SOLVENTS HAVING HIGHDIELECTRIC CONSTAWS
Oct.. 1961
1839
SOLVEKTS IIAVISG HIGH DIELECTRIC CONSTANTS. XI. ELECTROMOTIVE FORCE MEASUREMENTS OF TIIF, CELL Pt, 142: HCI ( m ) ; AgCI-Ag I Y S-AfETTIYLACETAMIDE A T 40°''2 B Y 1,I'J.C
n. I)\lYSOV.
r,ICI14I:D
c
SIZERIDAN l l S D IT4RTLEY
c
ECKsTROW
1)Ppcrrtmtrrl qf Chennstru, Wniversity of Kentucky, Lexington, Kentucky Recelied March IS, 1981
'I'he standnrd potenThe e.m f of the cell Pt, H2; IiCl(rn); iZgCI-Ag it- N-methvlacetnmide has been measured a t 40' t i d of the 51i\ e.-silver chloride elrrtiodr U R S fourid i o he 0 20.773 f 0 00035 volts. Activitv coefficients of HCI solutions in N-nieihyl:ic etamide a t 40" have t m n ralt dated for conrentrations ranging from 0 002 to 0 1 molal. The chloridc solution used for chloridizing the silver rlectrodes WAS free of bromide impurit,ies. A 1 molar HC1 solution was prepared with conductivity water by dilut.ing in an all-glass app:tr:rtus the middle portion of the second distillation of the constant boiling mixture of reagent grade IICI. Electrolytic Hp was passed through a Deoxo catalyt,ic piirifier and traps containing Ascitrite and Drierite and finally just, before entering the e.m.f. cell was passed over anhydrous magnesium perchlorate. The HCl solutions in N-methylacetamide were pre ared hy dipsolving a weighed quantity of HCI.2(CH3C8XHCHJ) in a weighed aitioiint of N-methylacetamide. The concentration of the solution in the e.m.f. cell was increased mhcn desired by adding enough stock solution directly to thc cell to give the desired concentration change. Electrodes.-The hydrogen rlwtrode consisted of platinrim foil 1.0 em. by 1.5 cm. attached to No. 18 €3 and S gage platinum wire fused iiito a yoft glass standard taper ground joint unit. Electrical contact was made with the e11:ctrodes by means of a copper wire dipping in mercury contained inside the glass support. The electrodes were cxlcxaned in 50% aqua regia and plated with platinum black ty tile customary electrolysis in a 3% solution of chloroplatinic acid in 1 M.HCI. The electrolysis was continued in 10% HtSOl to remove impurities arid to saturate the platinum black with Ht. The electrode waa thoroughly rinsed in distilled water and soaked in conductivity water. Finally, the electrode was rinsed in X-methylacetamide several times and stored in a dilute solution of HCI in Nmethylacfictamide rintii ready for use. The silver-silver chloride electrodes were of the thermalMaterials ---?i'-Rlrthvlacctn~id~~ was prepared hy the method deqcnhrd I,v Dawsori. WLlhoit and Seare.' The electrolytic type and were prepared according t,o the method a r ~ o m p l i ~ti~ hi :v set r r d frn(-tionsl of Harried,* Bntr~s7and Taniguchi and Janz.9 The elecfinal purification n freezings undt r ,AIL ntniosphere of dlxr nitrogen in whirl1 a trode bases were 2' mm. by 8 mm. spirals of about six turns uniform, gradual freezinc techniqiip gavc a material with of No. 26 B and S gage platinum wire. These were fused a conductivitv of 2 5 X 10-8 o h n i r l em.-' a t 40°, a densitv into soft glass stmdard t,aper ground joint units similar to of 0.9414 g./ml a t 40' arid a mrltirig point of 30.2'. (This those used for thr hydrogen electrode and with similar elecfreezing point is highrr than anv rwordctl previously for trical contact. After completion, the electrodes were stored i n a 0.05 M TIC1 aqueous solution for 10-20 hours this compound Hvdrogen rhloride gas was o h i n e d by dropping con- and then rinsed several times in N-methylwetamide. The centrated HzSOa onto warm reagent Krade KCI and then clcctrodes were soaked in a dilute solution of HCl in Npassing the gas throuzh a trap nt Drv Jce temperatimi. mrthvlacetamide for severx! hours and then rinsed three The white crystalline compound ~IC1.2(C~I,CONlTCFT,j times in the cell solution before use. Apparatus.-The electromotive force crll W R B an allwhich was used in preparing the wlutions for the clrctromotive force studies wa3 ohtained from the d r y IiCl gas glass type of the design recommended by Harned and hIorrison10 for use in non-aqueous solvents. and the purified I\'-methvlacetamide hv the method of Electromotive forre measurements were made with a Blirke and Ehlrckhalter Thip compound was prepared, purified and stored under an atmosphere of dry nitrogen. t.ype K-2 Tleeds and Northrup potentiometer. A General Silver oxide was prwipii ntcd from a solution of rcAagent Electric galvanometer was used in conjunction with the grade .4gNOa in conductivitv water hy the addition of a potentiometer; with very dilute solutions the sensitivity solution of reagent grade NaOH. The customary precaii- was rt 0.0001 volt, and with more concentrated solutions tions in preparation n r r e followed by a large number of the sensitivity was f O.ooOo2 volt. A Weston unsaturated type standard cell was used as reference. This cell was waahings.' cslibrat,ed against a cell of similar quality which had been (1) Taken from a. M.S. theais submitted by R. C. Sheridan. certified by the National Bureau of Standardx. The custom12) This work waa supported in part hy a research grant from the ary shielding was provided to guard against potential U. S. Atomic Energy Commission. leaks. (3) iM. Mandel and P. Decroly, Nature. 182, 794 (1958); Trans. .4ll measurements were made in a mineral oil-bath thermoFaraduy Soc.. 56, 29 (1980). stntrd to 40.00 31 0 03". -4dequat)e agitation was given (4) T. Pavolpoulos and H. Strelow, 2. phya'k Cham. (Frnnkfort), a, :t11 parts of the bath by mechanical stirring. 89 (1954). Procedure.-After careful washing and drying, the cell
Studies if platinum-hydrogen and silver- silver chloride rlc,*trodw have been made in aqueous and non-aqueou. sniiitions by many investigators. 11' only electromotive force studies in ~ i r w ga dielcrtnc constant greater than that of n-:iirr were made in formaniitlt by Mandel and Ilccrolv3 with a silver-silver cbhloride ciectrodc arid :IT-t'avolpoulos :aid Strelom4 with a cadmiiim -cndi.nii:m chklride rlrc.trodc. Althoiigti the electricztl eoi:tiiiet ivitics of niore than severity electrol? tt.i 111 h-methyI:tcetamide have been studied in tletai! 1 1 this I,,z'inratory by D:iwson and ro-morkei-s dimrig tlir 1n.t decade. no measurcmentc: i d i n : > to :I knnv ledge of thermodynrirmr properties { J f thwe solutions have been madr. I t was the p'rpw:C of ;his investigation to study the reproduc.ihi1it.i. oi the platinun~-hydrogrii and qiiver-silwr diloride ~ i ~ ~ t r o t in l e sN-methylacetamide solutions of €IC1 : i d to initiate stitdies of the thermodynamic propcr- of systems involving scllvents having high drc'ectnc constants. Dirlmtric const i n t of 1-mc~thylaretamide al 40° 1s 16.5 5 Experimental I
.'
(5) L. R. Dawson, E . D. Wilhoit and P. G . Sears, J. Am. Che7n. SOC.,
7 8 , 1569 (1956).
( 6 ) F. F. Blicke m d J. H. Burckhalter, ilid.. 64, 451 (1042) (7) R. G. Batsa, "Electrometric pH Determinations," John Wiley and Sons, Inc., New York, N. Y.. 1051. p. 200.
H. 9. Rarned, J . A m . Cham. SOC.,51, 418 (1929). (9) H. Taniguohi and G. J. Jana. 1. Eleelrochem. Sac., 104, 123 (1957). (IO) H. S. Earned and .I. 0. Morrison, Am. J . St+., 33, 161 (1937). (8)
L.
1830
R.D..~JVSOS, R.C. SHERIDAN AND H.C.ECKSTROM
0.210
E.M.F. DATAFOR HCl
TABLB I IN N-METHYLACETAMIDE AT 40"
(COR. TO
0.208 mHOl
Vol. 65
760 Mhl.) ISoell
Y=k
Run 1
,
0.200t
-
,
,
,
,
,
,
,
,
,
j
0.04 0.06 0.08 0.10 Molality of HCl. Fig. 1.-Plot of E' as a function of molality for hydrogen cthloride in 1-methylaeetamide at 40". 0
0.02
unit was purged with 1,.About 10 ml. of initial starting solution was placed in each saturator, and 28-30 ml. of this solut,ion was added to the U-tube from a weighing buret. This was sufficient to cover the electrodes completely. The depth of the hydrogen jet varied from 4.5 to 5.5 cm. The initial starting solution was used also in the hydrogen exit trap where the gas bubbled through approximately 10 mm. of solution before escaping to the atmosphere. The electrodes were placed in position after being rinsed in three test-tubes containing the initial starting solution. The cell then was placed in position in the oil-bath, and prepurified N, !vas bubbled rapidly through the cell for a t least 15 minutes before citarting the Hz flow and adjusting it to a flow rate of 1 t o 2 bubbles per second. The first e.m.f. reading mas taken 20-30 minutes after starting the €1, flow. Readings were taken a t 30 minute intervals until the potential became constant. Equilibrium was assumed to have been obtained when there was no detectable change in potential for 2 to 4 hours. After equilibrium was reached, the concentration of HC1 in the cell was increased by the addition of a given amount of concentrated stock solution of HCl in N-methylacetamide from a weighing buret. Ten to 20 hours were required for the cell to regain equilibrium aftpr increasing the concentration. The resistance of the potent>iometer was adjusted against the standard cell before each measurement and checked again aft,er she measurement. The positive lead from the potentiometer was connected to the iig-AgC1 electrode. -411 measurements were reduced to 760 mm. pressure of H?. The barometric pressure had been recorded during the potential measurements; the vapor pressure of N-methylacetamide a t 40" was estimated to be 0.2 mm. and the correction for t>hedepth effect of the hydrogen jet was in accordance with Hi.ls and Ives.11 Repeat determinations over the same concentrat,ion range wwe performed -dsing new stock solutions and newlj. prepared platinum and ,4g-.4gC1 electrodes.
Results The e.m.i'. values, corrected to 760 mm. of pressure, are given in Table I as a function of HCI molality for three independent series of measurement,s. TTal.ues of E' were calculated from. the equation'? E
+ 0.124264 log m - 0.01921T&i = E O . ~ ~ - A-~0.124264pm CI (1)
where the symbols have their usual meaning and
E' is used to represent the value of the left side of the above q u a t ion. 111) G . J. Hills a n d D. J. G. Ives, Nature, 163, 997 (1949). (12) This is t h e rxprpssion resulting from E = Eo 4.606 R T i F io? m - 4.606 R T / F Ion Y* whpre log Y * is assumed t o be ecqual t o
-
(-ay'm
+ Pmi.
the base 10).
T h e D n b y e - H i k k ~ constant I a has been evaluated If, a t -10' ,tml i i rcoirt! t o 0.1.7465 (for log to
0,002757 ,01659 .01770 .02631 .03500 .04869 ,06115
0,5248 .4286 .42543 .4042 1 .38889 .37107 ,35855
0.982 .969 .964 ,961 ,959 ,958 ,963
Run 2 0.00300 .01087 .01787 .02927 .03804 .04631 .04719 .05638 ,06619 .07588 .08543 ,09411 ,10225
0.5194 ,4515 .42495 .39888 .38518 ,37398 ,37382 .36344 ,35493 ,34748 .34109 ,33594 .33175
0.996 .967 ,963 ,953 ,945 ,956 ,941 ,954 ,952 953 ,953 ,952 ,947
Run 3 0.00301 ,01009 ,01155 ,02240 ,03223 .03723 .04141 ,05040 .05688 ,07624 ,09450
0.5205 ,45562 ,44818 ,41262 ,39330 ,38576 ,38033 ,36997 ,36326 ,34765 .33612
0.973 ,967 ,969 ,966 960 ,956 ,950 ,946 949 ,946 ,945
Discussion In Fig. 1, the values for E' are plotted against the corresponding values for m. Extrapolation of E' values t o zero concentration by the method of least squares gave a value for the standard potential of the Ag-AgC1 electrode in X-methylacetamide a t 40' Eo = 0.20573 f 0.00035 volt
The "best" fit of the data corresponds to a straight line with a slope of -0.033267. The value of p in eq. 1 is 0.26771. In mater a t 40°, E'A,-A,cI = 0.21208 volt,13 and in formamide a t 40°, EoA4g-AgC1 == 0.181 Utilizing the value of Eo,the mean molal activity coefficients, y + , of HCI in S-methylacetamide a t 40' were calculated by the equation given in footnote 12. These values are listed in Table I. The experimental results approach the values given by the Debye-Huckel limiting equation a t the lower concentrations. The activity coefficients of HC1 in N-methylacetamide decrease with increasing molality much iess than in aqueous solutions. For example (131 R . G Rates and V F R o n r r I ResPnrch Vatl Bur StandaTds, 63, 283 (1931).
oct., 19m
OXID.4TIOS O F FORMALDEHYDE BY
m ?+
n I. Y3
S -Methyla(etam1de 400
Waty13 40
0.01 ,967 .10 9 k7
0.01 .go3 .10 793
250
0.01 ,490 .10 ,332
H. ' ~ : L n l g u ~ :and i , i G , .I. .ranz, J . I V , ~ ciipw., ~ . 61, 688 (1057).
14)
NITROGEN DIOXIDE
183 1
The values for HC1 in ethanol a t 25' are given for romparison since no data are available at 40'. The properties of the cell Pt, H2; HCl(m); AgC1-Ag in N-methylacetamide are being studied in this Laboratory as a function of temperature and these studies are being extended to other solvelit sy3tems of high dielectric constant.
T)EUTl?KI1:JJE ISOTOPE EFFECTS I N THE GAS PHASE OXIDATION OF FORMALDEHYDE: BY NITROGEN DIOXIDE B Y DOXALDBARTON' Department o j Chemzstry, 1-niverszty of Mzchzgan, A n n .irhor, Machigun Re:ezned March 26, 1961
Ikriterium isot,ope effects in the kinetics of the gas phase oxidation of formaldehyde by nitrogen dioxide are reported a t 126". The competitive isotope effect for 1% CD20 is approximately 3.0. No exchange reaction forming CHDO is detectable under the conditions of the competitive experiments. By means of se arate experiments.with CHzO and CDzO the ratio of second-order rate constants is found to be 13.5 i 1.5. The C 0 : 8 0 ~ratio in the products decreases when CDzO is substituted for CH20. The kinetic and stoichiometric observations for 2 : l initial ?iOz:CH,O ratios agree substtintially with those in the literature. However, when the ratio NOz: CHzO is reduced the stoichiometric relationships change, favoring CO . I chain mechanism, in which hydrogen abstraction reactions are important, accounts for most of the observations
Introduction Previous studies? of the oxidation of CH20 by S O 2 revealed second-order kinetics when the initial ratio of' SO2:CI-I2Owas 2 : i . A constant ratio of C O : C O z was observed in the products. .kt 2 : i initial S02:CH2C) ratios the reactant' proportions were represented by the equ a t'1011 5CH20
+ 7x02 --+ 3CO + 2CO2 + 5CH20 + 7 N 0
For this and similar reactions,g rate-controlling bimolecular association reactions have been proposed. Rate-controlling hydrogen abstraction reactions also have been ~ r o p o s e d . ~Both suggest,ed mechanisms utilize a rate-controlling reaction between CHzO and KO2 followed by rapid destruction of the intermediates t,o give the required products. There appeared to be a significant' probability that the "elementary rcwtioii" hctween S O , and CHgO wts acwssibic. for an isotope study. Experimental i convent i m i i vacuiini apparn! I IS was uscd. Th(, procedure f o i storagti :m(I transfcir ol 1orm:rldehyde and nitrogen dioxide was similar to that dwcribed in reference 2 . The pressure in the reaction vessel t v w measurcd by means o f a quartz spiral n1:inornc~tc~rand I C~stt~rnd mcwi:r\miinomrtc~r. T l i ? rc3:trtion x w w l (1. nil., 8 'V r:itio 2 ( , m .- 1
)
~ 3 , sctnnst rii(
1 of 1'yri.s a n d Purroiinrlcd I)v a n
:tli;minun: cy1iritii.r i of :L reaction, n.t.i~nCOI t ( ' 0 . Sitric oside, in ( ' s e w of that, \x-:i~ rcamovetl at -160". It' no S O 2 WLS prwwit removed at -78' :tftc.r 1 1 trap ~ ~ zind contwits I t o staritl or-cvxiig'ht t o p r : i i i t polymrrizat icm of forni:iltlohyrit~. Ti' S O and S O ? ; L W presrnt the. ( 'O? so rr~movcd is cnrit:trriin:itcd hy S O . This may be riAmovcd 111. cvrling tliv -78" Iravtiori throiigh ~
~~~
.
(1) Department of ~ ( I I P I I I I S I I . ~ iind Cht-rnical Enginwring, [.nirers i t ) of Illinois, t7rhana. Illinois. I 2) F'. H. Pollz,rd a n d R. .\I. H.\Vyat,t. l'rans. FaiiirEay Scc., 46, i ( , O (1949;. ( 3 ) l a ) F. II. l'oll>~rda n d I
