overhead projector dernon~tra tion~ Teaching Photometry with Overhead Projector Experiments lvano G. R. Gutz, Luclo Angnes, Jalro J. Pedrottl institute de Quimica Universidade de S o Paulo P.O. Box 20780 01496-SSo Pauio, Brazil
Although a number of simple colorimetric photometers have been described in the literature (1-3) or can be obtained commercially, they are well suited only for use by small groups of students in the laboratory. In this article a filter photometer adequate for classroom demonstration is described. An overhead projector serves as the light source of the photometer. The light beam passes through afilter and a transparent cell containing the sample (or blank). Of the remaining light, part falls upon the photodetector positioned over the cell, and the rest is projected on the screen. The measured light intensity signal is simultaneously projected with help of a modified LCD digital multimeter. The selection of the best filter (wavelength) for a colored sample, the absorption of light of complementary color, or the relationship between concentration, path length, and light absorption (Beer-Lambert law and deviations) can be quantitatively demonstrated. Photometric titrations can be easily performed.
edited by DORIS KOLB Bradley University Peoria. iL 61625
an obsolete photometer), insertion of a 100-ohm potentiometer as a load and voltaee divider allows the measured ~ o t e n tial to be adjusted. Taking into consideration the increasing popularity of photodiodes as photo-transducers, especially in diode array spectrophotometers, they can be used optionally in the experiments. Good performance resulted by connecting a photodiode t o a 351 FET Operational Amplifier wired as current to voltage converter. A l-Mohm potentiometer in the feedhack permits the adjustment of the output voltage. Two 9-V batteries serve to power the Op Amp. Analogue voltmeters modified for overhead projection, like those previously described in this Journal (5,6),can be used to make the measurements. Better precision is provid-
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The experimental arrangement is shown in Figure 1.As a cell or cuvette, a flat, transparent Plexiglas box, about 10 X 8 X 3 cm (without the top) was used. T o improve the visualizationof the experiment on the screenand avoid spurious light on the photodetector, the projector stage is covered with a mask. The mask is made with asheet of white paper glued to a black one. The white face is turned down to the light source. The hole in the center of the mask is slightly smaller than the cell. This window can be narrowed with blacklwhite paper strips, when necessary. Near one of the corners of the mask, another window is cut out the size of the display of the digital multimeter. A set of a t least three optical filters (red, green, and blue) should be available. Colored cellophane films, suggested as filters in nonquantitative overhead experiments (4) are less satisfactory hecause they usually present a broader bandpass. An interference filter, with narrow bandpass, allows the realization of an additional experiment. In our case three optical filters, each one about 10 X 2 em, have been removed from an old EEL photometer. A violet (445-nm) interference filter of about 2.5 cm diameter was obtained from an obsolete amino acid analyzer. As photodetectors, photovoltaic or photoresistive devices can be used. Cadmium-sulfide photocells (Archer Cat. No. 276-1657), for example, can be adapted to one end of a black tube that prevents entrance of spurious ambient light. The tube is aligned above the cell and held in a fixed position with a clamp. A pair of wires is soldered to the terminals of the photoresistor and connected to the resistance measurement input of the multimeter. For photovoltaic devices (like a solar cell removed from a calculator or a selenium cell from
Figure 1. Arrangement of the overhead photoelectric coiorimeter: a, mask; b. cuvene; c, photodetector will?filter: d, potentiometer: e, multimeter: 1, LCD; g. screen.
Volume 68
Number 4
April 1991
325
Figure 2. Printed circun bard and flat cable wire connections for me external
Wavslsnpth Inm Figure 4. Spectra of the optical fllten used in the experiments: a, blue-violet filter:b. violet interferencefilter: c, green filter: d, red fllter.
directly to the points previously contacted by the display. At the other extreme, the wires were soldered to the printed circuit board. as shown in Fieure 2. The con& between t h i s board and the liquid crystal device was established through the two originally used strips of flexible transversely conducting material, maintained under some pressure with helu of an aluminum frame screwed on the hosrd, according to Figure 3. The assembled LCD is placed on the overhead, as previously indicated in Figure 1. The stability of the measurements depends on the maintenance of the positioning of the cell and photodetector during the experiments. he voltage fluctuation of the main power also changes the light intensity, but usually no stabilization is required.
Flgure 3. Lateral view of the assembled external LCD: a, top aluminum plate; b and h, screws: c and g, connection material: d, LCD: e, mask; f, frame: I, printed circult board. ed by a digital voltmeter that can be assembled with a transmission liquid crystal display (7). Projection of precise digital readout of voltage, resistance, and current measurement functions in various scales, convenient for different light detectors, was obtained here by modification of an inexpensive MIC-2200 digital multimeter with 3%digits. First, the instrument was disassembled and the liquid crystal display (LCD) removed. The mirrored tape glued on the back-plane of the LCD was carefully peeled off and substituted by a transparent film (cut out from an overhead olastic sheet). T h e next step ionsisted of making or buying a printed circuit board and cuttins a "window" in it to accommodate the display. Two strips of 0.5-m ribbon cable, one with 15 and another with 14 wires, were used for the electrical interconnecrion between the &ultimeter and the new board for the display. At the instrument side the wires were soldered 328
Journal of Chemical Education
Experiments 1. The described arrangement, without the photodetector and the multimeter, can be used first for some qualitative experiments and explanations about the subject. Depending on the level of the students, the class can begin with the projection of the colored filters together with their respective spectra, to illustrate the bandwidth and peak position. If available, an interference filter can be displayed too. Although the only difference seems to be the lower light intensity, the spectrum demonstrates the narrower bandwidth. Transmittance versus wavelength spectra of the filters can be previously obtained by hand or recorded with a spectrophotometer. A Beckman DU-70 was used for the generation of Figure 4. 2. A whole visible spectrum can be displayed by placing an interference filter wedge (8) (a Veril from Baush and Lomb is satisfactory) on an adequately sized mask at the projection plane of the overhead.Alternatively, the spectrum obtained with a transmission diffraction grating can he projected. However, the grating must be held between the head of the .oroiector . and the screen (not on the mark). In hoth rases,a low light level in the room is necessary. 3. Next, it would he interestinp, to demonstrate that our eyes interpret the mixture of red, blueviolet, and green primary colors as white. The filters are placed on the mask and after observation of the screen, a magnifying glass (large diameter), is held between the head of the projector and the screen, closer to the head. A suitable position is easily found in which the three colored light beams are focused on the same area of the screen re sen tine a nearlv white color. The filters can be covered one by &e with a'blark paber and the additive colors yellow, cyan, and magenta are obtained. 1. The relation between the color of ,olutionr and light abwrption of the complementary color can be illustrated by putting the large cell with a colored sample over the filters but with some area of
aliquots of the colored solution is done with a dropper or a mieropipet. The solution is stirred, and the measurements are made after the liquid stays quiet. I t is importantto explain t o the students that with this arrangement the increase of the solution volume in the cell has no effect because i t is compensated by the increase in the optical nathwav. 6. For the simplest arrangement i n d u i n g the photoresistor connected to rhr multimeter, usually, in the 21)-Kohm range, the obtained resistanre readings, R I ,can he convened into t r a n a n i t t a n r ~ by ralculating Rs.,.IRI, where R.,,,,.I, corresponds ro the resistanrr mennured for the hlank. usuallv watrr (Frcorres~ondstd IOIitimes the calculated value). The resistance in the ahsenee of light is usualIv . hieher .. than 20 Mohm. and anv. correction of the "dark current" can he disregarded. Graphs i,ftransmittance and ahrorharce vrrsua concentration (18r numher of drops added, can be pluttcd on the hlsckhoard or transparent graph paper and p~ojected.Linrar rali. hration rurves h a w heen
