The Effect of the Wave Length on the Photochemical Decomposition of Ethylene Iodide in Carbon Tetrachloride Solution Citation Nies, Nelson Perry (1935) The Effect of the Wave Length on the Photochemical Decomposition of Ethylene Iodide in Carbon Tetrachloride Solution. Master's thesis, California Institute of Technology. doi:10.7907/e8jg-kq17. https://resolver.caltech.edu/CaltechTHESIS:10102024-174620263 Abstract No abstract. Item Type: Thesis (Master's thesis) Subject Keywords: (Chemistry) Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Dickinson, Roscoe Gilkey Thesis Committee: Unknown, Unknown Defense Date: 1 January 1935 Record Number: CaltechTHESIS:10102024-174620263 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:10102024-174620263 DOI: 10.7907/e8jg-kq17 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 16787 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 11 Oct 2024 17:43 Last Modified: 11 Oct 2024 18:10 Thesis Files PDF - Final Version See Usage Policy. 4MB Repository Staff Only: item control page

THE PHOTOCHEMICAL DE CO MPD uI'r ION OF E':1HYLl!]{E IODIDE I N Cf:,R30N TETRACHLORIDE SOLUT ION Tl1e sis by Nelson P. Niea • In Par•tial Fulfillment of the Requirements for the De gree of Master of Science California Institute of Technology Pasadena, California 1935 THE EFF ECT OF 'fiIE WAVE LENCrTH ON THE PHOTOCHEMICAL DECO MPOSITION OF ETHYLENE IODIDE IN CARBON TETRACHLORIDE SOLUTION The :1.odine-smnsitized photoehern;!.cal decomposition of ethylene iodide in carbon tetrachloride solution was found by Seht.lroachar and VJ1ig1 to be expressed by the equation (l} when . the tra.nsriiissi.on of . the . solution is -small. . They advanced a mech&nism leading to the equation The prin1a.ry process of this mechanism is the absorption of a quantum by an iodine molecule, with subsequent dlssociation of this excited molecule into atoms. The decompooition is then a chnin reaction catalyzed by these atoms, the mechanism being identical with that of the thermal decomposition. Schumacher and Stieger2 studied the ef'f'ect of the \'tave length on the reaction, and fou.x1d that the constant of equation (1), divided by the \'18 Ve 0 1en.gth., was one sixth as great at 5461 A a.sit was at 4358 0 A. Since the eonvereence limit o.f the band spectrum of 0 iodine vapor is at 4995 A, t hey interpreted t hi s result a s bei ng due to a laek of dissociation of t he excited iodine molecules in the :: i and region. However, they overlooked the fact that the absorption coefficient of 5461 ¾s- much large1' t han that of 4358 A, and i'tr tvt-1 1 be shov1m in this paper t1w,t the : rate · does not ' .fa.:11 ' oi'f' greatlywhen the · absorption doefficient · a.nd tlie transmission are considered. Seh ur1achor and Y:iig subtracted the measured t hermal reaction from the total react:i.on to obta i n the photoche:mical reaction. 'l1hi s will also be shown to be incorrect, because of the close con~-,. eetion bo t ,,rnen t he mechanisms. Their reaction r.d.xtures were not st irred during the cleeompo::::i tion. and tho:h~ calculation s of the constant do:!:xn1ded on t h e assumption that the re a ctions rrere small. In the present paper are presented (1) resu1ts of measurements of t he absorption coeffi cients, ( 2) a consider•a tio:n of the correc- t ion for therrnal reaction, (3) a derivation of the dependence of t he rate on t he absorption coefficient and on the t1"'ansmission of t he solution, and {4) results of experiments on the photochemical decomposition. -3- Experimental The carbon tetrachloride was purified . 3 aeeording to the method of Polissar • The ethylene • iodide was prepared according to the method of Mooney and Ludlitm.4 , and contained a small amount of iodine. A weiglled araount was dissolved in a. weighed a.mount of the purified carbon tetrachloride. w/fh ff,,; fJ5u/{a fe The solution was titrated for free iodine/)and the concentra tion of the iodide calculated. In most of the experiments the iodine concentration was increased by adding iodine which had been resu'b limed from potassium iodide. '11he cylindrica l pyrex reaction cell wa.s 4.0 cm. in diameter, 2.1 cm. between the plane ends, contained 28 ce. of solution at room temperature, and had an air space of 25 cc. above the solution. It was always sea.led during an experiment, precautions being taken to avoid any decomposition of the carbon tetrachloricle during the seal-o:rr. It was furnished \1ith a vertical magl'letically 0perated plunger to act as a stirrer. The thermostat was so constructed that the reaction cell could be placed in a small air space surrounded by the vapor of a boiling liquid exeept for the path of the light beam. Since considerable time was required for the cell to come to the thermostat temperature. the cell was heated just to the right temperature before being placed in the thermostat. It was then left twent:r minutes before being illmninated. In the p reliminary experiments, with boiling water j_n the thermostat., the temperature of the eell may have been a degree low; in the later experiments,using carbon tetrachloride>it was 0 o 0 probably within .2 of 76.6. The experiments at 99 were corrected for the thermal reaction oceuring before illumination. At the end of an experiment 5 cc. of the reaction mixture at room temperature were pipetted into slightly aeidified potassium iodide solution and titrat6d with thiosulfa.te. The light source was a quartz mercury a.re~ usually taking 5 to 5.5 amperes at 45 to 55 volts. light was passed through a filter bo.B The and two lenses an§l a · beam was '·defined by a dia:pnr•agm.. After passing throu?,h the thermostat it was focused by another lens on a Moll micro-thermopile immersed in water.. The intensity was measured hourly du1~ing a run by removing the thermostat from the path of the beam. The linearity of the gHlvanometer deflection ·11ith the current through it was tested and eorreetions were applied where necessaPy. Suitable Corning glas s 0 filters were used to isolate the lines 4358 and 5461 A. w~dl-e · That the filters used absorbed all other/l lines was seen / ,,,,,,,,,,,· / 1,,, rr 1ermopi1e / Wa t _~ __ / Windr,ws End closed duri~g expieriments Scale 1/8 inch:;;; l ·cm . ,' ?G .fycr/ .J,, / Rea ction CeJJl i Diagram of the Optical System lens 8 dio p te1" / 'ccn,1. vap~ ,/ 11 nm. hole solution 8 diopter lenses Diaphragm 2 Li~,ht filters ouc1 I Shutter / 't -5in a. spectroscope. A 1 cm. :layer of a 10% solution of cupric chlOTide ; dihydrate served to absorb infra-red a.nd short ultra-violet. The Absorption Coefficients The absorption coefficients have been measured by several investigators, but the results are not very accurate. Measurements were , therefore, made with the same apparatus used for the decomposition. Since the values obtained agreed well among themselves,. they were used in i:r ef'erence to those found in the literature. '11hey were calculated from the equation where (I2) is the concentration of iodine in equivalents per kilogram of solution, ), the inside thicknes s of' the cell = 2 .• 1 cm.,. T 0 the tra.nsmiss.i. on of the cell containing carbon tetrachloride only, and T the transmission of the cell containing the iodine solution. The values obtained are given in Table I. Since they all involve the thickness of the cell, their absolute values may not be as accurate as their relative values. The average values at room temperature calculated using eoncentrations in mols per liter and decadic logarithms -6- ABSOH?fION COEFFIOIRNTS OF SOLUTIONS OF IODINE IN CARBON TETRACHLORIDE 0 0 5461 A 4358 A t .001'78'7 .00240 lQ0309 .00329 Y(ean '76.6° .00238 124.9 124.0 .004'75 123 •.7 .001544 .00198'7 .00309 .0039'7 Mean .000140 •.000300 .000511 .000599 l.21. 1 .t: 1. 5 .00340 I1iean 99·0 121.0 119.4 124.0 120.0 124.2 .± .5 123.5 125 .. 2 ...t.M· <">2 :-5 123.8 . .J • 123.7 .:t .8 1310 1258 128'7 1255 12'78:.t: 23 .000215 1223 .000307 .. 000430 1214 !ftean .000140 .0001'790 .000280 .000360 Mean 1191 1209 ±12 1210 1231 1206 1210 1214 ± 8 The mean values, calculated with the concentrations in equivalents per li t or and natural loga1•ithms., are: 76 800 84 820 85 840 fc75!:!ed/vdy are11 700 and 66 compared to the val. u e .s of Gillam ~ 6 and Morton ... :, 800 and 60; a.nd Ge t man · :, 800 and 100. 7 .. 0 Ghosh and Mu.rt hi give 417 for 5461 A.. The Correction fo~ Thermal Reaction For any eatalytic chain reaction i n which the thermal and photochemical :mechanisms a.re identical and t he photo chemica l rate is \proport:i.onal to the squs re root of. t he inte:nsi ty, the squ,'n"'e of the photochemi cal rate is e qual to the square of the observed rate minus the square of the indep endently observed the1"'mal rate 8 • the present ca se . fhis e.an ea sily be se e ~-, for The combined mechanism lp ( photoehemi cal) I2 + h✓ lt ( the17mal) I2 ~ 2 I ~ 2 C2 H4 I 2 t I 3 C2H4I ~ 4 C2I-I4 + I ----? 5 2I -"? leads to the expre ssion and ~ 2 I C2 H I +I 2 4 c2 H4 t I C2H4 I Iz -8- thermal rate 1 Then Therefore,. if the intensity of light is uniform throughout the solution, a correcti on of' only 2% shou.ld be subtracted if the independently observed therma l ra.te amounts to 20% of the total rate. On the other ha11d, if the light is very strongl y abs o1..bed so that the light rea ction takes place in a very t hin layer, the whole t hermal re acti on should be subtra cted . In intermediate si t71ati ons t h e corre cti on is uncertain. To avoid this unce rtainty, the temperatuPe of the final experiments was r-educed f1"om 99 0 . to 76.6 0 , for there the independently observed thermal r oacti on is only .5 to 3% of the total, instead of 3 t o 1OJ6 . No correction for it was made. ~f'fect of tho Absorption Coefficient 'l!he r ate of this react 1 on depend$ strongly on the absorption coefficient of the li ght used. An equa tion derivable from the following as s umptions has bee n given by Allmand 9 and expresses this dependence for t h e general case where the rate is proportional , to (Iabs )z . Consider a inn layer at a d i stance x :from t h e illumi nated. side of e. solut ion of thickness j, which iae.nti;r;ely i n t h e path of a uniform beam. Let ~~)x he the loca l r'd te of photochemical r e action in this layer,, Iabe the rate of absorption of radiation ( Einsteins ' per u.x1it v o 1u,11e per unit time) in t his I layer, Ix the intensity of the radiation ( Ei n stein~ ·-, per unit area per unit time) in this layer, f' some funci:i on of the concentrations, .!_ the concentration of the 11 {;, ht-absorbj_ ng con.stituent, o( the molal A l·-. t J..On • r • i ent 01"' _A ( n~ f' • it ion ~ -Ix -- I_ge ..-o(l,x) ~sorp coe f ::ic .vu~1.n · ·" , I o the i ntens it7 of radiati on incident on the solution ( Einsteins • per un:l. t a rea per uni t time) , l-md. ~~ the average ra t e o.f re a ction in the solution. Sssume (1) A and eoncentrat i ons of I•eactan ts are uniform t hroughout t h e solution. - -o(P._~ . (2) Beer's l aw (.Lx == -:Qe ) applies. (3) Rate at one lay er is unaffecte d by n Gi gh b oring la·yers of solution (diffusion rate f or i n terme d iate products is small). 'Ph on: In general the s olution will not be entirely i llumi na ted. If the beam has e.n area a and the so l ution a volume V, the s.verage 1"ate in the vessel will be Applying this equ a tion to the pre sent reac t ion, Jet; A ~ x = con centr•atl on of' free iod i ne, of tct al iodine, free and combined , f and K :::: . - ~• V -yki, 28 kg, dx dt ~ : : :. concentration ~ k2 ( s - x ) ~ .rhen 1 - (1 - e o<. .,/} ;;;.;.x ~ 2 ) (1.-e ~ (s-x) )(S-x} where a and b are t he iodine concentration s befo re and after illum:J.nation. Since t his expre ss:to:n cannot be integrated., the values of K were c alcu.1at ed from a. and b by the u se of Simpson' s Rule with four sections, or by the -'- prismoidal formula. in some experiments. This approximation introdu ces an err oP of les s than .5%. The galvanometer d ef1e cti ons were m.e.de proportional to I 0 by multiplying by the vm ve length. -11- The results of the measurements are given in Table 2, in wh ich Exp. is the m.1t1'l-.rer of the experi1:;'.lent, G. the galvanometer deflection tn centimeters, L1 t the tin1e of illmnination in minutes, S203 =- the iodine b e:f:ore a nd aft er illu.m.t nation e xpressed in ce. of .0054:-58 N thio-s ulfate requ j.red to titrate Sec. of the sclut5.on at ro om tcmr:erature , !,Q and T respectively the calculated transmissions of t he solution at the beg:tn..rdne and a.t the end of illumination, (C2H4 I 2 ) the initial concentrati on of ethylene iodide i n e quivalents (C2F14I2 ) per k iloer am 2 of solution . Alt hou e-.,.h tho 1.,esl.i lts a.t 99° arc not as l"elisble a-s those at '76.6°, a value of the t empe1~ature coefficient was ca lcula t-ed from t he mean val ues of K. 0 0 I t is l.'7 per 10° at 4358 A and J..6 at 5461 A, compared t o the va lue f ou.nd by Schumacher and -12- RESULTS OF THE KINETIC MEASURENiEH'i1S Exp. 6t s 2o3- 1nitial final 'f 0 T 0 " temperature 99: Wave length 5461 A,, .0018 30.0 .20 3.6'7 .'71 25 4.36 .00012 30.0 .20 5.22 .'71 28 2.77 .00053 40.0 .20 4.38 • 171 30 3.70 ,.,000019 45.0 1.26 6.32 .115 34 4.36 .000042 45.0 1.26 5.85 .115 35 3.88 5.,40 ~ .22 .1174 .20 2.60 5.05 10.4"1 .11'74 .11'74 6.29 6.95 2.58 3.09 2.110 60.0 60.0 60.0 9.79 5.05 .9'7 .67 .41 .41 .16 .18 0 3.42 252.0 2.39 39 3.19 40 3.11 2.51 2.66 2.75 6.16 .016 .00024 0 A, temperature 76.6°: 61.0 7.31 9.11 .2'7 .20 128.0 '7 .35 11.25 .zr, .14 116.3 3.18 5.80 .5'7 .36 241.0 3.18 6.45 .5'7 .32 138.5 3.23 6.'71 .5'7 .30 Wa ve length 4358 48 49 50 52 54 62 5.69 5.08 5.54 5.52 11ean Wave length 5461 A, temperature '76.6°: 37 5.19 90.0 1 .. 05 5.fi5 .16 .00012 .00092 38 5.29 60.0 1.05 4.05 .16 42 3.98 116.5 .'16 5.0'7 .2!1 .00016 43 4.47 125.5 .76 5.'70 .2'7 .000054 44 4.50 90 .. 0 1.06 4.'78 .16 .00026 3.95 154.0 3.01 8.30 .6056 51 .0000006 53 2.26 257.5 3.01 7.'79 .0056 .0000015 61 5.18 Wave length 4358 A- temperat'Ul"e 990.• 2.7'7 30.0 .20 1.10 .97 • 82 0 26 29 31 32 .11'74 .11'74 .11'74 .11'74 .11'74 1.31 270.5 2.54 245.5 2.28 225.5 3.,01 3.01 2.39 6.33 .59 9.58 4.68 .59 .,66 .33 .18 .44 6.28 .1174 6.28 Mean 6.45 t .. 11'74 1.'798 1.788 .1174 .11'74 .1174 .1174 1.843 .1323 .1323 ,-D.ll615 1.'1'1'1a 1.'740 .2€); 1.79'1 1.803 1.922 Mean 1.809 ±: .039 .1174 .1174 1.980 .1323 .1323 .1383 1.910 1.923 .13~3 l.08QB 1.90'1 1.962 8 .1323 .06615 2.080 Mean 1.945 :±:- .058 - hN'o,t used in calculating the mean. aintensity reduced by means of a fine screen. 1.a21 Discussion . of' .the Result s The value s of the specific rate are not greatly di ffe r ent at the t wo wave lengths . This indioa.ts ei t hat i odine molecules , under the conditions of these experiments, d.issoe:i.ate neai->ly as often i:> 0 when excited by 5461 Aas when e x cited by 435841 and t herefore in carbo" -lrdrt:,..dfor/de $0/vf-,oV) that ~t hel:"'e exists no threshold frequency for the dis s ociati on at the convergence limit of' iodine vapor• It is pro pos ed to Q.xplore the band :rn~ gion further by undertadng experiment s w:i.th yell.ow 1:1. ght . Aek;novrl ec\gm.ent I e.m i ndebted to Professor R. G, Dickinson, wh o suggested t his problem , f or mu.oh help and me.ny v a l uab le ti'Uggesti ons durins t h e p:.P ogr-ess of' this work. The absorption coe.fficients of solut:i.ons of iodine in ea1"bon. tet1"a.chlorid.e have been measured 0 at 4358 and 5461 A. o 0 At 20, for 4358 and 5461 A they ar e res pectively 121 and 1280; at '76.6°, 124 and 1210; at 99°, 124 and 1210, wh en the concentrati ons are expressed in equiva lents per ld.logrrun of solution and na tura l loge.1.. ith.ms a re 1.r ned. The r eJ.at:i. ve rates of the iodi ne- s ensitized photochenr'L ca l decomposition of ethylene iodide l:n carbon tetr9.chloride solution with wave 0 lengths 4358 and 5461 A have been determi ned• When t he eff"'ect of the absorption coef f icient is considered, the specific r ates are not greatly different at the two wave lengths, :l.ndica ting that under t hese ax.perimental conditions* iodt ne molecules excited with light of band re gion frequency (as far as A= 5461) dissociate nearly as often as t hose excited with li ght of the frequency of the re eion of continuou s absorption. -15- REFEREN CES 1schurnaeher and Wiig, z. physik. Chem. Bll 45 (1930) 2 schumacher and Stie ger, z. physik . Ch em.BE 348 (1931) 3 Polissar, J. Arri. Chem. Soc. 52, 956 (1930) 4 Mooney and Ludlam, Proc. Roy. Soc. Edinburgh, 49 160 5 Gillam and Morton, Pr oc. Hoy. Soe. Lor\don (A) 124 604 (1929) 6 oetman., J. Am . Ch em. Soc. 50 2883 {1928) 7-Ghosh and Murth1, z. physik . Ch em. B 26 8 (1929) 26"7 (1934) Style, Photochemistry, E . P. Dutton a nd Co ., Nev, York, p. 61 9A1lmand, J. Chem . Soc. ( 1929} 1558