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 T h e i m t l 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 001602 ,005338
0 ,0294 . (i293 ,6294 ,6294 ,0294
.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 ~ l28.00 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 t n : ~ . / c t n . ~ .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 t h e 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 u p 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.
NO TI?^
1114
Zehnder interferometer described elsewhere.2 The data of Lemoude3 for this system mere available for 1 1O , hiit two widely separated values of D mere reported a t w concentration of 4 mole yomethanol (1.30 and 1.89 X cm.2/sec.). In order to clearly defiiie the shape of the complete D-n: curve at 11O t\\.o supp1emeiitai.y points were obtained for this system a t metlianol concentrations of 0.477 mole 70( D = 2.875 X cm.2/sec.) and 1.572 cm.2/sec.). The data mole 01, ( D = 2.233 X q1~on-niii Fig. 1 indicate that the lower value reported by Lemoiide at 4 mole yo methanol is the moi e reliable.
a i
3.0
In the dilute methanol region in Fig. 1, a sharp cm.2/sec. occurs drop in D from 4.05 to 1.8 X as the concentration changes from 0 to G mole methanol. This probably is caused by a change in species from single methanol molecules of high mobility to polymers having greatly restricted translational freedom.4-6 111 this same region, t'lie activation energy for diffusion rises from 2.7 to 5.9 kcal.,/mole, indicating that energy equivalent to the breaking of a full hydrogen bond is involved. Further evidence that a high degree of ordering occurs in this region comes from the fact that the partial molal excess entropy of mixing for methanol drops abruptly from about 1.5 t o -2.0 e.u. as the concentration of methanol changes from 0 to 10 m ~ l e % . ~ pIt~ is also significant that E D for 0% methanol (2.7 kcal./mole) is only slightly above E D for self-diffusion of benzene (2.1 k c a l . / m ~ l e ) . ~ The difference, 0.6 kcal./mole, might represent, approximat,ely the interaction energy I)et8n.eeii henzene and methaiiol. 60
2 505
.O
L.
$
30, -.-_---_-___I--___ P 20 'Self-d/ffus/on of
u
P
"0
>-
DIFFUSION (E,)
40-
B
E i=
t
Vol. 59
pure bewene
S e l f - d f f w o n of pure methanol
10-
