T h e rational solution of this questioii aiid tlie rational accounting for all of the changes in energy aiid iiiattcr niay or ma!. iiot be difficult when the facts have been correctl!. aacertaiiied and stated. Rut the theor!- ad\-ocated in tlie paper referred to above is somen-hat startling. This theor!- is that the liberation of hydrogen from a given conihiiiation and coiiclitioii does iiot require the absorptioii of a fixed quantity of energy, liut a \.ariable quaiitit!., the value of which depends iipon the occliiding or absorbing capacit!. of a n adjacent body - not upon the energ!. el-olved or absorbed iii occliisioii, but iiierel!. upon the relativt. iiiass or- 1-oluiiie of h!.drogeii that a tinit of the adjacent bod!. is capable of occluding. T h e y assiiiiie that greater energ!. is required to liberate hydrogen i n coiitact with lead thaii is required to liberate it i n contact with platinuin, simplj. because tlie platinum has a greater occliiding capacity for Ii!-drogeeti than forlead. T h e results n.liicli would follo\v froiii mcli a tlieorj-, if i t could be established, are so iiioiiieiitoiis, aiid the sponsors wlio stand for tlie theor!- are of such eiiiinent authorit!., that I shall consider it unnecessar!. to apologize for g-oing rather ininutel!. into the details of tlie experiiiieiital facts and arguments, which seem to eiitirel!. refute it. In order that there ilia!. lie no iiiisuiiderstaiidina, it will be iiecesw-y to quote at soiiie leiigtli tlie stattiiieiits of S e r n s t and Dolezalek. The!. sa!. : “ The esplaiiatioii of this abiioriiial propert!. of lead, through which, e\-ideiitlj., is furnished tlie possibilit!. of making an accuinulator o f two volts, may easil!. be derived from the recently aiinoniiced theor!- coiiceriiing the geiieratioii of hydrogen iii contact with inetals, \vliich was deiiionstrated by the esperiinents of Caspari. * ‘ A4ccordii~gto this [theor)-] ‘ there is to he added to the
’
Reference is cited h y t h e authors to Caspari’s paper Ueber TVas5erstofientwicklun#”, Ztschr. pliys. Cherri. 30, H. I . , S9. Hnt ail examination of the paper reveals tlie fact t h a t the proof referred t o is only the suppositioii of Caspari. and is not a t all wu-rantecl 1)y the esperirnents lip describes. He ‘I
tiiiniiiiuiii electromotive force of decoiiipositioii of an electrolyte (free forination energy) a terin n-liicli takes into consideration a specific property of the electrode material, and which, as tlie esperiiiietits iiiake probable, is inversely proportional to the :ibsorbing capacit!. of the electrode metal for hydrogen. It seems, therefore, tliat tlie work, which is required for the formation of hydrogen bubbles 011 a metallic surface, decreases exceedingly with tlie [ increasiiig] occluding capacity ". From this assuinptioii the authors liaye concluded that, since tlie occluding capacity of lead for Iiydrogen is verj- small i n comparison Ivitli the occluding capacitJ- of platinum, the energy required to form hydrogen '' bubbles " on a lead plate is, therefore, greater than tliat required with a platinum plate and greater than the foriiiatioii energ!. of the decomposed coiiiponnd. I n other words, it is assumed tliat the quaiitit!. of energy reqaired to electrolj.ticall!- separate lijvdrogen gas froiii a specific coiiibiiiation and conditioii is not constant. but depends npoii a piiysical property of an adjacent bod>.. The!. also assiiiiie, incidentallj., that the liberation of h!-drogen with lead electrodes Is a secondary reaction, tlie decomposition of PbSO, being the pritnar!., that is, tlie electrochemical reaction. -1gaiiist these assertions and assuiiiptions I wish to place tlie following stateinents, n h i c h are held to be capable of e s periinental demonstration : I . Hydrogen gas is evolved electrolyticall!- in uiiliiiiited quantities froiii dilute sulfuric acid with electrodes of p i r e iiietallic lead when tlie electroinotive force is less thax 0.j volt. 2 . T h e electromotive force required to iiiaintain a steady curreiit between electrodes of metallic lead is less than 0.01 \.Olt. 3. T h i s occurs nithotit the ageiic!. or presence of lead s111phate iii the solution or on tlie cathode. 4. T h e electromotive force required to charge a n accuniula~-
-
~~
-
entirely ignored the electroclieriiical reactions taking place a n d the electromotive forces tlue to t h e corresponding energy changes. FIis inferences, thereftrre. c a n t ~ o t1)e called proof,
4
c.f.KPed
tor is in no waj. depeiident iipon or related to the evolution of hydrogen or oxygen on a lead electrode, nor is i t dependent upon the occliiding capacity of lead. T h e experimental demonstration of the first two of these propositions is of the iitmost importance, as it leaves the entire theory of Nernst and Dolezalek, not only without facts to stand on, but without any object or anomaly to explain. I n order to prove that with lead electrodes lijdrogen is evolved with less than 0.5 volt, it is o n l ~ .necessary to provide for a coiltintiation of the reaction between lend electrodes long enough to study the reaction and to measure the electroinotive force. T h i s requires a rather large anode surfacez becaiise the electrochemical actioii rapidly chaiiges the lead ,?‘node into an anode of lead sulphate. T h i s change results in an entirely different electrochemical reaction as soon as the entire available or exposed surface of the lead anode has been converted into siilphate. large inass of sniall, bright lead wire inaj. be used satisfactorily as an anode aiid a small, bright lead wire as a cathod e. T h e amount of lead sulphate foriiied on the surface of the anode 1 1 ~ .a given current is, zccordiiig to Faradaj-’slaw, proportional to the time the reaction contiiiues. &Asthe lead snlphate must cover the entire esposed siirface of the aiiode to a certain depth before the reaction is interrupted or superseded by a different reaction, we have, evidentlJ-, oiilj. to provide a sufficientlj. large area of aiiode surface, in order to coiitiniie the reaction at a fixed rate for an>-desired period of time. T h e evolution of hjdrogeii m n ~ -be observed for a fenseconds iii considerable volume at 0.j \.olt with an anode of lead ha\-ing- a n area of 0111~. tweiitj.-fi\.e square centimeters, if the lead cathode is reduced to a sinall sharp point. T h e most con\-enient aiid obvious niethod, lionever, of prolonging the reaction is to me as ail anode, lead in a state of molecular or verj. iiiiiiute division, subdivided chemical1j. or electroljdcally, siicli as is readilj- obtained in a charged negative accumulator plate, or in an electrolJticall5. deposited lead tree ”, 131. iising siich ‘(
ail aiiode and a cleaii strip uf sheet lead as a cathode, we are enabled to proloiig tlie deiiioiistratioii as long as there remains an:!. available metallic lead in t h e anode, or, in other words, until the accumulator plate is “sulphated ” or discharged. Iii all of the ixriotis esperiitieiital investigations referred to in the paper under consideration 110 attempt appears to have been made to really ascertain and to consider the chemical cliatiges successively produced b!. the current. iior the energ!’ changes, which must iiecessaril!. accoinpaiiy those cheiiiical changes. For example, electrodes of lead were interchanged with electrodes of lead sulphate or of platiiiiiin witliont colisidering that such a cliaiige would entirely a1ter the cheinical reactions. One important consideration, which \vas eiitirel!, igriored, is tlie fact that an electric current passing lietween lead electrodes in pure, diliite sulpliuric acid can produce at tlit. cathode, h!drogeii oiil!., aiid at the anode, lead sulphate oiil?.. Co.tisequently, tlie esperiiiieiits of these iiivestixators were iiot conducted under conditions which \voiild enable theiii to detect tlicse facts. If copper electrodes had been siibstituted for the lead electrodes, the!. noulcl have noticed a t once that the oiil!products are copper siilpliate at tlie anode and li!.drogeii a t the cathode. ITith lead electrodes the reaction is similar, except that lead siilphate, instead of copper sulpliate, is i:ecessaril!, formed at the aiiode. Of coiirse, the lead siilphate is iiiiicli more insoluble than the copper sulphate aiid, in a \.er!. tliiii la!.er, is less evident to the unaided eye on accoiiiit of being- 1111colored, but these n e l l - k n o \ ~ nfacts afford no groiind for s~ipposing that tlie lead sulphate is iiot formed nnder those coliditioiis. On the coiitrarj-, we 1ial.e abundant proof of the foriiiatioii of lead sulphate under these ideiitical conditions iii tlie discharge of a n ordinar!- lead accuiiiulator. T h e a\.ailaI,le or exposed metallic lead (in spoiig>-form) of tlie iiegatil-e plate of an accuiunlator acts as ail anode during discharge aiicl is thereb!. coni.erted directl!., witlioiit the foriiiatioii of ail!. gas, into lead sulphate. T h i s was proi.ed iiian!. years ago by Gladstone and Tribe, and has been corroborated li!. all snhsecjuent in\.estigati 011s.
TI’heii all of tlie exposed metallic lead of such an acciiiiiulator plate has been converted into lead snlphate, there is no longer any contact between the lead atid the electrolyte. Metallic lead is no longer the electrode. T h e conversion of the lead electrode into lead sulphate has produced a new apparatiis, iiew conditioiis, aiid a new reaction must occiir, if the current is coiltinued. -111 anode of lead sulphate has been substituted for an anode of lead, the mass of lead which remains unsulphated acti n g only as a conductor and helping to complete tlie circuit. -An!- electrocheniical action which iiow takes place at tlie aiiode can affect only the lead sulphate or the electrol!.te, the metallic lead having been entirely removed from the sphere of action. This change in the aiiode from lead to lead sulphate is L.ery rapid aiitl is completed i n a fraction of a second with a sinall current, lvlieii the exposed surface. is small, oLviiig to the i!isolubility of the lead sulphate, Ivhich liniits the reaction to a very t h i n , aliiiost inolecular esteriial la!-er. But when the lead is minntely powdered, so as to expose to the electrolyte a large proportioii of its iiiass, as i i i the ordinary accuiiiulator plate, the time and current required to coinplete the chaiige are proporti o m t e 1y great e r. If, after this cliaiige has been coinpleted, the current be continned in the saiiie direction, ail!. one of several electrochemical reactions may occiii-, including the following : I . T h e lead sulphate a t the aiiode iiia!- he con\.erted b? oxidation into lead peroxide, Pb02. or into lead persulphate, Ph(SO, 1,. 2. -4fter the aiiode has heen converted wholly or partly into lead peroxide the electrolJ-te iiia!. be decomposed with the forination of any of several osidatioii products, including oxygen, hydrogen peroxide, ozone, aiid persiilphuric acid. For each of these clieiiiical changes there is ail evolution or absorption of a definite quantity of cheniical energy. T h e cheniical energ!. of an electrocheniical change is always added to or takeii froin the electrical energ!- of the circuit and either increases or diiiiiiiishes the electromotive force between the electrodes, depeiidiug upon whether the cheniical energ!- of the
reaction is esotlieriiiic or endothermic. A n atteiiipt, therefore, to determine the electromotive force required to liberate liydrogeii b!. iiieasnring the potelitid difference lietn-een a lead cathode aiid an anode which clianges from metallic lead, firstl!. to lead sulphate aud then to lead peroxide, \vitliout considering. the eiiergg. of these changes or deterinining which change is taking place at the time of tlie nieasnrenieiit, ~ v o u l d appear to lie the lieiglit of absurdity. ?;either concentrations, osniotic prewires, nor iliat1ieinatic:il forniulre caii assist in such an 1111dertaking. It is evident from their results, as well as their statenleiits, that no iiieasiireinents \vere made bj. Seriist and Ihlezalek or b!- Caspari of the electromotive force reqriired to liberate IiJdrogen. with Ieod c~lcc~r-on'rs,that no inea:iirements nere coiiiiiienced until after tlie anode had ceased to lie an anode of lead. Caspari states, for example, that lie waited until tlie current hecame constant at a defiiiite \-aliie before making his ~iieasiireineiits, that is, iiiitil his anode \vas tliorouglil\. coii\,erted into lead peroxide. T h e fact that in their experiments the electroiiioti\-e force recinired to evol1.e hydrogen was l o w during a brief initial period seeiiis to 1iai.e lieeii noticed, hiit pirposeei!. ignored b!- all of tlie investigators, as though it were too insignificant a niatter to be \vortli!. uf ail!. serious coiisideratio~i. Yet it was cliiriiig this brief period oiil!. that the electrodes coiisisted of lead atid it was the electroinoti\.e force hetween electrodes of lead that formed tlie suliject of their investigations. T h e electroclieiiiical reaction lietiveen electrodes of iiietallic lead, wliicli results iii the formation of lead stilpliate, is a reaction e\.olt.ing \-erg. great ellerg!. (more thaii that reqiiired to liberate li\.drogen i, k i i t all of tlie reactions occurring after the m o d e has been converted into lex1 sulphate or lead peroxicle, absorb energ!.. Hence, the electroiiiotive force required to produce a curretit will be different i i i tlie different cases, whether an\. liydrogen is liberated or not. T h e energ!. required to liberate Il!-drog-eii froni a gi\.eii clieiiiical combination under given
8
C.. J . 1'Zced
conditions evidently corresponds to a fixed electrochemical electromotive force. Rut the electromotive force found experiiiieiitally between two electrodes, a t one of 7 1 liich hydrogen is evolved electrolytically, is merely the sum of this and all other electromotive forces included in the circuit between the two electrodes. T h i s observed electromotive force between tlie electrodes is not, therefore, a iiieasiire of tlie electromotive force required to liberate hydrogen. No method has yet been found of eliminating experimentally the electromotive force a t tlie other terminal of the electrolyte. T h e electromotive force between a n electrolyte and a single electrode has never been measured and we have no positive experimental evidence that any such electromotive force exists. T h e use of a so-called " norinal electrode" does not help 11s in the least. l y e are \ti11 nieasuring on 1y t h e elect roin ot i ve force bet w eeii t JYo el ect rod es. T h e only k n o n n soiirces of electromotive force (positive or negative) between two electrodes emersed in an electrolyte are the electrochen~ical,the Joule, the Tlioriisoii ( Kelvin), and tlie Peltier effects. T h e electromotive forces caused by the Joule, Thotiison, and Peltier effects are diie to energy introduced into or eliininated from the circnit in the forin of heat. T h e Joiile effect acts oiilj. in opposing ai1 equal or greater electromotive force. lVlien the teinperature of the s j stem is maintained constant and uniform, the intensities of these thernio-electromotive forces are proportional to tlie strength of the electric current. The). are, therefore, entirely eliminated when tlie teinperature is coiistaiit and uniforni and the current is zero. T h e electrotnoti\-e force caused by the electrochemical effect is, however, not dependent upoii the strength of the electric current, but depends only upon the cheinical energy of one univalent electrochemical equivalent of the reacting substances. It follo\vs, therefore, that, when tlie temperature of an electrochemical system is uniforni and coiistant and tlie current zero, the measured electroinoti\.e forcc is that due solely to the electrocliemical effect. I n the electrolj.sis of dilute sulpliuric acid between elec-
trodes of lead the electrochemical reaction results in the formaof lead sulphate a t the anode and hydrogen a t the cathode, the complete electrochemical change being represented by tlie equation, Pb - H,SO,Aq: PbSO, 2H Xq. * * * * * ( I 1
+
-
+
-
T h e energy evolved bj- this reaction is, expressed as the equivalent in tlie form of heat, 215~700- 210,100 = 5,boo (Bcrthelot) calories. T h e chemical affinity tending to cause this change to take place a t either electrode is equivalent to an electromotive force of
5,600 ~~
2
X 23,240
~
.-
0.12
&Isthis affinit!,
volt.
exists a t each electrode, the electrochemical electronioti\-e forces balance and can came no current in either direction. T h e system cannot of itself produce an electric current through a n e s ternal circuit. If, however, the slightest electromotive force be impressed upon the system froin an external soiirce in either direction through a closed circuit, the current call flow, producing lead sulphate a t the anode and hydrogen a t the cathode. In the experiments performed b j the writer, 0.38 volt was the lolvest external electromotive force, with which a n evolution of li!.drogen was observed, but it was found that a n electrotnoti1 e force of 0.01volt was sufficient to maintain a stead!, current. T h e soiirce of the electromotive force used was a separately excited dynamo. IYhen the anode has been converted into lead sulphate and the cathode remains metallic lead, the next reaction forms lead peroxide, hydrogen, and sulpliuric acid as follows : PbSO,
- HISO,-\q
--
2H - PbO,
-
2S0,hq
* * * *
* ( 2
T h i s reaction absorbs a n electromotive force equivalent to 21j,700-- 210,100--
(63,400 -23,240
~-
2 x
2 'Y
141.100)
-~
=-I . 7 3 volts.
Ozone, 0-,nil1 also be simultaneonsl\- formed by tlie independent electrolytic decomposition of the electrolj-te between the P b 0 2 anode thus formed and the lead cathode according to the equation, 3H,SO,Aq 6H - 0, - 3S0,Aq * * * (3 ).
..
- - --
T h e electroinotive force absorbed is equivalent to
As the electromotive force required to foriii ozone is 0.03 volt less than that required to form lead peroxide, there will always be some ozone foriried as soon as there is ail). lead peroxide to act as an anode, that is, as sooii as any cnrrent has passed. In practice i t is found that the lead peroxide and the ozone alwaj, s appear simultaneously when tlie operation is conducted a t ordinar!- temperatures. \Tit11 ail aiiode of lead peroside and a cathode of rnetallic lead we may have the formation of either lead persulpliate, Pb( SO4)*,hydrogen peroxide, Ho02,or periulphuric acid, HSC),, at the anode and lijdrogen at the cathode. T h e forniatioii of the lead persulpliate is r e p r e s e n t d by tlie equation, PbSO,
-&
H-SO,XCJ P b ( S 0 , ) . - 2H
-
X q **
- -. *
* I
4
Tlie foriiiatioii energj, of Pb(S04),does iiot appear to haye been published. Tlie forinatioii of lij-drogen perosicle with an aiiode of lead peroxide and a cathode of iiietallic lead is accordiiig to thy eqiiation, 2H,SO,-Aq 2H H 1 O , A ~ l7 2SO,Xq. * . * I j)
This absorbs on electroiiiotii e force ecjitivalent to 2
.: 2 1 0 , 1 0 0 -
(47,300 2
-
723.240
2
\
14',IOO,
1 . ~ volts. 5
T h e formation of persulphuric acid with an anode of lead peroxide and a catliocle of metallic lead is represented by the equation, 2H2S0,Xq 2HSO,Xq ?H * * * * * * 6
- -
T h e absorptioii is
Reactions
( I ) , (2), ( 3 ) .
(5). anti ( 6 ) are kiionn to occtir.
m-liile (4) is known not to ucciir to any appreciable extent, since lead persnlpliate is very soluble and its formation would carry tlie lead into solution, where it \vould be precipitated as lead sulphate, leaving persulphuric acid in solution. In all of the cases cited above the cathode is of metallic lead. If, instead of metallic lead, we use a cathode of lead sulphate and an anode of lead sulphate or lead peroxide, such as have in the charging of a discliarged accumulator, no current can pass without tlie reduction of the lead sulpliate of tlie cathode to metallic lead and the oxidation of tlie lead sulphate of the m o d e to lead peroxide, the reaction being, X C-~ 2PbS0,
-
PbO,
Pb
-
+ 2S0,Xq -
*
* *
9
.
* *
(7
i .
T h e absorption is
ITith the formation of lead and persolpliuric acid from a cathode of lead sulphate and an anode of lead peroxide we have H,SO,-lq T h e absorption is
~
Pb
PbSO,
-7
zHSO, Xq
* *
* * * * *
(8
In the reaction forming ozone and metallic lead from a cathode of lead sulphate and an anode of lead peroxide we have Aq
- \?PbSO,
gPb
-
3SO,Xq - 0,. .
.. . . .. . . 1 g )
The absorption is 3
215
-00-.(
3 141,100-'- 30,700) = 1.82 volts. 6 b 23,240 ~
In the reaction forming h\.dro,aen peroxide and nie:allic lead, we have PbSO, - H,SO,Xcl
Ph
- H,O,Aq
1
2S0,Aq.
.( I O )
T h e absorption is 2IS,jOO-- 2 1 0 , 1 0 0 -
_ _ ~ _ _
2
( 4 7 , 3- 0- 0 - 2
23,240
A
r4r,1oo,_ -
2
07 volts.
I2
C-. J. Reed
I n the above reactions it has been assumed that tlie sulpliiiric acid liberated from lead sulphate is alwaj,s liberated as SOIA4q,t h a t is, as 7 ~ 7 diZutc y acid. In ordinary practice wit11 lead accuxiiulators this assumption is not correct, as t h e acid used in the acciiinulator is not very dilute and the dilutioii of the electrolytically liberated acid cannot exceed the dilution of the acid into which it is liberated. T h e acid set free will, as a matter of fact, alwa!.s be considerabl!. stronger than the solution constituting the electrolyte, particularly in the interior of a spongy electrode. A correction ~niist,therefore, he added to tlie above results, which is the equi\-alent of tlie additional energy required to liberate the stronger acid. T h e total energ!- required to concentrate t h e most diliite acid to its ~naximiiiri densit! (specific graI.itj. of 1.842) is I :?S j o calories per equivalent (bivalent), correspoiidiiig to an 17,8jo electromotive force of 2
x 23,240
0.38 \volt. ?'lie iiecessar!. correctioii cannot, therefore, exceed this value. F o r a n y particiilar densit:. of acid the correspoiidiiig dilution heat e s pressed in calories and the eqiiivalelit correction expressed i n \-olts may be taken direct11 from the accompanying curve. T h e proper correction corresponding to the densit!. of the acid used must he added once if the reaction liberates free acid at only one electrode, as in equation (2), and must be added twice when the acid is liberated at both electrodes. as in eqnation ( 7 ) . =-
.\ssumiiig that in a n accumulator the density of tlie electrolyte is 1.20, the strength of the acid liberated will be a t least 1 . 2 0 aiid will probably be a t least 1.30 or 1.35 a t tlie point where it is liberated, t h a t is, i n the pores of the spongy electrodes. T h e correction corresponding to acid of this density is found froiii the curve to be 0.08 volt. .\pplyiiig this correction to the abo\-e reactions, we obtain in equation ( 2 ) 1 . j 3 ~ ' 0.08 1.81 vo!ts. ~
'3) (5)
1.70
(7)
1.845
(8) (9)
2.36
i IOJ
2.07
I.9j
1.82
0 . 0 8 =:
i
' 2
~
--
1.j8 2.03
0.08 Y 0 . 0 8 -7 2 . 0 0 0 . 0 8 _= 2.44
0.08 2 J 0.08 2 ,/
1.98
~~
~
2.23
"
"
' I
*, ' (
"
T h e formation heats used iii these calculations were tiiostlj, made at a ternperatiire of aboiit I j " C, aiid t h e electromotive forces are. therefore, correct for a n electrochemical system tnaiiitained a t that teinperatiire when t h e circuit is open or when t h e current is very minute. I t is scarcelj. iiecessar!. to add that the!, are entirely iii accord with those uni\-ersallj. olltained in practice. T h e increase i n the strength of the acid in tlie pores of tlie active material aiid t h e cotisequent formatioil of persulphuric acid a t the end of the charge accoiints satisfactoril!, for t h e high electromotive force reqnired. IYheii the electrodes of a n electrochemical s>.steni constitute al:jo the electrochemical reagents, as is the case with tlie lead sulphate, lead peroside, and metallic lead of a n accumulator, tlie decomposition tension of tlie intervening electrolyte is not necessarily a factor in determiiiiiig tlie electromotive force betbveen tlie electrodes. I t will iisually be a factor if t h e constituents of tlie electrol!.te are actually set free. Bnt it cannot be a factor if none of tlie constituents of the electrolyte are liberated the electrochemical action, because the decomposition cannot then ca.use the absorption or evolution of the foriliatioil energ).. T h e energy required to evolve the actual prodncts formed by electro-
I4
C. J . Rerd
chemical action a t the electrodes is always a measlire of the electromotive force tiecessary to do the work of chemical separation (or combination ) resulting in those products. If, for example, there were a dozen electrolytes intervening in series (separated by porous diaphragms) between the electrodes, the passage of the ctirreiit through the series would cause decomposition (interchange of bases and acid radicles) at both terminals of each intervening electro1j.t.e ; but, as the constituents of these intermediate electrolytes are not set free, the electromotive force required for such liberation is not imposed upon the circuit. T h e same is true of a single electrol>.te, such as dilute sulphuric agid, inten-ening between two electrodes, such as lead sulpliate, which furnish the liberated products of electrochemical action. In this case the products liberated are lead, lead peroxide, atid sulphuric acid, aiid the electroilloti\-e force required for the passage of the current is that which corresponds to the energ? required to separate these bodies oiil!, and is it1 110 wa!. dependent iipon the electroinotii-e force required to decompose any electrolyte or electrol!.tes that ma?. intervene. T h e clieniical changes a t the two electrodes occiir in series and the current can reach the electrolyte only by passing into and out of the lead sulpliate electrodes and decomposing this substance a t both places. If the current could have access to the electrol!te without p a s i n g in series throagli the electrodes and decoinposing them (as would be the case with platinum electrodes), the constituents of the electrolyte would be liberated aiid the electromotive force required would correspond to the energy of such liberation. Hut the current must cross the junctions between the electroll. te and the electrodes in series, and when these electrodes are of lead 5ulphate, they iiiiist undergo electrochemical decomposition. When all available lead sulphate on both electrodes of an accumulator has been converted into the products, lead, lead peroxide, and sulphuric acid, the electrodes can no longer supply the products of electroclieinical action aiid the>-must be supplied by the electrolyte. Oxygen, h>-drogen,ozoiie, hydrogen perox-
ide, and pursulpliuric acid then appear i n quantities corresponding to the total current. Bnt these products can theti lie liberated only by the passage of the current, and no current call pass without a n electromotive force exceeding the couiiter-electromotive force of the charged accutiialator n i t h a l l of the products that have been formed. IYe might as well expect a boat to move against the current of a river wlien its velocity is less t h a n that of the streaiii. T o prove that the liberation of lijdrogen on a lrad cathode is not dne to the priiiiary reductio11 of lead sulphate by the current and the secondary clieniical action of the reduced lead on the water or acid present, i t n-ill lie sufficient to show that the h! drogeii is liberated i n the same manner whether lead sulphate is present or not. T h i s may be sliowi by electrolyzing nit11 ai1 anode of platinmii and a cathode of clean, bright. metallic lead a :jol1ltioll of pure dilute sulphnric acid. .Altliough there is 110 lead sulphate present, either 011 t h e cathode or in tlie solntion, the passage of the current caiises an iiistantaneoiis evolution of li!.drogeii as soon as the lead cathode touches the solution, the quantity being liinited only 1 3 ~ .tlie cnrrent. I t cannot be supposed that the action of the current at the cxthode is to form aiid tlieii reduce lead sulphate. ;\s tliere is no other possible source of lead sulpliate, tliere cannot be an). present either before or during t h e passage of the current. Furtheriiiore, an examination of the acid surrounding the cathode iinmediatel\. afterwards shows no trace of lead sulphate or an)- other i i i i pnrities, the acid e\-aporating nitliout residue, tllough se\.eral handred ciihic centimeters of liyclrogeii ilia!. have been liberated. T h i s may be accoiilplishecl uncler practically the saine coiiditioiis with both electrodes of lead and an electroiiioti\.e force of 0.5 voit by using ai1 anode of finely di\.ided lead and snrrounding the cathode n-ith a porous cnp containing the piire acid. If tlie liberation of h!.drogen is n secoiidar!. action, that is, the chemical actioii of reduced iiietallic lead on the water or dilute acid, it is difficult t o see I i o ~ vthis secoiidar). reaction cat1
depend upon either the electromotive force or the current. Such a secondary reaction must depend only upon the presence of the reacting materials, that is, upon the presence of the reduced lead and the solution. These are always present in an accumnlator and ready to produce an!- possible secondary reaction as soon as the charging current f l o w , and they remain present after the charging current stops. T h e evolution of hydrogen. however, does not depend upon the presence of reduced lead in the solntion, but upon the current, stopping when the current stops arid commencing when the current begins, although there may remain a t all times an abundance of reduced lead. IThile it is undoubtedlj. true that reduced lead does act slowly on dilute sulphuric acid, evolving hydrogen and gradually discharging the charged accumulator, i t is equallj- certain that the evoliition of hydrogen a t a lead cathode during electrolysis does not depend upon the presence of lead sulpliate, any more than the evolution of hydrogen at a platinum cathode depends upon the presence of platinum sulphate.
