Wijs Iodine Numbers for Conjugated Double Bonds

I

Influence of Sample-Reagent Ratio

WILLIAM C. FORBES AND HARVEY A. NEVILLE, Lehigh University, Bethlehem, Penna.

90

RECEKT investigation of the catalytic dehydration of A castor oil by the authors involved a great many deter- minations of iodine numbers. The standard procedure for the Wijs method, calling for a standing interval of 30 minutes, mas used (1). The product upon which the determination was made contained in addition to single honds, both isolated and conjugated double bonds. Occasionally much lon-er results were obtained than would have been expected from the amount of Tvater evolved in the dehydration process. A check revealed that the size of sample used in these deter- minations was larger than customary and i t was suspected that this was the cause of the discrepancy. Extensive inves- tigations with tung oil by Ho, TTan, and Wen (S) and by K a n and Ho (5) proved that the time of contact, excess of reagent, and temperature were very important factors in obtaining concordant iodine numbers for this oil. Gardner ( 2 ) also has pointed out the importance of weight of sample in this determination.

A pertinent article by Smit ( 4 ) , describing the work of J. Boeseken and of J. Van Loon, showed that substances with conjugated systems act abnormally TJ ith Ki j s reagent. Figure 1, plotted from data given in this article, shows the variation of the iodine number with the weight of sample

I

‘Q S,//-IlNOlElC ACID \ (CONJWA TED S YST€M)

‘\ \

0 IO0 200 300 400 500

wr SAMPLE, MG-

FIGURE 1. VARIATION OF IODINE NUMBER WITH WEIGHT O F SAMPLE FOR ISOMERS 9,11- AND 9,12-LIKOLEIC ACID Superimposed is plot of per cent excess of reagent u s . iodine number

used for systems of two isolated double bonds and of t x o con- jugated double bonds. The former is represented by 9,12- linoleic acid and the latter by 9.11-linoleic acid. The differ- ence in the action of t,he tn-o systems to the Ki j s reagent is most striking. To show the effect of the excess of reagent, a plot of per cent excess of reagent 11s. the iodine number is superimposed upon the previous plot. The necessary data were calculated from those given by Smit. The juxtaposition of the two curves brings out clearly how dependent the result obtained with the conjugated system is upon the excess of reagent present.

PER CL-h’T fK€S REAGfN T - - - -

/M 80 a, 40 20 0 I I

I 140

I30 -- -*

W T. SAMPLE, MG - FIGURE 2. VARIATION OF IODINE NUMBER WITH WEIGHT

OF SAMPLE Castor oil dehydrated with 1 per cent sodium acid sulfate

Since the dehydrated castor oil under investigation con- tained conjugated double bonds (proved by its ability to undergo a Diels-Alder reaction), it \vas thought advisable to carry out a series of iodine determinations upon it, using a gradually increasing weight of sample. This was done for samples of castor oil dehydrated with 1 and n i t h 2 per cent sodium acid sulfate, and Figures 2 and 3 show plots of the data thus obtained. Again the dependence of the iodine number upon the excess of reagent present is clearly brought out.

The contrast in this respect between isolated and conjugated systems mas carried a step farther by comparing the iodine numbers of different weights of 9,12,15-1inolenic acid (three isolated double bonds) with those obtained from tung oil (China mood oil, three conjugated double bonds). Figure 4 shows the curves resulting when these data are plotted. Once more the isolated system gave fairly constant results, approximating very closely the theoretical value for three double bonds. The conjugated system, on the other hand, again yielded widely diverging results, a value two thirds the theoretical being approached. Tung oil, although not pure

72

FEBRUARY 15, 1940 ANALYTICAL EDITION 73.

triglyceride of eleostearic acid, contains enough to show the ture of isolated and conjugated double bonds. With an trend. This result may be due t o lJ4-addition to a con- isolated system the reagent would be exhausted when the jugated system, in this case adding either 9,12- or 11,14-: larger sample was used.

140

/ -CH-CH=CH-CH-CH=CH-

I 1 1

, ~ I

$ -CH-CH=CH-CH-CH-CH-

I I 1 1

9 10 11 12 13 14 -CH=CH-CH4H-CH=CH-

This mechanism would assume that it nas difficult, in the time interval used and with the excess commonly present, for the iodine to saturate the double bond between the two iodine atoms which have entered 1,4-, thus accounting for the limiting value of two thirds the theoretical.

Figure 5 shows a plot of the iodine numbers obtained from different 11-eights of raw castor oil. K i t h this compound containing only one double bond the weight of sample could be varied fivefold without appreciably changing the results.

I t is apparent from these data that the presence of con- jugated double bonds complicates the determination of iodine numbers by the Ki j s method and requires that some con- sistent weight of sample be adhered to for relative values. Even then there is the possibility of a large absolute error, particularly if three conjugated double bonds are present. If more or less sample n-ere weighed out than that selected as standard, a varying amount of reagent could be run in from a buret to keep the ratio of volume of reagent to weight of sample always constant. This would necessitate an extra calculation in order to convert the blank to the actual amount of reagent used.

A suggested rapid test for ascertaining whether a particular fatty oil contains a conjugated system of double bonds is t o use the same amount of reagent in determining iodine num- bers for 0.1-gram and 0.5-gram samples. If there is a great divergence in the results, n.e can be sure the system is con- jugated; if the difference is moderate, we probably have a mix-

r

t ~

80 0 100 200 300 400 500

n'% JAMPLP, MG.-

FIGURE 3. VARIATION OF IODINE KUMBER WITH WEIGHT OF SAMPLE

Castor oil dehydrated with 2 per cent sodium acid sulfate

\ -CH=CH-CH-CH=CH-CH-

I

I

I . 1 -CH-CH-CH-C!H=ZH-CH-

1 1 1

Summary

Iodine numbers obtained by the Wijs method for systems containing conjugated double bonds are strongly influenced by the excess amount of reagent present. Data are presented to show this effect for the conjugated systems (1) 9,11-linoleic acid, (2) tung oil, and (3) dehydrated castor oil. Contrasting data shov that the excess of reagent is of relatively slight importance for the isolated systems 9,12-1inoleic acid and 9,12,1.?-linolenic acid and for ram castor oil.

0 ' /30

0 /oo zoo .3m 400 Fm WT SAMPLE MG-

FIGURE 4. VARIATION OF IODINE I ~ U M B E R WITH WEIGHT OF SAMPLE FOR SYSTEMS OF THREE DOUBLE BONDS

Isolated (9,12,15-l:nole:o acid) us. conjugated ( 1 ung oil)

PER cmr EXCESS REAGENT---

a0 60 417 20 0 F 8 . o y I I r- t 1

1 , 106 200 300 400 300

a i , 80'

W T SAMPLE, MG-

FIGCRE 5 . VARIATION OF IODIXE NUMBER O F CASTOR OIL KITH WEIGHT OF SAMPLE

74 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 12, NO. 2

-4 procedure is suggested by mhich the ratio of volume of (2) Gardner, H. .i., “Physical and Chemical Examination of Paints, Varnishes, Lacquers and Colors”, 9th ed., p. 289, Washington Institute of Paint and Varnish Research, 1939

(3) Ho, K., I t a n , C. S., and men. S. H., ISD. EXG. C H E ~ Z . , Anal.

reagent to weight of sample is kept constant in order to ob- tain iodine numbers if conjugated bonds are present. Ed., 7, 96 (1935).

(4) h i t , W. C., Rec. trau. chim., 49, 539 (1930); citing Boeseken and Gelber, Ib id . , 46, 163 (1927), and Van Loon, J., Thesis, Delft, 1929.

(5) Wan, C. S., and Ho, K., ISD. Exo. CHEAI., hnal. Ed., 8, 282 (193G) ; cf. Wan, S. W. and Hu, D. B., J . Am. Chem. Soc., 61,

A simple test for the presence of conjugated double bonds is suggested, based upon the relative effects of excess reagent upon systems of isolated and conjugated double bonds.

Literature Cited 2277 (19393.

(1) Assoc. Official Agr. Chem., Official and Tentative Methods of PRESESTED before the Division of Pa in t and Varnish Chemistry a t the 98th Meeting of t h e American Chemical Society, Boston, hlass. Analysis, 4th ed., 1935.

Analysis of Sulfite Solutions Containing Selenium A Volumetric Method

R. C. SHA\-ER AND C. R . >ICCROSI(Y, Syracuse University, Syracuse, X. Y.

S THE course of a study of alkali sulfite solutions contain- I ing selenium a rapid and accurate volumetric method for the determination of sulfite and selenium v a s needed. The nature of these solutions was such that ordinary gravimetric methods could be applied only with difficulty because floccu- lent red selenium is precipitated on slight acidification. -1 procedure was worked out bg which it is possible t o determine the sulfite and selenium volumetrically in the same sample.

Use was made of the fact that red selenium in the colloidal condition can be quantitatively oxidized to selenious acid in the presence of a moderate concentration of acid by standard solutions of bromate, using the method of Coleman and blc- Crosky (W), according to the equation

2KBrOa + 3Se + 3H20 + 3H2Se03 + 2KBr

Preliminary experiments indicated that sulfurous acid is oxidized by the same reagent to sulfuric acid according to the equation

KBrOl + 3HzSo3 + 3H2S04 + KBr

Both reactions were found to take place in two steps, the initial step in both cases being the liberation of free bromine from bromide by the bromate. A study of the standard po- tentials for the following couples (3)

Se (black) + 3H20 = H2Se03 + 4Hf + 4e 2Br- = Br2(aq.) + 2e

H2S03 + H20 = SO4-- + 4Hf + 2e

Eo = -1.087

Eo = -0.20 EO = -0.740

indicated that in a mixture of sulfurous acid and selenium, the sulfurous acid should be oxidized by the bromate before the selenium reacts. This was confirmed by preliminary ex- periments, which also showed that if most of the bromate re- quired for the combined oxidation of the sulfurous acid and selenium present is added to the solution before acidification, the residual selenium can be kept in the colloidal condition and titrated to an end point with more bromate. Such a procedure precludes the loss of sulfurous acid as sulfur dioxide f i om the etrongly acid solution necessary for the oxidation of colloidal selenium by bromate.

The selenious acid in the resulting solution can then be ti- trated with standard thiosulfate and iodine by the method of Korris and Fay (5), as modified by Coleman and 11cCrosky (1). From the amount of selenium found by the latter titra- tion the equivalent volume of standard bromate solution is

calculated, and this is subtracted from the total amount of bromate consumed, giving the amount of bromate consumed in the oxidation of the sulfurous acid.

The two titrations on which the method has been based have been shonn to be applicable to the determination of as little as 0.1 mg. of selenium by the use of sufficiently dilute standard solutions (1, 2 , 3, 5 ) . However, the method given here will obviously be limited to amounts of selenium which will not require inconvenient and impractically large volumes of standard bromate solution to oxidize the sulfite content of the sample. For example, using a 50-ml. buret and a 25-ml. sample, the sulfite concentration will be limited to approxi- mately 0.009 molar (0.0356 gram of potassium sulfite per 25 ml.) in order to titrate 1 mg. or less of selenium with 0.01 N Ixomate solution.

TABLE I. REAGEXTS CONSUMED IN TITRATIONS OF A SULFITE- SELESIUM SOLUTION

Titration 0:1008 N KBr03 Init ial Iz Back- 0.1080 h‘ Saz- No. Init ial Tota l NazSzOa Titration 6103 Consumed

MI. M1. Ml. M1. A i l . 1 2 3 4 5 5 A 9

hv.

4 6 . 6 4 5 . 6 4 5 . 6 4 5 . 6 4 6 . 6 4 5 . 6 4 3 . 6 4 4 . 6 4 4 . 6

4 6 . 6 5 46 .66 4 6 . 6 4 4 6 . 6 5 46 .66 4 6 . 6 4 4 6 . 6 5 4G. 69a 4 6 . 6 5 46 .65

24 .60 2 2 , 5 0 2 2 , 4 9 22 .50 2 2 . 5 0 22 .50 22 .51 2 2 . 4 9 22 .49

3.10 0.79 0 . 8 2 0.79 0 . 7 9 0 . 7 8 0.80 0 . 7 6 0.81

21 S3a 21 80 21.76 21 80 21 80 21 80 21 80 21 81 21 77 21 79

a Omitted from averages.

For the titration of the residual colloidal selenium, an electron beam sectrometer, developed by Sullivan and Smith ( B ) , was used to determine the end point. This instrument allons a much more rapid approach to the end point and elimi- nates the use of carbon tetrachloride and iodine monochloride for this purpose as recommended by Coleman and AlcCrosky ( 1 ) . Briefly, the instrument consists of a potentiometric titrimeter in which a cathode ray tube (6E5) of the type used in the 1-isual tuning of radios is substituted for the milliam- meter commonly used in such circuits. By means of a volt- age amplification of 100, potential jumps of 100 to 200 milli- volts cause the instantaneous and full opening or closing of the cathode ray tube shadow (“sectrometer shadow”). The electrodes used in conjunction with this titrimeter were the self-polarizing bimetallic pair, platinum and tungsten. The chief advantage of the electron beam sectrometer is its ex-