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Chemistry 111: Section 105
TA: Florian Baumann
Particles You Can Sea: Aquatic Chemistry
By Joshua Turner Bram
Group Members: Michaela Caruso-Drennen, Bradley Boyland, Maston Casto
November 14, 2012
Pennsylvania State University – University Park
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TABLE OF CONTENTS
Title Page………………………………………………………………………….1
Table of Contents……………………………………………………………….....2
Introduction………………………………………………………………………..3
Procedure………………………………………………………………………….7
Results……………………………………………………………………………..9
Table 1: Calibration Curve Absorbance for Ca+2…………………………9
Table 2: Calibration Curve Absorbance for Mg+2………………………...9
Table 3: Absorbance Rates for Tested Water Samples…………………..10
Table 4: EDTA Titration Results (Molarities of water samples)……..….10
Table 5: AA Hardness of water samples…………………………………11
Table 6: EDTA Hardness of water samples……………………………...12
Table 7: pH and TDS of water samples………………………………….13
Figure 1: Absorbance at 422.7 nm vs. Ca+2 concentration (ppm)……….13
Figure 2: Absorbance at 202.5 nm vs. Mg+2 concentration (ppm)………14
Discussion……………………………………………………………………..…15
Conclusion………………………………………………………………….……18
References……………………………………………………………………..…19
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INTRODUCTION
Water hardness is often defined as the concentration of dissolved divalent cations,
principally Ca+2 and Mg+2, along with other polyvalent cations such as iron and
manganese (1,2). Water that is hard is thus considered water that has a high concentration
of these polyvalent cations, while soft water is considered to be water that has a low
concentration of polyvalent cations. Water that contains 0 to 60 mg/L of CaCO3 is
considered to be soft, 61 to 120 mg/L is moderately hard, 121 to 180 mg/L is hard, and
180 mg/L and above is considered very hard (6).
Water hardness is an important issue mainly in both industry and the household,
where harder water causes what is called scale formation in pans, pipes, boilers and
evaporators. Whenever the water that flows through industrial pipes and equipment is
heated, it evaporates and leaves its dissolved components behind on those surfaces (1).
Over time, those residues can create clogs in pipes that are nearly impossible to remove,
and if possible, are extremely expensive. In boilers, the residue forms a hard layer that is
inefficient for transferring heat, leading to poor energy usage. This also results in higher
company costs because more money has to be spent not only to replace appliances that no
longer function, but it also must be spent on further energy bills in order to overcome
heat transfer inefficiencies (3). Similarly, at home, mineral deposits from the divalent
cations will build up on dishes and in dishwashers, which can cause uneven heat transfers
that could potentially result in uncooked meat or eggs, which is a huge health hazard
considering the multitude of different bacteria that exist in food products.
When such issues arise at factories, those factories hire companies to analyze their
water hardness through numerous different processes, among which are Atomic
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Absorption spectrophotometry analysis (AA) and EDTA (ethylenediaminetetraacetic
acid) analysis. AA analysis is a more accurate method that involves analyzing the
concentration of specific cations (alkali metals, alkali earth metals, transition metals)
dissolved or suspended in solution. The process is based around the idea that different
elements have electronic energy levels specific to each atom, due to the differing charges
of the atomic radii and electrons. This means that in order for a specific atom to be
excited to a known energy level, light with energy matching a specific electron transition
must fall on the element. Since Ca+2 and Mg+2 are the divalent cations of interest, an AA
spectrophotometer with monochromatic light matching the energy of the electron
transitions for those elements will be used. Any Ca+2 or Mg+2 ions will absorb their
respective wavelength of light, and a photo detector on the opposite side of the apparatus
will observe a noticeable drop in beam intensity. The drop in intensity is then be used to
calculate the initial concentration of the divalent cation in the water sample by using the
Beer-Lambert law (absorption = abc, where a and b are both constants, and c is
concentration of the ions). Water is vaporized in the machine using a flame that breaks
down the water into its respective components, and then the light is shown through the
vaporized water sample. Then, an absorption reading is shown and used to calculate the
concentration of only one divalent cation (1).
The other main method to determine water hardness is EDTA, which is entirely
different and less accurate than AA. It involves the titration of a known volume of the
water sample by adding an EBT indicator and EDTA solution. The EBT, initially blue,
reacts with any Mg+2 to form a red color (the Mg+2+ replaces an H+, resulting in the
solution becoming acidic and turning it red). Then, EDTA is serially added until the
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solution turns back to blue, meaning that enough EDTA has been added to chelate all of
the Ca+2 and Mg+2 in the solution. The EDTA method actually measures the total
concentration of all dissolved and suspended polyvalent cations in the water, not just Ca+2
and Mg+2, so the EDTA method often yields higher concentrations than the AA method,
which is more precise because it measures the concentration of the divalent cations due to
their discrete energy levels (1).
One final method of analysis of water hardness that was performed was a process
to check the total dissolved solids (TDS) of the water sample. Essentially, the water is
allowed to evaporate, leaving behind its dissolved solutes. By analyzing the amount of
total residue solids left behind, a gauge of the amount of solutes in the water sample can
be determined. However, the leftover solutes are more than just the Ca+2 and Mg+2 (1).
Some of the more common techniques of reducing water hardness (softening
water) involve soap and cation exchange resins. Modern soaps are anionic surface-active
agents that break the surface tension of water (1). The anions will react with the divalent
cations to precipitate out some solid or grime that can then be more easily cleaned off.
With cation exchange resins, monovalent cations are exchanged for divalent cations by
running water through resins that contain the monovalent cations (1,3). The divalent
cations bind to the resin at the SO3- loops to form an ionic bond with the resin that is
more easily removed from the solution, leaving the soft monovalent cations behind in
solution. In both methods, divalent cations are precipitated out of solution because it
makes them easier to remove.
Four water samples were tested for this experiment. They were taken from the tap
faucet in room 105B Whitmore Lab, that same tap water run through a standard Space
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Saver Brita filter, Aquafina purified bottled water, and the water refill station in the HUB.
The expected results were that the Aquafina water would be “softest” because it
undergoes a process known as HydRO-7 that involves reverse-osmosis, ozonation, and
charcoal filtration to remove virtually all minerals in the water (4). Then, the Brita filter
water was expected to have the next “softest” water because it contains a filter that,
although not designed specifically to remove divalent cations, removes heavy metals and
chlorine, which would also remove some divalent cations. The water refill station was
expected to have relatively soft water as it does have some sort of filter, and is guaranteed
to be clean and of good taste (taste is often an good indicator of water hardness) (5). The
hardest water will be the unfiltered tap water that flows through old pipes that will have
accumulated a good deal of divalent cation residue over the years.
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PROCEDURE
In order to perform AA spectrophotometry, an aspirator tube is placed into the
water sample to be tested, which sucks up the water and atomizes everything in the
sample. After standard absorption values are measured by the photomultiplier tube
(PMT), the absorptions of the respective divalent cations are found using a calibration
curve. The calibration curve is constructed from calibration data from earlier in the lab
day that gives standard absorption values for different concentrations of divalent cations.
The calibration curve is then used to calculate the respective concentrations of Ca+2 and
Mg+2 cations (1).
In EDTA titration, a known volume of the water sample is adjusted to a ph of 10
by adding an NH3/NH4 buffer using a microburet of known drop size. EBT indicator is
then added to the solution, turning the solution blue because the indicator is in the HD2form at high pH (basic). Should there be Mg+2 present in the water sample, the solution
will turn red as the Mg2+ reacts with the HD2- to form MgD- and H+ ions, which make the
solution acidic and turn it red.
HD2- + Mg+2 + Ca+2  MgD- + H+ + Ca+2  CaEDTA + MgEDTA + HD2Blue
Red
(immediately) (eventually) (last) blue
Then, EDTA of known concentration is added using the same microburet of known drop
size in a serial manner along a 1 x 12 rack of clean wells. One drop of water, one drop of
NH3/NH4, and one drop of EBT indicator is added to each well, but then one drop of
EDTA is added to the first well, two to the second, three to the third, all the way until 12
drops of EDTA are added to the twelfth well so that a spectrum of titration can be
observed. When enough EDTA is present to chelate all of the Ca+2 and Mg+2 in the
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solution, the solution will revert back to blue as a colorless MgEDTA chelate will be
formed as the HD2- form of the EBT indicator is also reformed. By finding the first well
to turn entirely blue, the concentration of polyvalent cations (water hardness) present in
the water sample can be calculated (1).
The TDS analysis is performed by placing drops of the different water samples
onto aluminum foil and then placing the aluminum foil onto a hot plate, which evaporates
the water, leaving behind any polyvalent cation residue behind that can then be looked at
to roughly gauge a concentration of polyvalent cations. Each water sample is compared to
calcium solution residue of known concentration (1 x 10-3 M) to determine roughly how
much residue is present (1).
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RESULTS
Table 1: Calibration Curve Absorbance for Ca+2
Ca+2 concentration (ppm)
Absorbance at 422.7 nm
Check Standard (ppm)
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
0
Table 2: Calibration Curve Absorbance for Mg+2
Mg+2 concentration (ppm)
Absorbance at 202.5 nm
Check Standard (ppm)
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
0
These two tables show the absorbance values for known concentrations of divalent
cations, and then also show the check standards for those absorbance values. These check
standards were used to generate the calibration graphs and calibration curves used later to
determine the concentration of the tested water samples.
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Table 3: Absorbance Rates for Tested Water Samples
Sample
Dilutions
Ca+2 Abs.
Mg+2 Abs.
Tap Water (room 105B) (7)
2 DH2O: 1 sample
0.1570
0.1820
Brita Filtered water (105B)
None
0.1964
0.2742
HUB Refill Station (9)
1 DH2O: 1 sample
0.2444
0.2697
Aquafina purified water (10)
None
0
0
(8)
This table shows the absorbance values for the tested water samples with their respective
dilution factors also represented.
Table 4: EDTA Titration Results (Molarity of water samples)
Sample
Molarity
Molarity (Baking Soda
Molarity (Resin
(unsoftened)
softened)
softened)
Tap Water (105B) (7)
3 x 10-3 M
1.2 x 10-3 M
0M
Brita Filter (105B) (8)
1.2 x 10-3 M
1 x 10-3 M
0M
HUB Refill Station (9)
2.8 x 10-3 M
1.6 x 10-3 M
0M
Aquafina purified water (10) 6.67 x 10-5 M
2.5 x 10-5 M
0M
This is a table showing the results of the different EDTA titrations performed, with a
column devoted to the unsoftened water samples, a column devoted to the results from
the water samples softened with baking soda, and a column devoted to the water samples
softened with ion exchange resin. The molarities were calculated by using the equation
MEDTAVEDTA = MSampleVSample. The molarity of the EDTA was known to be 2 x 10-4 M,
and volume varied based on which well was the first to turn blue. Then, because the
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water sample had a consistent volume of one drop, the molarity of the water sample could
be calculated. For example, with the Brita filter water sample (unsoftened), the
calculation is shown:
(2 x 10-4 M)(6 drops) = (MSample)(1 drop)  MSample = 1.2 x 10-3 M
For the Aquafina purified water, the water hardness was so low that reverse titration was
needed in order to find the water hardness of the sample. For this method, more water
drops were added to the wells so that a concentration under 2 x 10-4 M could be found.
Using this method, the Aquafina water with 15 drops required 5 drops of EDTA to
chelate all of the divalent cations in the sample. Essentially, this is equivalent to 1/3 of a
drop.
Table 5: AA Hardness of water samples
Sample
AA Hardness (ppm)
Tap Water (105B) (7)
264.8 ppm
Brita Filter (105B) (8)
123.5 ppm
HUB Refill Station (9)
273.2 ppm
Aquafina purified water (10)
0 ppm
Water hardness values are calculated by setting the absorbance of each divalent cation to
their respective calibration curve equations. For Ca+2, the calibration curve equation was
y=0.0085X+0.01, and for Mg+2 the calibration curve equation was y=0.0159X+0.0083.
For the Brita filter data, the absorbance value for Ca+2 was 0.1964, so when set equal to
0.0159X+0.0083, X is found to equal 21.9 ppm for Ca+2.
0.1964 = 0.0085X + 0.01  X = 21.9 ppm Ca+2
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This value must then be converted to its equivalent concentration in CaCO3 parts per
million as it is the standard measure of water hardness. To do this, the 21.9 ppm is
multiplied by the molar mass of CaCO3 divided by the molar mass of Ca+2, (100g/1mole
divided by 40.0g/mole), which yields a water hardness of 68.7 ppm CaCO3 hardness for
the Ca+2 in the Brita filter water sample.
21.9 ppm Ca+2 x (100g CaCO3/1mole CaCO3) x (1 mole Ca+2/40.0g Ca+2)=68.7 hardness
This same method is then used for the Mg+2 to get the hardness of Mg+2 in the sample,
which is then added to the hardness of Ca+2 to get the total hardness of the sample. This
method of calculation is replicated for all samples.
Table 6: EDTA Hardness of water samples
Sample
EDTA Hardness (ppm)
Tap Water (105B) (7)
300 ppm
Brita Filter (105B) (8)
120 ppm
HUB Refill Station (9)
280 ppm
Aquafina purified water (10)
6.7 ppm
EDTA hardness was calculated by taking the molarity of the water sample, multiplying it
by the molar mass of CaCO3, and then converted to milligrams, which results in parts per
million (mg/L). For example, the Brita water sample calculation is shown:
1.2 x 10-3 M x (100.0g CaCO3/1 mole CaCO3) x (1000mg CaCO3/1 g CaCO3)=120 ppm
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Table 7: pH and TDS of water samples
Sample
pH before resin
pH after resin
TDS observations
Tap Water (105B) (7)
9
7
Faint white circle with
prominent ring
Brita Filter (105B)
7
3
Prominent white ring
(8)
with no filling in center
HUB Refill Station
6
4
Heavy, crusty textured
(9)
white circle
Aquafina purified
7
5
Virtually non-existent
water (10)
residue
The table above includes the observations of TDS residues and the pH of the solutions
before and after the ion exchange resin was added to soften the water sample.
Figure 1: Absorbance at 422.7 nm vs. Ca+2 concentration (ppm)
Absorbance (at 422.7 nm) vs. Metal
ion concentration (ppm) for Ca+2
Absorbance (at 422.7 nm)
0.5
0.45
y = 0.0085x + 0.01
R² = 0.9987
0.4
0.35
0.3
0.25
Absorbance
0.2
Linear (Absorbance)
0.15
0.1
0.05
0
0
20
40
Metal ion concentration (ppm)
60
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Figure 2: Absorbance at 202.5 nm vs. Mg+2 concentration (ppm)
Absorbance (at 202.5 nm) vs. Metal ion
concentration (ppm) for Mg+2
0.6
Absorbance (at 202.5 nm)
0.5
y = 0.0159x + 0.0083
R² = 0.9972
0.4
0.3
Absorbance
Linear (Absorbance)
0.2
0.1
0
0
10
20
30
Metal ion concentration (ppm)
40
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DISCUSSION
From the results, it can be seen that through EDTA analysis that the hypothesis
was confirmed, with the Aquafina purified bottled water being “softest” at 6.7 ppm (soft),
followed by the Brita filtered water at 120 ppm (moderately hard), followed by the HUB
refill station water at 280 ppm (very hard), and with the tap water from room 105B being
the hardest at 300ppm (very hard). For the AA analysis, similar results were obtained
with the Aquafina purified bottled water again being the “softest” at essentially 0 ppm,
with negative absorbance values being obtained, indicating that the concentration of
divalent cations in the water sample was lower than the concentration of divalent cations
in the distilled water (soft). This result was followed by the Brita filter water with 123.5
ppm (hard) and the HUB refill station water and tap water having similar hardness values
of 273.2 ppm and 264.8 ppm respectively (both very hard). This shows that the results
that were obtained were also similar across both EDTA analysis and AA analysis, which
indicates that little error was involved with either analysis method.
However, for all samples, the EDTA analysis should have resulted in a higher
water hardness value because it measures the concentration of all polyvalent cations, not
just Ca+2 and Mg+2. The EDTA does not discriminate against any polyvalent cation, so
other polyvalent cations such as iron and manganese would also have been measured for
the total concentration of the water samples. The AA analysis on the other hand is only
showing the concentration of the Ca+2 and Mg+2 cations, meaning that the AA analysis
results are probably more accurate than those of the EDTA analysis. Also, because the
EDTA titration method varied by 20 ppm if the observer was only off my one well, the
results are much less accurate and less predictable than the results from the AA analysis.
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Although the same result was obtained for the Brita filter sample through two trials of
EDTA, the AA analysis should still be more precise as well due to the consistent values
generated by the machine.
A 1982 geological survey of well water in the State College region actually
determined that the water hardness for most of the water provided to the greater State
College community was 230 ppm (CaCO3 hardness) (12). Because the State College
Borough Water Authority reports that most of the water provided to the community is in
fact groundwater that is pumped from wells and natural springs, this value, although
slightly outdated, gives a good baseline to compare the results of this experiment to (11).
Taking 230 ppm as the accepted value for the hardness of the tap water, the EDTA and
AA analyses yield percent errors of 30% and 15% respectively.
% Error=(Exp-Accepted)/(Accepted)x100%(300ppm–230ppm)/(300ppm)x100%=30%
The values could actually be more accurate though, because the survey indicates the
water hardness of the water at the well or spring source, while the tap water used for
analysis in the lab had to flow through old pipes that would have accumulated divalent
cation buildup over the years that would increase the hardness of the water.
The TDS results, although not entirely definitive, showed that the Aquafina water
had by far the least amount of polyvalent cations present, followed by the Brita, followed
by the Tap water and HUB refill station. However, the HUB refill station sample actually
appeared to have a higher hardness than the Tap water due to the more prominent white
residue left over after the evaporation of the water, which is in conflict with the results of
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the other analyses. It would have been easier though to have too much water dropped
onto the aluminum foil, which would have resulted in more polyvalent cations being
present and thus increased residue. When compared to the residue left behind by the 1.0 x
10-3 M Ca+2 solution, the Aquafina was the only sample that appeared to have less residue
while the rest had more. The Brita had a similar amount, which makes sense since the
experimental molarity of the Brita sample was 1.2 x 10-3 M, while both the Tap water and
HUB refill station had much more prominent white circles, indicating a much higher
concentration of polyvalent cations.
As for the Aquafina purified bottled water, the water hardness value is mandated
to be less than 10 ppm for purified water by the FDA, which they maintain that they do in
fact achieve (4). In fact, their testing of their water yields undetectable TDS in the
purified bottled water. According to the EDTA and AA analysis results, this fact was
corroborated as the EDTA yielded 6.67 ppm hardness and the AA yielded 0 ppm
hardness.
The commercial water-conditioning agent resulted in softer water for all four
water samples tested. On average, the hardness of the water samples was lowered 81
ppm, which is a rather significant softening percentage. The divalent cation exchange
process softened all of the water samples to 0 ppm (or essentially 0). Clearly, it was the
much more effective method of softening and is highly recommended for commercial
usage in plants to lower water hardness levels.
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CONCLUSION
This lab indicates several things about water hardness. Water samples with higher
water hardness (higher concentrations of dissolved solutes) will result in the formation of
white rings and circles in TDS analysis. Aquafina purified bottled water has incredibly
low levels of divalent cation concentration due to the intense purification processes
involved in the production of the water. The Brita filter water has medium levels of water
hardness due to the partial filtering of the divalent cations, while both the tap water and
HUB refill station water have relatively high water hardness due to the little to no
filtering of the water that occurs. To soften those waters, either a commercial watercondition agent or ion exchange resin can be used, although the resin is much more
effective and can essentially remove all divalent cations present in the water sample. The
hypothesis of the lab was more or less confirmed through the EDTA and AA analyses
that took place as a part of the lab.
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11. "What Is Water "hardness" and Should I Do Anything about It?" FAQ. SCBWA,
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