Determination of Unsaturation in Organic Compounds by Electrometric Titration BEN BRAAE’ Royal Swedish .4cademy of Engineering Sciences, Stockholm, Sweden
The mercury-catalyzed addition of bromine to isolated double bonds proceeds so rapidly that they can be determined by direct titration in the homogeneous phase. The end point is indicated electrometrically. As no excess of bromine is used, the risk of substitution or oxidation reactions is minimized or eliminated. The method is particularly suitahle for determination of double bonds in the
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H E bromine or iodine numbers determined by conventional titration methods do not always correspond to the true degree of unsaturation, owing to certain side reactions, especially oxidation and substitution processes which proceed simultaneously with the halogen addition and result in too high iodine or bromine numbers. This is of special importance in the analysis of mineral oils, Holmberg ( 7 ) has found that the iodine number of Swedish shale oils depends to a great extent on the type and amount of the iodine solution and the time of reaction. Many attempts have been made to eliminate oxidation and substitution processes in this procedure. RlcIlhiney ( I S , 1 4 ) employed an acidified bromide-bromate solution for determination of the bromine number. After titration of the bromine exces3 by means of sodium thiosulfate, he determined the hydrogen bromide formed by adding potassium iodide and iodate and titrating the iodine formed. The quantity of hydrogen bromide was considered to be a measure of the substitution. Other investigators [Jordan (9) and Lenis and Bradstreet (11)] found, however, that considerable amounts of hydrogen bromide mere formed upon the bromination of pentene, hexene, octene, and styrene, even if the total amount of bromine absorbed did not surpass the calculated value. The mechanism of the halogen addition thus seems to be considerably more complicated than originally presumed; the 1,2-dibromides primarily formed are probably unstable, and by splitting of a mole of hydrogen brornide are rearranged to stable unsaturated monobromides which do not react with bromine. In any case, the amount of h>drogen bromide formed cannot be conridered as a nieasure of the true substitution. Another effort to avoid side reactions has been made b> using a small, well defined excess of halogen solution and short reaction times, as in the bromine titration methods of Francis ( 6 ) ,Johnson and Clark ( 8 ) , and Lewis and Bradstreet (10). HoRever, the last-mentioned authors (11) have shown that substitution may occur even here, especially if the molerules contain hydrogen bound to a tertiary carbon atom. Lucas and Pressman (12) found that the bromine addition, R hen catalyzed by mercury salts, can be quantitatively performed within 3 minutes, even in the presence of conjugated bonds or triple bonds, which react rather sloaly Kith bromine. That these methods, in spite of their obvious advantages, are not yet generally adopted is probably due to the necessity of previously determining the approximate iodine number of the oil sample for calculating the bromine excess rt hich cannot be Present addrrdvrden
AR Separators Krnuska Idahoratonurn Ptorkholm
presence of easily substituted compounds. This is important in determining iodine numbers of hydrocarbons such as gasolines, lubricating oils,, shale oils, or turpentine. For technical purposes the method is specially suitable because of its simplicity and rapidity. No preliminary determination is necessary and the end-point determinatba is independent of the color nf the solution.
visually estimated if colored saiiiples are to be analyzed. In most cases it is therefore necessary to make several titrations in order to determine and calculate the exact bromine or iodine number. APPLICATION O F ELECTROMETRIC END-POINT OETERMINATION FOR BROMINE TITRATION OF OLEFINS
The mercury-ratalyzed bromint. addition to olefins proceeds rapidly that the double bond? ran be directly titrated with bromine. The solution does not contain any excess of bromine and t,hus there will be only a slight risk of substitjution and oxidation. The end point of the titration was indicated by the electrometric method proposed by E’oulk and Bawden ( 4 ) under the name of “dead-stop end point.” It was applied by Wernimont and Hopkinson (16) in determining water by titration with the Fischer solution ( 3 ) . I t depends on the potential jump occurring between two platinum electrodes when a slight excess of bromine is added to a solution coiitaining bromine ions. The titration device employed in the author‘s method consists of a rectifier and an amplifier built together and connected with a cathode ray t,ube magic eye. [The instrument was constructed and built by Bertil Ohlson, Stockholm, to whom the author is indebted for very valuable cooperation. Good results have also been obtained with an ordinar!. potentiometer circuit ( I ) . ] The apparatus is similar to that described by McKinney and Hall (15). The potential jump at the titration after amplification is made visible by the eye or measured by an ammeter (0 to 2 ma.), t,hus permitting thr determination of the end point of the titration. The solution must contain bromide ions. Sodium or potassium bromide is not suitable, however, for the end point indication is not sharp and the bromine consumption depends on the amount and concentration of potassium bromide used in the procedure, probahly because of the presence of Bra- ions. For the same reason a considerable bromine excess can be retitrat,ed with a few drops of sodium thiosulfate solution (of the same normality), owing to the reaction of sodium bromide with the free bromine. When hydrobromic arid was used as a source of bromide ions. no complications could be observed. The solution should also contain a catalyyt, such as a mercury aalt, for the addition of bromine. The author has employed mercuric chloride. I n neutral solution, however, the bromine combines with the mercury salt, forming a complex salt which prevents the bromine from taking part in the reactions a t the electrodes. This can be avoided by using a solution with high concentrat inn of chloride ions -for instance, by adding hydrodo
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ANALYTICAL CHEMISTRY
rhloric. acid to decomposc the cmqhx Siniultaneousl~. t h r wutrsr contained in the hydroehloi.ir arid catalyzes the addition of broinirit,.
'1.h cwreut ( ~ ~ r :.fter r w 0, 30, 60, 90, and 120 seconds intei~wc*t a t a single point, the cnd point. On further dtlition of broniinr thr current remains constant during and A h A L Y T I C 2 L I'HOCEDURE :titer the nholta rt,:+(tiiigperiod. As expected, the speed of the reaction ir differrwt for different olefins. Styrene absorbs the Methanol Solution. A . Fiftcwi niilliliters of hydrogrii h o inidti, specific gravity 1.38, 170 nil. ot' concentrated hydrochlorictromine excess i n less than 30 seconds, and oleic acid in about wid, spwific gravity 1.19, and 20 piwns of mercuric chloride arc' 60 cirwinds, wherws stilbene, having a doubIe bond between two diluted with pure methanol(frw from any oxidizable compounds \ phenyl y r n i i p . i , ~ q i i :i i h~n ~i i t 120 wc~orirls for the c w n p l p i c ~ t o 1 liter. This solution coirtqtrritls to 0.1 .V hydrol)i~~irui~~ :icai(l i t r i d 2 3' hydrncshlorir :tc*id. w i r t ion,
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F i g u r e 1. Titration ( : l i n e s for kXectrometrir Titration at 20" C. L e f t . 0.0642 gram of at)rene titrated w i t h 12.57 m l . of 0.0852 *V bromine solution. Iodine l o . 212; according to Hub1 P19 Center. 0.2030 gram of oleic acid titrated w i t h 15.80 ml. of 0.0851 R. bromine solution. Iodine ho. 84; according to Huh1 85 Right. 0.1099 p~rarnof e t i l h r n r titrated with 10.92 m l . of 0.1112 V Iwominr *elution. Indinr. Yo. 140: ralciilatrd II1
Bromine Solution. H . hbout 8 gIanis, equal t o 2.5 1111. oi' In routiiit. :uialyses 50.0 nil. of' t h e niethitiiol wlutiou are Immine, are diluted with carbon tc4rachloride to 1 liter. This pipetted into a beaker provided with electrodes and stirrer, a wlution corresponds to 0.1 -V bromide and is standardized in thv potent,ial is applied, and the amplifier is adjusted until the cathode ordinary way by addition of potassium iodide and titration with rap tube shows a clear green color. By the addition of a few sotliurn thiosulfate, starch I)&ig used as an indicator. If the c:udrops of broniine solution the inagic eye will be closed, .*howiiig a bon tetrachloride contains sulfur the bromine solution is not green ( w s s on n black background, which should he niaintained stable. The solution shcnlltl l i e kept i n tf:rrknrss and its normality for about 30 to 60 seconds. For this l)l:ink titration only 0.1 to checked every day. 0.2 ml. of bromine solutioii is requilrci. On t h e subsequent adThe titration is carried out : i s follovis: .$bout 1 to 3 millidition of about 1 to 3 milliequivalents of uris:itur:i,tetl compound equivalents of an unsatui,:tt(d c*ompoundm e weighed into a 150from a weighing pipet, tlir cross i n the eyt. \vi11 dis:tppear :tud turn ml. h i k e r . Accordirig to 1,uc~:is: i d Prtwman (12) the catalysis into a plain green field. The pipet is then weighed and the titrnlion with bromine solution is continued until the vross reappears of the bromine additioii to the double bond is best w h w tht, molar ratio of mercury to hi~omiiieis somewhat grwter th:in 1. in the eye. The cross should be visible during 30 to 60 seconds, a The proportion of merrury i i i the methanol solution cwwspontls time that is determined for t,he conipountl in question i i i the above-mentioned way. The bromine cloea no^ rt.:ic.t instantanet,o about 35 ml. of broniirit. solution. The quant'ity of the unously, :is t,he additiou is a nonionic3 reaction. To most unsat,urated compound to l r i ! weighed shoiild thus be equivalent to an amount of bromine solution not wding 3.5 ml. If the unsaturated compounds, however, ljromine is added fairly rapidly. saturated compound is riot easily di ved in methanol, it shoultl Thr bulk of the bromine solution may he added in one jet, after be 1)roughBinto solutioii by ~ildiiig( on t,etrachloritie OP C I ~ I ~ J ~ O -\r.hicli the last portion is added cautiously-e.g.. 20 drops per minute. By this method the complete tieterniination of the, forni. t)romiiw nurnl)rr can be arroniplishcd xithin leis than 10 minutrs By means of a pipet, 50.0 nil. of inrthaiiol solutioii L are i no'iv r. (Thrsolutionc.ontainsmercuricchloritie ) T1itw:tftr.r the two plat'inuni elertrntlrs r i i i ' e pl:icetl iti tht, beaker. Tlic, electrodes ure 0.5-mm. platinum ivir(1s :iritl inurt be eonipletely inimrrsed in the liquid, even if the stirrer is i r i use. They are coiiiircted witti the amplifier, and giveii :isuitiilde potential. The :ininieter is :idjustcd to zero position :tnd t h e olrfin is ~lo\vlytitrated tiy steinvise addition of t,he bromine solutioii in portions of about 0 . j nil. . i t the beginning of the titratiuii :t st,op watch is startect :inti the* If 1110re than 10 1n1. 01' I ) I , I ~ I ~ I I~~ o> l u t i o i i :tre i q u i r e d , t w o current between the electrotlcas is i.ea,d 011 the animeter at i l l ( , Iwphases will appe:tr toward tlic t,ntl ~ t the ' titration due to tho ginning and after 30, 60, 90, :tiid 120 seconds. T l i ~titr:ttioii is continued until the c~uiriwt r r i i i : i i w coiistrr~ritduring thv \vliole limited miscibility of thr. mrthanol solution with carbon tetrareading period. c~hloritle. Ho\\-ever, the sprc.cl i r f rca:tcbt ion lwtn-etvi bromiiit. :iri(l A hlank t,itrat,ioii witliout u n s a t u r a t t d conipouiitl s h u n s t h t ~rkfinis not influencwl thtJrrby. tlie blnrik consumptiori is tirg1igiI)k~-.orily :ilwut 0.1 t o 0.2 mi. of If other broniiii(.-coiisuiniiig rcw:tioIiu ai'(%taking p1ac-e siniulI ironiirie solution. taneouslj- with the :iddilion of l)i,oniiiir the curves of titration will be soniewliat differcmt from t hour shoivn abovr ; thus. thc Tlirs i~rsultsof this titratioii :ir(. i*oin9 10 1 1 12 13 14 15 16mi. 2 3 4 5 6 7 8 9m(. plrtrly repressed. .is illearly shown in Figf Figure 4. Titration C u r v e s l o r Klrctronletric Titrations ure 1, the end-point tlrtermination is sharp f.eft, 0.0782 pram of pinene titrated w i t h 15.76 m l . of 0.0852 .\ hromine solution. l o d i h r :it :I temperature o f - 15" C . .Is re.g:ird. Vo. 217; according t o Hub1 3 3 . -2O0 C. I styrrrie, the sitnit' iwult is obtained as in tho R i g h t . 0.0476 g r a m of styrene plus 0.0604 gram nf thiophene titrated w i t h 9.20 nil. bf 0.0852 1 Iwnminr *nlutinn. Iodine Yn. 209: itrcnrdine tn Hiibl 380. -15c C. : i t m n c i ~of thiophenr. For pinene the (leterminrd iutliitca tiuniht~r is 217 corresporiding to R c~alculitteclvalue of 186, whereas the t ~ r t ~ ~ ~ ~ I)?. t i t il i ci, r ~ :tativel?.. ~ ~ ~ ~ ~OII ~ ~ thc other, Iimrl, l Y J I l l ~ ~ l ~ U l l containing dS doubb~ Hiibl method gives 350. bonds conjugated to caihon~.lgroups> such as maleic acid a1111 Thew pwliniinar>, rxperiments sho~velearl! tllt: :idvantage of cinnamic acid, or triple bonds, such :IS ptienylacetylene, r w e t t h c ~ the t1irec.t titration of the double bond as conipared with the slon-l>- with l~romineand thrwfore thr double bond cannot mnventional nwthode. On t h r other h a n d . Pertain double bonds tli~tcvniined hy direct titr:+t,iou. FfJr wc2h unsaturated wi!ij ~ ~ i i t l on s
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ANALYTICAL CHEMISTRY
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V O L U M E 21, NO. 1 2 , D E C E M B E R 1949
has a tendency for substitution reactions. Table I1 shows the results of some titrations in the presence of easily sub( E n d point indication magic eye) stituted substances, such as thiophene Iodine KO.b y Electrometric and tetrahydronaphthalene. Here the Titration Easily Xormality Without easily conventional methods give misleading Suhstituted Bromine of At substituted Weight of results in contrast to the essily perCompound. Solution, Bromine At - IO0 compound Sample, MI. All. Solutioi 20' C. C. (Table I) Gram cia terial formed electrometric titration. Espe1 TetrahJ d i u 12.40 0.1154 3 n6 308 1 :vclohrxene 0.0590 - 15' C. cially a t low temperature-e.g., naphthalenc 310 12.48 0.1164 1 Tetrahydro... 0.0590 -the speed of the substitution reaction naphthalene 12.53 0.1154 1 Tetrahydro316 ... 0.0590 is so low that the determination of the naphtiialenf iodine number is not influenced, It is 1Ig. clear that a correct figure for the addi50 Thiopheiir 8.10 0.08521 224 214 .. . 0,0390 Styrt iiv 0.08521 50 Thiophenc ... 9.20 0,0&76 ... 209 tion of bromine can be obtainrd only if hI1. side reactions, such as substitution or 1 Tetrahg droY .75 0.09775 80.7 85.4 Olpir acid 0.1380 oxidation, proceed considerably slower naphthalene 1 Tetrahydro10.72 0,09775 91.0 ..I ... 0.1469 than the addition. This is not the case naphthalene 1 Tetrahydro14.07 0,09775 96.6 ..I ... 0.1809 with styryl-s-mercaptoacetic acid (CeHsnaphthalene 6.72 0.09175 I Tetrahydro. . . 86.6 0.0962 CH=CH-S--CH2COOH), in which the nauhthalene 0,09776 speed of oxidation of sulfur to sulfone 8.39 1 Tetrahydro*.. 8 5 . 6 0.1217 naphthalene goes on so rapidly that the double bond 84 3 10.87 0.09775 I. Tetrahvdro0.1602 ..e naphthalene cannot be determined. hlg. Table I11 shows the iodine number< 152 12.10 0,09775 .. 50 Thiophene 0,0990 of gasoline fractions from Swedish shalc 17.62 0,09775 197 .., 50 1 hiophene 0,1112 ... 128 13.07 0,09775 ,., 50 Thiophene .. ., .. 0,1259 oil, as determined by electrometric ti0.09775 ... 84.1 50 Thiophene 8.49 0.1252 50 Thiophene 10.32 n. 09775 . . 84.1 ,.. 0.1523 tration compared with those obtained 50 Thiophene 11.10 0.09776 86.4 0.1559 by the Hub1 method. The shale gasolinea are very rich in sulfur and have Fahle 111. Electrometric Iodine Xurnber Determination high iodine numbers. Normally the sulon Swedish Shale Oil Fractions fur clontent decreases with increasing boiling point of the frac(End point indication magic eye) tions. Accordingly, the difference hetween the iodine numbers Iodine N o_ . . ~ _ _ Boiling determined by the two methock decreases in the same order. A t Electrometric Electrometric Point, a t -IOo C. st 209 c. Huh1 0 c. low temperature lower iodine numbers are obtained than a t normal temperature, and the end point is much sharper and visible 54.7 69.3 ..
