
Discussion
The results from the AA testing do not reject the hypothesis, however the
results from the EDTA titration do indeed reject the hypothesis. According to the AA
results (Table 2), Spring Creek has the hardest water sample of 354.1 ppm because
the creek flows over very permeable and water-soluble rocks like limestone and
dolomite and most of the water comes from underground springs8. Lake Moshannon
has the second hardest sample because it also gets water from underground springs
and streams but the water does not flow over soluble rocks, it just sits for a long
period of time6. The next hardest sample came from Wilmington Ohio in a test pit.
The area mainly gets water from underground wells and springs so there still is a
presence of dissolved cations since the geology of Ohio is similar to the geology of
Pennsylvania with an abundance of soluble bedrock like limestone and dolomite9.
The softest sample was from Loyalsock Creek because the creek gets most of its
water from rain but it still flows over water-soluble rock sediment7.
Since the AA is much more accurate, there are many different errors that
could have happened during the EDTA titration and calculation process. Perhaps the
number of drops of EDTA was miscounted causing the solution to change color
earlier or later than expected. This is important because for each well is an
additional 20ppm so that could be significant if the drops were more or less than
what they were supposed to be. Another reason is that four different people tested
one water sample and one group member’s perception of when the well turned blue
might be different from when another group member thought the well was
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completely blue, which affected the accuracy. The main reason why the EDTA
results are significantly higher than the AA results is because EDTA reacts with all of
the cations instead of just Ca2+ and Mg2+. AA is done by a machine and has a high
accuracy and precision, while the precision for EDTA titration was high due to the
fact that the same well turned blue during both trials. For some group members, the
sample did not turn blue in the same well both times which negatively impacted the
precision and could have caused flawed data. One issue with AA was that equation
for the line of the Mg2+ Concentration vs. Absorbance graph (Figure 2) gave negative
numbers when solving for the concentration two of the samples. This means that
the concentration of magnesium is so little that it is not even relevant to the total
hardness so it ends up to be zero.
The AA works the best because it only tests for the calcium and magnesium
concentration, which is important because calcium and magnesium contribute the
most to hardness and scale formation in industrial use of water, water softening
only affects those two cations, and it is a much quicker and easier process (but much
more expensive). Although only the results of one sample treated with a softening
agent are shown (Table 1), all of the samples were significantly softer after being
treated with the softening agent.
Conclusion
Water hardness is not only important for personal use, but it also has a big
impact on industrial water use as well. Water that comes from underground springs
and flows over highly soluble rocks is much harder than typical stream water due to
the fact that it is in contact with the soluble sediment longer and has more time to
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pick up the dissolved cations. While stream water does still flow over water-soluble
rock, it still mostly comes from rain and is therefore softer. Water softening agents
are one common way to treat water to reduce hardness and get better personal and
industrial use. The type of rock that the water comes in contact with and the source
of water are the biggest factors when considering water hardness.
Reference
1PSU
Chemtrek: Small-Scale Experiments for General Chemistry. 2012-2013.
Stephen Thompson. Published by Hayden McNeal. Edited by Joseph T. Keiser.
Experiment 10: The Chemistry of Natural Water. Pg. 10.1-10.22.
2Spellman,
Frank R: The Science of Water Concepts and Applications, Second
Edition; CRC Press, 2007, 289-304.
3"Water
Hardness and Alkalinity." USGS Water-Quality Information:. U.S Geological
Survey, 13 Apr. 2012. Web. <http://water.usgs.gov/owq/hardness-alkalinity.html>.
4Trimble,
Stanley W: Encyclopedia of Water Science, Second Edition; CRC Press,
2007, 1327-1336.
5Thalmanand,
Katherine L, Bedessem, James M..Fierro, Pedro Ed: The Water
Encyclopedia, Third Edition Hydrologic Data and Internet Resources; CRC
Press,2007, 8.1-8.218.
6"Black
Moshannon State Park." PA DCNR . Pennsylvania Department of
Conservation and Natural Resources, 2012. Web.
<http://www.dcnr.state.pa.us/stateparks/findapark/blackmoshannon/index.htm>.
7"Loyalsock
Creek Watershed Profile." Loyalsock Creek Watershed Profile.
Susquehanna River Basin Commission, 2011. Web.
<http://mdw.srbc.net/remotewaterquality/watershed_profiles/loyalsock.htm>.
8"The
Spring Creek Watershed." The Susquehanna River Basin Hydrologic Observing
System:(SRBHOS). Penn State, 2011. Web.
<http://www.srbhos.psu.edu/srb_testbeds/springcreek.asp>.
9"Wilmington,
Ohio Water Department." Wilmington, OH. City of Wilmington, 2009.
Web. <http://www.ci.wilmington.oh.us/water.cfm>.
10Loyalsock
Creek Water Sample: Kaylene Killeen Lab Notebook.
11
11Spring
Creek Water Sample: Cooper Lacasse Lab Notebook.
12Wilmington,
Ohio Water Sample: Scott King Lab Notebook.
12