|
While you can see
with your eyes if water is cloudy or clear, most of the substances that
we are interested in testing water for are not visible to the naked eye.
So in order to test for the presence of dissolved solids in water, we
need to find a way “see” them. In this course, we will be using an
instrument known as a colorimeter not only to determine
the presence of specific dissolved solids, but also to quantify them, or
figure out their concentration.
Colorimetry is simply the measurement of color. One of the tests you’ll
be performing this week in lab is measuring the concentration of
chloride (chemical formula: Cl-1)
in the samples that you collected at the North Branch of the Chicago
River last week. Since the chloride dissolved in your samples is not
visible, we’ll need a way to make it visible. This is done using
chemistry. In general, a colorimetric procedure involves adding some
type of chemical (or “reagent”) to a sample of water. The reagent then
reacts with the substance we are trying to measure, causing a color
change in the sample. In the case of the chloride test, we add silver
nitrate to the sample. There is a chemical reaction that forms silver
chloride, which makes the water cloudy (the color change!) Since we not
only want to know if chloride is present in our samples, but also
how much
of it is present, the degree of color change due to the
chemical reaction must be related to the concentration of chloride in
the sample. The color change may be visible to your eyes, and we can use
the colorimeter to make a precise measurement of the degree of color
change.
A colorimeter passes light of a particular
“wavelength” through the sample. Using a detector, the colorimeter can
measure how much of the light has been absorbed by the sample. The
amount of light absorbed by the sample is related to the color change
caused by the chemical reaction. The amount of the absorption can then
be used to determine the concentration of the chemical of interest (in
this case, chloride).
For the tests we’ll be doing in this course, we’ll use an instrument
called a Smart2 Colorimeter. For each of the substances that we will be
measuring, a detailed procedure gives step by step instructions for
performing the test. In addition to this short introduction to
colorimetric analysis, you have also been assigned to read the
procedures for the analysis of chloride and sulfate (chemical formula:
SO42-)
for Friday’s class. While everything may not be clear at this point, by
reading the test procedures, you’ll have an idea of what to expect in
class this week. If you are curious about some other things, you can
access the operator’s manual for the Smart2 colorimeters at
http://www.lamotte.com/pages/common/pdf/instruct/1919manu.pdf
So
besides this short introduction to colorimetric analysis, don’t forget
to read the two test procedures also posted on the website: Chloride
test procedure and Sulfate test procedure. |
