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.
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