Chemical lnstrumentation S. 2. LEWIN, New York University, Washington Square, New York 3, N. Y.
T h i s series of articles presents a survey of the basic principles, choractrristics, and limitations of those instruments which 6nd important opplicntions in chemical worlc. The emphasis is on commerciallll available equipment, and approximate prices are quoted to show the order of magnitude of cost of the various types of design and construction.
16. Electroanalytical lnstrumentation (Continued) 'l'hopolarogrqhici~rstru~ncnt,s t,hat have hrrn disrussed to this point have involved thr mensunment of rcll current either :nt a disrrrt,e number of steady applied vokagcs [i.r., the manual polarograph and the "t,nst? polarogrqh], or as a function of :L los sly (linearly) increasing voltnge [i.e., the dr recording polarograph and the "rapid" polaragrephl. A different appro~~ that h ha8 cert,&inunique characteristics is hnsed upon the application to the dectralysis roll of n dc voltage upon which is superimposed u small alternating potmtial.
AC Polarogrophy
E curve. That ia, slope =Ai/AE, and AE = constant, hence Ai rr slop^. The usual dc polarographic curve shows a, current step or wave as a function of increasing applied eell voltage; the curve is close to horizontal (i.e., slope F-; 0) before and after this step, and the slope increases to a maximum as the half-wave potential is approached, falling toward zero after El/, is passed and the diffusion current plateau is approached. Accordingly, the ar romponent of the cell current goes through % peak instead of a step as a funetion of t h r applied voltage. This relationship between the de and ar polarographir mrves is shown in Figurc 30.
Figure 31. Compori$on of polorogromr abtained by the oc and dc techniques, illustrating the improvement in rerolvability of ciolely occurring reduction waver ar o consequence of the derivative-type record produced by the oc technique. [From Miller, D. M., Con. J. Chem., 34, 9 4 2 [1956).[
voltage, and the ac &mxg:m may I x afiectcd in a characteristic way. Far example, Figure 32 shors that the presence of a ~urfitce-activeagent produces current peaks on both sides of the region of the eleet,rorspillar). mmimum, a t plsws where the voltage fluctuation rcsults in periodic variations in the amount of surf:m adsorption arrurring, and henw in the doulde layer cilpsritnnre of the elertrode surfare. Thestudy of adsorption prap~rtiesthrough their influence on t h r ac polarogrnm has been termed tansamvrefru (iron) surfare lpnsion and nnmrlry).
Thc design principlr of a simple ac polarogrsph is illustrrttrd in Figure 20.
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E. potmtio, Figure 30. Comporisn of typical appearonce of o dc and on ac polorogram. E, is the "summit potential" of the oc polarogrom, and o c ~ v r sin the vicinity of the holf-wave potentiol of the dc curve.
Figure 29. Schematic diogram of an ac polarograph. An ac source i s placed in rerier with the ~ o l t a g ebeing topped off rn slidewire and this combination is opplied to the poiarogrophic cell.
If the cell potentid, E, is mused to undcrgo a periodic increase and decrease, by :m amount cqud to +AE, then the cell currmt, i, also variee. The fluctuation t,hxtis produced in the eell current depends upon the magnit,udes of E and AE, the frequcnry of the oscilla.tion, and the degree of rcversihilit,y of the electrode reactions as \wII as t,he diffusion cocficients of the eleetraxctive species. I n the simplest rase of a oneat,ep, rrwrsible reaction, the varihtion in cell current, A , is proportional to the diflermce in currents that would be observed when the steady potentials E- AE and E+AE are respectively applied to the rell. Hcnro, for s constant voltage amplitude, &A&, applied while E incresses over t,hc range of voltages of polarographic interest, t,he current amplitude is proportional to the slope of the familiar i versus
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Since the s c polsrogram consists of current m a i m s , it is often possible to detect with it the presence of species whose halfwave potentids lie so close together that the dc polarographie steps merge into what appears to be a. single wave. This is strikingly shown in Figure 31, which illustrates the detectability of TI+ in the presence of P b + + in the ac polarogram, whereas these species are undifferentiated in the de polarogram. The ac polarographir technique is more sophisticated than the dc approarh, and makes possible certain unique advantages. Because the parameter of interest is an ac signal, simple feedhackstabilized ac amplificrs can he employed, and a high sensitivity can he achieved. Since t,he voltage applied to the cell is continually fluctuat-
theelectrodeprocesses. If a surfsre-:i&e subnt,antr is orcsent in solution. its adsoru-
Figure 32. Tenrammetric curve [full line) due to presence of a wrface-active agent; dashed c u n e ,how the a c po1arogr.m obtained in the absence of the rurfoctont. EO is the electromaximum. In the region between A and B, the surfadant is adsorbed at the electrode/~~lvtioninterface, the capacity of the electrical double layer is decreased, and the oc base current is lowered. At the edges A ond B of the odmrption region, thorp peokr ore observed due to the periodic odmrptionl derorption processes brought about by the applied voltage. [Cf. Baue,, H. H., J. Electromol. Chem., 1 , 3 6 5 (l960l.l
Since the rurrent flowing in an ae polarograph has an alternating component, it is passihle to study the relationship of that osrillation to the exribing v d t ~ g fsignnl . in terms of either A ) amplitude, Lo frequenry and distrihutian of frequencies, and C) phase. The techniques drsrribed
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Chemicarl Instrumentartion above have heen concerned primarily r i t h the root mean squarr amplitude of the cell current flurtuxtion. Spveral imporb ant instrumental approaches have recently been developed which arc hased upon the frequency and/or phase dnt,ionships.
Second Harmonic Polarography I t has been pointed out previously thnt the current nhirh floas when a voltage is applied to a palarographic cell is composed of a capacitative, or condenser rurrmt, and a. faradair, or diffusion current. The former, arising from the charging up of tlre rlectrieal double layer a t the electrode surface, is w r y shnply related to tho applied voltage, and if the voltage alternates ait,h a freqnmc.y,f, the condenser current altrrnates with the same frequmry. Howevw, bhe faradsir rurrent involves much more complex phenomena, surh as the diffusion of el&ronrtive sperips to m d from the electrode, adsorption kinetics, and rates of electron transfer. Henrr, if n sinusoi,l;d voltage of frequrnry, j; is wpplicd, thr faradair rurrent rompanent dors not, i n goneml, hnvr n simplo sinusoidal shape. Thc inmdaic rurrent waveform is found to rwntain ronsidrrahlr contrilmtinns due to harmonics of t l original ~ frequency. This fact, suggests thnt if a tuncd omplifirr is emplayd, which passes only n Irequenry such as the serond hnnnonic, ?{. Imt rejects the first harmonic, j, t h ~~P : I S ured output signal will correspard principally to the iaradaic ltive romponmt. This is t,hr prirxiple of second hararonie ar polaroyraph~,. Figurc 33 s h o w t h ~ typirnl apl,earanrt. of s second harnwnir polamgram for a reversihlc drrt,rado pro(,rss. I t will be noted that t v o p e a k orcur, at approximately t h r two inflrrt,ion points of the :w (first harnmonir) peak.
Figure 33. Second harmonic polorogram (dqshed curve) compared with normal a< polorogrcm (full curve1 for 0.25 mM Cdtt in 0.5 M HCi04 solution. The alternating voltoge [From Bouer, war 15 mv r.m.r o t 2 0 0 c/s. H. H., J. Eledroonol. Chem., 1, 2 5 6 l19601.1
Thus, if the normal a r eurve is similar tn the fint derivative of the dr polarogrsm, then the second harmonic eurve is reminiscent of the second derivative. Techniques based upon the measurement of the second harmonic frequency should offer advantages in the detection of species of very similar hdf-wave potentials, in the deteet,ion of trace concentrations in the (Contintred on pan? A.522)
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Chemical Instrumentation presence of large concentrations of mare easily eleetrolyeahle species, and in the study of the mechanisms c! dertrodp proresses.
Phase Selective Polarographs T h e fnrndaic component of the cell current can also he distinguished from the caparitntive component in terms of t,he
phnw-s&ctivr drteetar circuit is e n ployed, i t should lw possihle t o amplify the, fsradrtic component r\.hile d i s c r i n ~ i w ~ t i n ~ against the c&cit,ativc companmt. This is th? principle of the phase selerliw volarooraoh which has heen dpsiened In. jessoi (Tiritish Pntcnt NO. B - I O T G ~ 1!14sj, , and which is shown in block diegram ill Fignm 34. A slidrwirr, P , and n n nc signal from n tmnsfor~ncr \\-inding :lrr
Figure 34. The phase selective poioragroph of Jetrop. The relay switch, S, alternotelf cmwdr the :ell output and o portion of the original voltage input rignoi to a detector bridge. A phore-rhiftw, H, is employed to alter the phore of the input until it cancels the ~apacitatirccompme+ of t h e :ell current; t h e read-out meter then shows the farodaic component. [Cf. Miiner, G. W. C., "The d London, 1957, p. 132.1 Principles ond Applications of Polamgrophy." Longman., Green ~ n Co., phase diKerence het,wrm t,hem. Sinrc the lat,ter is n simple rontlmser charging -urreat,, i t is !1O0 out of phase wit,h thr, mprrsserl voltsgt.. However, t h e fnradaic .wn.rnt in gencrnlly I , t t u r m O m d 45' out
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placed in series with the polsrogmphir crll. T h e alternating current from the r d l is frcl to t h r arnplifirr, A, and t h m wppli~rlto :I Iridgr circuit contijining. thv ~rr.:&out (('ortli,~rred on p a ~ r. I il41
Chemical Instrumentation n ~ e t w , G. Another ac signal from the aamr tmnsformw is fed to tt phase-shifting vircuit, H (eonsist,ing simply of s varilthlc resistor-condensrr :rrr;inwncrrt), and thence t o the sam? 1,ridgc A vibrating reed swibch, S, altprn:rt,ely connects the two arms of thp hridgr t o their respective sign&, vis., the cell output and the phaseshifted input volt:~ge. If the input voltage is ahifted hy !UI0, it will oppose and cancel the cap~citntivecomponent of the cell currmb, leaving only the f:tradsic conponrnt to produce a ddlertion of the mrter.
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Square Wave Polarography Another instrumental sppronch that i~ effectiw in dise~iminating against thp capacitstive colnponent o i the cell current is hased upon the use of a. step function for the wnwform of the impressed voltagv fluetustion. This is designated as squaw wave poiaroyraphy; Figurv 35 shows the form of thc square valti~gewave, and the forms of th? resulting condenser and far:,daic current waves. I t is instructive to compare this with the curves shown in Figure 24, which, however, refer t o n growin!, d m p and :< castnnt applied vokagr.
largely died away, hut the faradaie current is still suhstantial. Thus, if the current is mea~uredduring the time intervals, a (a. 100 microseconds), just preceding the occurrence of R voltage step, the measurement will he relatively free of the condenser current. Sinrr the square wevr corresponds to s voltage fluctuetion, fAE, tbho curve of current fluetuat,ion amplitudp versus cell potential will he similar to t h a t ohtained in ar polarography, i.e., peaks are observed in contrast to the rurrent steps or waves of da polarograms. As has heen noted hefore, ac pnlarogrsphy has the advantage
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h* GENERATOR
MODULATOR
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Figure 35. The voltage and current woveformr involved in square wave polarography. The full line current curve rhowr the copocitotive component of the cell current, the dmlhed curve is the foradqic component. The cell current is measured during the time periods, o.
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GENERATOR
Figure 36.
NCHRONISE
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Block diagram of Barker square wore polorogroph.
I t is evident that s hrief tinrr nftrr each v o l t a g ~step thr candrnser current hsa
that derivative-typ~curves are obtained, (Continued on page A527)
Chemical Instrumentation in which thr reduction of each rpducihle speries is displayed independent,ly of an,v species previously redurrd, and materials whirh are redurerl romplctrly irreversil,ly, such as oxyg~n, contribute signals or negligible magnitud~. Squ:re wave polarogr;tphy has ell these advantages, and ia addition the elimination of the eondmsrr current permits the mcnsurement of substances a t much loner concentrations. One type of rirruit design employed i l l a square w w e polwrogrxph is illn~t.mted in Irlock clingram in Figrwr 36. The square volb;~gewwvw irom a square wavr g ~ n e r x t o wrr r snprrimposed upon a linearly inrreusing voltage (in ;l n~gntivnsense, for cst,hodir. pol:rrogre~,hy) f l n r a. sawtooth ware g ~ n ~ r : ~ and t , o rapplied to the polnrographic rell. The nr camporwnt oi t h r cell current, is passed through ritpscit,ars to n synrhroniser, in whirh the signs1 is romp a d with a gating signal derived from ~ h oridnal c square wnvr. The gate opening is 100 microseconds long. The current, steps, freo of thr initial sharp pulses, are p m s ~ dt o a recorder for r e a d w t . A typical squitrt: w w e polnrogmm is drown in Fignrrt 37.
Figure 37. A typical q u a r e wove polarogram, obtained by the Mervyn-Horwell instrument with a solution containing 3 ppm Cu++, 7 ppm Pb", 2 ppm Cd++ond 200,000 ppm Znft.
A rommereially available square wave polsrograph is the Mervyn-Harwell instrument, made by Mervyn Instruments, Surrey, England (available in the U. S. through Instrument Corporation of America, Baltimore, Maryland; ca. $15,000).
Single Sweep Cathode Ray Polarograph
A unique instrumental approach to polmographic analysis is embodied in the single sweep cathode ray polsrograph, shown in block diagram in Figure 38. The (Continued o n page A529)
Chemical Instrumentation
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SUPPLY
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Figure 38. Block diagram of lhe Cothode Ray Polmrotrace, with which a 0.5-volt poloragrophic rcon it performed during the lost two seconds of the life of 0 single mercury drop, ond repeated for each ruccersive drop.
power supply is designed t,o produce a linear volt,sge rise of 0.5 v, which occurs during the last two seconds of t h r life of a single mercury drop, having a total lifetime of seven seconds. This voltage sran simultaneously is applied to the- X-axis input (horizontal deflection plates) of n cathode rap oscilloscope. The variation in cell current acrompan?ing t,his voltwe sweep isamplified and applied to the Y-axis input (vertirnl deflection plates) of the CRO. Thus, thp electron beam trares n graph of i verars E on t,he fare of t,he oscillosrope. An idealized rcprescntation of the form of the curve oht,nined for a reversible redudion is shown in Figure 39.
OW'
FALL
lD,FFUIlON ,CURRENT
.........-I ----.--... STAR,
POTLNTIAL
Figure 39. Form of the current-voltage curve observed with the single w e e p cathode roy polorograph for o reversibly reduced rpecier
It will he noted that this curve shows features t,hat are intermediate bet.ween those of dr and ae polarograms, i.e., the current rises to a peak as t h r voltage increasrs, and the peak oerurs in the vicinitv of t,he hxlfwave potential (as in ae polarography), hut as the voltage continues to increase, the current, subsides to the value that corresponds to thc diffusion current plateau ((as in de polaropraphy). The c u r r ~ npeak t is duo to t,he rapid discharge lf elect,rolyzshle speries present in the ~icinit?of the electrode surface which oc:urs when the potential rearhrs a sufficient nagnitude for the ~leetrodereaction to .ake place a t a rapid rate. As the elcc(Continued on page A530)
Chemical Instrumentation trode r ~ s r t i o n rontinurs, thc electrode surfare becomes depleted, and further current flow becomes limited by the mt,n of difusion of elcctroartive species to the deetrode from the hulk of t,he solution. A singlc swcop cathode r a y polarograph is nmnufartumd hy Sout,hern Instruments, Ltd., Surrcy, England (Palaratraec, $4600; avnilnhle in the U. S. through Standard Scientific Supply Corp., Ncw York 3, N. Y.). An example of an actual polarogram produced by bhis instrument is shown in Figure 40. The detachment of a
mercury drop from the capillary xt the end of its lifc produrrs a current pulse which is utilizod as a trigger t,o initi;,,t,c the ncxt valtagc sran. This aids in thc synchronization of the drop time with the voltage sweep rate, and produces a cathode ray trace that exactly rrpenbs i t d i every mven 8wonds ( i . . , with m ~ r y mcrrury drop). The starting potential of the dropping mercury rlertrotle mn Iw varied from +0.5 t,o -2.0 v relativc t o n mercury pool anode. The current sensit,ivit,y of the electron-tmrcd curvc can he varied from 0.0'2 microamperr to 0.125 milliampere, full-scale drllection. Sperial eirruitry is providrd for hnrking out,
Figure 40. Polarogrom obtoined with the Southern In=trurnent3 Cathode R o y Polatolrace. lfirrt peak) The solution contained O.O1yo Pb '+ and 0.002% F e i t I r e w n d peak) in sodium phosphate solution containing sucrose. The starting potential of the DME w w -0.25 v relative to o mercury pool anode, and the sweep i t toword more negative voltoger.
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Figure 41. The Cathode R a y Polarotrace, with electrode stand.
Electronic Differentiation of Polarograms
It was pointed out nlmve that t b nc ~ polarogram is like n rlerivativc of the rlc curve, but is not a true derivative, since most dretrode processes arc not rapidly enough rrvereible to rrtracc tbr dr curve .~sthe voltage fluctuates rapidly up am1 down. This olTrrs t h r advantage that thr rtc approach m a l w it possihl~to srpnrnte a rrversiblc elcctrodv process from an overlapping irreverdlh on?. However, thrrc are also udvm~tngrsin I l a r i a ~avnila1,le . a true derivative curvc, since i t prnnits an increase in the intorpretnbilit,v of thc data b ~ i n gread out ahont the system u n r l ~ r study, without changing the nature of thp electrode phenomena. A dc polarogrwm such as might be produced by a continuous recording rlc imlarograph, can be ~lrctronirallydifferrntiated by mcnns of x simplc N C circuit. such as (Conlinr~rdon page A532)
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Chemical instrumentation ~
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is shown in Figure 12. If a voltage, E,,, is applied to a series eomhination of a resistor and a capacitor, the capacitor starts to charge up to the applied voltage magnitude. If the input voltage is continually changing, then the capacitor never has time to reach the voltage of the input hefore the latter has changed to some new value. Hence, the capacitor is continually charging or discharging in the attempt to catch u p n i t h theinput voltage, and current flou-s in the circuit. When the input voltage is changing rapidly (dEldt large), the charging current is large, because the condenser has a large difference in volt,,r+. tr, rhnrev t.i) t o ; u h c n tlw input volt:tcr is r h . t n r i ~ rluwly l~ dk.' dl s n x ~ l l ) , tlw r o n h w r ~ a > t t m i is : ~not l 1r.r ld,ind E;,, and the charging current is small
Figure 42. If a varying input signal is applied to mn RC circuit, the condenser charging current is proportional to the rote of change of Eia. and hence E m = I X R = k X ldEinl/df.
Hence, the current is proportional to the time derivative of the input voltage function, and if the IR-drop produced hy this currcnt in flon,ingthrough the resistor, R, is monitored, i t will provide an output, signal, En,,, that is the derivative of Ei.. This principle of RC differentiation is med in several commercial polarogrhphs. A practical circuit that is used is shown in F i n r e 43. The capacitor CI is a large
Figure 43. The Leveque-Roth circuit for electronic differentiotion of a polamgram [J. Chim. phyr., 46, 480 (194911. When switch S ir dored, the recording golvonometer, G,drawr a conventional polarogrmn; with S open, o derivative curve is produced. CIis the differentiating copocitor; C? is o damping copasitor.
capacity (e.g., 2000 mfd) electrolytic condenser in s e r i e ~with the recorder, G, and passes current only in proportion to the rate of change of the voltage ( = i X I t ) acrossR,, which is due to the polarographic cell current. Condenser C2 is a damping condenser to reduce the excursions of the recorder occasioned by the growth and detachment of the mercury drops. Resistors R, and Rl together constitute an Ayrton shunt for regulating the recorder sensitivity. (Continued on page 8 5 3 4 )
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Chemical Instrumentation Elcctronie diRerentiatian of a varying signal produces some distortion hecnuse of thr lag between the input and the charging correut,. This distortion hceomcs greater, the longer the timc constant of thr dill'w entintor circuit, i . ~ . ,the larger thc valuc of I
