Name: Ankitha Miryala
TA: Saranya Pullanchery
Section: 106; Desk: 141
Room: 110B; Date: 10/22/14
Formal Report - CHEM 111
Experiment 10
The Chemistry of Natural
Waters
Group Members:
2
Matthew Montella
Kathleen Mcquiston
Elliot Dambaugh
Introduction:
Pure water is colorless, odorless and tasteless and is often called the universal solvent1.
Water is a very good solvent and picks up impurities very quickly. When water flows
through the rocks and sand it collects and dissolves some of the minerals from them.
Calcium and Magnesium are the most commonly dissolved minerals in water that make it
hard. The degree of hardness is proportional to the amount of dissolved multivalent
cations in it. Hence hardness changes considerably with respect to geology.
Water is used daily for many functions in our lives like cleaning, bathing, drinking,
temperature cooling for industry, scientific experimentation and agriculture2. Because of
the many functions, its important to understand the properties of water as polluted water
or hard water can affect and hinder many functions in our lives. Hard water affects all
cleaning tasks like laundry, dishwashing and bathing. Hard water may also prevent the
removal of soil and bacteria during cleaning as the divalent ions of Ca2+ and Mg2+ in
water constantly form residue with detergents and in the process does not dissolve dirt.
What is water hardness? 3
Hardness indicates the amount of multivalent cations preset in a water sample. Most of
the hardness is due to the divalent ions of Ca2+ and Mg2+ in water. Other ions such as iron
(Fe2+) and manganese (Mn2+) can also cause water hardness and are present in lower
3
concentrations. Water with greater values of hardness is called hard water while water
with lower values of hardness are called soft water.
Why is it important?
Hard water can change the texture of fabrics, our hair, glassware and other metals. It can
also cause clogging of pipelines in industry. Therefore treating hard water is important as
it can cause considerable damage to us. Contrastingly, hardness is required for certain
other tasks like preventing corrosion of pipes in boilers. Also, due to the presence of
Ca2+ and Mg2+ ions the water toxicity for marine life is controlled. The Ca2+ and
Mg2+ prevent the fish from absorbing metals such as lead, arsenic, and cadmium into
their bloodstream through their gills4. Also the presence of calcium and magnesium in
ground water is important, as calcium is important for plant cells and magnesium is an
essential nutrient for the formation f chlorophyll.
How do softening techniques work?
Hard water can be purified using different methods for avoiding the above problems.
Water is treated with the help of softening units, which remove the calcium and
magnesium present in it. Most water softening techniques use the principle of “ion
exchange” in which the hard multivalent ions are replaced with sodium and chloride
ions5. These sodium and chloride ions are loosely bound to the cation-exchange resins
and are replaced with the divalent calcium and magnesium ions during the chemical
reaction. The products formed are left in the form of a soap scum when we use detergents
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and cleaning agents forming divalent cations residue and subsequently removing water
hardness.
How do we measure water hardness?
The two common methods of determining the hardness of water are the EDTA and AA.
We worked on them in our lab. One major difference in EDTA and AA is that EDTA is
much inexpensive compared to AA, which uses expensive devices. One other difference
we observe here is the EDTA titration takes significant amount of time while the AA
method is done in seconds and is more accurate then the EDTA method.
Summary of EDTA and AA, how they work, what is different about them, why use two
separate methods?
EDTA works when we have Ca2+ and Mg2+ in substantial quantity. In this method we
first take the natural water and alter its pH to 10 by using buffer. Then we react it with
black T indicator to observe the color of the solution changed to blue. This happens
because of the high pH value. Then NH3/NH4Cl/MgEDTA buffer is added to the
solution in drops slowly, after which we observe the solution to turn wine red first. This
is the result of the buffer reacting with the Mg2+ showing us the presence of Mg2+. Then
on more titration with the EDTA, we see EDTA react with Ca2+, which again turns the
solution from wine red to blue color. The two reactions are as follows,
First the color is blue because of the indicator.
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EDTA4− + Mg2+ →[Mg-EDTA]2− - formation of wine red color showing presence of
Mg2+
When excess EDTA is added, blue color is formed again as Mg2+ is removed from the
indicator as shown below.
[Mg-EDTA] 2− + Ca2+ → [Ca-EDTA] 2− + Mg2+
Here [Mg-EDTA] 2− reacts with Ca2+ to again turn back into blue color showing the
presence of Ca2+.
Now talking about AA, which is a more accurate method, we use an Atomic Absorption
Spectrophotometer. Based on if the experiment is for Ca2+ and Mg2+, we send a light with
a particular energy level required to dissociate the particular molecule under inspection.
We pass the particular monochromatic light through a flame of the sample waters aerosol
being burned. Next we need to observe the amount of the light absorbed that is directly
correlated to the concentration of element in inspection. As the light dissociates the
molecule and the molecule will absorb it. Then this light is passed through a slit, which
only allows the wavelength of the light related to the element in inspection. Here the
machine has Photo multiplier tube, which examines the light and finds out how much
Ca2+ and Mg2+ is present in it. This gives us the measures of how much of these elements
are present in the water sample being tested.
My hypothesis
By specific analysis of the bathrooms and kitchen sinks and tiles, I could see there is calcium build up
along the surfaces and edges. I believe this is evidence that the water in University Park,
6
PA, is hard water with different mineral contaminants6. Most of the water we use is
groundwater7 and groundwater is highly susceptible to picking up Calcium and
Magnesium ions which are the contributors to water hardness. Unconsolidated sediments
vary widely in composition and, consequently have a large range of associated
groundwater chemistries. Limestone and dolomite, common in many of the valleys in
central and southeastern Pennsylvania, are composed of minerals that easily dissolve in
ground water. The longer the ground water flows through the rocks, the more the more
the time it has to dissolve minerals. Probably the most common problem with this is
hardness, which results from high concentrations of calcium. I am led to believe that the
water hardness is going to be in the Moderately Hard range having3.5-7.0 grains per
galloon of calcium and magnesium.
Procedure:
The experiment was performed based on the instruction from PSU CHEMTREK. First
we tested the water using Atomic Absorption (AA) Spectroscopy method. I have used a
1:1 diluted solution of my water sample throughout the experiment. After diluting the
water I filled two pipets with the diluted sample one each for testing magnesium and
calcium using AA method in the stalk room. The spectroscope gave the absorption values
for both of them and the values were noted down. The absorption value of calcium was
0.2274 and magnesium was 0.1536 for my diluted sample. The actual values would be
double this value as the dilution was 1:1. From the absorption values the concentration of
each of the ions was calculated. As my water sample was clear it did not need any
filtration before analysis.
7
Next I had to find the total dissolved solids in the sample. One drop each of the diluted
sample, distilled water and (1.0*10^-3) M conc. of Ca2+ were put on an aluminum foil of
dimensions 1inch*2inch and placed on a hot plate till all the water evaporated. The
amount of white solid that was left is the amount of dissolved solids in each of the
solutions. The result was that distilled water had no precipitate, the calcium solution had
a faint white ring and diluted sample had a heavier white ring comparatively.
Next was the EDTA titration method to find the hardness of water. First known solutions
of calcium and magnesium were used to experimentally determine the concentrations
using titration method. For titration first all the wells of a 1*12 well tray were filled with
one drop of EBT (black T), which was dark brownish ash in color. Now one drop of
NH3NH4Cl buffer was added to well 2 and well 3 and the color changed to blue. Next one
drop of 10-3 M solution of Mg2+ was added to only well 3 and the color changed to
purple/wine red. We use these above color changes for all the titration processes (i.e) if
there is Mg2+ ions present in our samples the color will change to purple/wine red
otherwise with only calcium it will remain blue as calcium does not react with EBT.
EDTA titration process8: A known volume of the natural water sample is taken and the
pH is adjusted to 10(blue color) by means of an NH3NH4 buffer. EBT indicator is added
to the solution. At high pH the indicator is in HD2- form, which is blue in color. If Mg2+ is
present in the sample the color changes to wine red. As calcium does not react with the
indicator at the start of the titration the color is wine red. EDTA solution is now added.
8
First EDTA reacts with Ca2+ forming a colorless chelate. As soon as enough EDTA is
added it begins to react with the magnesium indicator chelate to produce a colorless
MgEDTA chelate. As the Mg2+ is removed from the indicator, the indicator returns to its
blue form. This change from wine red color to blue color is the end point of the reaction
where all the Ca2+ and Mg2+ reacted with EDTA.
Following this process the titration was done using known concentrations of Ca2+ and
Mg2+. Then the calculated values were compared to the known values to observe the
accuracy of the EDTA titration process. Following the same method the hardness of my
diluted sample was determined by calculating the concentrations of Ca2+ and Mg2+
present in my sample. Finally, a commercial softening agent was used to soften my water
sample and concentrations of Ca2+ and Mg2+ were again calculated to investigate the
softening of water. Then the water was treated by the use of the cation exchange resin
and the titration was repeated and the results were compared with the case where no
softening agent was used.
Results:
Calculation of the concentrations of Ca2+ and Mg2+ from the absorbance values obtained
from the spectroscope in the AA method was done. Atoms have discrete energy levels,
which are unique to a specific atom. In AA, monochromatic light with ΔE of the atoms of
interest is projected through the sample and the atoms of interest absorb the light. From
9
these absorption values the data is calculated and the calibration is done. The wavelength
for Ca2+ is 422.7 nm and for Mg2+ it is 202.5 nm.
Ca2+ Conc. (ppm)
Absorbant Value (at 422.7nm)
1
0.0095
5
0.04242
10
0.08633
25
0.20014
50
0.37143
Table 1: AA Standards for Ca2+ from AA Operator - Brown
Mg2+ Conc. (ppm)
Absorbant Value (at 202.5nm)
1
0.0093
5
0.06096
10
0.11254
25
0.26743
30
0.30684
Table 2: AA Standards for Mg2+ from AA Operator - Brown
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Graphs are plotted using the above tables. The absorption values are wavelengths in
nanometers and the concentrations are in ppm (parts per million). A trend line was added
to easily see the best fit and to obtain an equation aimed at conversions for later use.
Absorbant value (at
422.7nm)
Calcium Concentration
0.4
0.35
0.3
y = 0.0074x + 0.008
0.25
0.2
Linear (Absorbant
Value (at 422.7nm))
0.15
0.1
0.05
0
0
20
Ca2+
40
60
Concentration (ppm)
Graph 1: Absorbance values versus calcium concentrations
Absorbant value (at
202.5nm)
Magnesium Concentration
0.35
0.3
0.25
y = 0.0102x + 0.0061
0.2
0.15
Linear (Absorbant
Value (at 202.5nm))
0.1
0.05
0
0
10
Mg2+
20
30
40
Concentration (ppm)
Graph 2: Absorbance values versus magnesium concentrations
11
From the graphs above and the trend line equations we can calculate the concentrations of
Ca2+ and Mg2+ are calculated from the obtained absorbance values in the AA analysis for
my sample.
For Calcium:
y = 0.00074x + 0.008
y = 0.2274
x = 19.919 ppm Ca2+
For Magnesium:
y = 0.0102x + 0.0061
y = 0.1536
x = 14.461 ppm Mg2+
Conversion to ppm of CaCO3:
For Calcium:
19.919 ppm Ca
2+
100 g CaCO3
1 mole
∗
= 49.8 ppm CaCO3
40g Ca2 +
1mole
For Magnesium:
100 g CaCO3
1 mole
14.461ppm Ca2+ ∗
= 59.58 ppm CaCO3
24.3g Mg2 +
1mole
Calculation of Molarity in ppm:
1 ∗ 10−3 moles CaCO3 100 g CaCO3 1000 mg CaCO3
100 mg CaCO3
∗
∗
=
1 liter of solution
1 mole CaCO3
1 g CaCO3
1 liter of solution
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Using the above formula:
49.8 ppm CaCO3 = 0.498*10-3M
59.58 ppm CaCO3 = 0.596*10-3M
Total Hardness: = 0.498*10-3M + 0.596*10-3M = 1.094*10-3M
Using the above formula Hardness of the water sample in ppm = 109.4 ppm
EDTA Titration of known concentrations:
Adding only Mg2+ solution: The first well that turned blue was well 6.
Moles of EDTA = Moles of Ca2+ ions
MEDTA*VEDTA = MCa2+*VCa2+
2*10-4 M *6 drops = MCa2+*1 drop
MCa2+ = 1.2*10-3 M
After adding Mg2+ and Ca2+ solution: The first well that had color change was well 10.
MEDTA*VEDTA = MCa2+*VCa2+ + MMg2+*VMg2+
Moles of EDTA = 2*10-3 M
2*10-4 M*10 drops = 1.2*10-3 M *1 drop + MMg2+*1 drop
MMg2+ = 3.2*10-3 M
EDTA Titration of my water sample:
The first well that changed color was well 8 during titration.
MEDTA*VEDTA = Mwater sample*Vwater sample
2*10-4 M*8 drops= Mwater sample*1 drop
Mwater sample = 1.6*10-3 M
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Calculation of Molarity in ppm:
1 ∗ 10−3 moles CaCO3 100 g CaCO3 1000 mg CaCO3
100 mg CaCO3
∗
∗
=
1 liter of solution
1 mole CaCO3
1 g CaCO3
1 liter of solution
Using the above formula Hardness of the water sample in ppm = 160 ppm
Conversion to units of grains per gallon:
160 𝑝𝑝𝑚 ∗
1 grain per gallon
= 9.28 𝑔𝑟𝑎𝑖𝑛𝑠 𝑝𝑒𝑟 𝑔𝑎𝑙𝑙𝑜𝑛
17.1 ppm
EDTA Titration water using commercial softening agent:
The first well that changed color was well 8 during titration. There was not any
noticeable difference in the hardness of water.
MEDTA*VEDTA = Mwater sample*Vwater sample
2*10-4 M*8 drops= Mwater sample*1 drop
Mwater sample = 1.6*10-3 M
EDTA Titration water using resin:
All the wells turned to blue starting from the second well. This implies that the water
turned very soft.
MEDTA*VEDTA = Mwater sample*Vwater sample
2*10-4 M*2 drops= Mwater sample*1 drop
Mwater sample = 0.4*10-3 M
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The results of the water sampling are from the samples obtained by four people in our
team including me. The first sample results were mine from a residential apartment in W.
College Ave in State College. The second sample results were of Matthew from
Michigan State9. The third sample results were of Kathleen from southeast PA10. The
fourth sample results were of Elliot from Pittsburg11. The data gathered and the results
obtained are tabulated below.
Method
My
Matthew:
Kathleen:
Elliot:
Sample:
Michigan
Southeast
Pittsburg
PA sample
sample
No Ring
No Ring
W. College sample
Ave, State
College
TDS
Distilled Water
No Ring
Ca2+
White ring Faint ring
Faint ring
Faint ring
not as
heavy as
sample
Heavier
Similar to
Similar to
Similar to
ring than
Ca2+
Ca2+
Ca2+
reference
0.2274
0.2137
0.1912
0.1862
Water Sample
AA
Ca Absorbance
Mg Absorbance
EDTA
No Ring
0.1536
0.0954
0.1094
0.0758
Unsoftened #drops
8
7
6
8
Baking Soda softened
8
5
5
6
2
2
2
3
#drops
Resin Softened #drops
Sample
Dilution
Diluted for AA?
1:1 ratio
No
dilution
1:1 ratio
1:1 ratio
15
Diluted for EDTA?
1:1 ratio
No
1:1 ratio
1:1 ratio
dilution
Table 3: Results of all the team members and the reference data for AA analysis and
EDTA method
Discussion:
From the above experimental results obtained I can say that my proposition during the
sampling plan was true in saying that water samples are going to be moderately hard.
From AA analysis the result was 109.4ppm and from EDTA it is 160ppm. These values
fall in the ranges of moderately hard to hard as given by the table below. Therefore, the
value obtained from AA analysis fall into the expected range and the value obtained from
EDTA titration is very close to the proposed hypothesis value. I think that the value
obtained from AA analysis is more accurate as the change for errors is less and precision
is high comparatively. Comparing the absorbance values from AA analysis and the
number of drops from EDTA titration we can see that the hardness of the water samples
of my group members are also in the range of moderately high. But the estimation of the
hardness of the water samples to be in this order PSU<MI<SE PA<Pitt was found to be
wrong. In fact my water sample was found to have the highest level of hardness.
Classification
mg/l or ppm
grains/gal
Soft
0 – 17.1
0-1
Slightly hard
17.1 - 60
1 - 3.5
Moderately Hard
60 - 120
3.5 – 7.0
Hard
120 - 180
7.0 - 10.5
16
Very Hard
180 & over
10.5 & over
Table 4: Hardness Chart12
During AA analysis I found that the absorbance value of Ca2+ was 32.5% higher than the
absorbance value of Mg2+. This means that there are lesser calcium ions present in my
water sample compared to magnesium ions. This can be understood from the calculated
values of hardness from the results section above. The hardness of calcium is
49.8 ppm CaCO3 that is lesser than the hardness of magnesium, which is
59.58 ppm CaCO3. Even with the EDTA titration process we can see that the amount of
calcium present is lesser than the magnesium present. The concentration of calcium
present is calculated to be MCa2+ = 1.2*10-3 M which is less than the calculated
concentration of magnesium which is MMg2+ = 3.2*10-3 M.
Further the total hardness obtained from the AA analysis is 109.4 ppm that is less than
the value obtained through the EDTA titration, which is 160 ppm.
The percentage difference is
160− 109.4
160
∗ 100 = 31.62%
This is because of the possible errors during the experiment. One of the main reasons
would be the size of the drops from the pipet during the titration process. As the size of
the drop is small by itself even little difference in size between drop will make a
difference in the number of drop required and therefore the well number. And with every
increase in well number the value in ppm will increase by approximately 10-15% and this
will affect the accuracy of the calculations and hence can cause error.
17
Also, we are only considering the addition of one whole drop. This may cause
considerable error because, if I noticed the color change in well 8 but the actual change
would have occurred even if I added only 1/10th of the drop. This means I measured the
hardness of an extra 1/9th of a drop. This causes a lot of difference and cannot be
controlled and will create an error. Also, mistakes noticing the change in color will also
add to the error. For all these reasons I think that the AA analysis technique is more
precise than the EDTA titration technique.
Figure 1: AA Standards
However, AA analysis has its own defects. We can see that the repeatability of the
instrument is not a 100% (i.e.) the value obtained keeps changing time to time and
18
therefore an average value is used for calculations. We can see from the figure above that
the check standard values vary from the corresponding standard concentrations.
Therefore this can also cause a considerable error. Also, while calculating the graph and
the trend line it is important to check that the x-axis is the concentration and the y-axis is
the absorbance value. Reversing these two would give and absolutely wrong value hence
causing an error. As we are calculating the concentration (x value) from the absorbant
value (y value), we have to plugin the y value to get the x value. Performing this
incorrectly can also lead to errors. Other sources of error common to both the techniques
are using incorrect formulae, incorrect units and incorrect calculations. This can be
attributed to negligence and misperception. Other sources of error common to both the
techniques are using incorrect formulae, incorrect units and incorrect calculations. This
can be attributed to negligence and misperception.
Conclusion:
Study of hardness of water is important because water is used for various tasks in our
daily lives and it is useful to know the causes of water hardness, the water softening
methods, the destructive affects of water hardness that will affect our lives. Therefore two
different techniques to measure the water hardness were studied during this experiment.
AA analysis and EDTA titration was performed to calculate the hardness of the water
samples of our group. The hardness values for the different water samples were
calculated successfully. The classification of the water hardness was found to be very
close to the expected results from the hypothesis, but the order of the hardness varied
19
from the obtained results. The hardness values of my sample from AA analysis and
EDTA titration varied but were close with a percentage difference of 31.62%. The
difference was attributed to the comparative precision of the spectroscope and the
minimal possibility of errors. On the whole the results obtained were very close to the
desired results but not totally accurate because of the possible errors described above.
Hence it is now well understood that the hardness of water will change will geological
locations because of the possibility of differ amounts of minerals being dissolved into
water along mountains and rivers and the ground soil composition.
References
1. “What is in the water”
http://www.denniswater.org/Public_Documents/DennisWater_WebDocs/faqs
2. “Water” http://www.newworldencyclopedia.org/entry/water
3. “Hardness”
http://www.globalwaterwatch.org/GWW/GWWeng/GWWhardnessEng.aspx
4. Wurts, William A. “Understanding Water Hardness” World Aquaculture, 24(1):
18. http://www2.ca.uky.edu/wkrec/Hardness.htm
5. “Water Softening” http://en.wikipedia.org/wiki/Water_softening
6. “Drinking water Contaminants” http://water.epa.gov/drink/contaminants/
7. Fleeger, G.M., 1999, The geology of Pennsylvania’s ground water (3rd ed.):
Pennsylvania Geological Survey, 4th ser., Educational Series 3, 34 p.
20
http://www.dcnr.state.pa.us/cs/groups/public/documents/document/dcnr_014598.
pdf
8. PSU Chemtrek, 2013-2014. Thompson, Stephen. Ed. Joseph T. Keiser. Plymouth,
MI: Hayden McNeil, 2010. Print. Pages 10-1 to 10-22.
9. Montella, Matthew, Chem 111 Lab Notebook, pp. 42-48.
10. Mcquiston, Kathleen, Chem 111 Lab Notebook, pp. 18-21.
11. Dambaugh, Elliot, Chem 111 Lab Notebook, pp. 23-26.
12. NebGuide. Water Resource Management. 1996, Revised July 2006. University of
Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources
http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=175