Chemical Instrumentation 5. Z
LEWIN, New
York University, Washington Square, New York 3, N. Y.
.'.am feature
displaced downscale by ss much as 10 times the full scale deflection of the cl~racteristics, and limitations of those instruments which find important galvanometer, or upscale by up to 1.5 applications i n chemical work. The emphasis is on commercidly available times full scale. eq~~ipment, and approximate prices are quoted to show the order of magnitude If a large average cell current is being of cost of the various types of design and construelion. bucked out by means of a compensator circuit, the magnitude of the current oscillations associated with the birth, growth, and detachment of the mercury k n o t affected. Hence, if compensa~ ~ o n t i n u e ~ drops ] tion ia employed to permit use of a higher sensitivity setting of the galvanometer means of a conventional galvanometer, The msoual pal~rographis, despite (or, shunt, these current oscillations become which reflects a light spot onto the photorather, because of) its simplicity, the the limiting factor in the precision with graphic paper. The displacement of the instrument of choice for a wide range of which the polarographie wave e m he light spot along the axis a t right angles to eleetroanalytical applications. I t is to be measured. Thus, to make the most of the the direction of chart rotation is proporpreferred in the quantitative study of compensation technique, it is necessary tional to the magnitude of the cell current. simple systems, for it offers the best to damp the excessive galvanometer Thus, the curve traced by the light spot on signal-to-noise ratio in current measureoscillations. This can be done simply the paper is equivalent to a plot of current ments a t any applied voltage. However, by connecting a large capacity condenser versus voltaae ~erwndicular - on mutually . . . in many cases a continuously recording (e.g., a 1000-mfd, Gvolt electrolytic BXW. instrument has unique advantages, parcapacitor) across the terminals of the The Model XI1 Polarograph ($950) ticularly in the speed and convenience galvanometer shunt. For general laboramade by E. H. Sargent and Co., Chicago of the data-gathering process, and the tory use, i t is important to be able to 30, Illinois, is designed in accordance with insurance i t provides that subtle or brief choose an approprirtte damping time the principles just described. The followeffects in the current-voltage curve will constant ( = R X C) for the type of ing are some additional features of this not beoverlooked. oscillations encountered, and to provide instrument. The light spot reflected for bucking out the small internal emf from the galvanometer is divided so that Photographic Recording DC (ca. 6 to 60 millivolts) present in comone half is directed to a, ground glass screen Polarographs mercial electrolytic eondenaers. For s. where it can be viewed by the operator, The first continuously recording polarosuitable circuit, see Philbrook, G. E., and and the other half falls onto a slit through graph was the photographic instrument Gruhb, H. M., Anal. Chem., 19,7(1947). which it enters the camera drum compnrtThis type of photographic recording designed by Heyrovsky and Shikata and ment. The camera drum is fitted with a polamgraph is a precise, reliable instrushown in schematic diagram in Figure 18. scale a t the end which protrudes from A motor-driven mechanism turns a ment. Since the current sensing element the instrument case, so that the operator shaft on which are mounted a drum is a sensitive galvanometer, the instrument can see the percentage of the voltage spun carrying a sheet of photosensitive paper is subject to the disturbing effects of being applied a t any moment. Thus, the as well as thedidingcontactor of a voltagevibrations, and should be carefully shockdividing resistor. Consequently, the voltmounted and protected from dust, fumes, age being applied ta the polarographie excessive thermal variations, etc. The cell a t any instant is proportional to the displacement of the chart from its starting position. The cell current is sensed hy
Thisseries of articles presenk o survey of the basic principles,
16. Electroanalytical lnstrumentation
Figure 18. lllvsfmting the design principle of the Heyrovrb-Shikota photographic recording polarogroph. The cell is D, the voltoge-dividing reristor is 8, the banery which server or the ~ r i m a wvoltme murce is H. the current sensor ir ;he goivanomiter E, which'refieds a light spot from the lamp G to the photographic paper on the drum C; tho golvonometer shunt is F.
Figure 19. The eledronis pen-recording polarogroph is o simple application of the mntinuous bolmco potenliometer recorder. The cell current I is opposed by the recorder slidewire current, leaving 0 residual current, IR, which i. senred or o difference voltage, VD, by the current detector amplifler. The balancing motor is activated b y the amplifler output and mover the slidewire contoctor until the differencevollage is reduced to zero.
is 0.006 microamperes per mm, and a 10position Ayrton shunt with ratios from 1:1 to 1000: 1 is provided. A compensatar circuit permits the current to be
principal disadvantages of this type of instrument stem from the use of the photographic process for the readdut. (Continued m page A447)
Volume 39, Number 6, June 1962
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A445
Chemical instrumentation ~p
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Consequently, considerable ~ R u r t has gone into the development, of the so-cnlled visual-recordin&, or pon-writing polnrograph.
Pen Recording DC Polorographs The dosign of electronic pen-recording polnrvgrnphs is based, in the case of most o f bhase instruments, upon the principle of the self-b:~lsncing pot~ntiometer rceortlrr. The main featuros of this type of instlxment design arc illwtrated in Figure 19. The cell current, which R o w in reiponsc to the voltage applied to bhe polaragraphio oell hy the polarizing slide-sire, is applictl to t,he recordcr slidewir~in opposition to t,he eurrcnt due to the s l i d t ~ i r ebntt,ery. The nct difference between thcsc current,~,I,!, is tho crror signal smsctl hy the currcnt dctcctor amplifier, the output of shicl, drivrs the balancing motor, which moves the slidewire cantactor in t,he direction that diminishes the error signal. The motor continues t o turn until the error signnl has been reducod t o wro. T l i ~ ~ s , t,hr slidewire is continuously and xutornst,irally bnlsnccd zgsinst the polnrocell current. A pen is mechanically linked t o t,he halancing motor, and dmws n T P C O I ~on B moving c l m t thnt reproduces the varktions in magnitude of the cell cnrrent. Since the rat? nf chart adw n e e is synchronized with t.he rate af scan of voltngcs on the polarizing slidewire ( t h r motor controls for thesc latter functions nre not shown in the diagram), tlrr record drawn on the chart is a hithfol plot of cell current versus applied voltage. Some ndditional featurcs of t h e electronic ppn-rpcording palamgraph circuit xrr d m m u in Figure 20. The slklewire
Figure 20. Schematic of the circuitry of a recording polorogroph. The poloriring voltoge ir obtained from PI; the recorder relf-balancing slidewire is PI; the rcole rpon adjustment is Pa. The =hopper which converts the dc cell current into on oc i g n d i. V; the amplifier is A, and the servo-molor i$ M. The polomgraphic cell is C.
PI provides bhe polarizing voltage for the polsrographie cell. The second slidewire, P,, is used t o provide the error signal for the amplifier, A . The net current flowing in P3 is chopped by the vibrator, V, into an nc signnl thnt is passed by the transformer, T, into the amplifier. The amplifier output drives the motor, W ,t o adjust the slidewire contactor until the error signnl has been reduced t o zero. (Continued on page A448)
The variable resistor R1adjusts the sensitivity of the smplifier, and the variable resistor PI varies the span of the recorder scale, i.e., the magnitude of the pen deflection for agiven cell current.
Leeds and Norlhrup An example of the type of recording polarograph just described is the ElectroChemograph, Type E ($2395) of Leedsand Northrup Go., Philadelphia 44, Penn~ylvania, 8ho1~r.n in Figure 21. The
Figure 21. The Leedr and Northrup ElectroChernograph. Type E, shown with the Polomtron electrode and cell orrembly.
of eleven full-scale ranges, from 0-1 microamperes to (t-100 pa. The zero position of t,he recorder pen is adjustable by means of a potentiometer control so that a part of the cell current can be bucked out, or to permit several successive runs to be made an the same length of chart paper by displacing each curve relative to its neighbors in order to allow intercomparison without overlapping and confusion of the traces. A four-position damping control permits the choice of four different RC-time constants far the damping of the pen response to input current oscillations (ef. Figure 23). The polarizing voltage scan is by means of a synchronous motor driving a slidewire contactor a t a rate corresponding to 200 millivolts per minute over a tn-o-volt span. The starting voltage may be set a t 0, +1, or -1 volt. The recorder response speed is 1 second for full scale travel.
Sargent A reeording instrument similar to the above in basic design is the Model XXI Polarograph ($2950) made by E. H. Sargent and Co., Chicago 30, Illinois. This instrument has, in addition, certain convenient features, among which are: twenty current sensitivity ranges, from 0.84 to 420 pa full scale; hand adjustment of the polarizing voltage t o any desired initial value; compensation current eontrols; choice of voltage spnn up to 3 volts. The period of one complete voltage divider rotation is 13.5 min. Two damping time constants are provided. (Cntinued on page A450)
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Chemical lnstrumentution S w g m t :md Co. also n~anuf:~ctwe several other polnrogmphic instrumerlts. The Model S V Polnrogrzph ($1585) shown in Figure 22 is a recent design h a e d upon tho Ssrgent Lshorstory Iteeonbr. Thiu is a less mpenfiive recorder than tlrosc employed in the instruments just ilrnrril>ed. The recorder pen s p e d is 10 scconds for full scale travel; the chart a.idt,h is 10 inches, and the basic sensit,ivit,y is 2.5 millivolts for full scale defiertion. The polarographie eireuibry eantairm rsarntinlly all the adjustmmt and r:ont,rol fentnrrs described nhovr, i n r l ~ d i n g ehnier of initid and span voltagrs, eurrrnt sensitivity, rl:~orl,ing, :ind rn~n~,onsnfian rrtrrent control.
Figure 22.
The Sargent Model XV Pdorograph.
An accessory designed for LIRP with the Modol XV l'olarogmph is the Micro Range Extmdpr ($260). This device contains srvrr:rl higli-rcsistmoe dropping ~wsistors,to giw Iighw rnrrmt sensitivities than nrr provirlwi with thr Modrl XV. Thc nddition:~I rul-writ ranges msdr avnilahle with this nreessory provide lull sc:~lrsensitivities as low an 0.025 pa. These great current srnsitivities are of value o d y in t h r rnc:wmrnent a l polamgraphic waves 01 ~ul,nt:mo~s nt ver,v low conrentrntions, r.g., in tht? ~rormmolnr range. At thmv law r:ollrmtrnbions, the caparitanre r.urrmt duo to rlurging of the electrir:d d o ~ h l rh y w a t the mercury /solution interface is as lnrgr or lxrgcr t,hm the f:~r:idair n m m t due to elretrorenct,ion of t h s~h s t x n c c ]wing dptertcd. I n order to w n o v r thc dfect of this eaparitnnrr currmt iron, the d:lt,n r e d o ~ on t the ch;~rt,this rhvirr p1.ovides n motor-driven control that solq,lies a compensation current which incrcnaes linearly an the edl valtagc increases, to bock out the simililrly inrre:uing rnparitanre nlrrent. Sine? merely hocl~ing out t h ~c a p w i t ~ r r rc ~ l r w u tw h i l ~wing n high awsitivity fat. rrcortling the d l eonw!t I lwve rxe~ssively large current oscill:ttions h e to the clrop gront,h. srvt.r;d xdditionnl stages of RC4li~mping are pmvirkd in t h i s necessory instrument,. Controls avr incorporated which pennit selection of the initial magnitude of the comprrmtion current as well as of its rate of increase with the palariringvoltngo. Thc utiliaat,iou of n lincurly increasing compensation ~:IITPL.II~ is hnsed upon the nssomption that tlrc eapnritive component of t,hr rrll rnrrent inerruses linearl,v with t,he npplid voltage. This is not trite
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Chemical Instrumentation nism, yet without the disadvantages of sluggishness and curve distortion that are the inevitable consequences of overdamping. The derivative feature is achieved by the use of a capacitor-resistor circuit that will be discussed in some detail later. I t has the advantage that the current steps or plateaus characteristic of the conventional polarogratn are transformed into current maxima, or peaks, and the resolvability of rlosely adjacent waves is greatly improved.
Other Manufacturers The Electro-Palariser ($1105) of American Optical Co., Buffalo 15, New York, employs a. 10 mv, 2.5 second, 5 inch chart width Varian servomechanism recorder with current sensitivities from 0.5 pa to 100 Ma, full scale in eleven steps. An initial polarizing bridge unit is provided to permit the starting potential to be selected with accuracy, and a separate span voltage bridge permits selection of the voltage span, and hence of the polarization rate. A four-position damping switch gives four different time constants of the recorder pen response, as shown in Figure 23.
Figure 23. Illustrating the effect of the damping control on the polarographic reod-out. The four traces shown were obtained with the some tolulion, at the m m e polarirotion rote, but with ~ ~ ~ ~ ~ a increased i v e l y capacitances connected O C ~ O S Sthe recorder input. Instrument used war the A 0 Electro-Polorirer.
The rRect of damping is not only to reduce the amplitude of the pen excursions; it also tends to diminish the height of the wave, to retard the current step, so that the half-nave potentid appears to occur later than it would without damping, and to change the shape of the entire curve. Therefore, it is generally desirable to enlploy the minimum damping consistent with easy legibility of the read-out and with the confinement of the pen excursions to the width of the chart paper. The Tinsley Polarographs, made by Evershed and Vignoles Ltd., London, W.4, England (available in the U. S. through James G. Biddle Co., Philadelphia 7, Pennsylvania) are based on the servomechanism recorder. The Mark 19 instrument has the usual controls, and includes damping, compensation current, and first derivative circuits. All control settings are made by means of step (Cmtinu~don page A45fi)
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Chemical Instrumentation switchos, t o facilitate reproduction of settings in repetitive work. The Polarecord ($2450), made by Metrohrn AG, Herissu, Switzerland (available in the U. 8. through Brinkmann Instruments, Great Neck, New York), uses a servomechanism recorder with 1-swond pen speed, 10-inch chart width, and a maximum current sensitivity of 0.025 &a full srale. I n addibion to the standard controls, i t provides a counter current control (ie., fired compensation current for suppression of previous waves), a capacitive current compensation, a ritngr of damping time constants, and a circuit for taking the fimt derivative of the polarographic curve. The Voltamograph of Cambridge Instrument Co., S p w York l i , S . Y., employs a moving coil graphic recorder rtnd is similar in design to the Polariter P03, alreadpdeserihed. I he pen-recording polarogr:qlhs described in the preerding paragraphs have been all-electronic instrument,^, in the sense that the cell rurrent is fed t o an amplifier and the output of this component drives either a moving coil writing mechs, nism, or a servo-motor. Alternative approaches t o the design of a direetwriting, visual-recording polarogrsph have been based upon the use of a conventional light beam reflerting galvanometer in conjunction either with a photo-cell and aperture and the read-out consists of the measurement of the resulting photo-
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t o a pen, producing an inked recokd oi the tracking motion. These approxrhes involvc more delicate, complex, and unreliable instrumentation than the allelectronic designs, and have not been commercially successful.
Timed Interval DC Polarograph It n 4 l he noted that much attention has been paid, in the design of recording polarographs, to the relative eontributions n ~ a d et o the cell current by the cxpaeitiv~ and iaradaic components respectivdy. In particular, the capacitive currmt is largely responsible for the pronounced oscillations in cell current attending the birth, growth and detach-
D E T A C H M E N T Or
DROP
Figure 24. Relationship beween the capacitive or condenser current, L, and the faradoic or diffudon current, ;D, which compose L e totol
observed cell current.
(('ontwzued on page A466)
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Chemical Instrumentation ment of mercury drops. The relationship h e t w c n theee two component curreuts is &own disgmmmatieally in Figure 24. Whrn the droplct is new, the cell currmt is principally capacitive, duo to the charging up of the elertriral douhlr! laypr nt the fresh int,crfaee, x h e r n a the diffusion current is small hecause thc surfnrr sren is small. .4s the drop grows, the rate of surface i ~ m 3inrrease beeomrs smallw, and the cnpnritive current falls "It, while tlrc diflusion current ineren~es. Toward tho cnd of the drop life thc dillusion cnrrcnt is lnrgr and uot inerensing mpidly, and the capacitive current is at it? ~~n:~llesf v~lue. This suggests thnt if the d l currcwt is mensurwl only for brief periods just hrforc thc detachment of each mercury droplct, the pol;~rogmphio d:~t:t will ho 1:rgely fme of the disturling dTwts of the condenser rorrmt. This is the l ~ i n r i p l e of the "%st (Genrmn: dcr 7'aslev = the key; l:~slen = to tourh) polrr~.ograph.," sholr-n ill hlark dingmnr in Figure 25. A oonventionnl po1nrogr:~phic cell ia employed, and n de polarizing voltage is i~pplied ns in the ot.h~r polnrogmphs pr~viously rleserilmJ. I n : ~ d ditian, ;ln nc cnrricr frequcnc:y from a sepnlatc power supply is pnssctl through the rrll. Thp det:whment of R n1ercor.v drop e a u c s the impcdnnre of the rcll to rise sharply. This crcstrs n pulse in the ac carrier signal, and an impulse is passed hhrongh the condenser, Cs t,o the "tast control" circuit. This contains an
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vontwtoi to h ~ ~ veverything r, i n nmliwsa far the next measurement. An example of the kind of polarograms produced hy this instrument is shown in Figure 26. The condenser current is tnurh smaller than in eonvmtiansl polaro-
CONTROL
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RECORDER
Figure 25. Block diagram of the Tott p&ograph. Condensers Ci and CI permit on olternoting voltage to be applied to the polarographic cell without affecting the diffusion current. When a mercury drop detaches, the sudden change in impedance of the cell causer on ac pulse to pow through C i to the Tart control circuit. The latter activoter a relay after a Rxed time delay, and covter the cell current to be recorded and the pdorizing potentiometer ond recorder chart to be odvonced for the next meoturement.
rlcrt,ronir;llly opcmtcd time dclny ~&y. .I fixed, lhief time interval after the a r pulsr l m hem received, t h r tast control i w t i v i l t ~R~relay n.hich closes circuits that perfonn the following sequence of functions: ( I ) me.wures the cell current misting at that instant, (2) plot this wrrent on x recorder, ( J ) disconnect tlrr cell current from t,he recard~r,(4) :~rlvancethc r~corderchart, and (6) advnnec the polarizing voltage slidewire
Figure 26. Polorogramr recorded with a Tast polomgraph. The rurrer correspond, from bottom up, to 11) 0.1 M KCL bare solution, 121 0.5 mM Cd++ in the bore solution, 131 1.0 mM Cdii, ond 141 1.5 mM Cd++.
grams, 2nd thr diffusion curwnt plateau is more horizontal. I t d l he noted that these effects have been achieved without the use of damping or of a compensation current,. Howcvrr, thr instrummt,ntion (Continued on page A4G8)
Chemical Instrumentation required for Tast polarography is much more complex than that which is employed conventionally. A polnrogritph of this design is the Selector Tat-Polarograph made by Atlas Werke AG, Bremen, Germany. It can he used to record either conventional or tast polarograms; the time delay period of the tast oontrol can he set a t any v d t ~ from e 0.4 to 3 secs.
a conventional polarographic cell in 10 to 25 mins. Hence, as much valtametric information can be obtained in 1 minute with rapid dropping, as ie collected in 10 to 25 mins under slower dropping conditions. The rapid dropping electrode causes stirring effects in the polamgraphic cell, such as are indicated in Figure 27, which
Figure 28. Comporiron of a ropid polorogram with a conventionoi curve obtained with the same system. Cell contained 0.5 mM NC in i 0.1 M KCI; weak damping [time mnrtont = 0.6 3ec.I war used in both cases. Drop time far the rapid polamgram wo. 0.25 sec.
The Rapid Polarograph A modification of the conventional dc recording polaragraph that is designed to improvc the usefulness of the instrunient for routine analytical work by decreasing the magnitude of the current oscillations and increasing the speed with which a complete polarogram is recorded h a heen developed under the name of rapid polamgraph?,. In conventional de polarography, the drop time (interval between successive drops) is of the order of 2 to 5 secs, and the time required for running a polarogram is from 5 to 15 minutes or more, depending upon the range of voltages scanned. In rapid polarography, an electromechznical relay strikes the dropping mercury capillary evory ca. 0.2 secs, causing a small droplet to be detached. Thus, a forced, rapid dropping of very small droplets is obtained. I n less than one min, as many drops are produced and detached in this w w as me involved in
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Figure 27. Stirring effects ore produced b y o ropid dropping electrode that ore not encountered in the u3e of a conventional dropping mercury electrode.
are not present with the conventional DME. This results in a diminution of the concentration polarization, and leads to an enhanced faradaic ourrent and smaller condenser current a t a given applied voltage. The appearance of a typical rapid polarogram is shown in Figure 28. It will he noted that the current oscillations are greatly reduced compared to a conventional polarogram, whereas the diffusion current wave is only slightly decreased.
The mpid polarographic tcchniqo
