4. Microcomputer. The microcomputer is a Commodore model 64 (C-64) with a model 1521disk drive anda Commodore color monitor. The 110 circuit is attached directly to the standard user port of this computer. The digitized signal from the pH meter is stored in a single memory location as a numher between 0 and 255. At regular intervals, based on the C-64's built-in clock, this memory location is interrogated and the number converted to a pH value based on an initialcalihration. The pH value can then be displayed, stored, and used to initiate, under program control, an appropriate action such as the addition of liquid to a chemical reaction vessel. 5. Liquid Injection Pump. The addition of liquid to s reaction vessel is accomplished hy means of a homemade syringe pump in which rotational motion produced by a unipolar stepper motor is converted to translational motion. 6. Pump Control Circuit. Digital signals generated by the computer are transferred to the stepper motor through an integrated circuit specifically designed for this purpose. 7. Reaction Vessel. For the experiments described in this paper, the reaction vessel consists of a simple glass beaker. The pH electrode is immersed in the solution and the solution itself is stirred continuously throughout the experiment with a magnetic stirring bar.
Sample Experlments and DlSCuSSlOn A methods program for the C-64 microcomputer was written in BASIC. I t provides an experimenter with a choice of four experimental methods: one for pH monitoring, one for controlling the pH of a solution in a chemical reactor, and two for titrating a solution (constant or variable volume titrant addition). Only the general features of the first two methods are discussed here in conjunction with two sample experiments. The discussion and examples of the automatic titration methods are omitted since similar descriptions have previously appeared in this Journal (1,6). pH Monitoring The hydrolysis of acetic anhydride (7)
(10, I I ) , acidbase neutralization (10, I2), and corrosion control (13). To demonstrate a simple elosed-looptype control of a chemical reaction, we selected the same reaction described above and pH as the control variable. This pravidesa system in which controlis direct (14, 15), through pH, and one that is adaptative (16) since the computer software can be used to adapt the system to different disturbances. The injection pump is filled with 3 M NaOH, and the experiment is initiated in the same way as in 1. The method program causes the computer to sense and store the pH, to compare the sensed value to a control point value of 3.7, and to attempt to hold the pH at this control point by adding base to the mixture as necessary. If the control point pH exceeds the sensed pH by more than a tolerance value of 0.1, the computer calls for the addition of one of three volumes of NaOH solution to the reaction vessel. If the difference between the control point pH and the sensed pH is great, a large volume (0.5 mL) of base is added. If the difference is moderate, a medium size volume (0.3 mL) is added, and, if the difference is small, only 0.1 mLis added. Although this isa relatively simple control function, it works quite well as can he seen from the data presented in Fienre 2h. The total volume added a t each " noint is orerented in Fieure 2c. I t is interest~~~~~ing to note that industrial pH contnd is ire. quently much looser than our tolerance ralue (9.121. In principle tighter control could he maintained hy use of a more sophisticated control function. However, the application of a rigorous control law to a nonlinear variable such as the pH of this reaction mixture would be considerably more eomplicated (11,16) and was not attempted.
.
.~~~~~~~~~~~ ..
Conclusions We feel that the experimental apparatus described gives very satisfactory results for the types of experiments in which we have tested it. Some of its limitations are: (1) the stepper motor maximum rotation speed, which limits the liquid addition to a maximum rate of 2.5 mL/min, (2) the electrode and pH-meter response time, (3) the computer software execution speed which is limited by the use of BASIC programming.
Additional improvements would include: the ability to control more than just the addition of a single solution, the ability to monitor more than one variable (in principle the computer system used should be canahle of monitoring. un. to eiaht variables s~multaneously~,and theabhty to monitor, ~nthe control mode, values that are far frvm the cmtrol set pomt.
Acknowledgment The authors wish to thank Charles F. Batten of the University of Houston for helpful comments and suggestions. This work was partially supported by Dow Quimica Mexicana and by Project INQ-048 of the Universidad Iberoamericana. The schematic diagrams of the circuits, description of the injection mechanism, parts list, and copies of the software are available from the authors upon request.
A Computer-Aided Optical Melting Point Device Michael Masterov' COOPBT Union for the Advancemem of Science & Art New York, NY 10003 Bredy Plerre-Louis Jamaica High School New York City Raymond Chuang2 Francis Lewis High Schwi New York City Most currently popular methods of determining melting points are effectively manual. While thermometers and heaters have improved, the most common detector remains the human eye (171. Alternate methods of determining melting points do exist. hut have not as yet become popular. Heat capacity methods like Differential Scanning Calorimetry (18)require large sample sizes (up to 1M) mg) and are too expensive (-$50,000) to use for routine melting point determination. A number of crystallagraph(Continued on page A76) ~
~~
is a convenient reaction with which to demonstrate the pH change that occun during the course of a simple acid-base reaction. For this experiment, 100 mL of distilled water is added to the vessel containing the pH electrode. The reaction is initiated ( t = 0) simply by pouring 3 mL of acetic anhydride into this heaker. Data acquisition is started at t = 0 from the C-64 microcomputer keyboard. The software for this method causes the computer to read and store the pH at t = 0 and at 10-s intervals thereafter until data acquisition is rerminated at the keyboard. During data srquiuition, the pH readings are dis~lnvedon the C-64 monitor. A plut of a typical set of data for this experiment is shown in Figure 2a.
pH Control The control of pH is of utmost importance in, for example, biological processes, e.g., fermentations, (8, 9), waste neutralization
Figure 2. Acetic anhydride hydrolysis: (a) monitoring. (b) pH control. (c) total volume added at each point.
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the computer bulletin bocrrd ic teehniques, e.g., NMR (19) and dilatometry (20), could he adapted t o melting point determination. hut none of these techniques detect the same effect as visual observaiion. Additionally, these techniques are also inherently expensive. A method that approximates visual observation involves measurement of light transmittance through a sample (21). A commercial device utilizing this principle is available (22). However, in cases where the solid is translucent or transparent, the detector electronics become both delicate and expensive. We should like t o reDort a n alternative uptical method that &ably determines meltingporntsrven when littleor nuchange in transmittanre occurs. No close-tolerance electronics are necessary, so parts cost can he kept under $300 in 1989 dollars. In this method, a capillary tube containingapproximately 3 mg of sample is placed in the path of an optical system where light passes through the sample and is projected onto an array of photaresistors. The image on the array depends on the configuration of the crystals and changes when they move or alter their morphologies. The array is interfaced to a computer, which calculates a rateof-pattern-change index based on the light intensity changes detected by the individual photoresistorsy The optical rate index, 1,is computed directly from the values provided by a n ADC using eq 1, where At!, and Ate, are the current and previous ADC values for the ith photocell. (1) I = Z(A,,, - A,oJ2 Noise values of I are typically under 300 units. Values of I two standard deviations above the mean of any given run ere considered significant. Any two significant values separated by less than one degree are con-
sidered part of the same peak, as shown in Figure 1,peak A. The broadest such peak is considered the melting range, and the centerpoint of the range is considered the meltine.. noint. Sienificant values far from anv . peak (singlr spikes! sornrtimrr occur as a result ofgas buhhler and mrchnniral diuturbnnres in t h ~ample. as shown in Figure 1, peak B.
-
Results The device was tested using 10 compounds, two of which, acetanilide and vanillin.. are USP meltine m i n t standards. A manual melting point of each sample was also taken by the rnpillary mrthod. The table cmtains a summary oi these resultr. For seven of the compounds, the manual melting range was entirely contained in the automatic range. The largest deviation in the other three was 0.4 'C. In all cases, the automatic melting range was larger than the manual, by a maximum of 3 "C. This is probably a function of device sensitivity. In repeated trials where an automatic melting point was run under manual observation, the device was always first to detect phase change. Where the range width is unimpor-
Scan Time (Minutes)
A76
Journal
of Chemical Education
~..
~
Conclusion Construction of the device described requires only simple, easily available components and modest techniques. In addition t o
Summary of Meltlng Polnta Results
AEetanllide Vanililn Oxalic Acid Dihydrate Resorcinol DL-Msndelic Acid 84uinolinol Benzophenone Acenapthene Benzii CyclohexanoneOxime s",,..-...*....--:-o"
m.."Lrillpslmulrr
Manual mp
Auto mp
115-118 82-83 101-102.5 111-112 119-121 73.5-74.5 48-49 94.5-98 95.5-98.5 90-92
114.2-116.7 79.8-83.8 100.8-102.9 110.7-112.2 117.8-121.1 73.1-74.7 48.3-49.4 94.9-96.6 95.7-97.1 89.9-92.0
~neralure ~efs= mpn
Compound
I>,
113-116 77-83 101-106 109-111 118-122 72-76 48-51 93-96.2 94.9-96 89-91
is. Za3a
ib, 26.36 ic, Zc, 3c ld.Zd.30 le.Ze3e if, 3f lg, Zf, 39 lh, 29, 3h li, Zh, 3i 11, 3j
Centerpolm w a n SD 115.6 82.1 102.0 111.7 119.8 73.9 48.8 95.5 96.8 90.9
0.12 0.44 0.27 0.15 0.41 0.17 0.15 0.16 0.14 0.25
Trials (Auto) 9 11 8 8 8 8 9 8 8 8 -
r.
oLiterafuremelting paint.
are
overlaps of values given
by CRC
(251, Shrlner 6 Fuion (26).Lange's (27)and me
manufacturer.
Reference pages given below: I-CRC: -C57: bC569; +C409: bC503: e--C367; f--C496:P C l E O ; hC65; cCl46:+C252 2-Shrine, 6 Fuson: -557; b604: -599; 6602: -595; t580: -577; h 5 6 0 S L a n g e ' s : e--7.84:b 7 . 4 4 8 : -7.570; b7.318: -7.475; 1-7.458: F7.134: k 7 . 8 2 : 1--7.131:+7.251
0 F l w e 1. Sample melting point scan-benzophenone; A. Melting range; B. Gas bubble spike.
tant and the only concern is a melting point, the centerpoint of the range should be used as i t remains constant from determination to determination. The worst case standard deviation observed for the centerpoint was 0.44 'C, and six of the 10 compounds tested had standard deviations under 0.2 'C. In addition, only two of the 10 cases had a centerpoint outside t h e literature melting range, and only one outside the manual range. The worst deviations were only 0.8 OC and 0.3 'C, in both eases for henzil. The device was also used to analyze sodium acetate 25 times. Figure 2 shows a typical result. Merck Index (23) reports a melting point of 58 'C for sodium acetate trihydrate and adds that a t 120 'C it becomes completely anhydrous. Sokolov (24) reports nonspecified transitions a t 58,118,130, and 239 'C. Roth e t al. report fusion a t 332 "C and a nonspecified transition a t 64 OC (24). No reference has been found for the other peaks shown in Figure 2.
io
35
Scan Time
105
(Minutes)
Figure 2. Sample scan of sodium acetate hydrate: A, 58 OC point: 8.64 'Cpoint: C. 118-12OoCpoint; omer points not found in literature.
r r l i r v i n g t h r tedium o f m e l t i n g p o i n t determ i n a t i m a , the d e v i c e s h o u l d i m p r o v e the
precision of these determinations by eliminating human judgement from the process. The device shows potential in the field of quality control. In addition f u r t h e r work is in progress to see if it can also he u s e f u l in the analysis of liquid crystal phase transitions.
Acknowledgment
The authors are grateful to John L. Bove, Chemistry D e p a r t m e n t , Cooper Union, for helpful advice with this work, to Michael Eilenfeldt and Kenneth J. Eltar for help with machining, and to the Dana Foundation and the Hehrew Technical Institute for partial funding of this work.
' S e n d inquiries to Cooper Union, ChE Dept. 51 Astor Place, New York. NY. 10003. Arm: M. M a s terov or J. Bcve. Detailed circuit diagrams will be provided on request. Currently st Cmper Union, NY.
1. Verbeek,A.A. J . Chem.Edue. 1985,62.687-688. 2. Vitz, E. W. J. Chem. E d u c 1986.63.803-804. 3. Williams, K. R.; Eyler, J. 8.: Colgate, S. 0.J . them. Edur. 1987.61.499400. 4. Arocs. P., Jr.: Ames. R. J . Chem.Educ. 1987,61,10171018. 5. Ghaff8ri.S. J. Chom.Educ. 1988.63.344. 6. Greenspan, P. D.; Burchfield. D. E.; Veening, H. J. Chem. Educ. 1985.62,68%690. 7. Greenberg, D.H. J. Cham.Educ. 1962.39.14&144. 8. veres. A ; Kurucz. L.; Nyeste, L.; pungor, E., Jr.; Kirchknoff, L.: Ha116, J. Hung Sci. Inrlr. L9RO,~, 5-9. 9. Jacob~.O.L.R.;Hewkin.P.F.;Whilo,C.IEEP~ocsrdinps 1980,127D141, 161-168. LO. Crsy, D. M. Leeds and Northrup Instruments Co.. 1984, Reprint C2-1043-RP 30.584. 11. Jacobs, 0.L. R.; Badran. W. A,: Proudfoot. C. s.: While. C. IEEPmc. 1987.131D(31, 1962W. 12. L0ve.J. Chem Eng. 1982, July, 2C27. 13. Dillon. C. P. Corrosion Control in the Chemical Process lndwlrias; MeGrsw-Hill: New York, 1986: p
1980: Chapter 7. p 183. 16. Custafsson,T. K.: Waller. K. V. AICHE J . 1986,32(21,
Belmont. CA, 1988; pp 761-766. 19. Lawson, K. D.; Flauft, T. J. J. Phya. Chem. 1965. 69, 4256-4268. 20. Vold, M. J.; Mecomber, M.: Vold, R. D. J. Am. Chem. Soc. 1941,63,168-175. 21. Hentan. D. P.: Howe, P. G. Con. J. Cham. 1955, 33. ,,a, .,,--..,n.. ."O.
22. Sorgent-Welrh Cofolog 133: Skokie, IL. 1985: p 820. 23. Windhuh. M.. Ed. The Merrk Index, 10th ed.: Rshwav. NJ. 1983: o 8407. M.: ~ranz&ini,P., Edn. Thwmadynnmic and 24. S&. ~ P I soits: pegamon: Transport P I D P ~ ~of ~organic Elmsford. NY. 1980: Chapter 1.2. lncludosRothsnd
"~,*".". .-. Q"b",".,
=.n...-o.
25. Weast. R. C.. Ed. CRC Hcndbook ol Chemistry rind Physics. 63rd ed.: Chemical Rubber: Cleveland,
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