Automatic Apparatus for Electrolysis at Controlled Potential JAMES J. LINGANE, Mallinckrodt Chemical Laboratory,
~ U I X Gexperinlente of by c ' ~ ' f ~ r o l y s i sRith a mercury cathode at a controlled potcninial (9)created the need for an apparatus that would automatitally maintain the potential of an electrode a t a constant value during the course of electrolysis, and t,hus relieve the experimenter of the tedious task of manually controlling the potential. Illgeniolls circuits for performing t,his function have been described recently by Hickling ( 2 ) antl hy Caldwell, Parker, and Diehl ( I ) , hut both of thew leavc something t,o he desired From the standpoint of simplicity and versatility. Hickling's circuit has the advantage of being completely elcctronic without moving mechanical parts, but it is practical only for electrolysis currents of a, fmv tenths of an ampere, and it controls in only one direction. The circuit of Caldwell, Parker, m d Diebl can handle large currcnts, but it i p limited to the use of a t,utal applied e.m.f. of 10 volts, and it also controls in only one direction-i.e., it prevents thr csathode potential from becoming more negative than a given value during the course of electrolysis, but i t does not coriect a positive drift. Both circuits require preliminary calibration before use. The apparatus de tmein ront,rols in both directions, it is applicable with applied voltages and currents of any magnitude, its component parts art! vommercially available and relativcly easy to assemble, and it (foes not require preliminary calibration. Thc essential features of the circuit are shown scheniat.irally in Figure 1.
Harvard University, Cambridge, Mass.
alternating current, and it is provided with a built-in gear t.raiti which provides a shaft speed of' 60 r.p.m. and a torque of 5 oz. in. (Holtzer-Cabot Electrical Co., Boston, Mass., T y p v as is used to (I,,RWC 2505). -4worm-gear crease t8hespeed further to 0.60 r.p.m. Mechanical connwtion to R1 is made by a belt drive with wooden pulleys, R*how r:xtio is 2.7, so that R1 rotatcs a t only 0.22 r.p.m. In addition l o i t simplicity the belt. drive has the advantage of serving as a siniplt. friction clutcIl, so that E, b(% mo,,c,ti manllally Tvithollt}l:l,.jllK to bother with locking deviws;. The rate of rotation of R, ihould bc m a l l to prevent o v e i h o t ing and hunting, and yet great enough SO that off-balance in the contiol circuit is corrected without delay. With one revolution in about 5 minutes hunting i- avoided, and the time requirrd to restore balance in the control circuit is zeldom greater than n h o u t 10 seconds and usually is considerably less. The time of rwpon5e between thc in.tant when thr galv:inom-
The total voltage applied to t,he electrolysis cell is adjusted by L: IOO-watt, 30-ohm radio potentiomrter rheostat, R, and indicated by the voltmeter 8, (0 to 15-volt scale). Rheostat R1 (General R d i o C h , Type 333-A) is poryered from the direct current house tnain, or with lead storage batteries, and a 50- or 100-ohm variable resistance, Rz,is provided for convenience in adjustsing the voltage across R1 to the optimum value for a particular expcrimcnt. For most purposes 10 to 15 volts across Rt are convenient, hut a smaller or larger value may be used in special cases. For some purposes it may he desirable to connect R, into the electrolysis circuit as a serirs resistance, rather than as a potential divider, but for the usual type of elcctrolysis tywrimcnt the arrangeiiicmt shown is most suitable. Thc electrolyzing current is read on the multiplc range ammeter, A . The range of the ammeter may he varied to suit thcy \)articularelcctrolysis conditions; for work with the mercury cathode under the conditions previously described (5, 4) two ratigcs from 0 to 0.1 and 0 to 1 ampere are most nsc~ful. The e.m.f. between the working electrode and the reference rlectrodc (usually a saturated r:ilornr,l electrode, S.C.E.) is opposed by a n e.1n.f. from a 100-ohm radio-type potentiometer rheostat, I?,, which may be pom-erd hy a single S o . 6 dry cell or a 2-volt lead storage cell. Obviously, I23 cannot be poivercd from the same source as R,, because this \vould create a short circuit hetween the c~lwtrolyzinyand control circuits. Thr opposing c..m.f. ("cat,hode potential") is read on voltmctcr Vz (0 to 1.5 volts), which should he calibrated and capable of being rend to 0.01 volt. A galvanometer relay, G, sctnsitivc t o -0.02 volt, (Kc5ston I3lcctric Instrument Co., Jlodel 30), is p1urc.d in series in the routrol circuit as shown. Off-balance between the cathode-reference vlectrode e.m.t'. and the opposing e.1n.f. set on R3 causes the gal\-&nometer relay to make contact either right, or left and opcbrntc onc or the other of the two clcctronic relays, K y (Z. Groincr antl ( catalog so,~ - 1 8 0 4 )\vllich , control the (jirection rot:Ition ( I f th(1 reversible synchronous motor, JI. In t u r n , the motor rot : r t i + rlicostat I f l until the totnl e.m.f. applied to the cell is dcc,r(,:,sc.tl or increased by such a,l that the catho(lo I,otent ial returns to the value set on &, and thus balance in the eontrol (airpuit is restored. The motor operates only n-hen the control rircnit is unbalanced. T h t - ,+iii:ill motor iiro(l rcqiiirtvl oiily R \ w t t r at) 110 volts
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Figure 1. Schematic Circuit Diagram of Apparatus for Electrolysis a t Controlled Potential 30-ohm radio potentiometer (General Radio co,, Type 333-A) R,. R,. 5 0 . ~loo-ohm ~ resu~atingre~istance R3. 100-ohm radio potentiometer (General Radio Co., Type 3 0 1 )
A.
qc,,,
M u l t i p l e range ammeter (0-to 0.1- and 0-to l-ampere scales)
V I . Voltmeter (0- to 15-volt scale) V2. Calibrated voltmeter with scale from 0 to 1.5 volts, capable of being read 0.01 volt
lo
C. Galvanometer relay sensitive to 6 0 . 0 2 volt (Weston Electric Instrument Co., M o d e l 30) Ry. Electronic relays set for normally open (E. Greiner and Co. Catalog No. E-1 804)
M. Reversible, synchronous electric motor with shaft speed 01 60 r.p.m. (Hollzer-Cabot Electrical Co., Boston, Mass., Type R W C 2505) S.C.E. Saturated calomel reference electrode Cell. Electrolysircell
332
or less
May, 1945
ANALYTICAL EDITION
333
relay makes or breaks contact and the instant that RIbegins or stops is only a fen- tenths of a, second. When the electrolyeis current is large, a relatively large chanyc 111 the total applied e.m.f. is required to produce a given changcx in the potential of the working electrode, because the major fraction of the total e.m.f. is dissipatd a? iR drop through t h cell, but near the end of an electrolysis n.hcn the currc~ntbecomes very small the potential of the working elvctrode changes virtually directly with the total applied e.m.f. Hence there is a grentvr tend(.ncy for the instrument to hunt its balance point near t h v ctntl of :tn electrolysis when the current is small. For this r(qisoii tlic. t,ot>alvoltage applied to R1 should not be larger than necessary, y o t,hat, R1 is nearly fully open a t the beginning of an expcri~nonti.e., so t,he increment of applied e m f . caused by momciit:try i)pcration of the motor will be relatively small. Hunting can also be eliminated by increasing Rt to decrease the voltage across R , after the current has dropped to a sm:i,Il value, but this usually is ary. With theat- precautions hunting was not a p o l ) Itm. (’oii-itlcr:Ll)le difficulty was t.spc~ric~ricc~l at, first with ntirkiiig (If the contacts of the galvanomcstrr rclay, eve11 tliouyh thc. ::lteriiating control current of the elccatronic relay-: :iniouiits to iitily ahout 2 niicioamperes aiitl thew \v:w no visible s p r k i n g . Tliis Jiffirulty wzhq e1irnin:itcbil complt~tc~ly l)y placing t i y o 0.25microfarad paper cnndeii,.ei.s, e i ~ c h0 1 which is iii 100-olmi resist:riicr to thc commnn centrr contart, awns. t l r t s gilvanomcter C l J l l t . , P t i as n h o ~ ~ - n By . trial thc conr1ensri.s niust he connected so Ih:it their ground sides hyar thca proper rehtioii t,o the ground sides of the control circuits of the elwtronic rcklays, citllcm-ise the condensers may leak current, to opcmte 6ht. rcJlays. If thc eli~ctronicrislays art’ provided with pilot lanip‘ coiuic,ctcd in ~i:iriilI~~l with tlio 1 o : i t l tt:rminals, t h r r must tie rcmovetl; ot,hctrwise I)oth circuits ill tliv motor will ti(, energiztd whrn (lit h t of ~ tlic. t ~ v orii t lie u i n t i.oll(,r ih opcrtitiiiy :itid its directiori. Siiiw Loth the total :i1iplied e.nl.f. arid tho potc~iiti:iI I)ct\\.c.c’n t \rorking electrode and the reforenre electrode arc rctatl directly oil voltmetcw VI ai111\-:, thc in.trrimrnt rcyuires no cnlibratioii c r i :Ill!’ sort beforc, US(#.
ircwrding to the “polarogrxphic. syrrthcsis” technique tlescrihed by Lingane, Swiin, :tiid Fic,ltls ($). i\lthough tllc inhtrunlent, W L ~ dc.igtwtl primarily for use with t h m~ c w u r y cathode, it is, of eourw, cxiually suitable for use with pl:iti~iim electrodes according to t 1 1 ~procedures developed. 1)). Sand ant1 his collaborators (t?), n-ith :III>- rombination of working clectroctc~and reftwnce electrode, a i i d f o r controlling the potential ol‘ an :iiio& I3y cnnnect.ing the ( w i t rol circuit cross the cathotic, aiitl a i i o d ( ~01 : i l l tslectrolysia cell thv iiist.iwnc:nt, ran be used to m i i t n i l t l t 1~i i : i I :rpplicd c..m.f., i f rlesii.c.cl.
To begin an espcbriniciit, switch Sa ih clostd firht, :~11d until T’? indicatw tho c*athoclrp~)i(~nti:il Ivhich it i t o rii:iintain. Tlic!n 11‘ i b t u i , 1 1 [ ~ h 1 i r k t o mro, and ,SI i 1Jiii:iIly SI is c l o s ( ~ it o act iv:itcs the. rwntrol rircuit, and t h r :XI)~ , : w : i t uis ~ left t,o itschlf until the elcctrolysis is coniplrtctl. At I!IV ( ~ i r t cif l :in e s p c ~ r i ~ ~ ~,S2 c ~shoultl nt, b c ~opcned to clis:c~oiinc:c~t tlics
A.S.T.M. Standards, 1944. Part I, 204i pages, Part 11, 1649 pages. Part 111, 2240 pages. Anieric:tii Society for Testing Materials. 260 Broad St., Philadelphia 2 , Pa. Pi,icc, $10 for each part: supplenieiit, 84 for each part.
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pourulr kiy cirrirloll(d potential elec:tl,olj
SUMMARY
,111 :ipp:tr:itu> i.G drrjcrihtd which automatically maintaius the' potential of an electrode conat,ant, clnririg electrolytic determimitions ; t n t l icpmttioirs. The instrunicsnt is sensitive t o *0.02 volt, i t i h :ipplirahlc wit,h i x i w i i t * :tiit1 applied voltages of ally niapiitude. iTs compom:nt parts arc. rommercially available and 1.1~11itiv~Iy i.:)*y to aseemblc, and it does not require preliminary c:~liiir:ition, .~lthouyht,ho instrument ww designed particularly for i * i ) i i ~ r o ~ l i ithe i g pot(~nti:~l of :I iiic’i’rur!. mth(Jde, it is equally .uir:il)l(t f c i r w c n i t h platinum or nthrr mtLtiil electrodes, and it [ ~ i i)(* i i c:niploycd witah :my r,nmliin:iti(iti cii working and referrnrc, t-l(i(*trodeP. LITERATURE CITED
I J (!ald\vell, C‘. W., Parker. 11. C.. m r 1 Diehl, H., I N D . l ’ ; s l r , C H E M .IS~L. .. ED.,16, .532 (19.24). ( 2 ) €IivkliiigsA , , Truns. Faratlay Soe.. 38,27 (1943). [ : 3 ) Linrane. J . J.: I X D . ESG. CHEXI.. ;\X.\L. b:I>., 16, 147 (1944). f4) Lirigatie, d. J., Swain, C. G . , and Fields, X l . , J. Am. Chem. Soc .. 65, 1345 (1943). ( 5 ) S:tiici, H . J. S . , “1‘;lectroc.lii.iriihti ~ n t lElectrochemical Analysie. Vol. 11. Gravimetric Electrolytic ;\nalysis”, London anti Ola~gow.Rlackir and Son, 1941, I
BOOK REVIEW
1118
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T l i c , scnaitivity of thc’ instrunicnt ic govcriicd primariiy by thcs inii~itisicsensitivity of thrt galvanometer w h y , and t)y the total ri-i;.tancc in thc m i i t rol circuit. With thv W r s t o n Model 30 u:tl\.:iriomctc~relay, aiid a. resistanre of 700 to 1000 ohms in t,he w i l t id rircuit, most of which wa thc bridge between the refI~I~I~IIIT clcctrodc and the rlectrolj solut,ion, the apparatus cont I ~ I I I I I Y I over a raiigc of slightly less than 0.04 volt-that is, an off~ 1 ~ 1 1 : 1of i i i0.02 ~ volt. in cither direction from the value set on V , \\:IC -ufficic.nt to a e t u t i t e the motor. This wnsitivity is more than :iinltlc for the grcvit majority of electrolysis experiments. For -omr purposcs it may be desirable to work a t a lower sensitivity, : ~ i i dthis can be obtained by increasing the spacing between the of the galvanomc~tc~r relay. anti )or shunting the relay *, tic ,separations of metals with the mercury cathode under the conditions previously described ( 3 ) ,anti has given very satisfactory service. I n n number of instance. quantitative separations of metals whose s t n ~ i d a i dpotentials differ hy only 0.20 volt have t lccn achiered. Tlio~cexperiment-: will b(1 described later. 111addition to purely analytical applications, the apparatus has i)rov(~iI w r y uscf’ul for t h c electrolytic preparation of organic com-
‘rhr 1041 BwIi of -LS.T.M. St:rn(lardn. issi~crli n three parts, cont a i n > in thc,ir IateFt approved form all thr hociety’s widely used specifications a n ( l re-ts for inaterials: 1285 bpwifications and standard tilethod>. . I l i -pecificntioiis. nhrtlier form:il standards or tentatire. a1.e givcii.
Pal t I, hIet,:~ls,deals with ferrrius : L I I ( ~ nonferrous metals (all .k >oiile E serial debigiiatioiis) exrept methods of chemical aiitl with general testing iiierliotls (I< serial designations). Par? 11, Xoriiiirt:dlic Matci,i:ilh, covers constructional ccnientit i o w ni;ttei.ial., i~i)iii*rete atit1 nggi’cKat,es, ninsonry building unith, wi.aniiv, pipe ant1 tile, therni:il ii~iitlntiiipinntcrials (all C serial desigi i i i t i ~ i n :~ i wood and woorl ~)re.*crrntivrs, puints, varnishes. and lacquers, rv:id inaterials, wateryri)ofing :tiid roofing materiak, soil(certain 1) aerial designations) ; general testing methods, thermoinetert. ( E serial designations). Part 1113Sonmet,allic JIateriills, C h i i r l a l , includey fuels, petroleum product.s, electrical insulating materials, rubber, textiles, soaps antl detergents, paper, plastics, water (remainder of D serial designations) : general resting methods, thernioiiieteis (E serial designations). . h i iiiiiovatioii is the inclusion of all eniergenc>-standards and emergeiiq. Rlternate provisions which h a w been widely used t o expedite production and procurement of important materials. Each part ha. a rompletr subject index (from 36 t o 44 pages). To kerp the books up to date, a supplement will be issued to earti part late i n 1945. A complete 200-page Index to Standards is furnished nithout additional charge and a copy accompanies the purclia-e of ea& part or complet,e set. ~ L I €3 I ~:iirtl
