Introduction
Water hardness is based on the amount of concentrated dissolved minerals that are
cations of charge 2+. There are many minerals that are dissolved in the water, but the two main
minerals that are typically measured in the water are Ca2+ and Mg2+. The harder the water is the
more 2+ cations are present. The reason that the water hardness is important is because hard
water creates a residue and scaling that is very hard and expensive to remove. The harder the
water is the more soap is needed to clean dishes, cloths, etc.1. This could cause a lot of trouble
for business that use water in their various products. For example a beer brewing company has
large boilers that boil water to a certain degree, if the water is really hard the minerals in the
water will create scaling that causes big problems when brewing beer. This problem can be
ratified with water softening agents.
Water hardness can be quantified is several different ways; milligrams per liter (mg/l),
parts per million (ppm), and grains per gallon (gpg). There are two main way to determine what
the hardness of a water sample. The first that is going to be talked about is atomic absorption
spectrophotometry (AA). The second is EDTA titration which will be talked about later on in
the document.
AA uses an instrument called a spectrophotometer that can measure the concentration of
a certain amount of mineral. The way this instrument works is a light is emitted from one end of
the machine that is the spectrum of either Mg or Ca, since these are the minerals measured in
water to determine hardness. It passes through a flame into a monochrometer. The flame is an
air-acetylene mixture that has within it an aerosol sample of the water sample 11. This atomizes
the metal cations Mg2+ and Ca2+. The light from the lamp passes through the flame and absorbs
1|Page
only the matching electronic energy levels as the lamp used. Say the lamp is a Ca lamp the light
will go through the flame that the monochrometer will pick up only that wavelength. The
wavelength is then bounced from mirror to grating to a PMT where the concentration is
measures.
EDTA titration is done by taking a sample of water, with Mg2+, and mixing it with a EBT
indicator. This will turn the water maroon because the indicator reacts with Mg2+ and not Ca2+.
To determine the hardness the EDTA solution is added, which reacts with Ca2+ creating a
chelate. Once the EDTA starts to react with the Mg it will begin to return to the normal blue
color. The point at which the color changes from maroon to blue is the titration point and the
hardness is calculated by using the equation MEDTA VEDTA = MCa2+ VCa2+.
The reason two methods are used to calculate water hardness is because the EDTA
titration accommodates for the concentration of all the 2+ cations that are in the water solution,
while the AA only accommodates for the Mg2+ and the Ca2+ concentrations. Both tests are
conducted so there is no bias to either concentration.
Zachary
Partner 1
Partner 2
Partner 3
Sample of
State
City
River
Pond
River
Tap
PA
NJ
CA
MD
Emmaus
Washington
Janul
Gathersburg
Table 1: Where the water samples came from
Table one shows where the water samples that were used in the experiment were taken
from and the location of their source. According to USGS Pennsylvania has generally soft water
of about 61 – 120 mg/l, Maryland and New Jersey had the softest water at around 0-60 mg/l, and
2|Page
southern California had the hardest of 181 – 250 mg/l 2. After seeing this information a
conclusion was determined that partner 1 and 3 would have the softest water, and partner 3
having the softest water because it is from the tap. Zachary would have the third softest water
and partner 3 would have the hardest water.
Procedure
First the hardness of the water was determined using AA. The samples of water were
collected, filtered and diluted if necessary, and put through the spectrophotometer to determine
the concentrations of Ca2+ and Mg2+. Next a test was done to observe the amount of total
dissolved solids that were in the water compared to two given samples. The three samples were
the water brought by the student, distilled water, and 1x10-3M Ca2+. Then the titration was
practiced on substances with known amounts of Ca2+ and Mg2+. This was used as practice for the
student before he/she actually started EDTA titration on their water samples.
Next the actual EDTA titration was done on the students’ samples to determine the
water’s hardness. After the EDTA titration was done twice on the water samples two different
types of water softening tests were done on the samples. First was the commercial waterconditioning agent was used, Arm & Hammer NaHCO3. After a viable solution was prepared
EDTA titration was done to determine the new hardness of the water sample. The second
softening procedure done was the cation exchange.
3|Page
Results
Absorbance Value (@ 422.7 nm)
Concentration vs Absorbance of Ca2+
y = 0.0096x + 0.014
R² = 0.9959
0.60000
0.50000
0.40000
0.30000
0.20000
0.10000
0.00000
0.00
10.00
20.00
30.00
40.00
50.00
60.00
Concentration (ppm)
Figure 1: Calibration of the Calcium spectrophotometer
Absorbance Value (@ 202.5 nm)
Concentration vs Absorbance of Mg2+
y = 0.0143x + 0.0197
R² = 0.9984
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
Concentration (ppm)
Figure 2: Calibration of the Magnesium spectrophotometer
4|Page
Zachary
Partner 1
Partner2
Partner3
Ca2+
Mg2+
Hardness Ca2+
Hardness Mg2+
Total Hardness
19.86
18.59
60.63
27.78
12.03
2.01
50.96
6.83
49.66
46.48
151.56
69.45
49.53
8.26
209.70
28.09
99.19
54.74
361.27
97.54
Table 2: Calculated hardness using AA in ppm
Zachary
Partner 1
Partner2
Partner3
MEDTA
VEDTA
VCa2+
MCa2+
0.0002
0.0002
0.0002
0.0002
7
5
5
9
1
1
1
1
0.0014
0.001
0.001
0.0018
Table 3: Calculated MCa2+ using the equation M1*V1 = M2*V2
Zachary
Partner 1
Partner2
Partner3
MCa2+
Concentration in ppm
0.0014
0.001
0.001
0.0018
140
100
100
180
Concentration in grains per
gallon
8.18713
5.84795
5.84795
10.5263
Table 4: Calculated concentration un softened in ppm and grain per gallon
Zachary
Partner 1
Partner2
Partner3
MEDTA
VEDTA
VCa2+
MCa2+
0.0002
0.0002
0.0002
0.0002
5
2
3
4
1
1
1
1
0.001
0.0004
0.0006
0.0008
Table 5: Calculated MCa2+ from the Arm & Hammer Baking Soda used to soften the water
samples
5|Page
Zachary
Partner 1
Partner2
Partner3
MCa2+
Concentration in ppm
0.001
0.0004
0.0006
0.0008
100
40
60
80
Concentration in grains per
gallon
5.847953
2.339181
3.508772
4.678363
Table 6: Concentrations that occurred after the Arm & Hammer baking soda was used to soften
the water samples
Zachary
Partner 1
Partner2
Partner3
MEDTA
VEDTA
VCa2+
MCa2+
0.0002
0.0002
0.0002
0.0002
1
1
1
3
1
1
1
1
0.0002
0.0002
0.0002
0.0006
Table 7: Calculated MCa2+ from the Resin ion exchange used to soften the water samples note for
all the VEDTA that are equal to 1 were actually less than one
Zachary
Partner 1
Partner2
Partner3
MCa2+
Concentration in ppm
0.0002
0.0002
0.0002
0.0006
20
20
20
60
Concentration in grains per
gallon
1.169591
1.169591
1.169591
3.508772
Table 8: Concentrations that occurred after the Resin ion exchange was used to soften the water
samples
6|Page
Dilution for AA
AA Ca Absorbance
AA Mg Abosorbance
Dilution for EDTA
EDTA Unsoften Drops
EDTA Baking Soda
Softened Drops
EDTA Resin Softened Drops
pH Before Resin
pH After Resin
TBS Observation
Zachary
Partner 1
Partner 2
Partner 3
n/a
0.2047
0.1918
n/a
7
5
n/a
0.1925
0.0484
n/a
5
2
1:4
0.596
0.7484
1:4
5
3
n/a
0.2807
0.1173
n/a
9
4
<1
8
3
Sample had a larger
ring with more
white residue in a
larger ring
1
2
3
Sample had
less white
residue in a
larger ring
<1
7
2
Sample had a
thicker white
residue in a
larger ring
3
2.50
2
Sample had more
white residue but in
a smaller ring
Table 9: Overview of all observations taken form Chem 111 Labratory Notebooks of Katie
Horneff, Luke Hess, Patrick Huang, and Zachary Horan 7 8 9 10
The equation below is to change the hardness taken from the AA machine that is in ppm and
change it to ppm hardness.
100 g CaCO3
19.86 ppm Ca2+ × [ 1 mole2+ ] = 49.65 ppm CaCO3 = 49.65 ppm hardness
40.0 g Ca
1 mole
The equation below is used to determine the molarity of the water solution; it was used he Table
3 to determine MCa2+.
MEDTA VEDTA = MCa2+ VCa2+ → MCa2+ =
MEDTA VEDTA
VCa2+
7|Page
The equation below is for determining the concentration of CaCO3 from EDTA titration; it was
used to determine the concentration in Table 4.
1.4 × 10−3 moles CaCO3 100 g CaCO3 1000 mg CaCO3
140 mg CaCO3
×
×
=
1 liter of solution
1 moleCaCO3
1 g CaCO3
1 liter of solution
= 140 ppm
The equation below is to convert the hardness of ppm to grains per gallon; it was used in Table 4
to determine the concentration in grains per gallon
140 ppm ×
1 grain per gallon
= 8.19 grains per gallon
17.1 ppm
Equations derived from PSU Chemtrek
Discussion
While using AA to determine the water hardness it was discovered that the softest water came
from Partner 1, the second softest came from Partner 3, the third softest came from Zachary, and
the hardest came from Partner 2. According to the hypothesis the only part that was wrong was
that Partner 3 had harder water than Partner 1. As stated above it was expected that the tap water
would be the hardest but the pond water from New Jersey ended up being the softest. After
further research is was discovered that since the tap water travels through all sorts of pipes it can
pick up bits of Ca2+ and Mg2+ that will make the water harder, so in this case the tap water might
be harder that the pond3. The pond water is fairly stagnant which would not bring much of any
metals into the pond than whatever might run into it during a rain storm so it is now relevant that
the pond should have the softest water. The hypothesis stated that the partners 1 and 3 would
8|Page
have the softest which they did, and that partner 2 would have the hardest and Zachary would
have the second hardest. The rest of this hypothesis was correct.
When looking at the EDTA titration of the water samples the findings were completely
different. The order from softest to hardest is now Partner 1, Partner 2, Zachary, Partner 3. This
is completely different, and the hardness are higher on average. Since AA only measures the
Ca2+ and Mg2+ it would read a lower overall hardness, but the EDTA measures all 2+ cations.
This could bring the hardness level up which would cause the data to differ. The way to measure
the EDTA titration is also not the most precise way because the VEDTA has to be a whole number
and it is taken from observation alone, when the experimenter thinks the color in the wells
change the right amount.
When the water samples were softened with the Arm & Hammer it changed the hardness
order by one person the order, from soft to hard, is now; Partner1, Partner2, Partner3, and
Zachary. The softening agent softened the water about 40 -80 ppm, and brings all the samples
into the moderately hard water category according to the department of environmental quality 4.
The Resin softened water softened the water so much that it was unable to determine the
exact hardness of the water sample. While using the EDTA titration the very first well was
turned to blue meaning that the hardness could be 20 ppm or it could be less. This would mean
that the VEDTA could equal one drop or it could be less but since the equipment is not scaled
small enough it has to be assumed that the softness is less than 20 ppm. This type of water
softener brings all the water samples down to soft water according to mecc.edu5.
The AA technique is more accurate because it measures just the Ca2+ and Mg2+ that is in
the water and can measure a smaller amount to get a more precise reading. The error in the AA
9|Page
technique is much smaller than the error in the EDTA titration. In the EDTA titration one
missed drop or a piece of sediment in the sample could give the wrong reading. If the water
sample was not filtered are diluted right it could give the wrong readings. Also the pipet used
could be defective and not be dropping the same size drops every time. All these are factors that
could go into giving a false or wrong reading of the hardness.
According to the Allentownpa.gov the water sample of Zachary was a little softer that the
average, the site states that the average is about 14.2 grains per gallon where Zachary’s was 8.17
grains per gallon 6. This could be due to that fact that the water sample was taken from a slightly
higher elevation than Allentown because it was taken on the outskirts of Allentown.
Conclusion
The hypothesis was generally correct for the AA but differed when it came to the EDTA
titration as can be seen in table 2 and table 4. The difference in the records for hardness using
the AA verses the EDTA titration was unexpected, and could possibly be redone to confirm
conclusions, but the fact that all the partners’ readings were all off would lead one to believe that
it is probably not just coincidence. The hardest water ended up coming from Janul, California
out of a river and the softest came from a pond in Washington, New Jersey. In conclusion the
north coasts of the US contain softer water where the southern coasts and central US contain
harder water.
10 | P a g e
References
1.
“Web Site Title” http://www.fcwa.org/water/hardness.htm
2.
“Web Site Title” http://water.usgs.gov/owq/hardness-alkalinity.html
3.
“Web Site Title” http://www.mwra.state.ma.us/04water/html/qual4concerns.htm
4.
“Web Site Title”
http://www.gallatin.mt.gov/Public_Documents/GallatinCoMT_WQDFactSheets/WaterSo
ftenersSepticsGW.pdf
5.
“Web Site Title” http://water.mecc.edu/exam_prep/hardness.html
6.
“Web Site Title” http://www.allentownpa.gov/Portals/0/files/Water/2009finalCCR.pdf
7.
Hess, Luke, Chem 111 Laboratory Notebook
8.
Horan, Zachary, Chem 111 Laboratory Notebook
9.
Horneff, Katie, Chem 111 Laboratory Notebook
10.
Huang, Patrick, Chem 111 Laboratory Notebook
11.
PSU Chemtrek Small-Scale Experiments for General Chemistry by Stephen Thompson
Hayden McNeil, Englewood Cliffs, NJ 2012-2013, pgs 10.1 – 10.22
12.
Chemistry 111 Student Packet Department of Chemistry Penn State by Joseph T. Keiser
Hayden McNeil, USA, 2012-2013, pgs 59 - 62
11 | P a g e