A Colorimetric Titration Experiment with Laser Excitation and Computer-Interfaced Endpoint Detection Manlsh A. Mehta and Richard F. Dalllnger Wabash College Crawfordsvllle, IN 47933 The subject of instrumentally oriented colorimetric titrations has received a eood deal of attention in this Journal (8-12). Colorimetric titration experiments are often performed in undereraduate laboratories because a varietv of instrumental components (light sources, detectors, etc.)can be studied in a low-cost, hands-on format. We have devised a colorimetric titration experiment that combines several attractive and instructive features, most notably the use of a laser light source and computer-interfaced detection. This experiment is assembled from items commonly found in most chemistry departments or inexpensively purchased. I t is suitable for an instrumental analysis lab or a special proiect in instrumental analvsis. " . and i t makes a colorfnl.~impressive lecture demonstration. The experimental apparatus is shown in Figure 4. The light source for the colorimetric titration was a simple helium-neon (HeNe) alignment laser. (We used a Spectra-Physics Model 155, which costs approximately $325, although any HeNe with any type of polarization may he used.) This laser provides a safe (Class 11) output of approximately one milliwatt a t 632.8nm. The laser output was directed through the sample cuvette and was focused such that the diverging beam exiting the sample filled the face of the photocell detector. This experiment can provide a student's first experience with the properties, use, and alignment of laser beams with virtuallv no safetv Students should. of . problems. . course, he instructed to use proper laser safety procedur& in this experiment despite the minimal hazard. The~hotocelldetector was constructed from a cadmium sulfide photocell (Radio Shack, $1.29), which exhibits a large resistance in the dark (-1 MQ) and a much smaller resistance when illuminated (-100 0). The detector responds to light transmitted by the sample solution, and thus the photocell resistance changes as the solution transmittance at 632.8 nm changes. The photocell was incorporated into a circuit, shown in Figure 5, containing a 9-V dry-cell battery and a 3.3-kQ current-limitinc resistor. This circuit orovides an,output voltage, measure; across the photoce~l,~ that is, pro~ortionalto the ohotocell resistance. This circuit is neck s ~ & since ~ the combuter interface card responds to changes in voltage, not resistance. The computer system was based around an Apple IIe
microrompt~ter.The i n t d a c r was the IMI ADALAH parkage (Inleractive Yicruware, Inc.. PO. Hox 771. State CoIlige, PA 16801; cost = $4'95) "sing the VID~SAMPLER software (13). This interface. which reauires no oroeramming to actuate save the VID~SAMPLERcommands;samples the analogue voltage output of the ohotocell detector circuit a t a user-adjustable rate of 0.018 to 20 times per second, performs an analosue-to-digital conversion. and sends the digital informationto one offour data buffers. The IMI VIDICHART graphics software permits interactive display of the data in theinterface buffers and includes several handy numerical functions for data massage. One of the attractive features of the VIDICHART graphics for this experiment is the capability to generate first-derivative plots of the titration curves. This allows for easy location of the titration endpoint. The data can also he stored on floppy disk for later manipulation, such as plotting with the IMI SCIPLOT package. We used this apparatus to examine some straiehtforward acid-base titrations. The analyte solution was placed in a 25mm absorption cuvette (total volume of a ~ ~ r o x i m a t e20 lv mL) and was stirred with a magnetic stirrkr. The titrant solution was added using a syringe pump (Sage Instruments model 341; cost = $750) that could deliver from 0.092 to 3.6 mL/min with a 10-mL syringe. The endpoint was detected by using a colored acidibase indicator in the sample cuvette. The only requirement for an indicator in this experiment was that i t show a change in transmission a t the red HeNe laser wavelength (632.8 nm) between the acidic and basic forms. In order to maximize this change. we chose indicators that have a yellow acidic form (red-transparent) and a hlue basic form (red-absorbing). There is a wide selection of these types of indicators covering virtually the entire range of pH transitions, including methyl violet, hromcresol green, bromcresol purple, hromthymol blue, n-naptholphthalein, cresol purple, thymol hlue, and thymolphthalein. Using a test soldtion ronralnmg several drups of hromthvmol hlue, tor example, we ohirrved the voltage outpur of our photocell detector to rhanee from 260 mV for the hlue form to 135 mV for the yellow form. This voltage change can he easily monitored hv ADALAB interface.1 A c o k n e t r i c titration was performed hy adding an aliquot of theanalvesolurion and 5-7 droosof indicator to the iuvette and delkering standardized titrant from the syringe. The start of the syringe drive and the start of the data acquisition routine were manually synchronized. Each titration curve took only 1-3 minutes to collect. Plots were generated both for the traditional sigmoidally shaped titration curve as well as the first derivative. We calculated the endpoint titrant volume from the VIDICHART-determined ~
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Our exoerlments were run in a darkened maximize the - - roam -. to ~. change in dark11ghl response. The experiment can oe run in a lighted room with some loss of s gnal-to-noise ratio, or the pholoceil can oe shieloed if tne experiment need be run in a I ghtea room. ~
Figure 4. Experimental apparatus. HeNe is helium-neon laser, L is biconvex lens. SD is syringe drive, MS is magnetic stirrer, SC is sample cwene. PD is photocell detector. IB is interface box and All is Apple lie computer.me laser beam oath is hatched.
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Figure 5. Photocell detector circuit. Lioht strikesthe ohotaceil. which functions pnoloresmor me c rc.d luncwms as a vo tage awder wllh me ompn valtagc V., proponmonal to the photocell resnslance A 9-V ary-cell battery pro" ~estneclrcblluol1aqe.andtherwllch Smay oelurnedon loconservetne banery when the detector is not in use. as a
Volume 64 Number 12 December 1987
1019
