Table 1. Instructor's Formatted Worksheet
-
(A)
(C)
(8)
Name Sample X DATA: Trial 1 Length of acid column in buret, mm Val of hydrogen. ml Temp of bath, C V. press of water Bar pressure, Torr CALCULATIONS Temp of bath. K 273.00 0.00 Acid column press 000 Press of dry H Tor VOI of dry H. ml 0.00 Wt of Mg in grams 0 Average w i of Mg Accepted Result Percent error
Table 2.
-
(A)
Name DATA:
(0)
(E)
Trial 2
Trial 1
273.00 000
0.00 0.00 0
Trlal 2
100
95
25.2 26.5 26.7
27.2 26.5 26.7
755
756
299.50 7.35 720.95
299.50 6.99 722.31
21.79 0.02364
0.02556
0
0.0251 100.00
23.56 0.0246 0.0251 1.99
Student's Data
(B)
(C)
Jackson
Sample # Trial 1
Lenglh of acld column In buret, mm Vol of hydrogen, ml Temp of bath, C V. press of water Bar pressure, Torr
(F)
-
(D) 36 Trial 2
84
80
41.2
45.4
25
24
23.8
22.4
759
759
Table 3. Computer-Generated Report
-
(A)
Name
(0)
(C)
(0)
Jackson
sample # Trial 1
Trial 2
DATA:
Length of acid column in buret, mm Val of hydrogen, ml Temp of bath, C V. press of water Bar pressure. Torr CALCULATIONS Temp of bath. K Acid column press Press of dry H Tor Vol of dry H. ml Wt of Mg in grams Average w i of Mg Accepted Result Percent error
84 41.2 25 23.8
36
Event-Drlven Data Acquisition: Using ADALAB with an Acculab Infrared Spectrometer
80
George C. Lisensky and Bryan A. Mehlhaff Beloit College Beloit. WI 5351 1
45.4 24
22.4
758
759
298
297 5.88
6.18
columns were calculated using VisiCalc functions and commands. The Droeram was then instructed to redicate the computationbo;tion of the lower part of columns E and F into the adiacent hlank ~ositionsof columns C and D. The zeros that appear in this part of the table result from the absence of figures in the data portion of columns C and D. This formatted sheet is for the "se of the instructor only and students are not allowed access to it. Table 2 is simply the datasection of Table 1. It is stored on a separate computer disk and is made available to students with blanks in columns C and D. Each student enters and saves his or her own data on this disk; sample data are shown on Table 3. Table 2 is stored in the Data Interchange Format (DIF). Table 3 is generated by instructing the program to transfer the student's data block onto the instructor's formatted worksheet (Table 1).Finally, the computer is instructed to print out the contents of Table 3. The advantages of the ahove program are manyfold. I t permits the thorough evaluation of alarge number of lahoratory reports within a short period of time. The program also enables both the teacher and the student to discover whether the source of error is due to poor data or due to computation errors. Our chemistry instructors have also found the program most helpful for assigning consistent grades. Our chemistry students were enthusiastic about using the comvuter. Our general chemistry students are, nt the present time, able to store a total of eight - addirional sets 01' data. They include percentage of copper in an ore, molecular weight of a volatile liquid, percentage of potassium chlorate in a mixture, molecular weight by the freezing-point depression, percentage of water in a hydrate, molar volume of a gas, normalitvof acid unknowns bv titration and redox determination of oxalic acid. For details concernine the formattine nrocedure and all of the VisiCalc commandsused, please &te to the above address.
729
730.7
36.2 ,03928
40.12 ,04353 ,0414 ,03859
-7.3
(DIF), which permits the user to transfer a full block of data from one file to another. The formulated worksheet permits the chemistry students to store experimental data and then enables the laboratory instructor to obtain a computer print out which contains the student's stored data as well as all calculated results and percent error. T o illustrate the use of DIF, I have chosen a simple general chemistry experiment: the indirect determination of magnesium. The unknown is first dissolved in dilute hydrochloric acid. The liberated hydrogen gas is collected, corrected to STP and then the weight of magnesium is calculated (3). Table 1 shows a formatted worksheet that is six columns wide. The last two columns, E and F, contain some typical laboratory data. The results in the lower section of these two
Interfacing mirroamputprs to srientifir instrumentation has berome relativrly straightfurward. For example, the rommonl\, used ADAIAB u~terface' for Aovle ~" .. wmvuters allows replacement of any chart recorder by the computer simply by connecting the appropriate voltage output from the instrument to the interface analog input pins. The independent variable is usually assumed to vary linearly with time and must then he calibrated using the known time required for an instrument scan. By contrast with such an approach we report here an example of data acquisition using event-driven timing. Many newer instruments have various digital signals available. For example, Figure 5 shows the external pin counections availahle on a Beckman Acculab 8 Infrared S ~ e c trometer (models 7-10 are similar). Connecting the appronriate Dins to the dieital innut and o u t. ~ unins .t of the ADA~ . A Rbkwd is not diyfintlt i n d allows for synshrunizution of the instrument .wan and data acuuisition. 1 I I h t a anluisition can be made to wait for theinstrument scan to Btart, eliminating the need to Dress a kev on the computer and the scan button on the spechometer it the same time. 2) Instrument scan can he continually monitored, causing data acqui~~
~~
ADALAB board from lnteractlve Mrcroware. Box 139. Slate College. PA 16804. ADALAB anoOUICKilOaretraoemarksof Interactive Microware.
Volume 63 Number 4
A~ril1986
323
SEPARATION:i8 COMPRESSION:8 PAGE 1/2
di9lt.l input
pin 1
Figure 5. Acculab Infrared Spectrometer pin connections in capitel letters. ADALAB interface board pin connections in lower case.
Figure 6. Expanded inhared spectrum of HCI gas. Digital data recorded every 2 wavenumbers, stored on disk, and then transmined to plotter.
sition to pause during the grating change a t 2000 wavenumbers until the instrument scan resumes. 3) The wavenumber pulse from the infrared spectrometer can he monitored, and the analog signal recorded when each wavenumber pulse is received. This gives a directly calibrated independent variable axis even if the chart drive is not constant or accurate. 4) Lastly, the computer can control whether the instrument chart also runs. When spectraare recorded in the computer, the cost of using expensive roll chart paper is greatly reduced. The ADALAB board with its QUICK110 software is capable of 20 analog-to-digital conversions per second. When connected with a Beckman Acculah spectrometer operating a t the normal (7.0 minlspectrum) and slow (22.0 rninfspectrum) scan speeds, this data sampling rate is sufficient to read the per cent transmittance, plot the value on the screen, and finish before the next wavenumber pulse. (Careful monitoring shows that no pulses are missed a t the normal scan rate.) Some of the wavenumher pulses are missed when using the fast (2.5 minlspectrum) scan rate.3 We use the fast scan to illustrate effects of data sampling rates to students. The full 12-hit resolution (-2048 to +2048) of the ADALAB analog-to-digital conversion is used for the transmittance values by first adjusting the spectrometer's "aux rec zero" t o -0.5V with the sample beam blocked and then the "aux rec span" to +0.5V with the reference beam blocked. The corresponding voltage range jumper on the ADALAB board is selected. The quality of spectra obtained by sampling at each wavenumber pulse is quite good. The spectrometer sends a pulse every 2 wavenumbers above 2000 wavenumhers and every 1 324
Journal of Chemical Education
Figure 7. Printed screen display comparing spectra of polystyrene (top) and polypropylene (banom). Separation is useradiustable vertical offset:corn pression is user%djustablenumber of data points per screen coordinate: page 112 indicates first of two pages is displayed.
wavenumher below 2000 wavenumhers, giving 2400 data points for the full 4000-600 wavenumher scan. Figure 6 shows a portion of an expanded spectrum of HCI gas at this sampling interval. Using the wavenumher pulse for the independent variable axis provides a more precise reading of wavenumher values than possible when using the instrument chart. For best repeatability, we use the PEN setting on the spectrometer to reset the initial position a t 4000 wavenumhers automatically. Running polystyrene as a sample consistently gave peak locations on our spectrometer (standard values in parentheses) of 3010 (3027), 2841 (2851), 1801 (1802), 1604 (1601), 1189 ( l l s l ) , 1039 (l028), 918 (9071, even though the resolution and wavenumber accuracy specifications for the spectrometer are eiven as "better than" 5 wavenumhers above 2000 and 3 wavenumbers below 2000 wavenumbers (4). This seems to indicate that the actual waveleneth accuracv of this system will differ somewhat from instrument to instrument. It is advantaeeous to store the data on disk as a hinarv data file. usingBeagle Brothers Pronto-DOS4,the 2400 integer data points can he stored or recalled in 3 s, while writing this quantity of data to a text file requires 80 s and uses twice as much disk space. Given stored spectra, a variety of manipulations are possible. These include scale expansions, overlays of known spectra, and even comparing peak locations to a list of known peak locations for functional group identification. We have found retrieval and nlottine- of soectra on a mul. tiple-pen plotter to provide better looking figures and easier interoretation of soectral differences. Students have been moresuccessful atidentifying a series of unknowns using this method. A set of four Applesoft Basic programs (using an Apple IIe with ADALAB board, Epson printer, Sweet-P Six Shooter plotter, and Grappler+ interface) to record a spectrum on disk, to recall spectra and find and identify peaks, to compare spectra using scale expansions and a screen plot or printer dump (see Fig. 7), and to plot spectra with wavenumbers for an arbitrarv wavenumher ranee and Daoer size on a plotter, are available from the authors. ~ a c program h works inde~endentlvto store or retrieve a hinarv data file. and all handle the change of scale at 2000 wavenumhers. ~ o ;
As noted by a referee, direct hardware triggering of analoptodigital conversion by the wavenumber pulse would allow fur faster scan rates, but this is not accommodated by the ADALAB system. Promo-DOS from Beagle Brothers, 3990 Old Town Avenue. San Diego. CA 92110.
these nroerams and samnle data files. as well as a similar set of prokrams for a Heckman 1)HG UV,vis spectrometer, send a blank disk and a check fur $10 to George - Lisenskv at the address above.
Microcomputer-Controlled Cyclic Voltammetry Robin J. Ontko, Raymond N. Rusrell, and Paul J. Ongrens Earlham College Richmond, IN 47374 Cyclic voltammetry is both a powerful tool in analytical chemistry and an attractive experiment for teaching a number of concepts in electrochemistry, as pointed out by a (5-8).This note number of recent articles in THIS JOURNAL describes construction of a simple CV system using a microcomnuter both for control of the cvcline voltaee and for " reading the response current. This system has been used for CV exneriments in analvtical and nhvsical chemistrv course; I t has also provided lab experience with interfacing and some basic electrochemical instrumentation princinles in more advanced courses. The essential components of the system are a conventional three-electrode potentiostat and an Apple I1 microcomputer with an Interactive Microware ADALABTM interface
Figure 8. Clrcuil layout for the C V system. INT: Interface connectors IDA: digital-to-analogoutput:AD: analog-todigitalinput).Op amp 1: voltage follower for driving voltage: op amp 2: current amplifier section. AE: suxilliary electrode: RE: reference electrode: WE: working electrode: C: 47 pi; R: 4.7 Kn: V: optional voltmeters.
card for D/A and A/D con~ersion.~A diagram ofthe circuit is shown in Figure 8. A reference SCE electrode, a P t wire auxillarv electrode. and a -4 mm2 P t surface electrode were assembled to fit into a 30-ml beaker equipped with bubbler and magnetic stirrer. The amplifier circuits were constructed follo.wing Dieienderfer ( 9 )using inexpensiw 711 operational umplifiers.The 2.7-K resiitur in the current amolifier section was chosen to lessen the chance of too large asignal entering the interface hoard, whose design limit is f12 V. This can be replaced with a variable resistor to increase the range of the instrument. For convenience, the circuit was assembled on an Op Amp 2 Designer breadboard (E&L Inc.), which also served as the f 15 V power supply. The voltmeters, dashed in as options, arestrongly recommended both as a quick indicator that the desired voltage cycle is actually appearing across the working and reference electrodes, and as a monitor on the current amplifier output voltage. Inexpensive digital voltmeters were used for this purpose. The f2-V input and output range settings of the ADALAB hoard are normally used. Output and input resolutions are 12 bit, including sign, so that there is no serious resolution problem for normal scan ranges of several tenths of a volt. The anolied notential is controlled and the current response is read through a simple Basic program that uses a machine language routine, QUICKI/O'M, to control the interface. A preliminary loop accepts a user-selected hold time to stabilize the system a t the initial potential of a cycle. A second loop is then used t o cycle the appliedpotential and to acquire 250 voltage readings from the current amplifier a t equally spaced times. he applied potential necessarily changes in steps in this system rather than in continuous fashion. In order to approximate a continuous change best, these steps are kept as small as possible and the voltage is then incremented several times between readings of output current. The program calculates the magnitude of each step from the cycle time (user selected), the total time required for 250 input readings, the time required for each output change, and the total voltaee chanee. A sten size of 1mV is typickfor scans such as that shown in ~ i & e 9. The part of the program necessary to obtain this type of data is about 30 'lines long; a full program listing is available upon request. In operation, the electrodes are immersed in the desired solutions and the control program is started to insure an applied voltage of 0 before the amplifiers are turned on. This avoids supplying an unpredictable voltage input and the possible consequence of a damaging voltage output. As an additional orotection feature. the . oroeram also returns the " drive voltage to zero if excessive currents are produced durine a run. The tvnical CV screen nlot shown in Fieure 9 mav hecompared with previous publkations in THISJOURNAL (6, 7). The program does not plot the first three points of each scan since these often show relatively large current transients due to capacitance effects. While a grid has been added for ease of locating peak voltages and currents, the student must supply the voltage limits, and use computed peak current values to interpret the vertical scale. This particular system can also take advantage of available software to add a labeled and numbered erid if d e ~ i r e d . ~ Given a microcomputer such as an Apple 11+ or IIe, and a eood dc Dower s u.. n ~ .l.v the . nrincinal additional cost of the &stem isthe ADALAH boar& about $500. For routine work, a rommerciallv availal)le unit surh a.i the Hiuanalvtiral Svstems Model &-IB provides advantages in term; of spekd, sensitivity, stability, and convenience a t a comparable cost. The present system, on theother hand, p r o d u r e ; ~sransof ~ reasonnl~lequaliry. and in addition exposes students lahoratory interfacing and some principl& of instrumentation.
..
-
Figure 9. Screen dumpof CV plot far rnPdK~[Fe(CN)el In 1 MKN03 (cf.ref. 7). Initial hold time: 20 s. cycle time: 22 s. Scale values added.
~Uihorto whom correspondence should be addressed. Sciplotter llTM from Interactive Microware, Box 139, State College, PA 16804. Volume 63
Number 4
April 1986
325
