Introduction to Colorimetric Analysis
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 semester is
the measurement of the concentration of chloride (chemical formula: Cl1-) in samples you’ll collect from
Lake Michigan, The North Branch of the Chicago River, and other places. 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!) How much silver chloride
forms depends on how much chloride there was in the original sample. So how cloudy the water gets
(or how much its’ color changes) also depends on how much chloride was in the original sample.
Another way of looking at it is 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, but 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, or how the light
passing through the sample has changed. 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). This way we can put a
number on (or quantify) the amount of chloride (or other dissolved solid) that was in our original
sample.