Two Readily-Constructed Instruments for the Teaching Laboratory Neil S. lsaacs University o f Reading, Whiteknights, Reading, RG6 2AD, England cuvet
The perennial problem of providing reliable instrumentation in the teaching laboratory on a diminishing budget is seldom solved by the substitution of cheaper models since their performance may he inadequate for the experiments to be performed. We report here designs for a colorimeter (absorptiometer) and for a polarimeter both of which may be constructed readily with average workshop facilities and whose performance is superior to many commercial instruments commonly in use in teaching laboratories. Both use light emitting diodes as light sources of various wavelength and photoresistors as the active light receptors, these components being both inexpensive and rugged. Stability and reproducibility are ensured by the use of the double beam principle. The principles of each instrument may be readily understood by the students who use them.
LED
\
holder
reference
sample
null
meter
A Double Beam Colorlmeter, Figure 1
The basic layout is shown in Figure 2a and the electrical circuitry in Figure 2b. Light from two LED'S mounted in parallel is separately allowed to pass through cuvets containing sample and reference solutions and on to two photoresistors which lie in two arms of a Wheatstone bridge. The resistance of the photoresistors falls, on increasing the illumination, with a linear characteristic. Shunts (Rl) are added to limit the dark resistance. Tubular cuvets serve best. Indeed, ordinary 5-ml sample vials are ideal since thev tend to focus the lipht. Thevare held i n s mount of b r a s , wi~,d,,or )>la.;tia-(e.~.,'Vl~fnrl, n laminated cloth-phenul-furt~~ddeh~de resin) which i~rovidesfor thetuu light to be kept separate. We place the optical and electrical systems in separate compartments to minimize any complications from spillage during use. The ceU compartment
a
recorder terminals
b
Qs
Figure 2. (a)Basic Layout of Calorimeter (b) Electrical circuit
Figwe 1. The finished absorptiometer;dimensions. 200 X 140 X 140 mm. The LED panel is interchangeable. The water Umrmostattingsystem is optional. Fw symbols, referto caption to Figwe 2
Components FR,, PR2 PhOtwesistors ORP12(Radiospares):dark resistance 10 M n , at 50 lux 2.4 k n , at 1000 lux. 150a LigM-ining diodes. high elficiency HewienPadard 5082-4658 (red). LED -4558 (yellow), -4958 (green) RI 6.8 k n 10 k n single turn potentiometer R, RS 500 single turn potentiometer 3 k n single tum wire wound potentiometer graduated from 0 to 100 R4 across full range. M null meter, center zero, about 2 k 0 resistance, 100 @afull scaledefiection S toggle switch 2 k 0 single turn potentiometer (dimming resistor) RE
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F g ~ r e4 CaI Drat on 01 tne aDsOrptiometer .s ng a n c r e s~ tale soln.on Aoaorpt ometer tco ormelerl read ngr *ere maae a n g a cyllndr CaI cbvet AOSomance read ngs were ob1a.neo at 530 nm in a spectropholometer bs ng a 1 cm plane cuvet and are propartlonai to concentration. A, using red LED: 8, using yellow LED
Figure 3. Emission ~haracteristicsof three LED's; (a) 4958 (green). (b) 4558 (yellow). (c)4658 (red).
being sealed as shown in Figure 2a. LED's are available in three colors, red, yellow, and green, with wavelength emission characteristics shown in Figure 3. A pair of each is mounted on a separate Lucite" panel, wired in parallel and equipped with a microplug which enables a rapid interchange of wavelengths to be made. The Operation of the Colorimeter With cuvets filled with solvent only and the appropriate LED oanel selected and connected. the lamn and hridee switcies are turned on. The halancing resistor, k4,fitted wiih a graduated ring from 0 to 100, is turned to 0 and the meter is nulled using "coarse" and "fine" variable resistors R2 and RB,respectively. The instrument is now ready for use. The sample cuvet is filled successively with a series of standard solutions of the absorbing substance. With the cell compartment closed, each solution is nulled by changing Rq and recording the readine each time. A wlot of these readings against conce&ation ( ~ i4)i should be iinear and gives a c&hration aeainst which to measure concentrations of unknown solutions 07 the same material. The instrument is found to be stable with time and is reproducible from day to day and may be used with solutions whose absorbance is well above 2. Several modifications have been found beneficial in extending the scope of the instrument. Since blue LED's are not available, a lamw board fitted with two small tungsten bulbs may be uied, a blue filter in front of each. ~rovisionmay be made for a socket into which to connect a chart recorder. The progress of a reaction may then he displayed as the out-ofbalance voltage in the meter arm (Fig. 5 ) . We also constructed a hollow brass cuvet holder through which water could be circulated in order to vary the temperature of the sample. 608
Journal of Chemical Education
Figure 5. Progress of reaction curves: (a) oxidation of isopropanal by alkaline perrnanganate, ambient temperature, paint by point plot. (b,c) oxidation of methanol by acidic dichromate b, 28% c. 42°C (green LED).' These curves were obtained usina a 5 mv chart recorder. .Caulion: Chomium compounds are known to have carcinogenic praperlies: when this experiment is run, always avoid skin contact.
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Thermostatting between 15' and 40" (*lo) was achieved by pussing the water irum a r a i d I ) I I C ~ F Ithrmxh the *dmplt. cumpnrtmmt t u waste and thc.rt.hv t o ohtnin a11 ehtirndtc uf Arrhenius activation energies. The colorimeter may he made considerably smaller than the model described and may be adapted to take an external power source (a stabilized voltage between 10 and 20 V is required) which would he advantageous if several such instruments were to be used in close proximity. Suitable power supply circuits using zener diodes are shown in Figure 6. A Polarimeter, Figures 7 and 8
Polaroid is often used as the polarizing element in student polarimeters but the angular precision obtainable, even with a split-field arrangement, is poor because of the lack of sensitivity of the eye to the purplish color of crossed polaroids and to the broadness of the intensity minimum which is sought.
Flgure 6. Two circuits lor suitable power supplies: rectlfler, silloon. (R.S. Rec 70) rec T transformer. 15 v. 3 va output Zener diode (BZX 15) 1000 wF, 25 v C4 0.22 WF C5 0.47 pF 4.7 k n R1 soild state regulator chlp No. 7815 S
PO*
/I
Figure 8. Details of the palarimeter design.
Figure 9. Method of obtaining crossed positions of analyzer and polarizer, both with and withwt an optically active sample in poshion by taking rotation readings at points on the light transmission curves symmetrically related to the mictoammeter reading-rotation curve. Figure 7. (a) Polarimeter unit dimensions 20 X 15 X 10 cm. la) View of interiol layout with lid raised.
These difficulties may be overcome by working on a balance point away from the point of minimum light transmission and by using a double-beam system to indicate the chosen balance point. Figure 8 shows the schematic layout of the instrument which may he compared with Figures 7a and h. Light from a
pair of LED'S (yellow will best match the 549 nm standard usually used) is directed a t the two photoresistors in the Wheatstone network. On the sample side, the analyzer polaroid is mounted in front of PR1 hut can be rotated to any desired orientation. The polarizer is mounted in front of the LED, L1, in the center of a 60-tooth sprocket. This may be rotated from outside the polarimeter by rotating the turning Volume 60
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mechanism of a 10-turn Helipot on whose shaft is a worm gear which engages the 60-tooth sprocket. Ten turns of the Helipot dial passes 1000 divisions on the indicator and produces a rotation of 60° so that an angular precision of 0.06O is obtainable. The sample is placed between polarizer and analyzer in a container such as that shown, made from glass tubing with microscope cover glass end windows attached by epoxy cement. The Operation of the Polarimeter
The lid is closed with no sample in and with analyzer and polarizer at roughly crossed positions. The reference polaroids, which serve to attenuate the reference beam are also roughly set a t the crossed position. PQis then adjusted so that, with the lid closed, a fairly high reading on the microammeter is indicated. Now, on rotating the helipot mechanism a maximum reading will be observed which corresponds to the true crossed position. T o minimize the difficulty of determining the position of a broad maximum and to eliminate backlash in the turning mechanism the following procedure is recommended. Approach the maximum position from several degrees away such that the microammeter reading is low and take a reading of the helipot when it reaches some arbitrary value, say, between 25 and 50%full scale deflection. Continue to rotate the polarizer until the maximum is passed and the
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Journal of Chemical Education
microammeter again reads the same value. Read the helipot setting again and take the balance (crossed) position as the mean of the two readings, Figure 9. Do not alter the reference polaroids further or this procedure will need to be repeated. Now insert the sample and repeat, always turning the helipot in the same direction when making a reading to avoid backlash. The difference between the mean balance positions with and without the sample, is the rotation, a due to the sample. One full turn of the helipot corresponds to a rotation of (360In)" ( n = numher of teeth on the sprocket, e.g. 60). The polarimeter may be calibrated using a standard sucrose solution for which
where [a]is the specific rotation (+66.4' for sucrose at 589 nm) c = concentration (gi100 ml of solution)
1 = pathlength (in dm)
One may determine specific rotations with a precision of about 0.5O and demonstrate the chiral properties of sugars, natural amino acids, tartaric acid, and alkaloids. The mutarotation of glucose may be shown to occur, catalyzed by acid or by base in water.
