, a-~icrocomputer
g y M . Mattson, T. R. Shepherd, and J. F. Solsky
Creighton University Omaha. NE 68178
Figure 5. Typical potentiomnric data fathe couiometric titratlon of 4.3 mg ascnbicacidwithbromhelCommcdne-64cmutw);-1nration curve: first derivative ~lat.
Inst&tional-quality infrared spectrophotometers have been an integral part of chemistry laboratory programs and spectral methods courses for several decades. Until the advent of microprocessing technologies these same spedrophotometers also were used extensively as research tools. These "older technolom" instruments have a few distinct advantages: they have pioved to be durable and reliable and are tvpicallv inexpensive to maintain. Unfortunately, they lack many bf thenice features of the newer spectrophotometers including: (1)the ability to save spectra on disk; (2) the ability to enlarge and display one or more regions of the spectrum of particular interest; and (3) the ability to subtract one spectrum from another and display the difference (for kinetics studies, etc.). We describe here a comDuter ~ t could be used to . r o- i e cthat upgrade most scanning infrared spectrophotometrrs to give them the feature* described above. The uparadina uroiect is reasonably inexpensive and could be c a r r i a out 6y an individual with no previous interfacing experience. In the Figure 6 we show the C-H stretching region of the infrared spectrum of polystyrene. Part A is the spectrum as Acculah 3 s~ectroohotometer. Part recorded hv a Beckman . B is an enlarged presentation of the same spectrum using our interfacine Droeram and the data collected while Part A was being recorded. Quality enlargements of a s ~ l spectral l window allows one to observe suhtle features that are often lost during a routine scan or t o monitor the course of a reaction bv carefullv com~aring recorded as a func-spectra . tion of time. The workine principle of the interfacing program is simple. The signaiis taken from the spectroph%ometer's output port (for a strip chart recorder) and fed directly into a 12-bit analog to digital ( A D ) interface card (Applied Engineering Co., -5320) placed in Slot #4 inside the Apple IIe microcomputer with 64K memory. Data are collected in 0.1 s intervals during an actual scan and immediately displayed on screen and ooked into memorv. Anv scanning infrared spectrophotomker with a signal o k p u i port couid be used in conjunction with our interfacing program. (Some modifications in timing loops would be necessary if a spectrometer other than aBeckman Acculab is used. Such changes involve program lines written in Applesoft Basic and assembly language.) When data acquisition is complete, the entire spec~
~
~
~~~
~~
~d
~~
-.
dehydroascorbic acid
The platinum indicator electrode senses the [ascorhic acid]/ [dehydroascorbic acid] ratio in solution prior t o the endpoint and the [Br2-]/[Br2]ratio after the endpoint. The potential drops rapidly during the beginning of the titration and then levels off. At the equivalence point, a very sharp rise is observed. By collecting the data directly into memory using the Apple II+/IMI ADALAB svstem. the endpoint of the titrati% is determined by the-computer by iocating the maximum of the first derivative of the data. Data are taken once every second. While this rate of acquisition may seem relatively slow, i t is sufficiently rapid for this determination and reduces the possibility of endpoint miscalculation due to random fluctuations of EMF. The plots generated with the Commodore system are shown in Figure 5; the first derivative is superimposed on the zero-order data. The reason for switching to the Commodore was that a complete data station comprising a monochrome monitor, a Commodore 64 Computer, a printerlplotter, and program cassette recorder costs only $550. The more elaborate (and user friendly) Apple II+/ADALAB system costs $6000. Results for this ex~erimentwere satisfactory. Reproducsamples yirlAed an unceribility t~~ranalysisof~scorbicacid tainty of i20).'This was orohahly caused by adsorhcd impurities on the surface o f the generator electrdde. Repetitive cleaning of the electrode improves the precision of the determination. User instructions for interfacing either computer system with this instrumental technique and the software are available upon request from the authors.
-
Acknowledgment
We wish to thank William J. Snyder of Bucknell's Chemical Engineering Department for the use of his Apple Computer and Robert J. Murcek for technical assistance. 890
Journal of Chemical Education
Figure 6. A portionvof Me infrared spectrum of palyslyrene ( C H stretching region) obtained from a Beckmsn Acculab 3 and recorded by the insnumen1 itself (Part 1)and by our sonware package and an Apple lle computer (Part 2).
