Chemical Instrumentation S. Z. LEWIN, New York University, Washington Square, New York 3, N. Y.
Thisseries of articies presents a suruey of the basic principles, characteristics, and limitations of those instruments which find important applications i n chemical work. The emphasis i s on commercially available equipment, and approximate prices are qwrted to shuw the order of magnitude of cost of the various types of design and construction.
16. Electroanalytical Instrumentation (Continued) The design of commercial electroanalytical instruments reflects the type of senice for which the device is intended, and ranges from simple, manual, inexpensive apparatus for occasional aamples or for teaching purposes, to elaborate, automatic, multifunctional equipment for rapid and repetitive or far special purpose analyses. The variety of designs possible is dramaticrtlly illustrated by a considwation of the types of commercially available polarographic instruments.
Polarographic Instruments A polarographic instrument is, by definition, any device which exploits the principles of polarograpby, and i t is natural to designate such an instrument as a "polarograph." However, the name "Polarograph" has been accepted by the U.S. Patent Office as a trademark of E. H . Sargent and Co., Chicago, Illinois, and
other manufacturers have avoided conflicting with this trademark. Hence, a variety of names have been introduced to designate instruments all of which are, in fact, polarographs. Examples of such names are: Electrochernograph (Leeds and Northrup); Elecdropode (Fisher Scientific); Voltamoscope and Voltamograph(Cambridge1nstrument Co.); Polarametrie Analyzer (American Instrument Co.); Polariter (Radiometer); Polarotrace (Southern Instruments Ltd.); ElectroPolarieer (American Optical Co.); Polarlog (Jarrell Ash Co.); Polarecord (Metrohn~). Palaragraphy was invented by Heyrovsky in 1922, and the design of a photographically-recording instrument was published by Heyrovsky and Shikata in 1925. The first ~ o l a r o e r a ~ htos become available in the u:S. were imported from Czeehoslovnkia by E. H. Sargent and Co. begin-
c
€5
ning about 1933. Since about 1939, Sargent has manufactured ita polarographs in its own plant. The first pen-recording polarograph was produced by Radiometer, Copenhagen, in 1938. The first U. S.built polarographic instrument was the Fisher Elecdropode, introduced in 1939. The simplest design of commercially available instruments is illustrated in Figure 9; the equipment consists of a battery, Es, in series with s. variable resistor, RI, to adjust the voltage span of the instrument, and a variable resistor, R*, which serves aa a voltage divider for applying a variable voltage to the polarographic cell. The cell current is measured by means of a galvanometer, G, the sensitivity and damping of which are adjusted by the shunt resistor, Ro.
The Slidewire If the values of the resistances and the characteristics of the galvanometer are properly chosen, the palamgraphically significant data, vis., i c e i t a8 a function of E,,u, can be read directly from the setting of the slidewire contactor on R1 ( =E..u) and the corresponding maximum excursion In of the galvanometer index (=i,n). order for the voltage being applied to the cell by the slidevire bridge to correspond to s. fixed linear scale inscribed on that variable resistor, it is necessary that the cell resistance be large compared to the slidewire resistance. The hasis for this requirement is illustrated in Figure 10.
CELLVOLTAGE
Figure 10. The polorographic cell conrtitvtes o rerirtonce, R r r u which i3 in porollel with a portion, RA, of the slidewire rerirtonce. Only if R r s u is large relative to R.4 is the voltage applied to !he cell given by the voltage divider relation,
E..v = IRnIR"
CELL
Figure 9. Boric circuit of simplest commercial polarographr. The battery voltage, Es i s Adjustment of R1 cantrdr divided into two ports, h and EX, b y the voltage divider, RI Rp. the .pan d voltages tho! can be applied to the poiomgraphic cell by the rlidewire contador.
+
+ RBI
X E.
The cell constitutes a resistance, R..rr, which is in parallel wit.h that part of the slidenire resistance being tapped off, viz., RA. Hence, the effective resistance of this pnr~llelcombination is: (Continued on page A356) Volume 39, Number 5, May 1962
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The total voltage applied to the slidewire, E, divides between the two sections demarcated by the rontactor in direct proportion to the resistances of those sections; vim.
If R,,, is large compared to RA, then
and the total voltage divides simply in the proportion Rr/Rn. Hence, if the slid*
wire is marked off into, e.g., 1000 equal divisions, the setting of the eontactor can be read as proportional to the cell volhge, or can he converted into voltage units simply hy multiplying by: (span voltage)/ 1000. The error involved in this direetreading approach is, then, proportional to the ratio of Ra ta R,,,. If RA is 100 ohms and Il,.u is 10,000 ohms, then RsI1 = 99.0 ohms, and EA will be smaller by 1% than i t would be if RGeliwere not connected t,o the slidewire. I t will be noted that,, in this example, the magnitude of En is 1% of that of R,zr. To reduce the voltage read-out error to 0.1%, i t would be necessary to make RA eoual to 0.1 0/, of R..tr. However, as the resistance of the slidewire is reduced, it is necessary to employ proportionately larger currents through this component inorder tomaintaina givcn total span voltage. Large currents are
undesirable because of their heating effects and the drain on the battery, Leading to instability, electrical noise, and short life of components. Hence, in eommereinl polarogmphs of this design, 8, compromise is npcessary, and a slidewire resistance in the neighborhood of 50 to 100 ohms is common.
The Galvanometer The other critical component in this type of polarograph is the galvanometer. I t must have a sensitivity adequate for the displav of small currents, yet must he rugged enough for service under average laboratory conditions, and must be eauabk oi followinz the ranid oscillations of
Figure 11. Oplicol design principle of a high. senritivity, golvmameler based upon o folded oplical lever arm of long total path length.
The type of galvanometer design used in theso polarogr~phsis illustrated in Figure 11. A coil of wire is mounted in the gap hetween the poles of a permanent magnet and is held in place by a suspension nirc: anrhored a t brackets above and below the roil. The sensitivity of the galvanometer is directly proportional to the number of turns of wire in the coil, the area of the mil, the magnetic field strength, the length of the suspension wire, and the optiral lever arm (i.e., the distance hrtween t,hr mirror mounted on the eoil and the read-out scale). It is inversrly proportional to the torsional moduhis (i.r., elasticity) of the suspension wire material and the radius of this wire. I n order to achieve strength and minimize sensit,ivity t,o vibrations, i t is necessary tn nse suspension wires that are not too thin or long; to compensate for this factor, whirh would decrease the sensitivity, the optiral lever arm is made very long, yet the entire inst,rument is kept to s. reasanahle size, by using mirrors to give s folded nptiral path.
Damping
If a current is applied to the eoil of a galvanometer, its magnetic field, interaetinp with the field of the uermanent marmrt. pr
s
,o
,,
Figure 16. Illustrating the we of a compenration current in poiarography. The polarogrm a t the left shows thot o rmoll mmount of rinc in the presence of a larger amount of cadmium giver a wave that connot be measured conveniently because the large cadmium diffusion wrrent requires a low renritivity rening d the gdvonometer shunt. If the cadmium wove is bucked out, the sensitivity con be increased, and the rinc wove con be magnified, as shown by the curve a t the right.
An example of the type of instrument described by Figure 15 is the Model 111 Polamgraph ($495) of E. H. Sargent and Co., Chicago 30, Illinois. The slidewire consists of a. ten-turn helical potentiometer linear in resistance within &0.1%. A single-turn potentiometer serves as the span voltage control, and this voltage is displayed on a 0-3 v voltmeter. The Ayrton shunt has ten positions with ratios ranging from 1: 1 to 1000: 1. The galvanometer sensitivity is 0.006 microamperes per mm. The galvanometer spot is read out on a curved scale (to eliminate the so-called "tangent error" produced when an angular deflection is read out on a flat scale) that is 315 mm long. The instrument bridge is batterypowered; the galvanometer lamp operates from a 110-volt ao line. The Fisher Scientific Co., Pittsburgh 19, Pennsylvania, manufactures a. manual polarogrqh designated as the Model 65 Elecdropode ($590). The compensation voltage is obtained from the rectified, regulated output of a silicon Zener diode circuit, powered by the 110-volt ac line. The slidewire bridge voltage is also obtained by rectifying the ac line. A mercury battery is provided for standardization of the bridge voltage; the galvsi nometer is used as the null-balance indicator for thisoperation. The Ayrton shunt has nine ranges, from 1:1 to 400: 1. The galvanometer sensitivity is 0.025 microamperes per division on an 11-in. flat scale (corresponding to 0.01 # a per mm on (Continued on page A37Z)
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wire rontsctor, and a self-balancing potentiomrtrir recorder (Model 6G Recording Acrrssary, W i 5 ) . A manual palarograph designed for rapid, routine analytical applientions is the Manual Polarogrnph A1650 (ahout $500) made by Southern Instruments Ltd., England (i~vailal)lein the U. S. through Standard Scientific Supply C o p , New York 3, N. Y.). The slideviro bridge control consists oi two dials which switch in fired inrre~nentsof voltage, in steps of 100 millivolts for one. and 10 millivolts for the other. This permits a given cell valti~geto be obtained quickly and reprodueibly, and facilitates repetitive analyses. A standard eell is included in the circuit so that the slidewire voltage can be standardized when desired, using the galvanometer as s. null-point indicator. A compensation circuit is provided ior t,he buekine-out of unnnntrd waves when major constituent. A feature ineorporsted in the design of this instrument that has not heen previously discusscd is the "counter current" circuit. In the palsrographic cell, the interface between tho mercury droplet and the electrolyte nct,s as a condenser ( r s paeitor), and each drop charges up just as a condenser eharges when connected across a voltage source. The quantity of elec-
lincarly increasing background, or "eondenser" current in a polarograrn, and leads to a pronounerd slope of the currentvoltage curve in the regions between polarographir waves. This eRert herarncs important when small waves are heing studied a t high sensitivity.
Figure 17. Illustrating the use of o counter-, or condenser-current control in polarogrophy. At high galvanometer ren.itivitier, the pronounced slope of the residual current may be troublesome, as in the core of lhe curve ot the left. It con be bucked out by applying to the gdvanorneter a linearly increasing current in opposition to the cell current, to give the result shown in the curye at the right.
The precision of estimation of the height of a wave can often he improved by bucking out the condenser current by means of n. "counter current" which is caused to increase linearly with the applied voltage.
(Continued on page -4374)
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Chemical instrumentation Figure 17 show the effect of the use of this counter-current technique in sharpening a polmographic wave. The countercurrent circuit involves the addition of a voltage divider across the slidewire bridge in such a way that a fraction of the voltage being applied to the polarographic cell sends a current through the galvanometer in the opposite direction to that of the cell current. The counter-current control adjusts the voltage divider ratio, i.e., it programs the bucking current. Other manual, galvanometer read-out polarographs that incorporate some or all of the features described in the preceding paragraphs are: Amineo Polarometric Analyner, American Instrument Co., Silver Spring, Maryland; Abresch Polarometer, H. Geissler Nachfolger, Bonn, Germany; Voltamoscope, Cambridge Instrument Co., New York 17, N. Y.
Bibliography DELAHAY,P., "New Instrumental Methods in Electrochemistry," Interscience Publishers, N. Y., 1954. LINGANE, J. J., "Electro~nillytical Chemistry," Interscience Publishers, N. Y., 2nd Ed., 1958. R E I L ~ YC. , N., COOKE,W. D., AND FURMAN,N. H., "Thee-Dimensional Model for Interpreting Eleetrometric Processes," Anal. Chem., 23, 1226 (1951).
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REILLEY,C. N., AND MCCURDY,W. H., JR., "Principles of High-Frequency Titrimetry," Anal. Chem., 25,86 (1953). REINMUTE,W. H., "Three-Dimensional Representation of Voltammetric Processes," Anal. Chern., 32, 1509 (1960). TAYLOR,R. P., AND FURMAN,N. H., "Constant Current Conductance," Anal. Chem., 24,1931 (1952).
HEYROVSKY, J., "Trends in Polarography," Science, 132, 123 (1960). KELLEY, M. T., and FISHER,TD. J., "Instrumentd Meththo of Derivative Polarography," And. Chem., 30, 929
BARKER,G. C., "SquareWave Polarography, and Some Related Techniques," Anal. Chim. Acta, 18,118 (1958). R. L., BARKER,G. C., and FAIRCLOTH, "Twin Electrodes in A. C. Polarography," J. Polarographic Soc., 1, 11
KOLTHOFF,I. M., and LINGANE, 3. J., "Polmogri~phy," Interscience Pubs., N. Y., 2nd Ed., 1952. LINGANE, J . J., "Operating Characteristies of the Sargent Model XX Visible Recording Polarography," Anal. Chem.,
(1958).
KELLEY,M. T., JONES,H. C., and FISHER; D. J., "Controlled-Potential and Derivative Polarograph," Anal. Chem., 31, 1475 (1959).
(1958).
BARKER,G. C., and JENKINS, I. L., "Square Wave Polarography," Analyst, 77,685-96 (1952).
BAUER,H. H., and ELVING,P. J., "Alternating Current Palarography," Anal. Chem., 30,334 (1958). CHARMT, G., ed., "Modern Electroanalytical Methods," D. Van Nostrand, Princeton, N. J., 1958. DELAHAY,P., and MAMONTOV,G., "Voltammetry a t Constant Current,'' And. Chem., 27,478 (1955). FEREEW,D. J., and MILNER,G. W. C., "Analyticd Applications of the Barker Square Wave Polarograph," Analyst, 80,132 (1955); 81,193 (1956).
FERREW, D, J., MILNER, G. W. C., SHALGOSKY, H. O., and SLEE, L. J., "A Comparative Study of Three Recently Developed Polarographs," Analyst, 81, 506 (1956).
18,734 (1946).
MEITES,L., "Polmographic Techniques," Interscience Pub., N. Y., 1955. MILNER,G. W. C., "Some Recent Developments in Polarography," J . Polzrtrographic Soe., 1 . 2 (1958). MILNER,G. W. C., "The Principles and Applications of Polmography and other Electromalytical Processes," Longmans, Green and Co., London, 1957. M ~ L E R ,0. H., "The Polarogrrtphic Method of Analysis," Chemical Education Pub. Co., Easton, Ps., 2nd Ed., 1951.
TAYLOR, J. K., "Examination of Absolute and Comparative Methods of Polerographic Analysis," Anal. Chem., 19, 368 (1917).
Nezt: Conclusion of the sumey of electroanalytical instn~nzentation.
