1112
NOTES
TABI~ IE EI,ECTRODX P O T E N T I A L , TcO?(o)-Ti:OJ -(nc[), 24" HnsOa, /
Icl'COl,
Ehtrode
J
1.00 x 10- 3 1.16 x 10-3 1 . 1 0 x 10-3
Pt coil Pt coil Pt coil Pt coil Pt foil Pt gauze Au coil Au coil
E.iii.f.,
...
0 . 5 x 10-3 0 . 5 x 10-3 2 . 0 x 10-3
1.613 x 1 . ~ 6 10-3 1.06 10-3
x x
volt,
6.65
-0.001 ,198 .100 .250 ,240 ,244 ,244 4 .223s
+ + + + + +
3.10 3.10 2.40 2.49 2.40 2.49 2.09
x x x
2.0 10-3 2.0 10-3 2.0 10-3 1.7 X
E O ,
calcd.
PH
- 0,734" - .741b
-
.i42c
- ,743 - .i33 - -,
-
, 4 9 3 1
-
,736 .7YGd Av. .738 2Z 0.003, volt Still slowly erinobling. * Equilibrium approached from negative side. c Equilibrium approached from positive side. This electrode W R S considerably more Iieavily coated with TcO, than the others. After equilibration had been obtained in nitrogen purified by heated copper, the measurements -were continued for 24 hours in a stream of argon similarly purified. The e.m.f. remained unchanged.
1.816 X l o w 3 1 . 0 3 ~x 10-3
- 150.6 kcal. mole-'.$ This, with the previously est,imated e n t r o p i e ~ gives ,~ -88.3 kcal. mole-1 as the free energy of formation of techiiet,ium dioside and -100.7 kcal. mole-' for A H . Using these values the standard potential of the Tc-Tc02 electrode is -0.272 v. According to these revised free energies A F a = -2.3 kcal. for the disproportionation reaction 3TcOi(c) + HzO(1) aq. _J 2HT~Od(acj) TcOo(c) This confirms the observed inst#ahilitJypreviously r e p ~ r t e d . ~The revised pot8entJial diagram for technetium is
+
+
-0.738 -0.8
I
Tc-
I
-0.272 TcOz-Tc03-
-0.q TcOi-
-0.472
OAK RIDGE, TENN.
A REVERSIBLE SILVER-SILVER CHLORIDE ELECTRODE FOR 95'37 (VOLUME) METHANOL-WATER -SOLUTIONS1 BY E. LEE PURLEE A N D ERNEST GRUNWALD Contribution f r o m the Department of Chemistru, Florida Rlate Unireruitu, Tallahassee, Florida Received M a y 23, 2968
Experimental difficulties with the silver-silver chloride electrode in organic solvents have been reported.2 In the course of recent e.m.f. measurements with the cell glass electrode /H+, C1-, other solutes, 05'%(vol.) meth,znol/AgCl-Ag
(I)
thermal silver-silver chloride electrodes prepared by the method of Anderson3 were observed to perform rather sluggishly and to require relatively long periods of time for equilibration, especially after exposure to dilute solutions of mixed electrolytes, e . g . , sodium chloride-sodium benzoate mistures. (1) This work has been supported b y a grant from t h e National Science Foundation. (2) (a) P. 9. Buckley and H . Knrtlry, Phil. Mag., [7] 8, 320 (1924). (b) H. 8. Harned s n d C. Caliiioti, J . Am. Chern. So?., 60, 2130 (1938). (3) N. J. Anderson, P1r.D. Dissertation. Deyt. of Clieiuistry, Univ. of Cliicago, Chicago, Ill., 1934.
E" is defined as the reference e.m.f. of the silversilver chloride electrode a t 25.0°, i.e. E* = E
+ 0.05015 log
CHCVI?/'
(2)
where E CH(CCI)
?I
= measured e.ni.f. in volts = stoichiometric hydrogen (chloride) roncn. = mea11 ionic activit.v coefficient
The equation -log ?/ = 1.G38 p'/%/(l
+ 2.42Gp'/z)
(3)
is employed to obtain activity coefficient^.^ Although values of E* differ for individual glass electrodes, E* has been found to remain satisfactorily constant ( h 0 . 2 mv.) and reproducible for a given glass electrode a t the ionic strengths employed (up t o ca. 0.025 M ) . Experiments with the Thermal Electrolytic Silver-Silver Chloride Electrode (Anderson Type). -With electrodes of this type E* was found to be satisfactorily constant when cell (1) contained 0.001-0.007 Ill HC1. It was, however, necessary to age the electrodes in 0,001 M HCl for 24 hours prior to measurements. Equilibration of the electrodes in the HC1 solutions was slow, requiring 30 to 45 minutes. After 12 hours immersion in the mixture: HCI(0.002 M)-ItCl(0.005 M ) , 2.5 hours were required for the electrodes to equilibrate in 0.001 Ai' HCI. After 6 hours immersion in the mixture: sodium benzoate (0.000 M)-benzoic acid (0.006 M)-soclium chloride (0.001 M ) , slightly over two hours were required for electrode equilibration in 0.001 PI HC1. After 45 min. the e.m.f. was still 3.0 mv. from its equilibrium value. These results are perhaps not surprising since the silver-silver chloride electrode is extremely porous and the silver chloride coating relatively heavy. 30 to 35% of the thermally deposited silver was converted to silver chloride. Experiments with a Silver Mirror Silver-Silver Chloride Electrode.-The silver mirror technique of depositing silver on platinum foil was employed. (See Experimental part.) A thin film of silver chloride was then electrolytically deposited on the silver surface. Electrodes prepared in this manner proved t o he T'ery sxt,isfactory. The e.m.f.'s for different sihrer mirror electrodes niid tlifferetit tdiermal elec(4) E. Grunwald, A n a l . Cheni., 9 6 , 10913 (19.54).
Oct,., 10.55
1113
NOTES
t>rolyticelectrodes i n cell (1) 1iai.e been found tjo (b) thickness of the silver chloride coatings. In agree to \vit8hin 0.2 in\.. 01' less. t i l all iiieasiii'e- Tahle 111 the equilihration times for different elecmeuts the eclriilibr:it~ioritime for the sil\.ei- iiiirror trodes of mryiiig sili,er chloride coatings are comelectrode lias been obseri.etl to be 10 to 15 minutes pared. and virtu;tlly inclepenclent of exposure to mixed TABLE 111 electrolytes. For example, the electrodes equiliEQUILIBRATION TIMES FOR DIFFERENT ELECTRODES IN h t J ereadily in 0.003 '11 HCI after the follo~vinges0.001 nf ficl AATER 12 HR. I>IhIERSION IS (0.nol /I[)posures KCl (0.001 M ) ~ I I X T U R I C S ( a ) 24 lit-. in soiliii.in 1wnzo:itr (0.0lCi . l f ~ - l ~ ~ ~ ncicl ~xoir Solvent, 05% (vol.) metli:tnol, 2:i.O'. (0.01 .I[) Fraction of Grluilibr%tior ( I ) ! 8 . 3 hi,. i t i KC'I (0.012 . I f ) Electrode .4g c o n l e i ted t.it!ie, Type to AgCI iuiri. ( r I I 111.. i t i w c l i i i i n foi,tii:ttr (0,001-I .1f )-t'oi~niir :i.citl Tlierinnl electrol>.tic 0 ,:30 180 (0.02sJ f ) - S : t C ~ I (0.00(i .If) Tlieriiinl electrolytic 0.17 70 Theimtl electrolytic 0.05 45 ( e ) 2-1 hi.. i t 1 distilled \v:ttci' IIirivr elertt,ol>~tic 0.15-0.25 10-1 5
Some t,ypiral data for HCI aiicl HCI-KC1 mist8ares 111 t'lie application of poteii tiometric tit)rirnet'ry are sho\vii i n Tnhle I. Ivitli cell (1) to solutiolis rich i l l oigaiiic soli.ent, it, TABLE I is clesiralile to lia1.e n clepeiitlahle elec*t,rnrleivliirli R 8 1 L l . E R ~ ~ l R R o R - ~ I , ~ t . T R o l , Y TSII,\'F:R IC ccliii I i I)ixtes mpitlly. The sil\.er i n i i m i , electrot le T R O D E F O R IIcI ANI) HC'I-IiCI l I I X ' l ' l - R E S n p p e n i ~tto ofi'er this a(lI.:uit'ztge. IN CHCI,
!)5% (vat,,) ~ I I C T H A S O L4~ 2r7.0"
.!I
CKCI,
0 . 00 12123 , 00 1'2'23
.If
-E*
0 ,0294 . (i293 ,6294 ,6294 ,0294
0 001602 ,005338
.oo
.00243ii , uo4o-L;
00 0 I O(i7
O0404T
(l.)
0,0012230 .on00 0 . 0295 i .02219 0.0001" (v.1 .4vrt,ngr niid st,niitlard deviation for 0 tlrtrrniii1:ttious : i t i liKri,eiit cmrcwt i'at ioiis iii this r:tiige.
The applicaliility of this electrode was further tested hy comparing ~ K values A for formic aiicl :ic*et,icaritlis i n cell (1) with values h:med on the cell ~ I A W e l w l r ~ x I e / " +, other. iioliites, I.)
nlPtlI:LI101
>%gc'l-,%g (4)
The t\\m sets of p K . 4 \ d u e s i n Table I1 are i n good agi,eemei;t. For compnrisoii, Tnhle I1 also colitailis smir results for acetic acifl \\.it11 t'he tjliei*mnl c:le:.ti-iiI~.tic~sil\.er-silI.er cliloric!e elec.tror!e. T h e Int,tei*results, being of the correct order of magiiitucle, exhibit8poor precision and artiially decreaseil steadily n.it,li increasing degree of neutralization, 111 80% niethauol it' has been readily possible to ol)t~aiii:t 1)i'er.ision of 1 0 . 0 2 i n tlie p l i ' ~ 'for ~ org t i i i v acids with t,lie thermal electlrolyt8ic electrode. 'The drifts For wetic arid i i i 95:& rnuttliniiol appear sigiiifiv:Liitj nntl cjiiit'e possihly result'etl from tliifirii I t,ies i I I o1)sen.iiig elect8rotleeq~iilihi~iiim, TABLE TI ~ ( l A l r . 4 R l i O SO F / I f < $
.krn
IX
\'.kl,UES
!In%
FOR
Fo~arrc!4cin
ANI) . 4 C E T I C
SiI\,er Cell 4
ni ir r ( B I. PIeo t rolyt io
+
Harried, J . A m . C h e m . Soc., 61, 41G (1939). ((i) "Hnndhooli of Chemistry mid Pliysios." 29th Edit,ion, Chemical Rilhher Piihlisliitig C o . , Clei.elntid, Oliio, t94.5, p. 2-192.
____
(\,or,,) \IETI-IASOL:25.0" ('ell 1 : .4c-.4i!rl E l r < , t r o d e :
Acid
Experimental.-l~~.in.f.'~ I ~ P I ' Piii~~:tsiit~cd ivit h n Bcckninii inodel C:S pH nietri, :irid W I T accurate t.o ivitliiti 0.1 inv. T h e cell roiisi*trd of a 180-nil. electt,olyt,icbenkci, fitted \ v i t , l ~ :t t ~ i ~ l ~ lriopper ~ t ~ r which held :L Bwkinaii fll90-42 glasr rlrrtt,otlr. Tho voluines of the test solutions rangrd fvom 2.5 to 50 nil. The cell \v:is suspended i n a large water thrrii)ost:it ~ n : t i i i t : ~ i tnt i ~ 28.00 l 0.02". Stin-itig of tlic soluI iotis W:LS arcoiiii)li4ird liy ninnunlly agitating tlie cell. The tliet~ni:tl-clertt~~lyti~~ silver-silver chloride electrodes ( Hni.tietl'* type 2); wrrr p i ' c p m t l by tlie method of Aidersott.3 Tlicb :tiiodization t,iine IKLS 3 to 3 . 5 hours and rrsultetl i n the coiivrt,siori of 30 to :35yoof the tlieimnlly deposited silvet, to silvci, chloride. Silver niii,t,oi~silver-silver c:hlot,iclc c,lecti,odes consisted of pI:ttinuni foil 0.001'' thick wltled to p1:ttiiium mire. The total electi~otl~s surface area \vas appi.osi~nately 2 cin.2. .\ftet, cleniiing w i t h concentrnted nitric ncid, concentrated :ttnrnoiiin nntl tlouhly dist,illed water, silver was deposit>ed 011 t he platitiuni foil hy the Roclielle snlts silvering process.8 (Sodium t:wt rate wns sullst.ituted for R.ochelle d t m s . ) The plntiiiuin foil n w srisprtttled vertic:dIy i t i t.lie silvering misturea. T h e ( , tl(bl)oritioiis of silver, rnch rcquiritig about oiir lioiir, \ v t > w n i : i i I ~ . This pmcedure rrsulted in the d ~ p o sitinti of cn. 0.8 rng. of silver per cni.2 of plntitium s~u~!:ice. T h c silver i : l i I i x i < l r cmitiiig W:LS produced by :~tiodiz:tt,ionfor 2 to :imitilit(+ i i i 0.05 J f liydroclilot,ic :wid a t ti ciit,retit, tlctisity of CU. 1.0 tn:~./ctn.~.By t>liesetechniqucs 15 to 2Sr, of the silver was cotivertcd t80silver chloride. The saturntetl Irolytic AgCI electrode iiniiicrsed in 9570 (vol.) niethanol saturated with KCI. T h r I:ittcr iiolution wns cotitaitied in a Pyrex tiibe (10 inin. X 100 mm.) fitted with a stnndnrd-taper inner joint nt, t,lie top. Liqiiitl jutictioii contact with the t,est solution in t h e crll w:ts innde tiia nn asbestos fiber sealed i n tlie bott,om of tlir electrode containc~r.
Tlirroinl
elect r o l v i c
Formic 6.li4S + 0,008 0,542 .... acetic 7.1303 =!= ,009 7.855 f 0.003" 7 . 9 0 to 7 ,i n h .4vernge and stnnclard deviation for 5 det,erminnt,ions i n which the 0; acid neutralized ranged up to 60%. 6 detei~niinntioiisby Dr. .4. L. Bacarella in this Laboratory i l l uliich the % acid neutrnlized ranged up tjo 8%.
Two important factors appear to influence the equilibration of the electrodes: (a) extreme porosity (ant1 large surface area) of the electrodes;
DIFFUSION IS THE SYSTEXI JIETH.\NOT,BENZENEL BYC. S. CALDWELL ASD A . L. BABE Drpai,tment o j C1iemii:al Eilgine?Ping, Uniurrsity of Il'ashinglon, 8 p a l t l e Ib'ashington Rrceiued illau 23, 1965
Diffusion coefficients for the systJem methanolbenzene have beeii measiired a t 2 i o with a Mach(1) This work was sii1)porterl in part by the Office of Ordnance Research, U. s. Army.
