Chemistry Of Natural Water

The purpose of this experiment is to explore the
hardness of the water on campus. Hard water has
been a problem for hundreds of years. One of the
earliest references to the hardness or softness of
water is in Hippocrates discourse on water quality
in Fifth century B.C. Hard water causes many
problems in both in the household and in the
industrial world. One of the largest problems with
hard water is that it tends to leave a residue when
it evaporates. Aside from being aesthetically
unpleasing to look at, the build up of hard water
residue can result in the clogging of valves, drains
and piping. This build up is merely the
accumulation of the minerals dissolved in natural
water and is commonly called scale. Other than
clogging plumbing, the build up of scale poses a
large problem in the industrial world. Many things
that are heated are often cooled by water running
thru piping. The build up of scale in these pipes
can greatly reduce the amount of heat the cooling
unit can draw away from the source it is trying to
heat. This poses a potentially dangerous situation.

The build up of excess heat can do a lot of
damage; boilers can explode, containers can melt
etc. On the flip side of the coin, a build up of scale
on an object being heated, a kettle for example,
can greatly reduce the heat efficiency of the kettle.

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Because of this, it takes much more energy to heat
the kettle to the necessary temperature. In the
industrial world, this could amount to large sums of
money being thrown into wasted heat. In addition
to clogging plumbing and reducing heating
efficiency, the build up of hard water also
adversely affects the efficiency of many soaps and
cleansers. The reason for this is because hard
water contains many divalent or sometimes even
polyvalent ions. These ions react with the soap
and although they do not form precipitates, they
prevent the soap from doing it’s job. When the
polyvalent ions react with the soap, they form an
insoluble soap scum. This is once again quite
unpleasing to look at and stains many surfaces.

The sole reason for all these problems arising from
hard water is because hard water tends to have
higher than normal concentrations of these
minerals, and hence it leaves a considerable
amount more residue than normal water. The
concentration of these minerals is what is known
as the water’s Total Dissolved Solids or TDS for
short. This is merely a way of expressing how
many particles are dissolved in water. The TDS
vary from waters of different sources, however
they are present in at least some quantity in all
water, unless it has been passed through a special
distillation filter. The relative TDS is easily
measured by placing two drops of water, one
distilled and one experimental on a hotplate and
evaporating the two drops. You will notice that the
experimental drop will leave a white residue. This
can be compared to samples from other sources,
and can be used as a crude way of measuring the
relative TDS of water from a specific area. The
more residue that is left behind, the more dissolved
solids were present in that particular sample of
water. The residue that is left, is in fact, the solids
that were in the water. Another, perhaps more
quantitative way of determining hardness of water
is by calculating the actual concentrations of
divalent ions held in solution. This can be done one
of two ways. One is by serially titrating the water
with increasing concentrations of indicator for
Mg++ and Ca++ (we will be using EDTA). This
will tell us the approximate concentration of all
divalent ions. This method of serial titrations is
accurate to within 10 parts per million (ppm) .

Another possible method for determining the
hardness of water is by using Atomic Absorption
Spectrophotometry or AA for short. AA is a
method of determining the concentrations of
individual metallic ions dissolved in the water. This
is accomplished by sending small amounts of
energy thru the water sample. This causes the
electrons to assume excited states. When the
electrons drop back to their ground states, they
release a photon of energy. This photon is
measured by a machine and matched up to the
corresponding element with the same E as was
released. This is in turn is related to the intensity of
the light emitted and the amount of light absorbed
and based on these calculations, a concentration
value is assigned. A quick overview of how the
atomic absorption spectrophotometer works
follows. First, the water sample is sucked up.

Then the water sample is atomized into a fine
aerosol mist. This is in turn sprayed into an
extremely high intensity flame of 2300 C which is
attained by burning a precise mix of air and
acetylene. This mixture burns hot enough to
atomize everything in the solution, solvent and
solute alike. A light is emitted from a hollow
cathode lamp. The light is then absorbed by the
atoms and an absorption spectrum is obtained.

This is matched with cataloged known values to
attain a reading on concentration. Because there
are so many problems with hard water, we
decided that perhaps the water on Penn State’s
campus should be examined. My partners and I
decided to test levels of divalent ions (specifically
Mg++ and Ca++ ) in successive floors of
dormitories. We hypothesized that the upper level
dormitories would have lower concentrations of
these divalent ions because seeing as how they are
both heavy metals, they would tend to settle out of
solution. The Ca++ should settle out first seeing
how it is heavier than the Mg++, but they will both
decrease in concentration as they climb to higher
floors in the dormitories. PROCEDURE We
collected samples from around Hamilton Halls,
West halls. In order to be systematic, we collected
samples in the morning from the water fountains
near the south end of the halls. We collected water
samples from each floor in order for comparison.

The reason we collected them in the morning was
so that the Mg++ and Ca++ would be in
noticeable quantities. We then went about and
tested and analyzed via serial titrations and via
Atomic Absorption Spectrophotometry. We also
obtained a TDS sample merely for the sake of
comparison, and to ensure that were in fact
dissolved solids in our water samples (without
which this lab would become moot). For the serial
titration, we merely mixed the water sample with
EBT, and then with increasing concentrations of
EDTA. The EBT served as an indicator to tell us
when the concentrations of the EDTA and the
divalent ions in solution were equal (actually it told
us when Mg++ was taken out of solution but that
served the same purpose). This allowed us to find
the concentration of the divalent ions dissolved in
solution. Based on this, we calculated the parts
per million and the grains per gallon for each water
sample. Finally, we took an AA reading for each
sample. This gave us absorption values and
concentration values for each of the two main
metals we were observing; Ca++ and Mg++. We
then plotted a graph of Atomic Absorption
Standards. These were values given to us by the
AA operator. These values helped us to calibrate
the machine. The parts per million that we find will
be based on plugging in the reported absorption
value into the resulting curve from the graph of
these values. The resulting concentration was used
as the final value for the hardness for that
particular sample. All calculations and conclusions
were done based on these final values obtained for
the concentration of Ca++ and Mg++.