computer series, 136 Leiv Jonn Stangeland Telemark lngenisrhsgskole 3900 Porsgrunn, Norway
The pH is measured, and the slope is calculated relative to the previous addition; The program determines the amount of base to add dependent on the slope, and this amount of base is added. The computer waits a reasonable delay time for mixing of the reagents and electrode stabilization.
Dennis M. Anjo California State University Long Beach. CA 90840
The use of microwm~uter-baseddata acquisition. contml, and analysis during experiments has b&me inireasinelv i m ~ o r t a n to t the curriculum of the instrumental a a s i s course. Laboratory microcomputers have bewme powerful tools for experimental chemists, and this trend will wntinue as these wmputers become more affordable. Experiments that employ the microcomputer to control a procedure directly give students a much greater breadth of experience than passive data acquisition experiments. This paper reports a relatively inexpensive project for an automatic titrator using a n MS-DOS microcomputer, where software controls the progress of the experiment through the rate of titrant addition. Similar stepper motorbased titration experiments have been presented in the past. Wilkins et al. ( I ) , Feld et al. (21, and Cornelius and Norman (3) have presented full-scale titration experiments using the PDP and Apple I1 computers. Long ( 4 )and Vitz (5) also have presented stepper motor-based titrant pumps. Feedback control of the titrant volume increments was not employed in these previous studies. This experiment can be used to demonstrate the principles of computer interfacing, motion control, and feedback wntrol. In this experiment the computer is employed to wntrol the addition rate of titrant based on the change in pH of the analyte system. Instead of evenly spaced measurements, this method gives a high density of data points near the equivalence point where the slope is steepest. This al-
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Figure 1. Schematic diagram of the titration system. 296
Tempe, AZ 85281
gorithm imitates the manual technique of titration where the measurements are widely spaced when the slope is shallow, but the measuremenk a;c very close in reglo-m of steep slope. ARer the initial addition of base (or acid, the pro~amfollowsa three-step cycle.
An MSDOS Microcomputer Controlled Titration System
SYRINGE
edited bv JAMES P. BIRK Arizona State University
Journal of Chemical Education
1
The students can vary the volume and time delay in this feedback loop in order to record the most accurate titration curve. Hardware An overview of the equipment employed in this project is shown on Figure 1.
Major Components an MS-DOS microwmputer, a 12-bit analog to digital w.nverter interface card (Real Time Devices AD500), a custom-built stepper motor interface card, a stepping linear actuator motor (Hurst 3602-0011, s 50-mL glass hypodermic sJlinge, a custom-builtmetal mounting frame for bath the stepping actuator and the syringe,and an ion meter with an analog output (Orion 811). The sample is placed in a beaker on a magnetic stirrer; the titrant is added to the sample through a tube from the syringe. The stepper motor controller and the metal mounting frame were constructed in-house and the details are given below. Stepper Motor Controller
An interface capable of pulsing out voltages with sufficient current reserve to drive a stepper motor was custom built. Commercial stepper motor controllers are available, but budget constraints led us to build our own controller. Commercial MS-DOS stepper motor controllers with adequate current reserve for an 8-watt motor were quite expensive. The custom card described below cost less than $80 to constmct. The interface card was constructed around the Hurst 220001 stepper motor control-integrated circuit, available in a standard DIP package. The wntroller circuit was installed on a JDR Electronics PR-1 prototype card. The prototype card dewded the board address and buffered the data lines to the wntroller chip. The controller allowed bidirectional operation of the syringe, which allowed filling without disassembly. The components we installed on the prototype board are shown on Figure 2, including the numbered lines from the integrated circuits. The external power supply was a 9-V battery eliminator (Radio Shack). Safety Note: All the leads between the external power supo. h . and the motor must be medium or heavv eauee wire (18 gauge,. The current to and from the mou,r s h d d n o t h e passed through the l i ~ h gauge t eandueror on rhr interfnw
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Figure 2. Schematic diagram of the stepper motor controller.
ion meter was attached to channel 1of the Real Time Devices AD500, an 8-channel12-bit plus sign analog to digital converter. This AD500 is a dual slope integrating A D which is efficient a t reiectine Dower line noise. The AD500 is a standard PCKT Lus cari installed in the microcomputer. The AD500 has an input impedance of 700 Mohm which allows direct connection to redox electrode systems without the ion meter. This input impedance is inadeouate for membrane electrodes, a n i the ion meter is necessary when employing ion-selective electrodes. Thesof;w&&itten for the titrations was set touse the 1-10-100 nnvzrammable eain feature of the AD500 to obtain the 'axikum numb& of significant figures. This allowed a resolution off 1.2, f 0.12 or f 0.012 mv a t the three respective gain values. The students should be informed that a 1-10-100 pattern is not the most efficient gain pattern for a 12-bit A/D because there are gaps in the resolution. There are three regions where the number of significant figures is less than 4: o n gain 1 below 1.22 V, o n gain 10 below 0.122 V, and 'on gain 100 any voltage below 0.0122. This generally will not affect the accuracy of a titration, but it limits the precision of direct potentiometry. Software Development The software was developed into three modules:
board nor through the light gauge wire wrap contacts. The heavy gauge wiring is shown aa heavy lines on Figure 2. The motor was deactivated between additions using the not-disable line (line#14 of the Hurst 220001). This was to prevent heat expansion of the threaded rod which would affect the volume calibration. The 74LS374 octal latch way used to buffer the data outout from the comouter data bus. Without the buffer the data output wouldbe pulsed up and down before the motor had time to respond. The 276-1801 quad nand gate was configured as a nor gate. This nor was used to enable the octal latch when the board data lines were active. See Cofion (61for details of the latch and enable circuitry Syringe Buret
Our present syringe buret is wmposed of a Hurst LAS 3604-001stepping linear actuator, a 50-mL syringe and an aluminum frame to anchor the components. The titrant is passed into the sample through a luer lock plastic tubing extension from the syringe. The buret has a step resolution of approximately 0.02 mL with the 50-mL syringe, but narrower syringes can be installed for greater resolution. The adjustable clamps on the frame allow the use of syringes from 5-100 mL in volume. The buret frame was wnstruded of an aluminum base with aluminum clamps for the syringe and the stepper actuator. The alumin& syringe clamps were designed to be horizontally adjustable, and Teflon inserts were made to fit a varietv of s-nees. ATeflon nluneer was attached to the threadei motor rid, and the piung& was supported by two steel rods that extended the leneth of the frame. The s t e ~ per actuator expelled the threa&d rod by turning a holldw threaded motor shaft: the end of the threaded rod must be secured from turning to expel the rod. The threaded rod was attached to the plunger with a set screw to prevent turning during the experiment. Data Input
We used an Orion pHlion meter model 811 for the measurement of solution potentials. The rewrder output of the
calibration of pH, calibration of the buret output, and the titration experiment. Each module is discussed below. pH Calibration Program
The output of the ion meter was kept in potential mode for the pH titrations. The potential response of the glass electrode was measured in a number of standard buffers, and a Nernst calibration curve was determined using linear regression. The slope, the intercept, and statistics were calculated and stored for use in the titration program. These parameters were determined immediately before titrations to minimize the effect of electrode potential drift on the OH measurements. Generation of a calibration curve frbm the direct A/D input minimizes the offset and eain errors of the A/D wnverter. The students should be warned that the electrode response is not linear over a wide range of pH values, and standardization is best carried out near the p& of the analyte. Volume Calibration Program
The output of the syringe was calibrated by measuring the mass of water exnelled out of the svrinee for 1000 pulses of the stepper ktuator. The millkte& per pulse were then calculated bv the software based on the densitv of water at the experimental temperature. The average of nlne determinations was 0.01864 ? 0.00001 mWsteo: the uncertainty is the standard deviation. Acid/Base Tifration Program
Initially the titration program displays a menu of the calibration data and other experimental data. The student can update the calibration data with a new volume per step value and both the slope and intercept of the Nernst calibration curve can be updated. The student also inputs both the delay time for mixing and the total amount of reagent to be added. The operator then goes to the buret filling routine, where the syringe piston can be moved using the keyboard armw keys. Volume 69 Number 4 April 1992
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Results The results of the titration of a 5.00-mL sample of 4.82 x lo-' M HCl solution with 0.100 M NaOH are shown on Fig-
ure 3, along with the first derivative plot. The points are widely spaced until the equivalence point is neared, then the points are a t maximum resolution. Similar plots are shown for a 5.00-mL sample of 6.34 x 10" M phosphoric acid solution in Fieure 4. The three-soeed aleorithm is sensitive enough to resolve accurately two phosphoric acid eauivalence ooints. without beine extraordinarilv slow. A study was done to compare the precision and accuracy of the computer-based instrument employing a 50-mL syringe with manual titrations using a 50-mL buret. Standard 0.1000 M HC1 (from a Merck Titrisol ampule) was titrated into 5.000-mL aliouots of 0.10 M NaOH. We soon found that the glass electrode (Corningmodel 91 combination electrode) took 30 s to reach equilibrium following each addition of acid. Sampling sooner between additions led to a systematically high equivalence point. Using the 30-9 interval between additions and the three-speed titrant addition algorithm, it took approximately 20 min to reach a pH far enough beyond the equivalence point to detect the first derivative maximum. For comparison it took a careful student approximately 30 min 6carry out the first titration with accuracy. Once the equivalencepoint region was known, the student took about 4 min & subsequent titrations. We are considering such a "smart" algorithm to be added to the titration program, which would allow the computer to add titrant rapidly to the solution until within 0.3 mL of a n exoeded eauivalence ooint. For seven consecutive titrations with the reagents the equivalence ooint for the comouter was 5.072 i 0.015 mL and the result of seven consecutive manual titrations was 5.081 f 0.015 mL. The uncertainties listed are standard deviations. We could find no significant difference between the two methods, and the precision limits were identical. The Basic programs were found to execute considerably faster when cornoiled from QuickBasic 3 t h a n from QuickBasic 4.5. The programs were run on a n 8088 microcomouter without a math coomcessor. QuickBasic 4.5 sets up a' virtual math coproces~orwhen one is absent in the microcornouter and this conceivablv causes the slowed execution, although other operations such as disk access were also slower. Both QuickBasic versions were much slower when run in memo& The exoeriment was successful in measurine the titration cum& of a number of strong and weak acids. The system could be easily modified,for redox or a precipitation titration. The cost of the entire system excluding the specific ion meter was well below $1000. The system was built around a n MS-DOS XT compatible microcomputer and components easily available from electronic suppliers. The
Hydrochloric Acid Titration
Phosphoric Acid Titration
The software was written to switch between three different titrant volumes added per measurement cycle; the amount added per step depended on the slope of the titration curve. We determined the slopes expected in the titrations using MathCad; for a titration of 0.1 M HC1 with 0.1 M NaOH added at 0.02 mL steps, the maximum and minimum slopes were 200 and 0.02 pWmL, respectively For acetic acid under the same conditions the maximum slope was 30 and the minimum slope was 0.04 pWmL. Based on these calculations. the three rates of additions were chosen. 0.5 mL uer addition when the slo~e was below 0.1: 0 2 ml. per nddlhon when the slope was hrrwern 0.1 and 1, 0 0" mL per odd~tmnwhm the slope exceeded 1
After each addition and pH measurement the slope was calculated between the present and the previous point. The amount of titrant for the next addition was determined bv the slooe between these two data points. More to fine tune additionrates cohd be added to the the titration. but as indicated below three rates are auite acceptable. We chose the slope limits for the three titration rates in a conservative manner, because the titration cannot be reversed if the slope suddenly jumps. In future versions of the software the students will be asked to calculate manually the slope at various points during the titration, and they will then input the~rchoicesofthr slope ranges for the three addition rates. The rate of addition can be accelerated for strong acids and slowed for very weak acids. Because the slope is determined in real time, the first derivative data is stored durim the titration, and no further calculations are necessary: The absolute value of the slope is stored so the equivalence point is a maxima for either acid or base titrations. The students should be warned that the discrete point by point first derivative should be plotted half way between the experimental points to avoid a frame shift error. The titration curve is plotted in real time on the screen and twical exoerimental errors can be spotted auicklv and corrected ear& in the titration. Atter the titration is finished a routine on the promam fmds the local maxima of the first derivative ploi wKich are the equivalence points. In all the acids tested, thc algorithm is at the 0.02-mL precision level when passing through the equivalence poi&.
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Volume NaOA (ml)
Figure 3. Titration curve and first derivative of hydrochloric acid 298
Journal of Chemical Education
Volume NaOH (ml)
Figure 4. Titration cuwe and firstderivative of phosphoric acid
svstem was considerablv less expensive than most commercial syringe pumps aione, andthe components may be used for manv other experiments. We have written alternate versions of the control code in modular form using Turbo Pascal and Quick Basic. Please contact L. Stangeland for information on the lhrbo Pascal software, and D. Anjo concerning the Quick Basic software. The titration data was stored in ASCII format disk files that can be loaded into spread sheets for publication quality graphs. Acknowledgment
We gratefully acknowledge the Royal Norwegian Council for Scientific and Industrial Research for a research fellowship and Telemark Ingeni0rh0gskole for a sabbatical leave. This work was supported by the Office of Research, California State University, Long Beach. We gratellly thank James McKibben who designed and constructed the syringe buret frame.
num: New Ymk, 1915. 2. Feld, W. A,; Shore, C. R.:P m , M. D. I n P o r s o ~ IC w & m in Chmmkfstn;Peter Lykos Ed.;Wiley: New York, 1981;p 88. 3. Cometius,R. D.; Norman, P. R. J C h . Educ. 1885,60,9649. 4.Long.d.W. J. ChemEdm. lML58.658. 6. Vitz,E. W. J. Chem. Edur 198663,804-806. 6. Cofhon, James W. The IBM PC Conneetion: S y k Berkeley, 1984.
Graphical Presentation of Acid-Base Reactions Using a Computer-InterfacedAutotitrator Michele E. lake,' David A. run ow,' and Meng-Chih Su Butler University, Indianapolis, IN 46208
Graphical presentation, with its traditional success in translating difficult concepts to sensible knowledge, remains as a basic tool-perhaps the most primary tool-for the understanding of science. In pursuing graphical presentation, one will probably deal with massive data sets. This can he awkward, if not impossible, without the help of computers. In the past two decades or so, microcomputers have redefined many aspects of science including the manipulation of huge data bases and graphics. It is now possible to paint a better picture of chemical processes and help students to learn chemistry more easily 'Present Address: Indiana University School of Medicine, lndian olis, IN 46202. 'Hogfeidt, E. Graphic Presentation of Equi\b"um Data In Treatise on Analyiical Chemistw, 2nd ed.; Kolthoff. I. M.; Elving, P. J; Eds.; John Wiley: New York, 1979: Pan I, Vol. 2. 3Leharne,S. J. Chem. Ed. 1989.66, A23S-AZ41.
Using Computers to Study Titrations In Real Time
Inspired by some theoretical attempt^,^? this work took on an experimental approach to show the intricacies of acid-base titrations. Although students have heard much about acid-base reactions, their attention has been focused on looking for the titration end point, which is signified by the color change of the indicator. The understanding of chemical changes in the titration process as a whole, particularly on a quantitative basis, has been largely neglected. The goal of this project is to help students see what occurs in the solution at everv . s i -d e step durinz the course ofa titration. Atitration is no longertreatedas the practice of seekingjust one point. It is treated as a lively chemical pmcess that changes continuously throughout its course. After some pilot work, the experiments presented here were incorporated into our sophomore analytical chemistry laboratory. With some basic training in computer use at the beginning of the semester, the students were able to carry out the experiments independently and perform data reduction on the computer. Data acquisition was accomplished by a computer-interfaced experimental setup on a point-to-point basis at the time reactions actually took place. The experimental pmcedure was controlled by a customized program. Several trials can be performed under the same conditions, thus allowing for statistical treatment on experimental results in the later data reduction procedure. In fact, class statistics were also obtained for an overall view of the experiment. Experimental The Equipment
The experimental setup consisted of an autotitration system and a computer-interfaced data acquisition system. The stand-alone autotitrator (Fisher ScientificModel 3951, originally purchased for other use, was connected to a Macintosh IIcx computer through a homemade logic gate circuitry. The circuitry, shown in Figure 1, was necessary for synchronizing the operation of titration and data acquisition. The titration rate was determined by a mechanical plunger driven by a stepper motor. It can he adjusted linearly on the dispenser module of the autotitrator. The pH changes that occurred during the titration were measured by a pair of general-purpose electrodes (Fisher Scientific Models 13-639-3 and 13-639-52), which were connected to the pH meter module of the titration system. Output from the autotitrator was translated to respective digital signals by a 16-bitresolution A/D convertor (Strawberry Tree Inc. Model ACM2-16-8)residing in the computer.
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Figure 1. Schematic diagram of logic gate circuitry in the experimental setup. Volume 69 Number 4 April 1992
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