Illustrated Guide to Home Chemistry Experiments

142 DIY Science: Illustrated Guide to Home Chemistry Experiments

LABORATORY 7 .5:

dETERmINE CoNCENTRATIoN of A SoLUTIoN By vISUAL CoLoRImETRy

Colorimetry is the study of color. As it applies

to chemistry, colorimetry is used as a practical

application of the Beer-Lambert Law (describing

the relationship between chemical concentration

and light transmitted; see your chemistry

textbook) to determine the percentage of light

that is transmitted by a solution that contains a

colored solute, and from that to determine the

concentration of the solute.

RIREEqU d EqUIpmENT ANd SUppLIES

£ goggles, gloves, and protective clothing

£ balance and weighing papers

£ spatula or scoop

£ pipette, 1.00 mL

£ pipette, 5.00 mL

£ test tubes (6) and rack

£ copper sulfate pentahydrate (~5 g)

£ distilled or deionized water

SBSTITUTIU oNS ANd modIfICATIoNS

• If you do not have a 5.00 mL pipette, you may use a
  10.00 mL pipette.


• If you do not have a 5.00 mL pipette, you may use
  the 1.00 mL pipette repeatedly to transfer 5.00 mL of
  liquids, with some loss of accuracy.


• If you have no pipettes, you may substitute a 10 mL
  graduated cylinder for the pipettes with considerable
  loss in accuracy.

Nowadays, most colorimetry is done instrumentally, using one
of the following instruments:

Colorimeter
A relatively inexpensive instrument—entry-level
colorimeters are available for less than $1,000—that allows
the light transmission of a solution to be tested with white
light or with a limited number of discrete wavelengths.
Older colorimeters use an incandescent white light source,
which may be used as-is or may be filtered to allow testing
transmission with colored light. Typically, such colorimeters
are supplied with three filters: violet-blue, green-yellow, and
orange-red. Newer calorimeters often substitute colored
LEDs for the incandescent light source. Before use, a
colorimeter is calibrated by filling the sample tube, called
a cuvette, with distilled water and setting the colorimeter
to read 100% transmission with that cuvette in place. The
cuvette is then filled with the unknown solution and used
to determine the percentage of light transmitted at one
or several wavelengths. Colorimeters are widely used in
industrial processes, such as wine making, where high
accuracy and selectivity is not required.

Spectrophotometer
A spectrophotometer is a more sophisticated (and much
more expensive) version of a colorimeter. Instead of using
only white light or light at three or four discrete wavelengths,
a spectrophotometer simultaneously tests a sample at
many wavelengths. Inexpensive spectrophotometers may
sample every 10 nanometers (nm) or 20 nm over the visible
spectrum from 400 nm to 700 nm. Spectrophotometers
used in university and industrial chemistry labs typically
sample every nm (or better) over a range from 200 nm in the
ultraviolet to 800 to 1,100 nm in the infrared. If a colorimeter
is a meat-ax, a spectrophotometer is a scalpel. The many

data points determined by a spectrophotometer can be

plotted on a graph to yield a “fingerprint” of the transmission

characteristics of a particular solution over a wide range of

wavelengths. By comparing this fingerprint to the fingerprints

of known compounds, spectrophotometry can be used to

identify unknown compounds in solution, rather than just the

concentration of a known compound.

Despite the ubiquity of colorimeters and spectrophotometers

in modern laboratories, such instruments are not required to

obtain useful data by colorimetry. Colorimetry was used long

before such instruments became available, by employing one of

the most sensitive instruments available for determining color:

the Mark I human eyeball.

In this lab, we’ll use our eyes to determine the concentration of a

solution of copper sulfate by colorimetry.