edited ~- - - bv -
JAMES P. B I R ~ Arizona State University
computer series, 166
This may provide new life for instruments that are inoperable except for the source and optical bench. This approach simplifies a n earlier published method of interfacing to the Spedronic 20 (7).
LlMSport (VI): Optimizing Computer-Driven Photometry with the Spectronic 20 and Pipetronic I Ed Vitz Kutztown University Kutztown. PA 19530
Experimental
Construction of Pipetronic Cell Compartment
We have designed a computer-interfaced photometer t h a t is much more easily constructed t h a n the "Blocktronic."' costs $10-$15. and is canable of absorbance measu r e m e n t s for t h r e e wavelength ranges. With o u r "GAMEREAD"oromam r 1 I the measurements are not limited to the eight-b; precision of the IBM gameport BIOS. The associated software revorts the absorbance readinnautomatically in a spreadsheet cell a s described in previous L l M S ~ o r tarticles (1-5). The measurement will be printed in the color of the incident light chosen by the user in spreadsheet versions t h a t allow i t (e.g., Lotus 1-2-3 V2.3+); that is, if the source is the red LED with emitted light centered around 636 nm, the sample absorbance value will be printed in red on the spreadsheet. While a recent article (6)has demonstrated that the Blocktronic is more than satisfactory for most photometry in educational laboratories, the Pipetronic offers several improvements. In addition. the detector assemblv can be olaced dircetlv in the filter holder of a spectroniE2 20 so i h a t absorbance measurements can be recorded with the same software.
'
Temps, AZ 85287.1604
The Piuetmnic is constructed from standard 314in. W C pipe fitti&, available in most hardware stores, a s shown in Figure 1.A"TeeZserves as the cell compartment, and a 1/2-in. x 314-in. female-tc-female adapter is connected to the vertical branch of the'%en, to serve as a holder for the 1/2-in. cuvettes or 13x 100 m m test tubes. Some brands of test tubes fit without modification. while some reauire that the a d a ~ t o rbe sanded slightly enlarge the i n t e k d diameter. A lenpth of 314-in. PVC o i ~ is e cut to conned the '%e" and a d a ~ t o so r that the test2;bes protrude 118 in. from the adaptor for easy removal. For best ~erformance,a conical recess is drilled in the interior bottom of the Tee" to center the c u ~ e t t eThis . ~ is done with a If-in. drill inserted through the adaptor while it is mounted to the "Tee", taking care not to drill entirely through the bottom of the "Tee". W C 314-in. end caps are used to cover the cell compartment. A~hotocell~ is mounted in a #1 or #2 one-hole rubber stonper,hhile one rrdL?.een5 and one h l u e q E D are mounted in a U1 or #2 two-hole rubber stopper. Threceolor LED's ant now
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Project SERAPHIM. Department of 314" W C Cap Chemistry, University of Wisconsin-Madison, 1101 Universitv Ave.. Madison. WI 53706. I Reoistered trademark ofthe manufacturer, ~ i l o nRoy Company, 820 Linden Ave., Rochester, NY 1462. This was suggested by Roger Bacon (western Carolina Univenity)during the recent 314" F x 11.2" Adapter NSFKutztown University LlMSport Workshop. Photocells are available from Radio Shack or Circuit Specialists, P.O. Box 3047, 314" x 1-314" W C Pipe Scottsdale. AZ 85271-3047 (Item #J4-805. CdS eal. T h i oriainal Block: .. .~hotcidl. - ~ - - -, 51.40 ~-~ Ironic used photocells siniar t i ~ a &VT=/O~ Photoconductive Cell, available from Newark Electronics. 4801 N. Ravenswood Ave., Chicago. IL 60640; Phone: 312-784-5100 (Item # 43~850,$1.73 ea.). Newark Electronics has 314' ' Cap many regional offices in the US. RedIGreen LED's are available from DigiKey. 701 Brwks Ave. South, PO Box 677, TMef River Falls, MN 56701-0677;Phone: 800 344-4539 (Item # P392, $0.90 ea). The LED's should be the three-lead style, size T 1.314. # 1 1 . ~ 0 1 ~ CdS Plutocdl "Blue LED's are available from Marlin P. #Z PHole Blue LED Jones & Associates, P.O. Box 12685, Lake Fnbber Stopper Rubber Stopper for cwette Dimple Park, FL 33403-0685; Phone 407-848-8236 alignment (Item #4784 OP. $1.50 ea.). or DigiKey (Item #IOICR-ND clear blue. $2.80 ea. or Item # 100CR-ND diffuse blue. $2.80 ea.). or from C~rcuitSpeclallsts (Item # L53BC. $1.95 ea). Figure 1. Diagram showing construction of Pipetronic. ~h~ LED'S should be 5 mm (T 1.314 size).
$8
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Volume 71 Number 10 October 1994
879
available and could be used to simdifv the desim further. although they increase the total cost*anirequire &at the entire device be renlaced if one of its elements is destroved. LED'S with ditfuiing epoxy bodies were used to obtainthe results in this paper. The stoppers are inserted in 1-314-in. lengths of 314-in. PVC pipe, which in turn are inserted in the horizontal branches of the '%en. PVC 314in. end caps are used to protect the LED and photocell assemblies, if desired.
LFT pin 2 or pin 4 2N2222 Red Green or Blue
Electronics We use three 2N2222 or similar switching transistors8 to direct the current to one of the three LED's, a s shown in Figure 2. Each of the three LED's is driven by a circuit like the one shown. In turn. the transistors are controlled bv the auxiliary LPT port that we use a s a general digital output ~ o r(I). t or bv the dieital output of the Acautek board i4). kircuit board-mount< k[Ql p~tentiometers~control the intensity of the LED's. The ohotocell is connected to the game port, so that its resiitancr! can be determined with ~ the high recision hs our "GAVEREAD" r e ~ l n c e m e nfor standard IBM gameport BIOS (I). Spectronic Modification The detectorhulb compartment of the Spectronic 20 is opened, the photodetector tube is removed, and the top end of the #2 rubber stopper holding the photocell is slid into the grooves intended for the filter holder? The photocell is connected to the IBM gameport in the usual way (1,3). Performance Measurements Simple calorimeters or photometers that use test tubes or cylindrical cuvettes suffer certain intrinsic limitations because the cuvettes have walls that are slightly irregular in shape and thickness, have variable internal diameters, and cannot be held in a reproducible or predictable geometric relationship with the incident light beam. Furthermore, photometers that use a n LED without a monochromator as the incident beam cannot give absorbance values that compare to those obtained using instruments that typically have incident bandwidths of 10 nm (or significantlv less). I n view of these limitations. we subiected the pipet"ronic and Spectronic 20 modification to what we consider appropriate tests. Stock solutions of red (three concentrations) and green (one concentration) food ~oloring'~ were prepared so that they had absorbances between 0 and 2 at thrwn\vlcngths of interest. Green bod colors were used for measurements 3t 400,486, and 640 nm, while red food colors -~~~~ ~ ~ were - used ~ for- thr~s~! ~ at 450., 500. , 570.. and 585 nm. Figure 3 shows spectra of the dyes obtained on a Hewlett-Packard 8451A1' diode array instrument, and Figure 4 shows the emission spectra of the LED's. The latter spectra were obtained by placing the LED in the cell compartment of the 8451A, and using a battery and potentiometer to adjust the level. The "Intensity" mode of the ~
~
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Figure 2. Schematic diagram of circuit for controlling LED's with the auxilliary printer port. The circuit for each of the three LED's is identical. instrument was used to record directly amplified output of the uncalibrated diodes, but the values for the peak maxima and widths a t half height should be reliable. The maxima are 496,563, and 636 nm. and the widths are 99.21. and 36 nm f i r the blue, green, and red LED's, respectively. The S~ectronic20 was used for all absorbance measurements for comparison purposes in this paper. Each of the four stock solitions was g s s i p e d a n Hrbitrary relative concentration of 1.0. and diluted with distilled water to give solutions of relative concentration 0.8, 0.6, 0.4, and 0.2. These solutions and distilled water blanks were used in a series of six Milton Roy (Cat. No. 33-17-80) or Bausch & Lomb (Cat. No. 33-29-27) 112 in. Spectronic 20 cuvettes to obtain the Beer's Law d o t s used to evaluate orecision a n d reproducibility of measurements. A significant amount of error i s introduced by using different unmatched round cuvettes, but this regimen was judged to reflect typical usage of simple photometers and thus to be more appropriate than more rigorous regimens that would require using a single cuvette oriented carefully for all measurements (our students actually use 131100 mm test tubes rather than cuvettes). Furthermore, since simple photometers do not allow precise wavelength selection, the ~ i ~ e t r o nwas i c used to measure absorbances under the farfrom-ideal conditions that mav arise in the laboratom. For example, measurements were made where sample absorbance was low, or where the source band and absorbance band overlapped only in a'narrow region where both were sloped steeply. ~ o t u1-2-3 s regressionanalysis was used to calculate the coefficient of determination (or *goodness of
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Spectra of Food Colors Green and Red. Two Concentrations
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Full Spectrum (RGB) "Rainbow" LED's (#[)IS-1024-102.$7.50) are available from Ledtronics, Inc., 4009 Pacific Coast Hwy, Torrance, CA 90505; Phone: 310-534-1505. Transistors are available from DigiKey (item #PN2222, $0.20 ea), or from Radio Shack (Item 276-2009,MPS222A transistor, $0.59 ea., or Item #276-1617,assortment of switching transistors, $1.98 for 15). 9Thisdesign evolved from work of Steven M. Kellner (St. Michaek College) and Rosemary Fowler (Coney College) during the recent NSFiKutztown University LlMSport Workshop. lo McCormick Assorted Food Colors, PO. Box 208, Hunt Valley, MD 21030-0208.The dyes are FD&CRed #40 and #3, Blue # I , and Yellow #5. " Hewlen-Packard Co.. 3495 Deer Creek Rd., Palo Alto, CA 94304; 800-227-9770.
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Journal of Chemical Education
0
100
200
300
400
Wavelength, nm Figure 3. Spectra of red and green food colors.
500
600
700
Spectra ol
Blue, Green, and Red LEDs
rL
(menucall lmenu)
lmenu
480 nm 565 nm 640 nm Calibrate blue range (afl)(put resp,O,O,@Ipt(888,14))(wait@now+@time(0.0.3)] (L~I (L~I (~r) REM: menu choices for 565 and 640 nm have bean deleted for clarity. Lb
350
400
450
500
550
600
650
(getlabel Place blank in spectrometer and press 0 Absorbance..resol (off)(put resp,0,0,@lpt(888;2))(wait@now+@time(0,0.3)) (put AminB,O,O,~GP(2)]-(CALC](indiwte Ostring(AminB.O)] Have instructor adius (if aminBt3Ol(~etlabel speclrometer!!!.rei~{indicate)(quit) (getlabel Push then measure Absorbance with AIt-A (indicatel(on1
700
Wavelength, nm
Figure 4. Spectra of blue, green, and red LED'S.
rare like Lb) !a
fit"), RZ,for the linear regression of optical absorbance on relative concentration. Pipetronic Performance Pipetronic measurements are made by first using the Alt-L. "caLibrate" command to select the a ~ ~ r o a r i aLED te color from the calibration menu, placing a iiani in the cell holder. and recording the "I." (100% transmittance) game . Absorbance measport reading by urements are made with Alt-A, selecting the desired wavelength, and pressing . The macros supporting these commands are shown in Figure 5. The intensity of each LED is adjusted individually with its associated potentiometer before the initial calibration so that the response of the CdS detector is within the allowable range. If an "I," measurement outside allowable limits is obtained during calibration, the macro prompts the user to "Have the instructor adjust the spectrometer." Another concern is to for - ~ allowine ~ - -- ~ ~ -enoueh time for the LED'S and ~hotocells o~ reach a stable reading, especially during calibration. I t was found that turning on all LED'S for a few seconds, then turning off all but the selected LED before the measurement helped to hasten the stabilization for a CdS photocell that had equilibrated to darkness. Some CdS photocells mav be faster. or exhibit less of a "memorv" effect. Once the calibration routine has been run for particular LED source. t h a t source is turned ~ e r m a n e n t l von for subsequent absorbance mwsurcrne&. One of the greatest obstacles to repeatabilitv ofmeasurements in ~ l & k t r o n i c - t ~devices ~e is the d i f f d t y of positioning the cuvette re~roduciblvin the incident light beam. a cuvetie containing distilled water To tescthis was positioned in the Pipetronic, and the gameport reading was measured repeatedly while the sample was illuminated by a chosen LED carrying a measured current. The procedure was then repeated, except that the blank absorbance (log Vl,) was recorded. Next, the blank absorbance was measured repeatedly, and the sample holder cover was removed, the cuvette was removed, rotated, and replaced, and the cover re~lacedbetween measurements. For comparison, the absorbance of a sample was determined re~eatedlvwithout disturbing the s a m ~ l e and . finally, the'absorbince was measured repeatedli, each time after rotating the sample a s above. LED currents were measured with a digital multimeter to determine the effect of LED intensity. ~
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a
(off)(menucall absmenu)
absmenu
480nm 565 nm 640 nm Choose blue incident light with wavelength 480 nanometers (if aminB~l3O#OR#aminB~7Ol(getlabel "Please recalibrate: I: to begin".resp)(Lb)(on)(quit) (Ba) (Gal Fa1 REM: menu choices for 565 and 640 nm have been deleted for darity. Ra
(off)(put resp,O,O,@lpt(888,8))(wait@naw+@time(O,O,t)) (put bufferRa,O,O,@GP(Z)l- . (wlc)lrvavalR--(on)(CALC]
REM: Ga and Ba are like Ra.
Figure 5. Lotus 1-2-3macros for controlling the Pipetronic. Spectronic 20 Performance
To make a measurement on the Spectronic 20, a blank samole is inserted and the right knob is adiusted to pive a n "I," game port reading between about 70 a i d 90. he Alt-L C'caLibrate") L l M S ~ o r tcommand d i s.~ l . a v sthe value and records the selerterj value automaticall\: Clnckwise rotiition of the Spectronic 20 full-scale adjust knob moves a Vshaped incident light beam occluder to increase 10and decrease the gameport reading. The absorbance of a sample is then read with the Alt-Acommand. The macros sumorting these commands are shown in Figure 6.
..
\L
IoM(~etiabe1 Place blank in spectrometer and press (Record 0 Absorbance)..resp) (put Amin,O,O,OGP[2))-[CALC)(indicateBstdng(Amin.0)) Piease sdlust right knob (it aminc70XOR~amin>90)(~etIabbl to get readins between 70 and 90i!!.resp](indicate]{on~(q~n~ (getlabel Push then measure Absorbance with Alt-A.,respJ
a !
(ofll(if amin~7orORxamin~9o)(geti1bbl"Please recalibrate: press to proceed',resp)/L(an){~uifl (put buffera.O.O.QGP(2)J(caic)/luaval--(on)
Figure 6. Lotus 1-2-3macros for controlling the modifiedSpectronic 20. Volume 71 Number 10 October 1994
881
Absorbance vs. Relative Concentration PipetrOnic Blue LED, or450 nm 0.71
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Relative Concentration (Red Solution) o Pipetronic 0 Spectronic 20 Figure 7. Beer's Law plots for red food coloring solution measured with the Pipetronic (R2 = .995),or Spectronic 20 (R2 = ,998) at 450 nm. Results and Discussion
Performance of Pipetronic We used two tests of performance; first, the linearity of a Beer's Law plot, and second, the repeatability of a single absorbance measurement. Beer's Law dots generated with the Pi~etronicand S w tronic 20 are shorn-in Figures 7-10. ~ i & e 7, for exakple, shows that the Pipetronic with a blue LED performs simii ca t 450 nm for a red food coloring larly to the ~ ~ e d m n20 solution (R2 and intercept a r e ,995 and 0.024 for t h e Pipetronic, 0.998 and 0.00048 for the Spedronic 20). Similar results are obtained for the green food color solution. Figure 8 compares the Pipetronic to the Spedronic 20 a t 485 nm. In Figures 7 and 8, the wavelengths were chosen for measurement on the Spectronic 20 so that absorbance values were similar to those obtained on the Pipetronic. Figures 7,8, and 9 disolav mostlv random errors in the Pioetronic measuremen& d;e to v&ations in cuvette Figure 10 shows the effect of non-random errors that led to a convex-u~ward curve for the Pipetmnic. This is probably due to the Loinciden= of a steep shoulder on the absorbance band with a shoulder of awide incident light band. Linearization methods are described below. The results of the repeatability tests described in the experimental section are shown in Table 1. All standard des a t i o n s are for six repetitions. Two generalizations can be made from the table: (1) non-repeatability in positioning the cuvette leads to the large errors. Absorban& measureAbsorbance vs. Relative Concentration Pipetronic Blue LED, or 485 nm
Relative Concentration (Green Solution) 0 Pipetronic 0 Spectronlc 20 Figure 8. Beer's Law plots for green food coloring solution measured with the Pipetronic or Spectronic 20 at 485 nm.
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Journal of Chemical Education
Absorbance vs. Relative Concentration Pipetronic Green LED, 585 nm 0.35 0.3
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Relative Concentration (Red Solution) 0 Pipetronic 0 Spectronic 20 Figure 9. Beer's Law plots for red food coloring solution measured with the Pipetronic or Spectronic 20 at 585 nm.
ments typically have a n error of less than 1% (standard deviation of six samples), but rotation of the cuvette increases the error to 2-5%. And (2). the LED'S must be driven with sufficient current to keep the gameport readings for the blank low. Low intensities generally decrease the repeatability of measurements and-increase the time required for the CdS photocell to stabilize (see below). Because cuvette is the limiting factor for precision of measurements, the standard deviation for measurine the absorbance of a sample when the cuvette is rotated pGbably is the best measurement of the precision of the Pipetronic, and it is around 0.01 absorbance units. Linearization of Pipetronic Results Two methods can be used to interpolate concentration values easily from a curved Beer's Law plot like that in Figure 10. The first method is to fit the absorbance measurements to a function by regression, then to use the parameters to calculate concentrations from absorbances. Table 2 shows the results of a Lotus 1-2-3 polynomial regression of order 2 (Abs = .2734 C-,1208 CZ+ ,001398) that has R2 = .998. The regression is done by specifying the cells containing C and CZvalues as the X Range, and the Absorbance values a s the Y Range. While Lotus is capable of higher order regressions, these gave inferior fits. The second method is sunerior for cases where i t is deemed necessary to present students with the ideal Beer's Law behavior for a solution that is not well behaved a s a result of instrument artifacts (broad incident light Absorbance vs. Relative Concentration Pipetronic Red LED, or 660 n m
Relative Concentration (Green Solution) 0 Pipetronic 0 Spectronic 20 Figure 10. Beer's Law plots for green foodcoloring solution measured with the Pipetmnic or Spectronic 20 at 660 nm.
slow with equilibration to a changed level sometimes requiring several seconds. The II I1 time required depends on the intensity, the LED Color and Current: magnitude of the change in intensity, and Color lB!ue IGreen IGreen lRed IRed the direction of the change (equilibration to Current I 1.99 1 3.28 1 1.37 1 0.48 1 0.1 I 0.05 bright light is faster than to dim light). The approach to equilibrium is approximately Effect of Rotatlng Blank in Cell Compartment on GP Reading: first order with half life of 3 4 s if the intenAvg. 1 150.8333 1 99.66667 1 106 1 360 1 89.33333 1 202.1667 sity is very low, and much faster if the inten%Std.Dev. 1 4.165321 1 2.141513 1 4.458317 1 8.706166 1 0.834354 1 5.70746 sity is high. For this reason, the calibration Std.Dev. 1 6.282692 1 2.134375 1 4.725816 1 31.3422 1 0.745356 1 11.53858 and measurement macros, shown in Figure 6, reject I, game port readings above 90, because the low incident intensity will lead to Reproducibility of Repeated Blank Absorbance Measurements: very long equilibration times for samples Avg. 1 0.007359 1 0.001428 1 0 1 0.003761 1 -9.4E-06 1 0.002725 %Std.Dev. 1 20.16331 1 141.4214 1 #VALUE! 1 11.70453 1 -30483.8 1 35.23099 with ahsorhances approaching 1.0. There is Std.Dev. 1 0.001484 1 0.002019 1 0 [ 0.00044 1 0.002853 1 0.00096 also a more pronounced contribution due to stray light. The macros also reject 10 game port readings helow 70 for the blank so that Effectof Rotating Blank on Absorbance Measurements: isnot lost. 1 0.011619 1 0.016121 1 -0.00471 1 0.005152 1 -0.00838 1 -0.05227 precision Avg. Fortunately, the CdS photocell fives satis%Std.Dev. 1 58.99928 1 50.27585 1 -517.925 1 678.4931 1 -89.8351 1 -49.7145 Std.Dev. 1 0.006855 1 0.008105 1 0.024375 1 0.034959 1 0.007527 1 0.025984 factory resuits, because replacing the slow CdS photocell with a fast phototransistor leads to other problems. The optics of the Repeated Absorbance Measurements on Same Sample: Spectronic 20 lead to a focussed beam that 1 0.387595 1 0.364062 1 0.147601 1 0.166575 1 0.147933 1 0.114174 must be intercepted by a fairly large phoAvg. % Std.Dev. 1 0.826947 1 0.381875 1 6.62E-07 1 1.278723 1 1.121855 1 1.303496 todetector. A phototransistor, with its very Std.Dev. 1 0.003205 1 0.00139 1 9.77E-10 1 0.00213 1 0.00166 1 0.001488 small active area, shows great variations in response when a cuvette containing distilled Effectof Rotating Cuvette on Absorbance Measurement: water is rotated in the sample holder. For ex1 0.379352 1 0.38361 1 0.136829 1 0.168278 1 0.149651 1 0.1 12051 ample, series of 10 gameport readings for a Avg. % Std.Dev. 1 2.374739 1 3.323863 1 5.644169 1 2.544852 1 3.926553 1 3.533559 single blank showed standard deviations of Std.Dev. 1 0.009009 1 0.012751 1 0.007723 1 0.004282 1 0.005876 1 0.003959 up to 38%. Standard deviations of approxiRepeated readings (sets of 6) without disturbance of the cuvene, or repeated readings when the mately 10% could be obtained by measures cuvene is removed, rotated, and replaced are shown. Average readings and standard deviations are such as roughening the convex epoxy lens of aiven for a CdS Dhotocell connected to the IBM QarneDort driven by a non-standard B l O S Raw game- the phototransistor, mounting it behind a port readings f i r blank samples, or absorban& readings (log loil) are reported. Data are given for "frosted" diffusing screen, or changing the illuminationwith blue, green, and red LEOS each driven at two currents. distance between the phototransistor and cuvette. If the device were not required to perform with little attention to detail, a debandwidth centered on the steep edge of the sample absign incorporating phototransistors may be possible, espesorption, for example). The method above might be appmcially if several phototransistors were incorporated. By priate for students in instrumental analysis, but it is probcomparison, however, a CdS photocell typically produced a ably an unnecessary distraction for beginning students. series of gameport readings with a standard deviation of The second method linearizes the output of the Pipetronic 1.7% or less, which is similar to that shown by the Specfor a articular t w e of solution with the template shown in tronic 20 itself. 'Table 3. ~ a m e ~ oreadings % are entered in &mn 0 , rows Over most wavelengths and concentration ranges, the 2631. for the concentrations in column M. Ideal aameport response of the photocell is quite linear as demonstrated readings are calculated in column R by an adaption of the equation1 = I&, where cis the concentration from column. Table 2. Lotus 1-2-3 Regression Analysis of Beer's Law Asecond-order polynomial regression is used to calculate x Plot (Pipetronic Data) Shown in Figure 10. coefficients and intercepts that are then used in cells N35 and N36 to linearize the raw gameport readings for the blank and sample in cells N33 and N34. The absorbance is Regression Output: calculated in the range avalR by use of the standard equaConstant 0.001399 tion, using the linearized gameport readings. Std Err of Y Est 0.003228 R Squared 0.998182 Performance of Spectronic 20 Modification No. of Observations 6 Measurements made with a CdS photocell mounted in a Degrees of Freedom 3 Spectronic 20 filter holder may be very precise, because the blank (A=O) may produce a game port reading of 75, and samples of increasing absorptivity produce values rangingup to over 750 when measured with the LIMSport BIOS (1).Ahsorhance values thus have a precision of no worse than log(75176)-log(75/75), or 0.006 units, ignoring effects due to cuvette positioning and nonuniformity. The precision would be significantly worse with the standard IBM Gameport BIOS. Because the replacement BIOS expands the range of the gameport to 16 bits, one might wonder why lower intensities aren't used to increase the size of the gameport readings and thus increase the precision. The reason is that the response time of a CdS photocell is Table 1. Reproducibility Tests for the Pipetronic Photometer.
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Volume 71 Number 10 October 1994
883
Table 3. Lotus 1-2-3 Regression Analysis of Ideal Gameport Readings on actual Gameport Readings Made with the Pipetronlc, with Macros Which Linearize the Beer's Law Plot Shown In Figure 10.
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M
N 0 Regression Output:
P
Constant Std Err of Y Est R Snl~areri .. of Obsetvations rees of Freedom
961.98068 7.0359127 0.9903341
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1
Slo Err of Coef.
Q
I
-19.8653410,1129588 14.43859621 0.0211925
R
S
by the R2values for the Beer's Law plots shown in Figures 11-14. Figure 11,for example, shows that the CdS sensor in t h e Spectronic 20 performs a b o u t a s well a s t h e standard detector a t 400 nm a s indicated by the coefficients of determination and intercepts for Beer's Law plots for solutions of green food coloring (.999 and 0.0007 versus ,999 and 0.017). Figures 12-14 show s i m i l a r r e s u l t s for o t h e r wavelengths. Variations on the Pipetronic Design
The distance between the cuvette and the LED's and photocells critically affects the intensity of light measured by the photocell, and, of course, so does the depth to which the devices a r e inserted into the rubber stopper holes. The easy assembly of the Pipetronic encourages . . . -. . . . . -. experimentation. LED's can be dif661 = Raw Gamepoll Readina 1'or I-do5 aminR I fuse or clear, and clear LED'S can be bufferRa 123 = Raw Gamepoll Reading made diffusing by roughening their aminL 89.004291 = (N33.$0$22)+(N33"2'$P$22)+$P516 convex surfaces with sandpaper. BufferL 227.49678 = (N34'$0$22)+(N34Y'$P$22)+$P$16 Rather than using rubber stopper avalR 0.408022 = -@LN(N35/N36)/2.3 mounts, Radio S h a c k panel mounted redlereen LED (276-025). Absorbance vs. Relative Concentration and a Radio Shack chrome LED hold& (276-080).with blui 400 nm LED, may be mounted in a drilled PVC end cap. Plastic lenses may be incorporated into the design. The intensity of the LED's can be controlled and optimized for the absorbance of the sample. For example, would it be possible to determine accurately the absorbance of samples with A> 1 if the intensity of the source were increased? Different colors of LED can be kept in rubber stopper mounts and easily substituted in the cell holder. Yellow LED's with a broad (-70 nm a t 80 mA) peak centered around 615 nm, several colors of red LED, and LED'S with colored lenses are all available, and may allow optimization of some determinations.
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Relative Concentration (Green Solution) o Speckonlo 20 A CdS Figure 11. Beer's Law plots forgreen foodcoloring solution measured with the Spectronic 20 at 400 nm, either with the standard detector or a CdS Dhotocell. Absorbance vs. Relative Concentration 500 nm
Relative Concentration (Red Solution) 0 CdS A Spectronic 20 Flgure 12. Beer's Law piotsfor green food coionng soldtlon measured wltn tne Spectron c 20 at 500 nm, ether wttn the standard detector or
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Journal of Chemical Education
l 2 Phototransiston are available from Newark Electronics (Item #MRD310, $1.13 ea.. or Item ECG3036). l3Siemens (G3416)and other silicon photodevices are sold by the Electronics Goidmine. P.O. Box 5408. Scottsdale.AZ 85261; Phone: 602-451-7454.
Absorbance vo. Relative Concentration 570 nm
Relative Concentation (Red Solution) 0 CdS + Spectronic 20 Figure 13. Beer's Law plots for green foodcoloring solution measured with the Spectronic 20 at 570 nm, either with the standard detector or a CdS photocell.
A Spreadsheet Approach to Determining the Degree of Distortion in Five-Coordinate Compounds
Absorbance vs. Relative Concentration 640 nrn
Craig D. Montgomery Trinity Western University 7600 Glover Rd. Langiey, B.C., Canada, V3A6H4
Relative Concentration (Green Solution) 0 CdS + Spectronic 20 Figure 14. Beer's Law plots for green foodcoloring solution measured with the Spectronic 20 at 640 nm, either with the standard detector or a CdS photocell. Phot~transistors'~ with verv fast remonse times mav be substituted for the photocells; and they are available with either flat or convex lenses. There is a dramatic d~fference in phototransistor resistance when the rubber stopper mount is rotated from measurement to measurement for the same sample, or when the cuvette is rotated, due to the small active area of the phototransistor, and focussing effect of round cuvettes. This may be obviated to some extent by choosing phototransistors integral wide-angle lenses, or by using two or more phototransistors in parallel or series as the detector. Silicon photodiodes may provide the ideal combination of large active area and fast response, but they are current1 quite expensive unless purchased from surplus dealers. I? Concluslons The Pipetronic is an easily wnstruded, inexpensive photometer for educational laboratories that gives reasonable absorption measurements over three wavelength ranges. The awxiated LlMS~ortsoftware allows control from a latus 12-3 spreadsheet k t h automatic display of results in spreadsheet cells in a wlor that refleets the incident light color. The CdS detector assembly from the Pipetronic can be used easily in a Spectronic 20, and LIMSport soRware can be used to acquire data from the modified instrument. To obtain a Lotus 1-2-3 spreadsheet tutorial for wnstructmg the Pipetronic, and wpies of the macros used to control it, send a blank disk and prepaid mailer to the author. Literature Cited 1.W z . E.: Reinhard. S. J. C h .Edue. 1885.70.245. 2. Wz;E.J. Cham.Educ. 1992,6!3,744
3. Re. E. J. C h .Edue. 1885,70.63. 4. Re, E.; Reinhard, S. J. Cham.Edue. 1885,70,758-761. 5.Wz.E.;Betta,T A. ZIMSportuersuspHDataAcq~ition:AoInerpensiveRobeand Calibration SoRware,'J Cham.Edue., in press. 6. Berkka, L. H.:Clark, W.J.; White,D.C.J. Ckem Edue.. 19%69,691.
The stereochemistry of five-coordinate compounds, of both main gmup and transition metal elements, is of considerable interest because of the wide range of geometries observed.Although a trigonal bipyramidal (TBP)geometry would be predicted if consideration was given only to VSEPR arguments, other factors such as incompletely filled d subshells, in the case of transition metal complexes, and ring strain can result in distortion. Holmes (1, 2) has quantified the degree of distortion from an idealized TBP geometry for a series of phosphoranes in the solid state by considering dihedral angles and found that the distorted structures tend to lie along the Berry coordinate (3).That is, the distortion is towards a square or rectangular pyramidal structure (SP, RP). This has been used to support the Berry pseudorotation mechanism, as opposed to the turnstile mechanism (41, as the means by which these phosphoranes exchange sites in a TBP arrangement. When considering five-coordinate stereochemistry in either a main group or transition metal inorganic chemistry course, it is often stressed that there exists a delicate balance of energetic factors that results in the favoring of either the TBP and S P structure over the other. Typically [Ni(CN)51&is referred to since both structures are observed in a single crystal (5).Therefore it would be a useful exercise for students to calculate the degree of distortion for a series of compounds and thereby see the subtle interplay of factors affecting pentacoordinate stereochemistry. Furthermore, because these two Idealized structures are related by the Berry pseudorotation mechanism, such an exercise could illustrate the adherence of such structures to the Berry coordinate versus the turnstile coordinate. However, calculating the distortion using Holmes' method of dihedral angles, given fractional atomic coordinates can be rather difficult for a crystal system that is not tetragonal, cubic, or orthorhombic. This paper illustrates how Holmes' method may he adapted to a spreadsheet, given only the five bond distances and ten bond angles around the central atom. Dihedral Angles and Distortion The idealized TBP and SP geometries are shown in the figure. In addition, Holmes alsn refers to a rectangular pyramidal geometry (RP), for structures of spirobicyclic compounds having two five-membered rings.For these struc-
7. Nagel, EdgarH. J Cham.Edue. 1880,67,A75.
Table 1. Dihedral Angles for Idealized TBP, SP and RP Structures (1). Dihedral Angle. Si
TBP
SP
RP
AB
53.1 53.1 53.1
76.9
76.9 0.0 76.9
BC AC
0.0 76.9
TBP ldealized trigonal bipyramidal (TBP) and square pyramidal geometries Volume 71 Number 10 October 1994
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