Figure 1
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Figure 1
INSTRUMENT , TECHNIQUE, SPECIFICATIONS
This device, one of many colorimeters, is reproduced with permission
from Chemistry with Computer ( published by Vernier
Software). It can be used instead of the commercially available
Spectronic 20 for many simple, colorimetric experiments.
The device uses fixed, single-color LEDs as the source of light. The
detector is a low-cost cadmium sulfide cell. The sample cuvette is a
simple test tube. The body of the device is made from a single block
of wood. Holes are drilled to accommodate the LEDs, detector, and
cuvette. The signal is monitored with either a voltmeter, a computer
interface, or CBL device. Samples are placed in the test tube, which
is inserted in a central hole in a block of wood drilled
perpendicularly to holes for the LED light source and cadmium sulfide
detector.
Red, green, yellow and blue LEDs may be interchanged to adjust the
desired wavelength for the experiment.
The device is powered by a 6 V DC source, either from the CBL or by
an external battery. The voltage measured on the cadmium sulfide cell
is directly proportional to the percentage transmission of light
through the cuvette and sample. The device is made from the following
from
All Electronics:
red LED (LED1), $0.15;
green LED (LED2), $0.20;
yellow LED (LED3), $0.20;
3 conductor cable, (3C/S22), 1 m, $0.42;
DIN inline, male (DIN-5), 0.75;
10 K potentiometer (LTP-10K), $0.50;
2 alligator clips (MTL-10), $0.50;
180 resistor (RES-180), $0.05.
from Jameco Electronics
Photocell, Part #120301, $1.39 ea
from Flinn Scientific:
5 test tubes (100 x 13 mm);
4 rubber stoppers (#5), $1.35.
from a hardware store:
block of wood, 4 x 4 x 5 in., $0.25.
from a photo developing store:
black film canister.
Power source: computer, CBL or external 5 V battery.
Estimated Cost of device: $6.26.
Distributors
All Electronics Corporation, PO Box 567, Van Nuys, CA 91480,
USA (tel: 800/826-5432).
Flinn Scientific, 131 Flinn Street, Batavia, IL 60510, USA (tel:
708/879-6900, 800/452-1261)
Jameco Electronic Components
1355 Shoreway Rd
Belmont, CA 94002
(415) 592-8097
Fax: (415) 592-2503
Order Line: (800) 831-4242
*Blocktronic plans have been around for years. This one is a
modification of plans provided by permission by Vernier Software in
Chemistry with Computers for Apple II and IBM
computers.
Fabrication
Figure 2. Photometer
1. Cut a piece of wood 4 x 4 in., at least 5 in. in length.
2. Drill two 1-in. holes to a depth of 1 in. exactly at the center of
opposite faces of the block. These will hold the LEDs and light
sensors when they are mounted in stoppers.
3. Drill a 3/8-in. hole through the center of the 1-in. holes
completely through the block. This will be the light path.
4. Find the center of the top of the block and drill a 5/8-in. hole
(for the test tube slot) to a depth of 3-3/8 in. This hole should
intersect the 3/8-in. hole for the light path.
Make sure the 100 x 13 mm test tube fits snugly, but not too tightly,
in this hole. The bottom of the test tube should rest well below the
light path.
5. To the left and right of the light path, drill two 1/8-in. holes
completely through the block to the other face. These will hold a 5 V
line and a ground line to the LED.
6. Call one face the 'front'. At a point 1 in. above the 1/8-in. hole
on the right, drill a 1-in. hole to a depth of 1/4 in. This will hold
the potentiometer.
Figure 3
7. To attach the LED or photocell, first reduce the size of the rubber stopper as shown in Figure 4.
8. Drill two 1/32-in. holes through the stopper, about 1/4 in.
apart.
9. Force the leads of the LED or photocell through the holes.
10. Mark each lead of the LED with its polarity. The longer lead is
the positive one.
11. Heat sink one of the leads on the photocell with an alligator
clip, since excessive heat can damage the photocell. Solder one end
of the 180 resistor to one lead of the photocell as shown in Figure 5
and Figure 7.
12. Insert the photocell in the hole in the front of the wooden
block.
13. Cut a miniature jumper cable in half.
Strip 1/4 in. of insulation off the end of the wire and thread the
end of the wire through the 1/8-in. hole in the back of the block so
that it comes out of the left hole (D) when facing at the front of
the block. The clip should be on the negative lead of the LED.
14. Wrap the wire around the other end of the resistor and solder
it on.
15. Strip 1/4 in. of insulation from the other half of the
miniature jumper cable. Push the end of the wire through from the
back to the front, in the other 1/8-in. hole.
16. Solder this wire to the center terminal of the potentiometer,
point (E), Figure 7. Solder a short stip of wire from the other lug
on the potentiometer to the free lead on the photocell.
17. Hot glue the potentiometer to its hole (F) as shown in Figure
5.
18. Cut a hole in the lid of a film canister, large enough for a test
tube to pass through it. Screw or hot glue the lid upside down, over
the test tube slot in the wood block as shown in Figure 8.
Procedure
1. Apply 5 V to the photocell side of the resistor, and
connect the center terminal of the potentiometer to the ground as in
Figure 9.
2. Measure the signal with a voltmeter from the other lead on the
photocell to the ground.
3. To connect to a Vernier Software computer interface or CBL unit,
construct an interface cable as shown in Figure 9 or use the voltage
leads supplied with the CBL unit.
Teacher's information
This device can be used for the following activities:
a. Beer/Lambert Law experiments
b. the determination of free chlorine in various water samples
c. mouthwash dye analysis
d. the determination of the iron content of vitamin supplements
e. colorimetric equilibrium experiments
f. light stick kinetics.
References
Holmquist, D. and Volz, D. (1991), Chemistry with
computers, Apple II/IBM , Portland: Vernier Software.
Holmquist, D. and Volz, D. (1995), Chemistry with CBL ,
Portland: Vernier Software.