Determination of Iodine Numbers I)OKOT[IY .J. lfISCOX, Dorninion Department of .dgriculture, O t t a w a , Canada licate determinations were made on each sample by the tlvo methods.
HE iodine iiunibcr of oils and fats has long been uscd a> :i Tmeasure of their unsat'uration. The Wijs method, romnionlj used, requires 30 minutes' absorption time. In 1939 the use of mercuric acetate to speed the reaction, introduced by Hoffman and Green ( g ) , cut the absorption time to 3 minutes. Later Norris and Buswell ( 3 ) investigated the use of mercuric acetate with Hanus solution; they used a reaction time of 3 to 5 minutei. In all the investigations of t.he use of mercuric acetate eniphasis has been placed on the t.ime factor. However, when hundlrds of determinations have to be made, the volume of the solutions used also becomes a major factor. Sorris and Buswell ( 8 ) h a w shown in the case of t'ung oil that when mercuric acetate is u s d the Wijs values vary only insignificantly in the reagent exr c w range of 30 to 225%. If the action of mercuric acetate is intieprndent of the presence of a large excess of iodine, it should be possible t'o reduce the volume of Wijs solution from that normally used. Further, as the accelerated reaction leaves littlc time for the escape of halogens from thc solution, stnppwecl flaqlis niiglrt not ht, riecwsary.
Because of the presence of ricinoleic acid, mercuric acetate rannot be used to determine the iodine number of castor oil (3,6). The iodine number of a sample of this oil was 85.3 by the .l.O.A.C. method and 91.4 by the new method. However, a w n p l e of sulfonated castor oil 50% neutral had an iodine numtwr of 30.1 by the A.O.A.C. and 29.5 by the new method.
Table I .
Iodine Sunibers of Oils, Fats, and Fatt) 4cids
(Each value i, mean of three determinations) Sample Iodine Sumberc and ___ i.O,.I.C. S e w method Ilitference Variety Crop T e a r Flaxseed 170,8 170.1 Royal 1945 to 7 -0.7 182.7 183.4 Redwing 1945 183.3 183.1 +0.2 Dakota 1945 170.3 -1.8 168.5 Bison 1945 -0.2 187.1 187.3 Crystal 194.5 182.8 Dominion 1945 182.3 +0.5 184.1 -1.1 183.0 Royal 1946 -1.1 191.2 192,3 Redwing 1946 190.9 190.7 +0.2 Dakota 1946 -1.4 180.2 181.6 Bison 1946 195.5 -0.5 195.0 Crystal 1946 186.9 -0.3 186.6 Dominion 1946 200.3 201 .o -0.7 Gossamer 1946 Norfolk Queen 1946 193.0 +0.7 192.3 -1.5 186.8 185.3 Custera 1946 189.5 190.4 +0.9 Vie toryn 1946 192.8 192.8 0.0 Viking" 1946 -0.6 180.7 181.3 Noveltya 1946 Soybeans Early Blackeye 1944 126.9 127,9 -1.0 134.5 -0.4 134.9 Goldsoy 1944 135.4 -1.3 Manitoba Brown 1944 136.7 133.0 133.2 -0.2 Pagoda 1945 131.7 132.6 -0.9 Kabqtt 1945 AIanitoba Brown 1945 -1.8 135.4 137.2 Mandarin 130.7 131.3 1945 -0.c La Peslon 132.4 130.9 I945 t1.J AI-229 130.8 128.3 1945 +2.5 Lincoln(" 137.5 -0.4 137.1 1946 1Iandarin" 128,3 -1.8 130.1 1946 Richland0 126.1 127.9 -1.8 1946 Harman" 134.2 134.3 1946 -0.1 Le Platon 138.1 137.0 1946 +1.1 Kabott 141.2 139.8 1946 +1.4 141.3 1946 141.2 -0.1 Pagoda Sunfiower Sunrise 1944 127.7 129.8 -2.1 134.5 3975 1944 132.4 +2.1 3977 1944 125.6 125.1 -0.4 3978 1944 128.8 130.3 +1.5 Sunrise" 1945 132.5 131.9 -0.6 Mennonite" 1945 135.7 136.0 +0.3 Sunrise 1946 138.2 136.5 -1., Safflower C D 3477" 1945 155.2 154.5 +0.7 1945 151.6 151.7 Type 6" -0.1 Pusa 2" 1945 135.3 133.4 +1.9 Simlaa 1945 147.9 150.3 -2.4 149.2 Karron" 1945 150.9 +1.7 Miscellaneous Peanut oil 102.5 103.2 Commercial -0.1 Corn oil 123.6 121.9 Commercial f1.7 Palm oil 8.5 6.9 Commercial +1.6 Olive oil 39.7 39.4 Commercial +0.3 42.3 44.6 Seal oil Commercial -2.3 93.6 92.1 Rapeseed oil Commercial t1.5 112.5 109.1 Croton oil Commercial +3.4 Cottonseed oil Commercial 106,6 103.2 +3.4 Oleic acid 52.3 53.1 Commercial -0.8 Palmitic acid 54.6 53.1 Commercial +1.5 Stearic acid 4.0 4.9 Commercial -0.9 Butter 35.8 36.1 Commercial -0.3 Lard 55.0 53,5 Commercial +1.5 Lamb f a t Con~rnercial 42.6 42.8 f0.2 Total 8018.3 8017.9 +0.4 Mean 133,6 133.6 0 . Oil extracted o n Goldfisch apparatus with i.'etroleum ether a3 soi\ent. ~~
s;zr 3
0
1%
SPSFLOVER
1 3
SOYBEAN
40
eo
i z 0 1 6 0 2 m a o d ~ 3 2 0 3 6 0
PER CENT WlJS SOLUTICN ABOVE THEORETICAL
F i g u r e 1.
Effect of \-olume of Wijs Solution Iodine Numbers
.
011
With these points in mind the folloxing method \vas developed: Approximately 0.1 gram of the oil or fat was weighed and dissolved in 10 ml. of chloroform in a 250-nil. Erlenmeyer flask. Ten milliliters of Wijs were added, followed by 5 ml. of 2.5% mercuric acetate in glacial acetic acid solution. After 5 minutes ( 6 minutes for oil from flaxseed) 5 nil. of 15% potassium iodide and 25 ml. of water were added. The excess iodine was titrated immediately with 0.1 N sodium thiosulfate using starch as an indicator. This method was compared n i t h the official Wijs method of the A.O.A.C. (1) on a large number of samples. For this investigation W j s solution was prepared in 18- to 20-liter quantities and kept in a stoppered bottle in the dark. Smaller volumes of 3 to 5 liters were transferred from this stock to a paper-wrapped bottle as required. It has been found in this laboratory over a period of 4 years that Wjs solution need not be prepared every 30 days as recommended by the A.O.A.C. Allowing for small variations due to temperature, the titer of blanks remained the same for 6 to 8 months. Norris and Buswell (4)found that Wijs solution could be kept well over a year. Samples of oil-bearing seeds, chosen a t random from groups grown across Canada, and commercial oils and fats were used in the investigation. The oil was pressed from the seeds on a Carver laboratory press and the iodine numbers m-ere determined the same day. From a few samples the oil was extracted on the Goldfisch apparatus using petroleum ether as the solvent. Trip-
679
680
ANALYTICAL CHEMISTRY
The results of the iodine number determinat,ions are given in Table I, which lists the means of the triplicate iodine numbers determined by the two methods and the differences between them. There is no difference between the means of the methods. I n s3me individual samples the difference is relatively large but a st'atistical analysis of all the results showed t,hat these differences are not significant. The reduction in the volume of Wijs solution did not, affect the accuracy of the determinat,ion to any significant extent. The st,andard error of a single mean, based on triplicate determinations, was calculated for each type of oil. I t was the same by both methods for soybean ( *0.7), saffloiver ( + 0 . 7 ) , and t.he miscellaneous group ( *0.5). For flaxseed by the .4.O.A.C. method it was 1 0 . 9 and by the modified mercuric acetate method *0.8, whereas for sunflower it Tvas *0.7 and * 0.3, respectively. With the volume of Wijs solution reduced to 10 ml. it seemed advisable to determine how far t,his vias above the theoret,ical requirements and how close to the theoretical the volume could be brought without affecting the iodine number. For this purpose composite samples of freshly pressed oil from soybeans, safflower, and flaxseed were used. The iodine number was determined in triplicate on 0,100-gram samples, using varying amounts of Ki js solution. Five-minute absorption periods were used except for oil from flaxseed, where 6 minutes were allowed. From the values obt,ained using 25 ml. of Kijs solution the theoretical volume required for 0.100 gram of each oil was calculated. The results of this experiment are presented graphically in Figure 1. The equations for the lines were calculated from the first five values for flaxseed, six for soybeans, and all for the safflower. The lines have been extended in the graph only as far as these values. The position of 10 ml. of V-ijs solution is indicated on each line by the vert,ical broken line marked X . The line extends well beyond this point in each case, so 10 ml. of Kijs are more than sufficient for the satisfactory determination of iodine numbers. The regression coefficient was calculated in each case and was insignificant,. The lines, for all practical purposes, are parallel to the axis. Inspection of Figure 1 shows that for iodine
numbers of 200 no excess of Kijs solution is present. Consequently when only 10 nil. are used, the weight of sample must be reduced i f the iodine number is over 200. In all but the safflower samples the value of the iodine number decreases when the volume of TTijs solution closely approaches the theoretical volume. Thiq decrease may be partly compensated for by an increased time interval. Khen the smallest volume of Wijs solution 11-as used in the experiment n i t h Aaxseed and the time was increased to 10 minutes, the value of the iodine number \$as increased from 181.5 to 183.7. K i t h soybeans. the increase was from 127.1 to 128.2. SUMMARY AND COR-CLUSIONS
The mercuric acetate method of determining iodine numbers was modified by reduring the volume of Kijs solution and using unstoppered flasks. Comparison n-ith the official Kijs method of the Association of Official Aigricultural Chemists on a large number of samples shon-ed no significant difference between the results obtained. The effect of decreasing amounts of Kijs solution on the value of iodine numbers !vas investigated. K i t h nonconjugated fat's a,nd oils 10 ml. of Wijs solution are sufficient for 0.1-gram samplrs, provided the iodine value is not more than 200. ACKhOWLEDGBIENT
Alcknowledgmentis made to the Adanis Chemical Company for the supply of sulfonated castor oil. LITERATURE CITED
(1) Assoc. Official h g r . Chem., Official and Tentative Methods of .Inalysis, p. 495 (1945). (2) Hoffman, H. D., and Green, C. E., Oil and Soap. 16, 236 (1939). and Buswell, R. J., IKD. ENG.CHEM?., A N ~ LED., . (3) N o r m , F. Ai.. 15, 258 (1943). (4) Ihid., 16, 417 (1914). (5) Skell, P. S., and Kadlove, S. B., Ihid., 18, 67 (1946). RECEIVED September 17, 1947. Contributlon 139, Division of Chemistry, Science Service, Dominion Department of Agriculture.
New pH Indicator for Titration of Sodium Carbonate Disodium 4,4'-Bis(d-amino-l-naphthyla~o)-2,2'-stilbenedisulfonate JIICH-IEL TAKhS, D e p a r t m e n t of W a t e r Supply, Detroit, Jllich.
'--'
HE dye resuking from t.he coupling of 1 mole of 4,4'-diTaminostilbene-2,2'-disuiionic acid with 2 moles of @-naphthylamine is assigned the formula by Schultz ( 4 ) . hccording to this SO3H
"2
c> I
I
X=N
XH1
SOjH
I
CH=CH
(_7>
'
N=S
p H of 3.8, t,he indicator lends itself advantageously to the titration of 0.2 S and 0.5 S sodium carbonat'e solutions on the one hand, or alternatively, to t'he direct titration of proportiorlate amounts of t,he solid salt. INDICATOR PROPERTIES
a
authority, the dye is known in the color trade as Hessian Purple N extra and Direct Purple. A study of the dye's properties discloses that it may function profitably as a tit'rimetric indicator in the p H region near 4.0. The defects of methyl orange in&cat,or in the acid titration of crtrbonat,es have long been recognized. UP to the present, the remedy has consisted principally in modifying the indicator through the addition of inert dyes like cyanole xylene FF ( 2 ) . Occasionally, bromophenol blue has been substituted for methyl orange in this tit,ration (6). More recently, modified methyl yellow indicator ( I ) has been proposed for this titration. Each indicator Dossesses Deculiar merits and. handled discriminatingly. -_ .provides advantages over the older methyl orange dye. Inasmuch as the sharpest color change of disodium 4,4'-bis(2-amino-l-naphthylaeo)-2,2'-stilbenedisulfonateoccurs a t a
The dye was prepared by the tetraazotization of 9.5 grams (0.025 M ) of 4,4'-diaminostilbene2,2'-disulfonicacid, Eastman Kodak T4614, and coupling with 8.0 grams (0.06 M) of @-naphthylamine,Eastman Kodak 174 (5). The amine was first dissolved in 50 ml. of glacial acetic acid and distilled water was added with stirring to a 100-ml. volume. Coupling was accomplished in this 50% acetic acid solution. The disodium salt was formed by grinding a weighed quantity of the dye in a mortar with the calculated volume of 0.05 Av SOdium hydroxide and diluting to the proper concentration. Two concentrations of the dye were tested for indicator efficiency: 0.5 and 0.1%. Both solutions have a deep red color. Two or 3 drops of the 0.5% solution suffice for every 50-ml. volume titrated. In the case of the 0.1% solution about 10 drops are
n e ~ ~ ~ ' , ' ~ ~ ~ h a~sirupy , ~ ~consistency, ~ ~ ~ ~presenting ~ s e sa minor problem of handling, On this account, most of the titrations reported in this paper were performed with the 0.1% sohtion. The color characteristics of the indicator were tested using sodium dibasic phosphate-citric acid buffers ( 3 ) . These buffer solutions shoxved the alkaline color of the indicator to be a delicate
