Gordon R. Coe, Ralph D. Conlon,' and Raymond E. Dessy
University of Cincinnati Cincinnati, Ohio
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Construction of
Simple Refractometer
A chemical instrumentation experiment
The construction of a simple refwctometer, as here described, can be used as another example of an experiment to teach basic concepts of instrumentation.* The total cost, abont 860, should not be prohibitive. Even budget-squeezed smaller institutions can justify this against the acquisition of an instrument of wide utility. We estimate that two students could build one instrument in abont 12 laboratory hours. A 10-cm piece of 8-mm diameter Pyrex rod is bent into approximately a 90" angle, and then the outside of the angle is polished to a f l a t . V h i s flat is then sealed into a standard 6-in. Pyrex test tube about 4 cm from the bottom so that the flat will be in contact with liquids placed in the test tube. One potential limitation to this refractometer is that a fairly large sample is needed (roughly 10 ml), but this should not he a major restriction since the sample is not altered in any manner. An alternative is the substitution of a small volume ultraviolet cell for the test tube, but this is more expensive. Actually, any small Pyrex vesselis satisfactory since the size of the cell is only critical in the sense that a larger volume cell requires more sample. Since a variety of solvents will be used as samples, the seal should be made of glass rather than chemical cements. To one arm of the Pyrex rod an incandescent light is sealed with a clear cement; to the second arm is sealed a CdSe detector cell. The detector is a photocell (Clairex Crystal Photocell CL-604L) that is extremely sensitive in the red and near infrared region of the spectrum, and is available from the Clairex Corporation, 8 West 30th St., New York 1, N. Y., or through most electronic supply houses. The entire apparatus ((Fig. 1) is then painted black to isolate the system from the surrounding light; the test tube is stoppered. The light from the bulb passes down the Pyrex rod to the ground flat which is now the interface between two media (the glass rod and the sample solution). As long as the refractive index of the rod differs from that of the sample, a portion of the light rays will be reflected back to the CdSe detector. The phenomenon may be better understood by considering Figure 2 where glass and a second medium are shown with a normal constructed a t the interface. A single ray making an angle i with the normal passes through the glass and meets the interface, passes through, and emerges in the less dense medium making a new angle r with the normal. A portion of this ray is also reflected a t the same angle i. In the 'Present addreas: Proeter and Gamble, Ivorydde Technical Center, Cincinnati, Ohio. THIS JOURNAL, 39,147,338,611 (1962); 40,73 (1963). =Thisservice can be rendered bv comoanv or eve" anv o~tical " glass rnanufmturer.
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case where the medium is air, the ratio sin r/sin i represents the index of refraction of the glass as de6ned by Snell's law. No consider the situation where the angle i is increased toward 90". As we increase the angle i, the angle r also increases until it becomes equal to 90" a t which point light no longer passes into the medium. The ray that makes an angle i' with the normal so that r equals 90" is defined as the critical ray, and the angle i' is the critical angle. When the angle i is increased past this critical angle, all the light is reflected from the surface back into the first medium. This total reflection occurs only when the light passes from the denser to the rarer medium. We return to our case and see that since the angle i is contant as well as the refractive index of the glass, the refractive index of the medium affects the value of r, and in addition, the amount of light that is reflected back into the glass rod and onto the CdSe detector. This relationship between the refractive indexes forming the interface and the magnitude of reflected light is Fresnel's principle.'
Figure 1. Pyrex rod prism sealed into the sample cell.
Figure 2. Roy diagram showing interface. Roy travels from denre to less dense medium.
The detector is simply a photo cell in which a resistance change is produced that is proportional to the amount of incident light. This resistance is measured by balancing in a Wheatstone bridge, using a battery and a galvanometer. The schematic diagram is shown in Figure 3. The attenuator is actually a decade resistor for values of resistance between 1 ohm and 10 K ohms, and is used as a sensitivity control for the instrument. The instrument may be standardized by measuring the resistance ~roducedbv s a m ~ l eof s known refractive index and constructing a standard curve of resistance
' For an excellent presentation of Fresnel's principle see, for exM., JR.,LEMON, H. B.,AND STEPHENSON, R. J., ample, FERENCE, "Analytical Experimental Physics," 2nd rev. ed., University of Chicago Press, Chicago, Illinois, 1956, p. 510-38. Volume 41, Number 6 , June 1964
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versus refractive index. A typical curve constructed by using mixtures of methanol and benzene is shown as Figure 4. A characteristic of this instrument is that it can be used with high precision only when the refractive index of the Pyrex rod (1.517) is considerably greater (0.04) refractive index units) than that of the sample medium. Photocell Golwno-
Attenuotor
IntolOKn
Decade
Constont Voltqe Transformer Incondescent r.
Lamp
Figure 3. Schematic diogrom of resistance bridge circuit and incmderced lamp power supply.
At low values of sample refractive index the accuracy of the instrument is quite good (=k0.0005 refractive index units) but the precision faUs off a t sample refractive index values above 1.47. The useful range of this instrument is therefore from 1.00 to 1.47. This limitation is prohibitive to the use of some compounds such as the aromatic hydrocarbons but still wiU allow the determination of a great many compounds. Glass rod of higher refractive index (up t o 1.7) which can be used with the high refractive index liquids is available from Corning Glass Works. The light source is simply a small incandescent lamp (6 v, 0.20 amp) powered by a constant voltage transformer. I n order to have a reasonable precision in the photocurrent, I, produced by the CdSe photocell, the voltage, V , applied across the terminals of the lamp must be regulated quite rigidly as may be seen by the relationship: I = KV" where n has a value between 3 and 4 for an incandescent lamp.6 After the line voltage passes through the transformer i t is then sent through a variable resistor t o load the constant voltage transformer t o its rated output to insure optimum regulation, and to adjust the bulb to the most sensitive response of the photocell. The latter condition is met when the lamp is operated below rated voltage. Such operation greatly extends the useful life of the lamp. The lamp intensity is adjusted so that the resistance of the photocell is about 3-5 K ohms with water in the sample cell.
15v+ 10 turn Pot
Figure 5.
1.32 250
350 450 550 650 Resistonce of Photocell (Ohms)
750
Figure 4. tolibration curve for refroctometer showing photocell resistonce as a function of romple refractive index.
338 / Journal o f Chemical Education
Cell
I i Schematic diagram of voltage bridge circuit.
As an additional or alternative experiment, the resistant bridge may be replaced by a voltage bridge and the CdSe photocell by a barrier-layer cell. The schematic for the voltage bridge is shown as Figure 5. This additional exercise would give the student experience with both photoconductive and photovoltaic cells. V I L ~ DH., R.,MERRI~T, L. L.,JR.,AND DEAN,J. A,, ''Instrumental Methods of Analy~k,"D. Van Nostrand Company, Ino., Princeton, New Jersey, 3rd ed., 1958, p. 31-2.
