Thiocyanogen Number and Its Application t o Studies on Lard LAMRENCE ZELENYAND C. H. BAILEY,Division of Agricultural Biochemistry, University Farm, St. Paul, Minn. vapor with suction. Pipet 25 ml. of the thiocyanogen solution into each flask. (Caution: Do not use mouth on pipet.) Keep the flasks in a dark place for 17 hours. Add 20 ml. of a 15 per cent potassium iodide solution to each flask and a t once titrate the liberated iodine with standard thiosulfate solution, using starch as an indicator. The thiosulfate solution is actually standardized in terms of iodine rather than in terms of thiocyanogen equivalents. Record the thiocyanogen number in terms of iodine number, as the iodine equivalent of the standard thiosulfate solution is known. Duplicate blanks should be run for each series of determinations
T
HE technic for the quantitative determination of the addition of thiocyanogen to fats and oils was developed by Kaufmann (6). Thiocyanogen in solution behares very much like the halogens; in fact, it is sometimes called a pseudohalogen. I n reactivity i t stands midway between bromine and iodine; it is freed b y bromine from its salts, but i t liberates iodine from iodides. It is immediately decomposed by water, the end products being HSCK, HCY, and HzS04. I n most solvents it polymerizes more or less rapidly; turbidity appears first, followed by the precipitation of a yellow amorphous mass. Thiocyanogen adds only to certain of the double bonds of the unsaturated fatty acids, while iodine and bromine add to all such double bonds. For example, thiocyanogen adds to only one of the two double bonds of linolic acid, while i t adds quantitatively to the one double bond of oleic acid. It follows, then, that for fats containing only these two unsaturated fatty acids the difference between the iodine number and the thiocyanogen number will be a measure of the linolic acid content. Thiocyanogen solution suitable for fat analysis is prepared b y adding bromine and an excess of lead thiocyanate t o anhydrous acetic acid and shaking the mixture until it is decolorized. The lead bromide and excess lead thiocyanate are filtered off through a dry plaited filter, giving a perfectly clear, colorless solution of thiocyanogen. A solution about 0.05 -11 of ( S C Q is most suitable. Kaufmann (5) found that the tendency for thiocyanogen t o polymerize was less in acetic acid solution than in other solvents. It is of the utmost importance, however, that all reagents and glassware used be absolutely dry, as traces of moisture greatly lessen the stability of the solution. Kaufmann (5) prepares the anhydrous acetic acid by distilling glacial acetic acid over phosphorus pentoxide and collecting the distillate boiling a t 118-120’ C. Barbour (1) prepares the acid by refluxing glacial acid with a slight excess of acetic anhydride. Thiocyanogen solutions in acetic acid prepared by Kaufmann’s method seem t o be the more stable. Properly prepared solutions lose less than 1 per cent of their titration value during the first week after preparation.
EXPERIMENTAL RESULTS Thiocyanogen and iodine nuniLers were determined 011 four samples of lard (a normal lard and three consecutive stages of hydrogenation of the same lard). These samples were furnished b y the Research Laboratories of the Institute of American Meat Packers under whose auspices the work was done. The thiocyanogen and iodine numbers were calculated on the basis of the mixed fatty acids. From the data obtained were calculated the percentages of linolic acid, oleic plus iso-oleic acids, total saturated acids, and total unsaturated acids. These percentages are based on the total fatty acids and on the assumption that no acid of higher unsaturation is present (probably not strictly true, as traces of linolenic, arachadonic, and possibly other unsaturated acids are present). As both the iodine number and the thiocyanogen number of oleic and iso-oleic acids are 90, while the iodine number of linolic acid is 180 and the thiocyanogen number is 90, the following calculations may be made: Per cent saturated acids = (90 - thiocyanogen no.) x 100/90 Per cent linolic acid = (iodine no. - thiocyanogen no.) X l00/90
Per cent unsaturated acids = 100 - yo saturated arids Per cent oleic acid = 70unsaturated acids - 70linolic acid TABLEI. CHANGES13 L~NOLIC A N D OTHERACID CONTENT OF LARDWITH PROGRESSIVE HYDROGENATION THIO P.Y A. N O. -
IODIXEG E R DirNO. O F NO. O F FERMIXED MIXED ENCE
EXPERIMENTAL PROCEDURE I n determining thiocyanogen numbers on lard i t was found necessary to modify Kaufmann’s technic in two respects. The lard is washed into the reaction flask with anhydrous ether, the ether being subsequently evaporated off. This leaves the lard in a thin film on the bottom of the flask, thus exposing a large surface to the thiocyanogen solution. This procedure is necessary because lard is relatively insoluble in acetic acid. The reaction is allowed t o proceed for 17 hours, the 5 hours suggested b y Kaufmann being insufficient. The following technic gave satisfactory and consistent results: Prepare an approximately 0.05 J4 thiocyanogen solution according to Kaufmann’s directions, using the utmost care in having all reagents and glassware absolutely dry. Wash aproximately 0.2-gram samples of lard into 250-ml. glass-stoppered Erlenmeyer flasks with 20-ml. portions of anhydrous ether. Evaporate off the ether on a warm sand bath, removing all ether
LARDS.4MPLE
FATTYFATTY(I A C I D S .kCIDS SCN)
Omxr
+
Iso- LINO- UNSATU-SATUO L E I C ~ LIC ACID ACID
% Original, not hydrogenated Partially hydrogenated Hydrogenated more than meceding sample Hydrogenated more than preceding sample 0 From partial
%
49.1 11.8
RATED ACIDS
RATED ACIDS
%
%
60.9
39.1
65.5 65.3 61.4 61.5 56.9 56.9
54.9 54.8 54.8 54.5 54.2 54.1
10.6 6.8
53.1
7.6
60.7
39.3
2.8
57.0
3 1
60.1
39.9
51.0 51.1
50.1 49.9
1.1
54.4
1.2
55.6
44.4
reduction of linolic acid.
The results (Table I) show clearly that during the hydrogenation process one of the two double bonds of linolic acid is reduced almost completely before the reduction of the other double bond or of the double bond of oleic acid. The saturated fatty acids show no appreciable increase until practically all of the linolic acid has been reduced to an isomer of oleic acid. The same thing has been shown b y Barbour ( 1 ) for the hydrogenation of cottonseed oil. Barbour used 13 samples 109
