ON IRREVERSIBLE CELLS. BY A . E . T;\\'l,OR.
T h e present paper is a continuation of the work of Baiicroft' 011 single-liquid polarizable cells, with especial reference to the effect of the negative ion oti t h e poteiitial differeiice lietween an electrode and t h e electrolyte in which it is iiiimersed. Tlie electromotive force of a cell, having for electrodes an!. two of tlie metals Mg, Zii, Cd, Sii, PI), or Ei in solutions of chlorides, l)roiiiitles, iodides. sulpliates, nitrates or acetates, does not seem to lie in an)' wa). a fuiiction of tlie negative ion. TLrlien, however, mercury serves as one electrode a variation with the negative ion always appears. Measurements were made with zinc ant1 catliiiiuni in coinhination with mercury in chloride, liroiiiide, iodide. nitrate, sulphate and acetate solutions. T h e results ol>taitietl \yere conipared with the values given by Pasclieii' for the single potential differences entering into t1ie:;e same cells. Tlie ineasureiiients of Pasclieii show i t i every case a variation with the negative ion for the sitigle potential clifferelice. T h a t these variations do not appear in the electromotive forces of the cells in whic1i.tn.o of the iiietals Mg. Zn, Cd. Sn, PI, or Bi are used as electrodes, is explained I)!, the fact that for each negative ion the term due to it has the saiiie iiuiiiericnl value atid the same sign for each metal, but varying for the different ions. So, in the electroiiioti1.e force, which is the si1111of the two potential differences taken in opposite directions, this term tliie to the iiegative ion clisappears. \\'it11 iiierciirj., however, the value of the term seems to he the same as with t h e other metals iiieiitioiied but with opposite sign. 111 conseqiieiice of this clifferelice in sign, there appears in the electromotive force of a cell. iii wliich iiiercury is oiie of t h e electrodes, a term due to the negative ion and having a \ d u e equal to ~
~-
.~
~
'Zeit. phys. Chetii., 1 2 , 259 ( 1S93) ; Physical Revie\\., 3 , \Vied. A n n . , 4 3 , 590 (1891),
250
(1S96).
twice that appearing in tlie single potential differences. With platinum the numerical value of this term is 110 longer the same. T h e behavior of copper, in this respect thus becoines a matter of interest, and the first part of this paper is given to an in1,estigatioii of cells composed of Zn, Cd, Hg and Cu in solutions of chlorides, iodides and sulphates. T h e cadmium first used mas takeii from the Uni\-ersitJr niuseuni and later some pure cadiiiiiun was obtained froin Einier and Amend. Measureiiients made with the various cadmium electrodes on tlie cell, Cd 1 chloride 1 Hg all agreed very well among theiiiselves, and further agreed with the iiieasureinetits of Bancroft on the same cell. 'I'wo sticks of pure copper, previousl>, used 11). Prof. Bancroft iii electrical measurements, served as copper electrodes. T h e mercury used was purified according to the method giveti by Ostwaltl iii his Phjlsiko-cheniische i l f ~ s s i i i g e n ,#. I 00, and gave extreme1 J. sat isfactory results. Iinpure iiiercury was repeatedl!. shaken with dilute sulphuric acid to which was added a few drops of potassiuiii hichromate, and after being carefully washed with distilled \vater, was allowed to run in a fine streaiii through a long column of ten pelcent. nitric acid. T h i s last process was repeated several tiiiies. T h e pure mercury finally obtained retained a perfectly bright surface during a period of several months, in fact until it was all used. Especial care was exercised in the preparation of tlie potassiuni salts used. T h e potassium chloride hac1 been prepared 111- being sis tiiiies precipitated from a concentrated solution of tlie salt 1)). hydrochloric acid gas, aiid had been preiviousl>. used to tleteriiiiiie the constaiit 01' a conductivity cell. I t had also been conipareil irith potassiuni chloride used by Dr. Kortright iii concluctivity work, and i n Iiotli cases the same constaiit mas obtained for the cell. T h e sulphate first used probably contained a trace of some other salt for it ga1.e low values ; but on being recrystallized several ti iiies very satisfactory results were olitained. T h e deterininations of the electroinotive force were made with a small Lippmann's electrometer according to the well known coiiipensation method of Poggendorff. A Latinier Clark cell served as a normal element. T h i s cell was compared from time to time with another Clark cell, and the two were always found to agree within
at least a millivolt. Two Leclanchk's having a combined electromotive force of about 2.6 volts were used as a working cell. These cells were very constant, although they slowly changed to the extent of about three or four per cent. T h e value of the Clark in ternis of the Leclaiichk was determined two or three times a day. Tlie single-liquid cells measured were made u p in little vials of about tnwit).-five tiiilliiiieters iii tlianieter and seventy-fire millimeters i n height. These vials were provided with paraffined corks into which the electrodes were tightly fitted. In case that mercury was used as one of t h e electrodes, a glass tube, into which a plati i t i t n i wire had lieen fused, passed do\vii through tlie electrolyte arid made connection with tlie iiiercury. Great care was taken to keep the cells perfectly clean, as slight inipitrities have a verj. marked effect. \Vlietierer tlie cells were made u p they were first very carefully \ \ d i e t 1 for a long time with distilled water : and each time, the electrodes were carefull!- scraped or cut with a sharp knife to elisitre a bright clean surface, then washed with distilled water and wiped with filter paper to remove an). loose particles of metal. T h e bottles used for the solutions were first thoroughly washed with distilled water and tlien 1)oiletl out with steaiii. In all cases the niaxitiiuiii reading of tlie electroniotive force of the cell was taken, for all tlie cells measured were fouiid to increase rather qiiickI\- to a iiiaxiiiiuiii, which reniaiiied constant for a time. Vast of the cells reached a ~iiaxiriiiitiireading in about ail hour, while sotile of the cells, especially the sulphate cells, might require t\vo or three hours. In a few cases tlie tiiaxiiiiitiii was reached in fiiteeii ur tivent). iuinutes. However, the time for ail!. particular cell \vas not in tlie least constant. Throughout tliis whole paper tweiitietli nornial solutions are alnTa\-s w e d when no concentration is specified. Tlie cells first tiieasitretl were potassiuiii chloride cells in whicli C u , Hg and Cd ser\.ed as electrodes. The cells are all measured in volts, the value of tlie standard Clark being taken a s 1.434 volts a t rs°C. Each value represents a different cell, a series of readings being taken, and the tnaximum which remains constant being given. T h e cells are always niritteii,
TABLE I. --n/ Electrodes
Cd I Hg
11 --
Cu 1 H g
Ctl
~
cu
0.821
0.255
0. j 4 6
0.251
0.568 0.566 0. j 6 S 0.568 0.561 0.557
0.812 O.SI3 0.818
20
K C1-
0.814
0.815
e. g., Cd 1
20
'
0.254 0.254
KC1 1 Hg, so that the current
I
iti
the cell runs from
left to right, tlie anode thus being written first. A list of nieasurements is given rather than a simple average, iii order that the variation of tlie various cells may lie seen. A greater accuracy than 0 . 0 1 volts is not claiiiied for the results, although it s e e m probable that the accnracy is somewhat greater thaii that. I t will be seen that the cells obey very well Poggendorff's lan., iii that tlie \.slue of the cell Cd 1 Hg is very iiearly equal to the siiiii of the values given for the two cells Cd 1 KC1 1 Cu arid Cu i KCl 1 Hg. It ~vould seem tlierefore that the averages give \ d u e s which differ but a few iiiillirolts from the true values of tlie cells iiieasuretl. A t any rate tlie maximuin variation observed woultl 1iai.e little or. no effect upon the general facts observed. T h e sulphate cells in table I1 were measured. T h e values of these cells were much harder to determine than the corresponding ones for tlie chloride cells. T h e slightest impurity has a very large effect. A great man). iiieasureiiieiits were made with a sulphate solution, which I supposed to be pure. T h e results, however, clicl not agree very well aniong theniselves, nor would they add up according to Poggeiidorff's law. Tlie cell Cd 1 Hg was iiot equal to tlie siiiii of the two cells Ccll Cu and Cu 1 H g . Tlie salt was thereupon purified by repeated crj.stallizatioii and satisfactor!. nieasurements were obtained. A very slight aiiiouiit of sulphuric
0. j40
1.03; 1.037 1.029 7.033
0.309
0. f
i n .the solution.’ Especially ivith like concentrations of salts having tlie sanie anion. are t h e saiiie xxlues obtained. From these measureiiients of Ostwald aiid Pascheii it is rather difficult to determine whether the potential differetice varies with the concentration of tlie electrolyte or’ not. T h e variations are iiot a t a11 large except i t i certain cases, and ma). i n general coiiie withiii tlie limits of experiiiiental error. Further, if we consider tlie tivo cells Cd 1 KC11 Hg a n d Cu I KCI Hg which were fouiid to lie invariable o\-er a ver!. \ride range of coiicentratioti, we ma!. coiiie to otie of t n o conclusions with regard to the single potential differences. Either the!. do iiot i.ary a t all with the concentration, or t h e variation is equal in each case a n d coiisequeiitly does iiot appear iii tlie electromotive force. Baiicroft adopted tlie first view, h u t later work 011 111). part seems to poiiit t o the latter alternati\,e as tlie more proliable of the two. Further, what do the single potential cliffereiices show witli regard to tlie effect of the tiegati1.e ioii ? Does t h e iiegative ioti have ail effect siiiipljr oii iiirrcur). anti copper. as the nieasireiiieiits on the cells might indicate ? Froiii the iiieasureiiieiits of Pasclieii. Baiicroft saw that n i t l i Cd, Hg ant1 Zn the negative ion always liad an effect. Further in passing from the chloride to the iodide a difference of aliout o. r j volts, aiicl from tlie hroiiiide to tlie iodide a tliffereiice of 0.08 volts \vas to be oliserved. S o w i n measuring tlie Z n H g or Ctll Hg cells n cliffereiice of 0 . 3 volts was noticed betweeti tlie chloride and iodide cells, and 0 . 1 6 volts het\veeii tlie chloride aiitl broiiiitle cells, while Zii I Cd was tlie saiiie for chloritleh, Iiroiiiitles aiid iodides. This sho\vetl that the effect of tlie iiegati1.e ion \vas tlie sanie on all the metals coiisideretl. hut thnt with iiierciir). tlie sigii was differeiit- T h e ii~iiiibersassigiietl I)!, Ilaiicroft a s Ijeiiig tIie iiiost pro1)sl)le values for Z i i . Ctl niid Hg i i i chlorides hroiiiicles a i i t l iodides are given i i i Table LyIK. In atlditioii to these values;. I liave calculatetl fro111in!. iiieastireiiients the values for the sulphates and for copl’er i i i chlorides niitl sulphates. i i i tlie followiiig iiiaiiner : Cd 1 K,SO, I H g = I ,035 Cd 1 KISO, 1 cu=o.7-’s CCI K2Cl H g =o. S I ,i Cd 1 K,SO, -0.365 1 (SO,-Cl) =0.220 so,1 Cu ‘0.363 so,--cl =o. I I O ~
~
’
‘Ivied. .Itin. 43, j 7 0 ~605 ( rS9r 1.
16
C u ( K C l 1 H g =0.252 CliHg =0.562 c u I c1 =0.3 I O
T.ULE VII. -
-.
~-
Solution Chloride
Sulpli ate Bromide Iodide
~
211
1 Sol.
Cd 1 Sol.
Sol. Hg
0.255 0.365
0.562 0.672 0.4s3
o.sy9 0.699 0.507 0.436
0. I
74
0.104
Sol. I Cu 0.310
0.363
0.410
I t will he seen that with copper the value due to the negative ioii is not the same iii \xlue a s with Zn, Ctl and Hg. I t has, h m v ; eL'er, the same sigii a s the term for iiiercmy, but a smaller value. In passing froiii a chloride to a sulphate, with copper there is a difference of h i t 0.053 volts instead of 0.I I O volts, so that when coiiibined with Zn, Cd or Hg the electrotiioti\.e force a1waj.s appears to be a function of the negati1.e ion. T h e following talde will give the values o1)tained for the singleliquid reversible cells, Zii 1 Hg, Ccl 1 Hg, Zii i Cd, Zii 1 Cu, Ccl I Cu aiicl Cu 1 Hg, from actual observation aiid by calculation from Table l r 1 I . T h e first tliyee series of cells were measured by Bancroft, the last three by nij.self, as were also the sulphates.
TXBLE 1-111 Zn 1 Hg Solution
calc.
oh.
Ctl i Hg calc.
Zll I Ctl c:k.
01,s.
obs.
0.81;
o . S r g 0.334 037 1.035 0.334 o . S I 4 o.LiI.5 0 , 3 3 2 0.657 0 . 6 j g 0.333
Chloride Sii 1ph a t e Iodide Bromide
0.S17 I
0.333 0.334 0.331
0.333
~
cu Solution
011s.
Cd 1 c u obs.
calc. ~
Chloride S u 1pli ate
0.897 1.061
Cu I Hg calc. ~
01,s.
0 . jG5 , 0. j 6 j 0 . 2 5 2 0.252 0.728 0 . 7 2 8 0.309 0.306
I t is a n ea$). matter to give a formula for these cells if we consider the single poteiitial differences macle up of two terms ; one due to the metal antl tlie solvent, a n d the other due to the negative ion. Now we have found that in all cases except with copper the tertii due to the negative ion is int1epei:deiit of the metal considered. So, for the potential differences Zti 1 K,SO,, Z n 1 KC1. Cd 1 K,So,, Cd I KCI, Hg 1 R.,SO,, Hg 1 KCI, Cu 1 K,S@,, Cu 1 KCI, we may write --!+a, .J+6. 6+-a,6 + 6 , C--a, C-b, D--n, and D--6,. W e would then have for the cells Zn 1 K,SO, 1 Cd and Zii 1 KC1 1 Cd the forniulz ,-l+n --6--n or ,-i-B, antl .-f+6--6-6 or -4-6 for the electromotive forces ; that is, tlie values of t h e two cells are equal and iiidependent of the negative ion. For the cells Cd 1 K,SO, I Hg and Cd 1 KCl j H g we sliould have B+n--C+tr or 6 - - + a n , aiicl 6+b--r+b or 6,-C+rB. T h e two values are not equal and a term due to the negative ioii conies in, which is twice the value of the term due to the negative ioii in tlie single potential differences. Similarly we sliould have for Cu 1 K,)SO,1 Hg atid Cu 1 KC1 [ H g , D--a,- C+a or D--C+tr-n, and D-6,-LC'+6 or D-C+h-B,. In view of the iiieasiireiiieiits of Baiicroft on S n , Pb, Bi and Mg combined \vitli Cd or Zii i n chlorides, bromides, iodides, sulphates, nitrates, acetates, carbonates and oxalates, i n which lie found that the electromotive force is a function of the electrode metals and independent of the negative ions, we may conclude that with all of these metals the term diie to the negative ion lias the same numerical value atid the same sign, and consequently disappears when the metals are combined in a one liquid cell. With mercury the values tile vai.ious iiegati1.e ions are the same as with the metals just eiiumerated biit having opposite sign. With copper the tertii due to the negative ion has tlie same sign a s with inercur!., lmt lias a siiialler numerical value than the corresponding teriii with mercurj.. II'itIi platinum tlie term due to the negative ion h a s not the satlie value ; arid fiirther cells, in which platinum ser\.es as a n electrode, va r!. \vi th the coiiceii trat ion. Very little work has been done on the ineasureiiient of single potential differences with reversible electrodes. Le Blanc' made a ~~
~~
~~
'Zeit. pliys, Cheni. L
~
12,
345 (1Sg3).
18
A. E. Taplor.
few ineasureineiits by determining the decomposition point of certain normal salt solutions. Neuniann' has niade quite a list of measurenieiits on the potential difference between a metal a i d iiornial solutions of sulpliates, chlorides, acetates and nitrates of these metals. These iiieasureiiients were made with the aid of a normal niercurycaloniel electrode, consisting of mercury in a normal potassiuiii chloride solution to which an excess of calomel was added. Ostwald gives the value 0.560 volts to this normal electrode, and its value has been found to be extreinely constant by Coggeshall.' The method of determining the single potential differences mas by ineasw i n g the electromotive force of a cell composed of the calomel electrode coiiibined with the metal and electrolyte whose potential difference was desired, and subtracting the value 0.560 volts from this, From these single potential differelices may be obtained by addition the values for the electromotive force of reversible cells ; and the values thus obtained for reversible cells agree very well with the values directly nieasnred. Now let us further consider the single potential differelices involved in one or two concrete examples of a cell composed of two metals and two electrolytes c. g. Zn 1211SO, 1 Hg, CI,KCl I Hg, Cu 1 CuSO, 1 Hg,Cl,KCl I H g and Z n I ZnSO, I CuSO, 1 Cu. W e then have in the first case the four potential difference : A. between Zn and the electrolyte ZnSO,. B between ZnSO, aiid KCI. C between KCl and Hg. D between Zn aiid Hg. This last value D is negligible. T h e value of the cell is then '4+B+ C, similarly for the second cell we have .4,+Br+ C, and for By subtracting the second from the the first the third A+B,--.iZ,. value.4-A,+ (B-B,) is obtained, and this was found experimentallj. to agree with the formula A+R,-J,. T h e assumption is made from the forniulz given by Plaiick and Nernst that the iyalues B , B, and B, are very small, but I do not know that there are any nieasurements that make this entirely justifia1,le. I t is easily seen that the only assuniptioii whicli need be made, that the sum of the first IIhid. 1 4 , 2 2 5 (1894). 2Zeit. phys: Cliern. 17,62 ( ~ S g g ) .
two cells shall give the third, is that B,=B-B,. Now if B2 is small or zero then 6 atid 5,iiiay 1iaI.e any values provided that they are eclual or nearl!. so. I f , however, tlie \ d u e s 6 and -6, are not so sinall as to lie neglected, the single poteiitial differences of Neunianti are incorrect bj. just sucli an amount. T h e following values arc given 11). Neuiiiatui a s the absolute potential of tlie metals in their norinal salt solutions.
TABLE: IX. ~
Metal
~~
Sulphate
~
,
Chloride ~~
~
TI
Fe
co
Ni Pb
Metal ~
~~
~~~~
Sulphate ~ _ _ _
~~
~~~~
Bi As
RJIg AI M 11 2 11 ccl
~
_ _ ~ _ _ _ .~~ _ - ~
-0.490
0.162
0.114
0.1jI
-0.315 -0.550 -0.085
SI1
cu Hg Ag Pd Pt A 11
Chloride _____
-0.376
Sb 0.503 0.174
0. j 2 4
'
-0.585 -0.g80
-0,974
- I .066 -1.140 -1.356
I t will be seen that the sulghates and chlorides do not vary tnucli. A s tlie solutions are all norinal, Neiitiiann explains the differences by the fact that tlie dissociation cannot be assiunecl to be complete, antl consequently the osiiiotic pressures of the cation can not be assumed to be equal. To prove t h a t on sufficient dilution the apparent effect of the anion will disappear completelj~,he iiieasiired thaIliuni in solutions of tenth, fiftieth and hundredth noriiial tIiaIIiuni salts. IVith the huiidredth iiortiial solutions the sanie value within a niilli\.olt was obtaiiiecl in every case, aiicl further a very satisfactor!. increase of potential with dilution. T h e nieasurenients are not as convincing a s they might lie if, the various halogen acids had been used, in which in other cases the largest effect due to tlie negative ion has been oliserrecl. H e did measure Iij.tlrochloric antl li\drofluoric acid salts, litit tlie remaining salts are salts of organic acids ; and Ostwald previouslJ. found that all the organic acids gave about the same value, and that very near to sulphuric acid. IJrhat influence the fluorine ioii iiiay have in other cases 1 do
A . E. Tqdor.
20
not kiiow, as I think 110 other iiieasureiiients 1iai.e been made with it. I inade a few iiieasurenients on one-liquid cells reversible with respect to one of the electrodes,
TABLEX __
~-
Electrodes
Electrol\,te
Concentration __-
~
Cd Hg ca Hg c(1 H g Cd H g cu Hg
c11 cLl
Hg Hg c u Hg Cd c11 ca c u
~
Cd CI, KC1 Cd so, KISO+ c u so, c u so, H,SO, K,SO, Cd c1, KC1
11 2 0
11 2 0
n
IO 11 2 0 11 2 0
IO 20 11 2 0 11 2 0 11
n
11 2 0
and fouiid that in every case they agreed 1.et-y nearly with the values obtained from the cells in which potassium salts served a s electrolytes. T h e most natural conclusion to draw is that the single potential differences are the same in corresponding cells, that is. Cd 1 Cd C1, is the same as Cd 1 KCI etc. W e are practically forced to this if we consider the potential difference at the cathode itidependent of the concentration or to be practically independent of the nature of the cation, when this is not the same as the electrode metal, as the 1i;easureinents of Pasctieti would indicate. However, the values which would thus be obtained for single potential differences a t reversible electrodes .do not at all agree with those obtained by Neumann, especially in the case of copper. bIy values for copper are about the same as those obtained b!. Ostwald for the free acid. [ To he ro/rliii/rc~f.]
