Topics in..
.
Chemical instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers o j ~ mJOURNAL s by calling attention to new developments in ihe t h e w , d e w , or availability of chemical laboratoy instrumentation, or presenting useful insights and explanations of topics that are o j practical imporlance to those who use, or teach the use of, modern instrumentation and i n s t m n t a l techniques. The editor invites correspondence from prospective contributors.
LXIV. Laboratory Integrators Galen W. Ewing Probably the best known applicatibn of an integrator in the analytical laboratory is in measuring the area beneath a. peak in a chromatagram. Another example is found in nmr spectroscopy. A similar need appears in any instrumental method where the magnitude of a. signal is proportional to the magnitude of the desired quantity, and in which the signal is scanned with respect to time or some other independent variable. An integrator is also required in coulometry, particularly in coulometrio titrimetry; for this applicrttion it usually goes by the name coulometa. In addition to these obviously chemical uses, integrators appear as internal components of a number of instruments. I t is an integrator which supplies a voltage proportiand to time (a ramp voltage) in a polarograph. Integrators are fundamental to multichannel analyzers and computers of average transients. They can be employed as noise rejection filters and in the measurement of electrical noise. The methods by which integration can he performed are conveniently classified as nonelectrical, electrochemicd, eleotricd analog, and digital.
Figure 1. Plonimetsr. The fulcrum is placed a t some convenient Rxed point on the drawing board or table.
is a. mechanical device consisting of two h m connected at a. swivel joint. One of the bars (the "pole arm") is free to rotate around s. fixed point or fulcrum. The second bar (the "tracer arm") hems at its fm end a stylus with which to trace out the curve-whose ares. is to be integrated. At the swivel point there is a carriage, rigidly attached to the tracer arm, in which is mounted a wheel with its axis parallel to the tracer arm. The wheel rests on the paper and turns in response to any component of carriage motion normal to the tracer arm, while slipping over the paper without turning if motion is prtrallel to the m. It can he shown m&hemstically that the number of turns
of the wheel (indicated on a counter) gives directly the ares. enclosed by the curve, when the stylus has completed tracing its periphery. Clearly none of the above mentioned methods are appropriate where high accuracy is desired. There are a number of mechanical devices which can effect integration. A p parently the only one to he utilized extensively in lzhorrttory instrumentation is the hall-and-disc mechanism. [For other devices, see Ref. (I).] The balland disc 'device is exemplified by the unit manufactured by Disc Instruments as an accessory to he installed in nearly any strip-chart recorder (Fig. 2). The device consists of a rotating disc in friction contact with s. hall, which in turn makes contact with s. roller. The roller rotates as a. result of power transferred to it by the ball and disc. The ball is held in position by a retaining frame which can move radially across the disc. This linear motion is taken from the pen oarrirtge of the recorder, and the rotation of the disc is synchronous with the p%per feed. Mathematical andysis shows that the rotation of the roller is proportional to the area. beneath the curve being traced on the recorder.
Electrochemical Integrators The classical silver coulometer consists of a pure silver anode suspended inside a platinum crucible which acts as cathode, filled with a solution of silver nitrate. The weight of silver deposited on the crucible during an experiment represents the timsiutegral of the current passed, hence the quantity of electric charge involved. Many other chemical systems lending themselves to coulometry have been described (2). The ultimate measurement in chemical
Non-Electrical Integration The area. heneath a curve can he estimated geometricdy, as by counting squares on graph paper or approximating a triangular fit to a Gaussian curve. The latter operation can he facilitated by a. specid transparent plastic overlay distributed hy Carle Instruments under the trade-name "Basic Wedge Integrator." A similar technique involves cutting out the area to be integrated and weighing it. If this procedure is to he followed, it is recommended that a xerographed copy he cut and weighed rather than the original recorder chart. This not only preserves the original record, hut improves precision, since copy paper is usually heavier than chart paper. Area measurement can be instrumented by means of a planimeter (Fig. 1). This
-I Figure 2. Principle of the boll-ond-disc in legrotor. Tho second lidled ball is not essential, but merely a design convenience. The rotation
of the mller is by o"rpirol.in, rpiral.out" cam and oduoter an auxiliary pen to record the numbor of revolutions. (Dilc inrtrumentrl
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p~
coulometem may be gravimetric, as above, volumetric (as when a gas is evolved), or colorimetric (applicable if a light-absorbing substance is formed or destroyed). Another type of chemical integrator depends on the transfer of mercury from one electrode t o another identical one (Fig. 3). The two mercury columns are held apart by surface tension in a finebore glass capillary. The gap between the electrodes is completely filled with a solution of a mercury salt. Current passing through effectively transfers mercury from mode to cathode, so that the aqueous gap moves toward the anode. The position of the gap can be gaged visually against an engraved scale, or more precisely measured with a micrometer microscope. These simple and inexpensive integrators are widely used as elapsed time indicators in life and performance testing and similar long-term applications. I n one representative model the gap moves 2.5 cm in 1 hr for a current of 1 mA, or in 10' hr if the current is limited to 1FA. One model is provided with a closefitting metal sheath (Fig. 4). Connection with alternsting current provides an output directly proportional to the length of the ungrounded electrode. I have seen no report of this integrator used as a coulometer, hut it should be so applicable, within its ratings, particularly for lengthy experiments with small currents. Texas Research and Elect,ronic Corporation manufactures s. device known as a solion which can integrate current as a function of time by electrolysis of an iadine-iadide solution between platinum electrodes (3). The liquid-filled glass ~~
~
A B C
Figure 3. (0) An eladmchemicol tronsfer integrator. A and C are electrodes of the same metol; the g a p B contains an aqueous solution. (b) "lndoshmn" elopred-time meter. (Curtis Inrkuments, Inc.)
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Figure 4. A mercury elapsed-time integrotor orranged for electrical reodovt. (Curtis Inbumentz, 1nc.J
cell (Fig. 5) is provided with four platinum electrodes, designated respectively as the input (I),shield (S), readout (R), and common (C) connections, R and S being perforated. Negative current i passing through the cell from I to C causes iodine to accumulate in the space between C and R, its amount given by Faraday's law as proportional to Q, the quantity of electricity,
Q
=
J-idt
The output meter connected t o R sees 8. current io which is proportional to the iodine concentration, hence t o Q,
io
=
KQ
=
K f idt
where K is a sensitivity factor. The purpose of electrode S is t o prevent diffusion of iodine across the cell. The accuracy of the solion integrator is of the order of 1% of full scale, which can be improved to 0.27& by special selection.
Electrical Analog Integrators A familiar special-purpose integrator is the watt-hour meter which provides the basis for our electricity bills. This consists of a low-inertia motor which rotates a t a speed determined by the current passing through its windings. (It only measures power because it is operated a t constant voltage.) The motor is geared t o a dial indicator as readout. The same principle is utilized in a laboratory integrator, primarily for coulometry, in which a low-inertia, permanentmagnet motor is electrically in series with the load (e.g., the coulometric titration cell). The armature of the motor drives a. mechanical counter that can be calibrated to read directly in milliequivalents of chemical effect.
Figure 5. Four-electrode rolion connected a= an integrator. (Texas Research ond E k f r o n i c s
Gorp.)
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Chemical Instrumentation
Figure 6. Operational omplifler integrotorr: (01 for small surront inputs, Et, = -Il/C) f li. dt; fbJ for voltage inputs, E.,t = -11 /RCl f Ern
dt.
1
Undoubtedly the most versatile analog integrator for laboratory instrumentktion is provided by an operational amplifier with capacitive feedback (4, 5). Figure 6 shows two modifications of this circuit. One or the other can he applied in any situation where the signal t o be integrated can be obtained as either a current or a voltage, and this is true of nearly all instrumental methods. These integrators are easily designed and constructed by amsteurs, since operational amplifiers (and their power supplies) are readily available and inexpensive. The range of currents over which the integrator of Fig. 6a will operate can be varied by selection of the capacitor, and extended t o large currents by inclusion of a. shunt resistor across the input. The voltage integrator (Fig. 6b) can be ranged by adjustment of either R or C via multiposition switches. The output in either case can be read on a voltmeter or recorder. [For more details, consult Ref. (6) and literature from manufscturers of operational amplifiers.]
Digital Integrators For many purposes, particularly the evaluation of chromatogram and similar peaks, the most convenient integration is provided by a digital integrator. This instrument is based on a voltage-tofrequency converter, an elementary example of which is shown in Figure 7. If the signal t o be integrated is a current, from a high impedance source, a preamplifier converts it t o an equivalent voltage. This voltage then chargy a capacitor C, through resistor Rl. U" designates a. component with a breakdown potential such that when the rising voltage on C, reaches a critical point, U abruptly starts t o conduct, discharging the capacitor. The sudden change in potential is conducted through coupling capacitor Cz t o a digital counter. [Component U can be a neon lamp, a small thyratron, a unijunction transistor, or a thyristor (6).] The number of counts registered per unit time relates directly to the magnitude of the input current. The count is summed over the time during which the peak is scanned. A chromatography integrator may use, for example, a. conversion factor of 1000 counts per millivolt per second (referred to the voltage a t the output of the preamplifier). The voltage may vary
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Table 1.
Chemical Instrumentation
Compound Pro ane i&tme %-Butane Butene-1 ;so-Butene trons-Buten+2 cis-Buten-2 1.3-Butsdiene -.
Comparison of Precision of Integration Methods (71
HXWX Planimeter Triangulstion '/rH Cut & Weigh Avz. rel. Avg. w rel. Avg. s rel. ATE. c rel.
Ball & Disc Avg. rel.
0.04 4.48 14.01 18.52 8.12 20.18 16.11 18.19
0.04 4.57 14.05 18.73 8.07 20.21 16.06 18.28
.-
20 1.65 5.64 3.29 5.67 6.49 3.66 2.03
0.05 4.79 13.70 18.52 8.55 20.32 15.83 18.25
14 8.77 4.53 4.75 3.74 3.25 1.58 1.81
0.04 4.52 13.56 18.66 8.56 20.07 16.41 18.17
12 3.76 1.62 3.05 2.22 2.04 3.71 1.65
0.005 5.04 14.99 18.40 8.01 20.09 15.85 17.58
14 1.98 2.80 1.22 2.12 1.39 1.45 1.19
.-
.
Digital Avg. rel.
18 0.03 10 0.44 4.56 0.88 1.00 14.06 0.36 2.46 18.13 0.27 2.60 8.24 0.49 1.14 20.16 0.20 0.68 15.95 0.38 0.71 18.27 0.49
YIeDlSlOn-
-
(c rel.)
Figure 7. A voltage-to-frequency converter. For the nature of component "U" see text.
during an experiment b y a. factor of 10' or more, which corresponds to a counting frequency between 1 Hz and 1 MHz, so t h e ent,ire circuitry must be strictly linear aver this rather large range. Figure 8 shows a typical digital integrator for chromatography.
4.06%
4.06%
There are other electronic principles usable for digital integration, corresponding generally t o t h e various types of digital voltmeters; see for example, Ref. (6). A major advantage of digital integration is that it is easily incorporated into computer systems of overall d a t a reduction and sometimes control of t h e analytical tools, matters beyond t h e scope of this review.
Relative Merits
Figure 8. A digital integrator for chromotompplicatiang the Auto-Pro 30. (Beckmoo lnrtrumentr, I n 4
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2.58%
1.74%
1.29%
0.44%
Exoludea propane peak.
Each of the devices and methods for integration have their merits. At one extreme, we encounter negligible cost for t h e geometrical methods, accompanied by greater expenditure of operator's time, as well as lower precision. At t h e other
end of t h e scde is the high degree of precision and convenience of t h e digital iinte grators, which are, however, quite expensive. M a n y operators find t h a t one of t h e intermediate types, such as t h e hdl-and-disc integrator instslled in a r e corder is satisfactory. For some purposes, of course, short-cut methods may be a p propriate. In coulometric titrimetry, for example, s. coulometer can often be r e placed b y a constant current source and timer. Table 1 gives t h e integrated results of a. chromatographic separation of volatile hydrocarbons evaluated b y six techniques (7). T h e relative precisions and the times required for evaluation may be taken as representative.
(Conhued a page AS40)
Chemical instrumentation References (1) Ho~zsaoor,W.
(2)
(3) (4)
(5)
(6) (7)
G.. "Instruments for Mensurement and Control." Reinhold, N. Y., 1962. 2nded.: p. 128-134. E w r m , G. W.. "Instrumental Method8 of Chemical Anslysis," MoGraw-Hill, N. Y., 1969, 3rd ed.; p. 323-325. Hnno, R. M.. and LANE,R. N.. J . Eladrochem. Soo.. 104. 727 (1957). ~ ~ 1 P..550. V ~ a s o e .B. H.. and Ewma, G . W., *'Anslog and Digital Electronics for Scientists." Wiley-Intersaieoce, N. Y.. 1979. KAY,B., and H ~ n x o sJ. , L..Hewldt-Pockard J O U ~ O ~ao . (7), 2 (1969). GIGL.J. M., HZTBBARD. J. R., and DOPRE. G. D.. Tech. Bull.. 101-71 (1971), Vidar AutoLab. Mountain View. Calif.
(n,
List of Manufaclurers Planimeters Keuffel&Esser Co., 20 Whippany Rd. Morristown, N. J. 07960 Royson Engineering Co. 101 N. Penn St. Hatboro, Pa. 19040 Technieon Chromatography Corp Research Park Chauneey, N. Y. 10502 Mechanical Disc Instruments Div. of Finnigm Instruments Corp. 2701 S. Halladay St. Santa Ans, Calif. 92705
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General Precision, Inc. 6511 Mort,on Grove. Ill. 60053 .-Royson Engineering Co. 101 N. Penn St. Hatboro, Pa. 19040 Genmetrical Carle Instruments, Inc. 1141 E. Ash Ave. Fullerton, Calif. 92631
Electrochemical Bergen Laboratories, Inc. 60 Spruce St. Patemon, N. J. 07510 Curtis Instruments, Inc. 200 Kisco Ave. Mt. Kisco, N. Y. 10549 Mast Development Co. 2212 E. 12th St. Davenport, Iowa 52803 Texas Research and Electronics CI 6612 Denton Drive Dallas, Texas 75221 Electrical Analog A. R. F. Products, Ine. Grtrdner Rd. Raton, N. M. 87740 Elcor Div. of Halliburton Co. 2431 Linden Lane Silver Spring, Md. 20910 Esterline-Angus Div. of Esterline P. 0. Box 24000 Indirtnapolis, Ind. 46224 Infrared Industries, Inc. P. 0. Box 989 Santa.Barbara, Calif. 93102
Leeds & Northrup Co. North Wales, Pa. 19454
DC Mob? Acromag, Ino. 30765 Wixom Rd. Wixom, Mich. 48096 General Precision, Inc. 6511 Oakton St. Morton Grove, Ill. 60053 Instron Corp. 2500 Washington St. Canton, Mass. 02021 Perkin-Elmer Corp. 800 Main Ave. Norwalk, Conn. 06852 Roekwell Mfg. Co., Republic Div., 2240 Diversey Pkwy. Chicago, Ill. 60607 Constant Current, Timed A. R. F. Products, Ine. Gardner Rd. Raton, N. M. 87740 Leeds & Northrup Co. North Wales, Pa. 19454 McKee-Pederson Instruments P. 0 . Box 322 Dandle, Calif. 94526 Metrahm, via Brinkmann Insts., Inc. Cantiawe Rd. Westbury, N. Y. 11590 Sargent-Welch Scientific Co. 7300 N. Linder Ave. Skokie, 111. 60076 Digital AutoLab Division of Spectra Physics 655 Clyde Ave. Mountain View. Calif. 94040 Beckman ~nstruments,Inc. 2500 Harbor Blvd. Fullerton, Calif. 92634 Buxeo Electronics, Inc. W. Woods Rd. Sharon, Conn. 06069 Fischer & Porter Co. County Line Rd. Warminster, Pa. 18974 Halmar Electronics, Inc. 1544 W. Mound St. Columbus, Ohio 43223 Hewlett-Packard Co. Rte. 41 Avondde, Pa. 19311 Infotronics Corp. 2475 Broadway Boulder, Colo. 80302 Inseo, The Instrument Co. P. 0. Box 3770 Santa Bmbara, Calif. 93105 Kent, via Andtech, Ine. Blue Hen Industrial Park Newark, Del. 19711 NesterIFaust Div. of Perkin-Elmer Corp 2401 Ogletown Rd. Newark, Del. 19711 Perkin-Elmer Corp. 800 Main Ave. Norwalk, Conn. 06852 Royson Engineering Co. 101 N. Penn St. Harboro, Pa. 19040 Seientific Industries, Ino. 150 Herrieks Rd. Mineola, N. Y. 11501 Varian Aerograph 2700 Mitchell Drive Walnut Creek, Calif. 94598
