The Determination of the Concentrations of Sugar Solutions by Laser Refractometry Elvin Hughes, Jr., and Vaughn Jelks Southeastern Louisiana University, Hammond, LA 70402 Diane Landry Hughes Loranger Hlgh School, Loranger, LA 70446 The laser has provided the physical scientist new techniaues for studvina the fundamental pro~ertiesof matter. A review of current research activities was-recently published in this Journal.' Many of these techniques are now being used in research laboratories. A discussion of chemical refractometry is available in the text by Willard, Merritt, Dean, and Settle.2 The authors of this text provide fundamental mathematical equations and vocabulary necessary for measurine the refractive index of various substances. of this article is to describe a simple, semiThe auantitative. analvtical method for determinine the amrox(mate concentrations of sucrose solutions. he-intendof the authors is to ~ r o v i d ean experiment that utilizes readily available equipment and a simple system and demonstrates some of the steps involved in developing an analytical method. This experiment can be easily demonstrated to students in a classroom. A variety of other solutions, for example ethanollwater. could he used as alternatives. Traditionallv. most small, lbw-powered lasers have been underutilized: This exoeriment. we hooe. . . will orovide another a~olication for small lasers.
Ficlure 1. Jiq for holding microscope sildes,
p3y-Y
1.25 X 1.25 X 5
in. Epoxyata, b,
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chalkboard 1 2'
LASER Experimental The equipment needed to perform laser refractometryconsists of the following items: a low-powered HeINe laser, a hollow prism, and several standardized solutions. (Caution: Follow the safetv reeommendations ~rovidedbv the manufacturer of the laser. and avoid loukingdirectlg at the laser beam.! We made a mall prism hy using epoxy rocrmrnt fhrrr mirrosropeslidrs logrther in thr shape oian equilateral rriangle. A jig, ahuun in Figure I . nna used t*, assure thnr the slides remained in the proper configuration during the euringof the cement. The prism was then cemented to a glass plate that served as the base of the prism. An alternative to the prism is presented in another section of this article. Place black cardboard baffles at points a and b to absorb the reflected laser beams. Align the laser and prism as shown inFigure 2, and fill the prism with water. Mark the location where the refracted beam strikes the chalkboard.Remove the water from the prism with the aid of a syringe or a pipet. Fill the prism with a 10% (wlw) solution of sucrose in water, and mark the location where the heam strikes the chalkboard. Repeat this procedure with 20%,30% 40% standardized solutions. The refracted beam will strike the chalkboard at a different point with each different solution. Figure 3 shows the variation in the degree of refraction with the concentration of the solution in the prism. Use a meter stick, and measure the distance between the ooint at which the refracted beam strikes the tmard and the point at which it strikes the h a r d when 11 travclx throughonly water.Thisdiutance uill be referred tuas the displacement from water.
Figure 2. Alignment of laser and prism.
chalkboard +
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Verdleck. Jim (symposium organizer). J. Chem. Educ. 1982, 59, 440. Wlllard, H. W.; Merritt. L. L., Jr.: Dean, J. A,: Settle, F. A. Insfr~mental Methods OfAnalysis. 6th ed.; Wadsworth: Belmont. CA. 1981; pp 403-412.
Figure 3. Refraction of laser beam by various sucrose solutions Volume 65
Number 11
November 1988
1007
Dlsplacement and Concentration Data % Sucrose, by Weight
Displacement from Water (cm)
0.0 10.0 20.0 30.0 40.0
0 11.9 26.5 45.9 71.6
Figure 5. Erlenmeyer flask as alternative to hollow prism.
obtains will depend upon the geometry of the apparatus.) Two sucrose-water unknowns were prepared that contained 25.0% and 35.0% sucrose bv weight. The dis~laeementsof the refracted beams were measured,-and from the calibration curves the concentrations of the unknownswere determined to be 24.9%and 35.090,respectively.
Alternative To Use of Hollow Prlsm
% sucrose, by weight
To a first approximation, a small (-50-mL) Erlenmeyer flask adeauatelv annroaches the shave of a nrism. In this case. the beam w r ~ i ~ a l and l y onr siould use a meterstick in a will be re&&l vertical r~mfigurationto mensure the displacemenr of thr rrfractrd twam. Figure 5 illustrates the use of an Erlenmeyer flaskasa snmple container.
Figure 4. Dlsplacement vs. sucrose concentration, The table lists the displacement of the refracted ray relative to the water as a function of concentration. All measurements were made at 24 OC. Analysls of Unknowns Figure 4 shows a plot of displacement of the refracted ray versus percent sucrose. (The actual numbers that another experimentalist
1008
Journal of Chemical Education
Dlscusslon This laser refractometry technique can be used a t all levels of chemistry instruction. T h e authors have used it in high school laboratories, freshman college chemistry, and in instrumental analvsis. Manv institutions have small. lowpowered HeINe iasers for demonstration purposes. his experiment could easily be performed in those environments.
