Why is it important to know the quality and condition of water? Water is a necessity for
human life. Billions of liters of water are used daily and it is only natural to ensure its quality to
maintain health and prevent sickness. People don’t realize that water contains many different
substances that cause it to be “hard” and “soft”.
O) is not solely made up of hydrogen and oxygen. Most water contains many
trace elements, specifically divalent cations, which alter its purity. These trace elements
contribute to what we call the “hardness” of water. Water obtains elements and minerals such as
through different sources of solutes; gases, erosion of rocks, specifically
sedimentary rocks, rainwater containing sea particles, and man-made pollutants (12). Hardness
of water was originally determined by its ability to precipitate soap (1). Trace elements are
involved with precipitating soap, chiefly Mg
act as the prime precipitators. Mg
are the two main divalent cations that constitute the total hardness of water (1). As
quantities of Ca
increases, so does the hardness (12). Water hardness can range from
zero to hundreds of milligrams per meter (1).
In the lab “The Chemistry of Natural Waters”, tests were conducted to selected water
samples from State College, PA to calculate the concentrations of trace elements found in thw
water. The specific dissolved divalent cations we tested for were Ca
. To do this, we
used two methods; the use of Atomic Absorption Spectrophotometry (AA) and EDTA titrations.
EDTA titration is a form of determining the hardness of water. First a buffer must be
) to a natural water sample adjusting it to a pH of 10 (12). Eriochrome Black T,
an indicator known as EBT, is added to the water sample giving a red wine color (12). Before the
addition of the water sample, the indicator is blue. This color change from blue to wine red is due
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to the presence of Mg
(12). By adding a chelating substance (EDTA), Mg
removed from the solution (2). EDTA (the chelating agent) binds to Ca
first and once complete,
it binds to the Mg
ions (2). When both of these divalent cations bind with the EDTA, they are
removed, and the indicator turns back to its blue form (2). This form of removing water hardness
expels the permanent hardness ions (2). EDTA can be found in many cleaning agents such as
soaps and detergents. The chelating agent removes these cations resulting in softer water.
Although EDTA titration is popular to test for hardness because it is easily conducted, it is not
the most precise.
Atomic Absorption Spectrophotometry is another method used to test the hardness of
water (3). An Atomic Absorption Spectrophotometer (AA) is used to quantitatively test for the
presence of elements in water. To determine what element is present in a substance, a
monochromatic light is emitted from a specific hollow cathode lamp (in our case, a Mg
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This note was uploaded on 09/23/2008 for the course CHEM 111 taught by Professor Keiser,josephthoheckman,kimberlyproia,michaelantsanders,rebeccalo'neill,ryanshaw during the Fall '07 term at Penn State.