DETERMINATION OF ORGANIC PEROXIDES Evaluation of a Modified lodometric Method CHARLES D. WAGNER, RICHARD H. S3IITH. AND EDWARD D. PETERS Shell Decelopment Company,Emeryville, Calif. A n iodometric method for peroxide determination has been modified and extensively tested. The modified method accurately determines peroxides present in autoxidized olefins but, in common with all other methods, it gives only empirical results when applied to peroxides formed in conjugated diolefins.
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Experiments proved that under the conditions of the method iodine does not add to mono-olefins, and that in the absence of peroxides it does not add to conjugated diolefins. The method has been found more accurate and more widely applicable than indometric methods employing acetic acid as solvent.
ODOMETRIC methods as well as ferrous ion methods have Modifications found advantageous are: (1) replacement of been widely used for the determination of peroxides in oxidized potassium iodide by sodium iodide which is much more soluble in organic materials. Most of the proposed iodometric methods inthe reaction mixture and makes possible a higher concentration volve reduction of peroxides by iodide ion in acctic acid solution of iodide, tending to increase the reaction rate and to decrease with or without addition of a strong mineral acid. The xellpossible addition of iodine t o unsaturated materials; and (2) known procedures of Wheeler ( I 6 ) , Lea ( 7 ) , Marks and Blorrell elimination of water from the reaction mixture to avoid the low ( I I ) , Franke and Jerchel (S), Liebhafsky and Sharkey ( 8 ) , results obtained on autoxidized diolefins in the presence of water Taffel and &vis (15),Stansby ( I C ) , and French, Olcott, and Mat[Lips, Chapman, and McFarlsne (9) noted that presence of water till ( 4 ) are examples of such methods, some of which use chloromarkcdly decreased results obtained on fats by an iodometric form or carbon tetrachloride t o aid in dissolving the sample. method, and Liebhafsky and Sharkey (8)observed that water reKokatnur and Jelling (6) proposed a procedure employing isotards peroxide reduction by iodide.] I n order to avoid possible propyl alcohol as solvent, in conjunction with a small amount of interference by oxygen in the experimental work, the reaction Itcetic acid and potassium iodide. Nozaki (12) has recently remixture was kept under a blanket of carbon dioxide, and sample ported a procedure in which acetic anhydride is used as the and reaction mixture \\ere deaerated prior to analysis; however. 4olvent. such precautions have been found unnecessary for general use. Many differences of opinion exist concerning the reliability of Experiments using the modified iodide method show that reiodometric methods. It has been well established that many jults on known hydroperoxides are accurate, since wide variation peroxycarboxylic acids, diacyl peroxides, hydroperoxides, and in reaction conditions produces little change in results which are already close to theoretical. The fact that peroxide was not volaother peroxide compounds can be determined quantitatively by tilized by passage of carbon dioxide gas was proved by tests using iodometric methods, although a few reports exist doubting the accuracy of certain iodometric methods on such simple peroxides lert-butyl hydroperoxide. I t was found that appreciable amount* as cyclohexene and Tetralin peroxides. The most common critiof water must be present in the mixture at the titration end poini in order to avoid high results due t o the slowness of reaction becism has been that iodine liberated in the reaction disappears by addition t o olefinic double bonds; this assertion has been bascd tween iodine and thiosulfate and consequent overtitration, especially when the titer is small. This effect had been noted by Lieb upon dependence of results on sample size with certain materials hafsky and Sharkey ( 8 ) while studying a method utilizing aretit and upon test experiments involving reaction of free iodine rather than triiodide ion, Wheeler (16) found that when iodide ion was acid as solvent. Variation in reaction conditions produced little change in results present iodine did not add t o certain unsaturated peroxide-free oils, but proof was lacking that triiodide did not react with olefins on materials containing no autoxidized conjugated diolefins, indiin the presence of peroxides. Panyutin and Gindin ( I S ) formucating that the method is accurate when applied to such materials, this is consistent with recent observations that mono-olefin perlated a procedure by which they measured iodidc added, iodine oxides and initially formed ether peroxides are, in fact, hydroFound, and iodide unoxidized, thus hoping t o compensate for peroxides. However, results on diolefins and ascaridole showed iodine addition, but the procedure itself was so different from the wide variation with changes in reaction conditions, indicating that usual procedures that information presented in that report did such bridge-type peroxides are not determined accurately by this not prove or disprove iodine addition under normal conditions method. of iodometric methods. Another criticism which might be directed a t iodometric Evperiments have demonstrated that under the conditions of methods, particularly .those employing strong acid, is the possithe method iodine does not add to mono-olefins and that, in the bility of reduction of organic compounds other than peroxides, absence of peroxides, it does not add to diolefins. One expeiiment including possible reduction of any alkylene diiodides formed by indicated that iodine might be absorbed by diolefins in the presaddition of iodine t o olefin. Atmospheric oxygen is known to ence of diolefin peroxides. The sodium iodide-isopropyl alcohol method was found to be as useful in all case9 as sodium iodidecause high results with most iodometric methods, particularly in acetic acid methods and t o have the advantages of somewhat the presence of strong acids, and methods employing sodium bimore general applicability and comparative freedom from intercarbonate or solid carbon dioxide ( I , 8) were designed to overcome this defect. The Kokatnur-Jelling method (6) was reported not ference bv atmospheric oxygen to be influenced by atmospheric oxygen. For general use the method proposed by Kokatnur and Jelling EXPERIMENTAL ANALYTICAL PROCEDURE (6) has been found the most suitable of the iodometric methods lnto a 250-ml. Erlenmeyer flask equipped with a gas inlet tube tested. [Since this work was completed, Nozaki ( 1 4 ) has reported introduce 40 ml. of dry isopropyl alcohol, 2 ml. of glacial acetic that use of acetic anhydride in place of acetic acid overcomes the acid, and up to 10 ml. (usually 5 ml.) of the sample, containing up to 2 milliequivalents of peroxides. Connect the flask to a disadvantages inherent in the use of the latter as solvent.] 976
DECEMBER 1947
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Eflect of Absence of Water on Accuracy of Iodometric Titration
Volume of Calculated Titer, Jtandard Iodine 0.1 N X a A O a Solution M 1. 1M1.
Titer Found, 0.1 N NazS2Os 5 ml. of Hx0 added .lf I . .If 7.
lowing it to autoxidize. It consisted of about 70% 2-methyl-1,3pentadiene and 30% 4-methyl-1,3-pentadiene. Isoprene, redistilled, was 97.1% pure (determined by freezing point). Diethyl ether was anhydrous lferck reagent'.
Effect of Water on the Titration. I t was found that the rraction between sodium thiosulfate and triiodide ion is rather slow 50 11.53 11.57 11.56 unless 5 t o 10% of water is present in the reaction mixture. 25 5.77 5.75 5,78 10 2.31 2.81 2.37 When the titration volume 1.02 0.51 2 0.46 ___ is reasonably large, sufficient water is furnished by the thioTable 11. Effect of Manipulative Variables in the Sodium Todidelsopropyl Alcohol sulfate solution but with 10% Method on Results for Known Peroxidesa titers water must be added Hydrogen before titration. This effect Tetralin tert-Butyl Peroxide, Benzoyl Peroxide, % b Hydroperoxide, % b %b Peroxide. % b Ascariis shown in Table I, which 5 ml. of dole, % b 5 ml. of 5 ml. of 5 nil. of summarizes experiments in AnH@ before AnH20 before H2.0 be,fore AnH20 before AnVariable hydrous titration hydrous titration titration hydrous titration hydrous which known amounts of I! Reaction time standard isopropyl alcohol 2 min. 103.7 97.3 96.7 96.5 29.7 100.2 98.8 7.3 5 min. 105.1 97.1 96.5 96.5 29.6 100.2 98.0 11.2 solution of iodine contained 25.7 29.4 loo., 98.5 96.0 15 min. 102.4 97.3 95.8 in the reaction mixture were Sample size titrated hot, using a con95.1 29.9 104.0 98.3 5j.6 96.2 103.7 2 rnl. 105.7 96.0 29.4 100.7 98.1 25.7 97.3 95.8 5 ml. 102.4 ,stant total amount of reac10 ml. Q6.7 97.2 96.2 96.2 , . ... tion mixture [s ml. of standirnount of iodide0 2 grams 102.2 96.9 96.3 29.4 99.4 98.8 25.1 ard solution iodine in is+ 100.2 28.6 7 grams 99.4 98.1 97.5 29.7 102.3 propyl alcohol, (50 - 2) ml. of Effect of 5 ml. of H10 Bdded a t s t a r t 87.2 96.0 100.7 3.7 isopropyl alcohol, 2 ml. of Added after reflux 97.0 96.0 98.5 18.4 glacial acetic acid, and 2 N o HnO 102.4 95.8 98.5 25.7 grams of sodium iodide]. b:tfect of heat 30-min. reflux Accuracy of Method and before I - added 87.4 P3.5 R4.2 2Q.O 84.3 86.2 23.5 0-min. reflux Effect of Manipulative varibefore I - a d d e d 102.4 Y7.3 Ll5.8 96.0 29 4 100.7 98.5 25.7 ables on Results. The known a Dissolved in benzene (c.P. thiophene-free), a b o u t 0.1 ,?'except hydrogen peroxide, which was diluted with water peroxides Were tested to study b Per cent recovery based on 100% purity. "Anhydrous" refers t o anhydrous conditions of experimental method. Other colunina of d a t a obtained b y adding 5 ml. of water t o reaction mixture just before titration. the effect of variations in 0 Method modified: sodium iodide dissolved by refliixins in i-opropyl alcohol-acetic acid. follon-ed by heatine to reflux, a n d addition of sample through condenser. Carbon dioxide used in usual way before a n d a f t e r refluxing. heating time, reaction time, _____ .;ample size, amount of iodine, and amount of water (Table 11). These tests indicate thai reflux condenser and pass carbon dioxide gas through t'he mixture (1) the amount of iodide is not critical, provided a large excess is for 3 minutes. Stop the carbon dioxide flow, hcat the solution to present, (2) reaction time and sample size are not critical for reflux, add through the condenser 10 ml. of isopropyl alcohol peroxides tested (except for ascaridole, which reduces slowly), (3 saturated with sodium iodide, and heat the mixture a t gentle only the ascaridole analysis \vas markedly affected by the presence reflux for 15 =t0.5 minutes, Resume the carbon dioxide How, disconnect the flask from the condenser. and titrate the Contents of water during the reaction, and (4)except for benzoyl peroxide immediately with 0.1 N sodium thiosulfate to the disappearance the peroxides art: stable toTvard heat. Consistent results close of the yellow color (use o.ol N thiosulfate for very low peroxide to theoretical n i t h the hydroperoxides indicate that the method concentrations). is accurate for tlieir determination. Additional support for thi+ EXPERIMENTAL conclusion is given by consistent results of 94% obtained on 8 $ample of cy, cy-dimethylbenzyl hydroperoxide prepared by autoxiMaterials Used. Tetrahydronaphthyl hydroperoxide (Te tralin dation. peroxide) was prepared by air oxidation of Tetralin a t 75', .:rystallization, and recrystallization. a,cy-Dimet hylbenzyl hydroThe experimental procedure was applied Fith variations t o tltW peroxide (cumene peroxide) was prepared from cuniene by a autoxidized materials, with results shown in Table 111. WirL method similar to that-of Hock and Lang ( 5 ) . Hydrogen peroxide materials containing no conjugated diolefins, results were affected was a sample of Baker's C.P. 3oy0 (in water), assaying 29.9%. &&Butyl hydroperoxide was prepared by careful distil!ation of little by changes in the experimental conditions, whereas reLnion Bay State material, and analyzed close to 96.5y0 by the sults with autoxidized isoprene and methylpentadiene were critiprocedure given above. Bcnzoyl peroxide was obtained froin cally dependent on the conditions employed. The diolefin peroxLucidol Corporation. Ascaridole, obtained from Eastman Kodak Company, was said. to be better than 99% pure, and analyzed , ides react
