652
17. s. GREELEY,
w.T. SMITH, ,JR., R.IT.STOUGHTON
. ~ N Di r .
H.
LIETZKE
Tol. 64
ELECTROJTOTIT'E FORCE STUDIES IS ,lQUEOT'S SOLUTIONS AT ELEV-4TED TFXPERATUKES. I. THE STAXDAKD POTESTIAL OF THE SILVER-SLLVER CHLORIDE E1 ECTRODE'
s. GREELEY,JVILLIAM T. SMITH,J R . , R24YM0ND w.STOUGHTON AND 31. H. LIETZKE
B Y I~rCHARD
Contribution from the Chemistry Division, Oak Ridge National Laboratory,2 Oak Ridge, Temnessee, and the Department of Chemistry, Unzcersity of Tennessee, Knoxczlle, Tennessee Received A'ovember 6 6 , 1969
The electromotive force of the cell Pt-H?(p) IHCl(m) IAgCl-iig was measured a t temperatures from 25 to 200" and in some cases to 275' using hydrochloric acid concentrations from 0.005 to 1.0 m and hydrogen pressures of about one atmosphere. A closed, static, high pressure system was used which consisted of a fused silica vessel contained tightly in a steel autoclave with a Teflon interliner. The measurements were reproducible to within f 0 . 5 mv. from 25 to 225", 1 2 mv. a t 250', and f 5 mv. at 275". At 25 to 90" the results agreed satisfactorily with previous investigations. Calculations were made of the standard potential of the silver-silver chloride electrode and the resulting values may be expressed by Eo = 0.23735 -. 5.378.3 X 1 O w 4 t - 2.3728 X 10-612volts with a standard error of fit of 0.19 mv. from 25 to 200".
Introduction The s tandard electrode potential of the silversilver chloride electrode has been measured from 0 to 60' by Earned and Ehlers3e4and from 0 to 95" by Bates and Bower,j but no measurements of the standard potential a t higher temperatures have been reported. Since the silver-silyer chloride electrode shows promise of being useful as a reference electrode a t temperatures well above it was decided to determine its standard potential to as high a temperature as possible. Also, it was of interest to study the behavior of the hydrogen electrode a t elevated temperatures. Therefore, the cell was inveatigatcd over the concentration range 0.003 to 1.0 m HCI, the tcniperatnre range 25 to 255", and at hydrogen pressures of about one atmosphere. Experimental The Autoclave Assembly.-The primary containmrnt vessel for the sollition was a rrlindricnl liner made of firsd silica for corrosion resistance. It fitted into a Teflon inteiliner which in turn fitted snugly into a type 347 stainless steel autorlave. The Teflon interliner, having a relativelv high coefficient of thermal expansion, expanded as the autoclave was heated and filled the space between the steel body and the quartz liner rompletely . This prevented condensntion of water vapor between the liner and the body. A 111) on the Teflon interliner served as a gasket for sealing the autoclave head to the autoclave body. The total volume of the autoclave available for solution and vapor was 160 ml. Corrosion of the silira liner and other silica parts was measured during several tests and it was found that the rate mas low enough to be neglected as a source of error. For instance, in 0.0075 m HC1 the low in weight of all of the. silica eyposed to solution in a 36 hour test to 225" n-as 0 3 mg. The Teflon intrrliner W:LS also a possible sourrt' of (1) Based on a tk.esis submitted b y R. 6 . Greeley in partial fulfillment of t h e requirements for t h e degree Doctor of Philosophy, University of Tjnnessee. Presented a t the 135th Aleeting of the American Chemical Society, Boston, hIass., April 6, 1959. (2) OBerated for l,he United States Atomic Energy Commission by the Union Carbide Corporation. (3) H. S. Harned and R. 1%'. Ehlers, J . Am. Chem. Soc., S 4 , 1350 (1932). (4) H. S. I-Iarned and R. W. Ehlera, ibid.. 66, 2179 (1933). ( 5 ) R. C.Bates and V. E. Bower, J . Research .Yatl. Bur. Standards, 53, 283 (1954). (6) M. €1. Lietzke a n d J. V. Vaughen, J . Am. Clcem. Sor., 77, 876 (1955): It. IV. R o g c h o ~ ~ d h r i rnnd y C. F. Bonilla, J . EZrctrocAmm. ,Fat.. 103, 241 ( 1 9 X ) ; V. I'raabk, Werkslstoga u . K O r T O S < O 7 L , 9 , 524 (1955).
rontamination since it was observed previously a t this Laboratory that Teflon evolves low molrvular weight material a t temperatures :is low as 150O.7 Consequently t.he interliners were hc:ated to 275' in a trial rim before use in an a c p d test. 1he autoc.l:tve heed was machined from commercially pure titariiiim and is shown in detail in Fig. 1. .4 valve T Y ~ Sinst:tlled directly in t,he head which dlom-ed the vapor space to be evaciiated through the stainless steel rapillary tube. Hydrogen was also vented through this line during the initial bubbling. -4platinum capillary titbe connecting the autoclave interior with the pressure gauges was inserted through the cenber of the titanium valve stem and was sealed in at, the top of the valve stem with a Teflon gasket and a steel nut. The platinum capillary tube was c,onnected a t its other end t o a two-way, two-stem autoc:lave valve mounted outside of the oven which in turn was connected t,o the hydrogen supply system and to the pressure gauges. The electrode leads were passed through the bomb head as shown in Fig. 1 and xere ronnected to the electrodes by firm rrimping. The titanium autoclave head was resistant to corrosion hy the vapor above the acid solutions under all conditions except during the tests with 0.5 and 1.0 ni HC1. Above 225" with the former and 125' with the latter soliit~ior:an increase in pressure with time a t constant temperature m u notired. The estent of the reaction was followed by the change of pressure and e.m.f. with time and the resiilts were rorrwted as discwsed below. On opening the autoclave after the two tests in which rorrosion occurred, a white deposit, determined spectrographically to be TiO?, was found on the titanium. Therefore the corrosion rciiction wts assumed to be Ti 2H10 +TiOz 2H2 (.$I
+
+
Agreement was obtained between the observed pressure increase and the pressure increase calculated from the change in solut,ion conrentration due to the loss of a.ater in accordance with reaction (A). The titanium aut,oclave head was held ont,o the autoclave body by eight steel lugs support,ed in a stainltw stwl c a p screwed onto the aiitoclave body. The lugs wtPd 011 :L Delleville spring washer designed so that the thermal e s l m Pion and contraction of the Teflon gasket (the lip on the Ti.flon interliner) was matched by comprcwion :md cspansion of the spring.* The Oven and the Measuring Equipment.-The autoc h v e was placed in a large, cylindrical, nlnminiim 13lork inside an elec-trically heated, forced-draft oven which wvns controlled to within & l o . Tithin the autoclave this ten;perat>ure fluctuation was smoothed to less than 1 0 . 0 0 1 . The temperatsure of the autoclave head was measured with a platinum resistance thermometer inserted in the thermowell shown in Fig. 1. The thermometer had been (4t m t e d by the National Bureau of Standards at the i w ,
-
(7) W. C. Waggener, Oak Ridge Nationnl Laboratory, private corn-
miinicntion, ( 8 ) .J. 0 . :\lri~errnntj .A. T.nwlo. Trnnr. A m . ,%r, Nw!,. Gngrs., 68, 305 (1 0:lfj).
steam and sulfur points and the ice point was rechecked before each run. Pressures were measured using two Heise precision bourdon gages connected with water-filled stainless sterl capillary tubes to the valves and to the platinum capillary discussed above. One of the gauges had a range 0 to 250 p.s.i.g. and an accuracy of 0.25 p.s.i. It was used a t temperatures to 200". The other gauge had a range 0 to 1000 p.s.1.g. and arrirracy of 1 p.s.i. The gauges were calihrated using a Conso1id:tteti Electrodynamics Corp. primary pressure standard of the pneumatic dead weight type. The e.m.f. was measured with an A4ppliedPhysics Corp. Pdodel 30 vibrating reed electromet,er and a Rubicon type B potentiometer, and was displayed on a Brown recording potentiometer. The e.ri1.f. values were precise to f0.02 mv . The Electrodes.-The hydrogen electrode was made to fio:tt a t the surface of the solution in order to come quickly to equilibrium with the hydrogen in the vapor phase. A I-in. lengt,h of 24 gauge platinnm wire was attached to a projection on a fused silica float with fine 40 gauge platinum vzire and formed around the float so as to be held a t the siirfnce of the solution and parallel to it. Another fine p1:itinum wire was welded onto the 24 gauge wire in order to connect it with t,he platinum lead wire in the autoclave h a d . The platinum was platinized as directed by Bates.$ On sever:tl occasions the floating elect,rode was compared with a foil elect,rode, prepared in the usual way,'O in dilute hydrochloric acid saturated vith hydrogen. The two types cbf electrodes exhibited the same potential to within f 0 . 0 2 rnv. The silver-silver chloride electrodes were of the thermal type." For the first serirs of t,ests, which went only t o 200", a four-inch length of '/a in. dia. 0.d. soft glass tubing was used t o support the platinum wire of the electrode. Six electrodes were prepared simultaneously, and, aftrr cooling, they were immersed in hydrochloric acid of t.he same composition as that to he used in the test, and intrrconnected. The solution was heated to 75" and allowed to cool overnight, as suggested by Ashby, Croolte and Datta.'* The next day the electrodes were within f 0 . 0 5 mv. of each other. Two electrodes reading within 0.02 niv. of the average were chosen for the experiment and fast>enedonto the platinurn leads through the autoclave head. I t was found t,hat dilute hydrochloric acid at'tarkrd the soft glass electrode support a t 250". Therefore, in the succccding series of tcst,s, which went to 275", the soft glass hltler was omitted and the entire lengt,h of t,he platinum n-ire cleetrode was coat,ed wit.h the silver-silver chloride tlqiosit. Only onc p:tir of these elrctrodes was made and thry merc used for the entire second series of rxperimrnts c'ovcring tho range 25 to 2 i 5 ' . Thermal cycling apprnrcd to improve the characteristics of these electrodes. With cart being taken to avoid mechanical strain, the results a t all tcmpcratures in common with the first series were entirely coniparablc. -4silica tiibr containing a sintered silica disk was p l m d Excess solid :I roirnd each silvrr-silver chloridr rlertrode. si1vt.r chloride was placed inside the silica tube prior to each 1 un The Solutions and Materials.-All solutions wrre made from condiictivit> water; all apparatus was given final rinsings with the ssmcx; and all chemicals were recrystallizrd or washed in condiictivity water as dremed necessary. Thr h\-drorhlorir acid solutions wrre made up by weight dilution. from twice-distilled constant boiling hydrochloric acid which liar1 been analyzed gravimetrically and found to agree with vztlues given by Foulk and Hollingsworth.'3 Silver oxide was made according to the procedure given by Bates" and was wtshcd twentv-five times with distilled water and ten times r(ith condiirtivity water. Silver chlorate was made accordiiig to thr proredtiregiven in "Inorganic Syntheses."'5 Silver (9) R. G. I h t e s , "Electrometrie pII Determinations." John Jf'iley and Sons, Inc., New York, E.Y., 1954. p. 167. (10) Ref. 9, p , 166. (11) C. K. Rule and V. K. LaMer, J. A m . Chem. Soe.. 68, 2339 (1936). (12) J. H. Ashby, F. M. Crooke a n d S. P. D a t t a . Biochem. J.. 66, 190 (1954). (13) C. W. Foulk a n d If. Hollingsmorth. J . Am. Chem. Soc., 45, 1223 (1923). (14) R. ( 2 . N R t E s , ref. 9. p. 206.
P t ,APILLA'C"
THREE SUCH FITTINGS PER H E A D -
-uuE
c-2
SILVER SOLDER-
STEEL FOLLOWER SOAPSTONE GLAN
Fig. 1.-Titanium
bomh head for use with autoclave assembly.
chloride was made according to the procedure given Ly Zimmerman'e except that the starting material was Baker and Adamson reagent grade silver nitrate. The absence of bromide from all solutions and silver compounds was checked by the method of Pinching and Bates.17 Electrolytic hydrogen obtained in commercial cylinders was passed over Ascarite, Drierite and a platinized catalyst bed before passing through two bubble towers containing solutions identical to that under test and then into the ai~t~oclave. General Procedure.-After the autoclave containing the electrodes and solution had been put into the aluminum block in the oven and all connections made, the vapor space was evacuated and hydrogen was admitted carefully. The process was repeated twice and then hydrogen was bubbled through the solution for several hours (titration of thr solution before and after this procedure in several trials showed that the concentration changed less than one part per thousand). When the chart record of the e.m.f. showed that equilibrium was attained, the valve in the head of the autoclave was closed, the hydrogen supply was shut off, and the valve to the pressure gages was opened. An initial reading of temperature, pressure and electromotive force was taken, and then the temperature was raised to the nest level desired. After equilibrium, the second set of readings was taken and the temperature again raised. After the final set of readings had been taken a t the highest tempernturr, the oven was cooled to room temprrature and another set of readings taken. I t should be noted that the autoclave was entirely static during the test and that hydrogen was bubbled through the solution only at the beginning of the test, not during any of the subsequent measurements.
Results and Calculations Temperature and Concentration Range.-The first series of tests vias conducted on seven solutions from 0.005 to 1.0 m HC1 at 25, 60, 90, 125, 150, 175 and 200". Several duplicate tests were made and the e.m.f. d u e s taken a t the same temperature were reproducible to within about * 0.4 (15) D. C. Nicholson and C. E . Holley, Jr., "Inorganic Synthesca," Edited b y W. C. Fernelius, Vol. 11, 1st Ed., McGraw-Hill Book Co., New York, N. Y., 1946, p. 4. (16) W.Zimmerman, 111. J. Am. Chem. Soc., 7 4 , 8 5 2 (1952). (17) G. D. Pinching and R. G. Bates, J . Research A-atZ. Bur. Stand. ards 37,311 (1946).
mv. In one test, nieasurenients were made during both ascending and descending temperatures with good agreement b e t w e n values taken a t the same temperature, but otherwise measurements were made only during ascending temperatures (except for the final measurement a t room temperature). Agreement between initial and filial 1-alues at 25" averaged 0.5 rnv. In the second series of tests, measurements were extended to include 22.7, 250 and 275" (the 60 and 90" points generally were omitted', with nine solutions from 0.005 to 0.5 m. h 1.0 m solution was measured to 200". Molality and Ionic Strength of the Solutions.It waq found that, besides the two tests in which corrosior, occurred, the e.m.f. did riot stay roilhtant after thermal equilibrium had been attained a t the h:gher temperatures, but showed a steady, linear drcreasc with time. This effcrt was noticed by A n d e m n ~ and ~ ~he attributed it to the reduction of silver ion b y hjdrogen. This reaction both increased 1he arid concentration and decreased the hydrogen pressure according to the reaction '/?€I?
+ ..\gC1 --+ Ag + HC1
(R)
The rate of the reaction increased with increasing temperature and with decreasing acid concentration. Since 110 prorision was made for sampling the solution clt the time of each measuremelit, it mas found necessary t o calculate the concentration of the solution from the slope of the recorder trace of e.m.f. us. time. Therefore each measurement of temperature, pressure and e.m.f. was delayed until the slope of the recorder trace had been established after attainment of thermal equilibrium. Table I is a list of the rate of change of the e.m.f. and the total extent of the change at each temperature for several of the tests. The change in e.m.f. was assumed to be due entirely to reaction B a t 0.01 to 0 2 m and to reaction A at 0.5 and 1.0 m, since no corrosion mas observed below 0.5 m and since the estimated rate of reaction €3 a t 0.5 and 1.0 m was negligible. The total extent of the change depended on the time the system was held a t each temperatiire as well as on the rate of change. The values licted in columns three m d four of Table I nere corrected for the change in e m f. due to the increase or decrease in hydrogen pressure arid the correvted rhange in e.m.f. thrn was ronverted to the chanty in molality during thc period of nieasuremcnt t)ythe equatioii
These changes in molality are liqted i n column five of T:tble I and were cumulative during each run. 11o;alitp changes during temperature chnnpcs ivrrc calculated using an average value of the e.m.f.time slop? determined at the upper and l o w r temperatures. No cahanges in concentratioii orcurrcd helo-vv the 1on.ei:t temperature listed for enc$h concentration. A check on the calculation at each temperatiirr during a test was obtained by adding up all the (18) N. J. knderson, P h D. thesis, University of Chicago, Chicago I l l , 194.5. p. 12-17.
RATE .4ND JZXTENT O F
TABLE I REACTIOKS 11 . j N D B DURIXG SEVERAL TESTS
Rate Concn.. m
0.01"
0.05"
0.1"
0.2"
of ctianpo
T ~ n i j ~ . , of c.m.f. OC. mv.,'hr.
175 200 225 2 50 200 225 250 275 200 "5 2(iL \ I t l ) i FIT OF ihle to mifirm the calculation. The data of Eo" = E + b l Pollitzcr and Ptrehelz4taken at 50 and 70" mould Temp OP a R, of t \ tnv in-1 OC. EO, v. in! indicate the crror to be considerably less than that 25 0 22233 0 08 $0 132 0 011 1 3 O 22 Idculatetl. 60 1068 23 - 025 028 G 0 k1 Calculation of Eo" --The further treatment of 1696 LO - 125 OLi 7 0 :iG the data n-a3 iiniilnr to that of Rate< and B o ~ e r , ~ 00 125 1330 1 1 - llfj 013 7 0 2'3 ind a Dehye-Htickel equation in extended form 150 1082 10 - 230 010 S O 21 \vas u6ed 0
U I J ~
/31
-
log (1
+ O.O3tiO-l~/I)-t-
Est
(3)
where p0 == dwsity of water, I> = dielect,ric constant 'of water,25T = absolute temperature, I = ionic jtrengt'h of solution = r n ~ ~ c l S.A.$CI (ie.,the sum 'of the molalities of HCl and &%gCl), A = denominator coefficient = 50.2904 ( D T ) - ' / ? d ,d = ion size parameter, B = linear term coefficient, log (1 10.03604m'1= corm. from rational to practical scale, Ext = estended turns of Gronwall, LnRIer and
+
+
( 2 2 ) A I . I[. Lietzkr. ".in O R A C L E Code fot I.cnst Hqii:irrs." Oak Ridge National Laborntor).--CF-5~-2-20, Feb. -1, 1959. ( 2 3 ) J. 13. Hildcbrand a n d R . L. Scott, "The Solubility of Konelecbrolytes," 3rd e d . , Reinhold Publ. Corp., New York, N. Y., 1950, Chapter XIV. (24) F. Pollitzer a n d E . Strebel, 2 . p h g s i k . Chem., 110, 708 (1924). (25) C , . C . j \ k r l l d arid FT. I. Oshry. J. A m . C h e m . SOL-.,72, 2844 1 1 YL7Oi.
17 5 LOO 225 250 275 a
u
0708 17 0348 30 - 0051 34 - 054 2 16 - 000 4 74 = standard erior
- 271 - 412 - 578 -1
1
-0 05
1113 01-1
0 0
,i(j
11
68
021) 12 I2 15 2.3 20
LO 2 37 3 80
Smoothed Values of the Electromotive Force.- For the purpo-c of comp:irisoxr with previour :uid powiblc f u t u e nork, imoothed \ ,~luc=- of the rlwtioniotive force up to 0.1 m IIC1 n.ere c~,rlc~ul:~tcd from the Eo and B I d u e s using equations 4 and 3 'l'hcic values arc liqted in Table 111. I t should I)e iiotcd that a solution originally exactly thc ronw11tration listed Till not have the same c~o~icentratioii (26) T €1 Gronnall J 5 8 (1928)
X K LaRIer a n d I< S a n d r e d I ' / ? w ? L L 29,
(271 E R Cohen 1i A1 C l o n e a n d J I\ 11 I>riinond The Tundariiental Constants of P h j sics '' InteisLipnre P n l ) l ~ s h e ~Isn c Ne\% Yorh N. Y 1957.
TABLEI11 SMOOTHED VALUESOF THE ELECTROMOTIVE FORCE OF Pt-H2( p)lHC1(na)lAgCI-Ag A T O N E ATMOSPHERE HYDROGEN PRESSLRE Eamooth
7
(L
"Its) 175'
m
250
60'
900
125'
1503
0 001 ,002 .005 .0075 .Ol .02 .025 .05 ,075 .1 .2 .5 1.0
0.57904
0 5953 ,5565 ,5055 . -1831 ,4672 .4295 ,4174 ,3802 ,3587 ,3435 ,3063 ,2546 ,2124
0 6036 ,5617 ,5063 ,4820 ,4638 ,42313 ,4107 .3i05 .34i3 ,3310 ,2900 .2357 ,1907
0 6063 ,5620 . ,5023 .-I758 ,4571 .A124 ,3982 ,3543 ,3290 ,3112 . 268.5 ,2085 .1614
0 6014
0 5897
,5571 .4052 .4674 ,4478 4007 ,3856 ,3396 ,3132 ,2948 2-!09 ,1871 ,1384
,5474 .-I846 ,4559 ,3354 ,3859 ,3701 ,3215 ,2936 ,2742 ,2260 ,1635 .I128
.ii4415
.ax38 ,47530 ,46411 .*I3017 ,41932 ,38576 ,36619 ,35228 .3,1891 ,27245 .23340
at elevated temperatures due to reaction B and hence will not give the electromotive force listed. However, for short times at temperatures up to 200" and for solutions greater than 0.005 m the difference is not large. The solubility of silver chloride was taken into account in calculating the values for Table 111. Also listed in Table I11 are smoothed electromotive force values at 0.2, 0.5 and 1.0 m. These were obtained simply by averaging the experimental values a t each concentration and temperature, after correcting to even molalities by the term 2RT/F In (mobs/meven)where necessary. Discussion The values of Enmeasured in this study are compared in Table IV with those obtained by Bates and Bower.5 It can be seen that the agreement a t 25" is excellent, ie., within 0.01 mv. Bates and Bowcr critically Teviened the data a t 25" reported in the literature and discussed the surprising 0.18 mv. discrepancy between their data and those of Harned and Ehlers. Taniguchi and Janz28attributed this discrepancy to slight differences in thermal strain of the silver-silver chloride electrodes. Since thc electrodes used here were in a sense thermally annealed by heating and cooling in solution, it would appear that our data confirm the value of Bates and Bower. I n order to compare the data at higher temperatures, the En values of Bates and Bower mere extrapolated in two ways. Their measured values were fitted by the method of least squares first to a quadratic function of thc centigrade temperature to give
TIIE
CELL
-
2000
225"
2500
2750
0 5698 ,5308 ,4689 ,4407 ,4185 ,3673 ,3508 ,3002 .2713 ,2514 ,2025 ,1309 ,0853
0 5422 ,5067 ,4474 .4183 .3970 ,344i ,3277 ,2758 ,2463 ,2261 ,1751 .0968
G 504 ,471
0 474 ,443 390 363 ,342 290 ,272
415
337 366 314 29; 246 218 200 143 037
,219 ,189 . I68 .098
.0?4
TABLEIV STA~YDARD ELECTRODE POTENTIAL OF THE SILVER-SILVER CHLORIDE ELECTRODE Temp.,
"C. 25 GO 90 125 180 175 200 225 250 273 300
Measured. this study
0.22233 ,1908 ,1696 .1330 .lo32 .0708 0348 - ,0051 ,034 090 I
-
,-----Bates Measured 0 2224 0.22234 ,19649 ,1965 ,1695 ,1897 ,1330 ,1033 O7bG .0.'349 - ,0038 Eq. 8
.
-
,043
090 ,138
-
and BowerEq. 6 &I. 7 0 22232 0 22225 19648 ,19654 ,1G96 ,169ii ,1331 ,1324 ,1040 ,1020 ,0725 .0886 ,0387 ,0319 ,0080 0028 - ,035 ,051 .OQi ,146
-
.0i5 I17
For comparative purposes the following equation was obtained by fitting the measured Eovalues of this study over the range where thc data were the more accurate and numerous, 25 to 200" E @= 0 . 3 7 5 5
- 5.3783 X
t
- 2.3i28
X
i2 ( 8 )
T'alues cal(~11attdfrom tht equations are shown in Table IT and it can be seen that the measured values fall between the two extrapolated values except at 250'.
The standard error of fit of the data from 25 to 200" to this equation was 0.19 my. The smoothed values of the e.1n.f. a t 25, 60 and 90' were cornpared with Harned and Ehlers4 and with Bates and Bower6 as shown in Table T'. As can be seen, the agreement is satisfactory. Part of the discrepancy between the present results and those of I-Iarned and Ehlers a t GO" is that d w%s taken to be 6.0 instead of their valuc of 4.3 A. at go', 7 A. was used instead of thr. vnluc of G A. used by Bates and Bower. The agreement with the data of Bate. and Bowcr is particularly valuable since their study was over a wider temperature range than any previous work and they used many more cells a t each temperature. Since their electrodes were of the thermnlc)lectrolytic type and their methods somewhat different, support is leiit to the view that sybtematic errors in the preqtiit study were minimizctl. One previous investigation of the cell at elevattd temperatures was that of Roychoudhurg and B ~ n i l l a . Their ~~ e.m.f. values mere compared with dues calculated using the Eoand y*3nvalues
(28) H. Taitigiich and 0 J. Jana, J . hlertrockem. SOC. 104, 124 (1957).
(29) R . N. Roychoudhury and C. F. Bonilla. J . L'lectrochern. 5 o c . 103, 241 (1946).
E o = 0.23683
- 6.2004
X 10-4t
- 2.5241 X
10-ot2 (6)
sild also to a n equation involving a T log T term in the absolute temperature to give EO
=
-0.0GKi4 - 0.003i3157' log 7'
+ 0.01021i7'
(i)
A+,
TABLE V COMPARISON O F RESULTSWITH THOSE OF PREVIOUS INVESTIGATORS 7
Temp.,
HC1 concn.,
25
0.005 .01 .05 .1 .2
60
0.005
OC.
m
1.0
.01
.05 .1 .2
This study
0.49838 .46411 ,38576 ,35228 .31891 .23340 ,5054 ,4672 ,3802 .3135 ,3063 .2124 .50G3 .4648 .3705
(volts) Harned and Ehlers
Esrnooth
0.49841 ,46416 ,38587 ,35239 .SI871 .23328 .SO49 ,4667 ,3797 ,3425 .3055 .2123
Bates and Bower
0.40840 ,46412 .38579 ,35233
.5052 .4669 .3707 ,3428
The cell used in this study is ordinarily referred to as a cell "without liquid junction." However, ill the present case the solubility of silver chloride became appreciable as the temperature increased and since the silver chloride was prevented from diffusing into the main body of solution by the silica frit there was a definite liquid junction. The junction potential was assumed to be negligible in relation to the other effects noted above since the concentration of dissolved silver chloride was generally small in comparison to the hydrochloric acid concentration. For instance the solubility of silver chloride a t 200" ranged between 0.001 and 0.002 nz .
d further consideration, viz., the difference in ionic strength between the solution within the silica tube surrounding the silver-silver chloride clectrode I .o and the bulk solution, was neglected for the same 00 0.005 ,5063 reason. The ionic strength of the solution within .O1 .4648 the silica tube was used in the calculations. Of .05 .3703 course, the actual ionic strength depends upon the .1 .a10 .3304 species of complex ions which are present and these .2 .2906 are not known a t the higher temperatures. I .o ,1907 In addition to the junction potential caused by of the present study and found to agree within diqsolved silver chloride, a further junction potenabout 5 and 10 mv. a t 100 and 150", respectively. tial would have been caused by reaction B if the HCl However, the difference between calculated and ob- produced had remained within the silica tube. served values was 35 niv. a t 250" and in the op- However, it is believed that the high mobility of posite direction from that calculated by Lietzke3I hydrogen ion, particularly a t the higher temperain his interpretation of their data. The discrep- tures, and the presence of HC1 in the vapor phase ancy might be due to the fact that they used silver mould have allowed sufficient transport of HCl to chloride electrodes chloridised on the surface only. prevent any significant difference in HC1 concentraThis surface coating could have dissolved a t 250" tion between the solution in the silica tube and in giving a n unsaturated solution of silver chloride in the bulk solution. hydrochloric acid. The Debye-Huckel equation in extended form Roychoudhury and Ronilla found a discrepancy was found to be entirely applicable over the range from the theoretical effect of hydrogen pressure on of temperatures studied. Straight lines were obthe electromotive force. This effect was attributed tained for I.:nttus. I that were within the limits of exto an effect of hydrogen on the silver-silver chloride perimental error and which readily allowed extrapelectrode which Lietzke suggested might be due to olation to infinite solution. However, the magnihydrogen electrode sites on that electrode. I n the tude of the ion-size parameter, d , a t the higher tempresent study in an experiment in 0.05 m HC1 a t peratures was so large as to suggest that d in these 150" the Nernst slope was obeyed upon increasing cases is simply an adjustable constant at each temthe hydrogen pressure from 1.5 to 3 atm. How- perature and bears no relation to ion size, Lietzke ever, Roychoudhury and Bonilla's pressures were as and S t o u g h t ~ nfound ~ ~ that for solubility calculahigh as 40 atm. tions up to 200°, when the denominator term in the If hydrogen electrode sites were established on the Debye-Huckel expression for the activity coefsilver-silver chloride electrode the electromotive ficient TYas set equal to (1 A , where As was forces would indeed be lower than in the absence of independent of temperature, better agreement was hydrogen. Since Anderson'* found that a platin- obtained with experiment than with the full esized platinum electrode covered with finely divided pression (1 5 0 . 2 (~D T )-11%v'pal). silver still gave the same potential as a fresh platAcknowledgments.-We wish to thank Drs. R. J . inized surface, it would appear that the hydrogen electrode reaction can readily occur on a finely Raridon and Kurt A. Kraus for making available divided silwr surface. However, whether hydro- to u s their data 011 the solubility of AgCl in HC1 gen electrode sites exist on the silver-silver elec- a t elevated temperatiire
