Determination of Acetyl Number of Fats and Oils Modification of the Method of Roberts and Schuette KENNETH HELRICH AND WILLIAM RIEM4N 111 School of Chemistry, Rutgers University, New Brunswick, N. J . hydroxide solution, much tedious and time-consuming operation is avoided. Reduction of the total titration to a volume under 50 ml. makes it unnecessary to fill the buret twice or to use a special, large buret. Addition of part of the hydroxide before boiling the anhydride in the flask has also been eliminated. Removal and shaking of the sealed tubes after they have been in the oven 10 minutes have been omitted from the procedure. This not only saves 10 minutes in the heating time, but also Aiminates the possibility of a decrease in oven temperature when the tubes are removed and replaced.
HE method of Roberts and Schuette ( 6 ) for determining the Tacetyl number of fats and oils is by far the simplest and soundest in principle. However, it can be simplified by changes in the manipulative technique and by allowing greater latitude in the reagent specifications, without sacrificing its advantages. REAGENTS
Sodium hydroxide, 0.50 * 0.05 N (carbonate-free), standardized with potassium acid phthalate. Acetic anhydride, redistilled. Phenolphthalein indicator, 1%. PROCEDURE
Table 1. Acetyl Numbers
Prepare reaction tubes by drawing out a 30-cm. length of 12mni. glass tubing until the middle is const,ricted t o about 6 mm., then cut the tubing into two halves and seal the large ends. Weigh an emptjytube, add 1.0 to 1.2 grams of acetic anhydride from a pipet, and reweigh. Similarly add 2.8 to 3.2 grams of sample and weigh again. Seal the h b e , shake, and then heat for 1 hour in a vertical position in an oven a t 180" * 5'. Cool and break t,he t,ube, and pour the contents into a 500-ml. Erlenmeyer flask containing 50 ml. of water. Rinse the tube first with hot and then with cold water until the volume in the flask is about 200 ml. Bring the mixture just to a boil under a reflux condenser, let it cool for 5 minutes, and then rinse the condenser. Cool, and titrate with approximately 0.5 S sodium hydroxide, using 10 drops of.phenolphthalein. Run blanks on the acetic anhydride in order to standardize it. The rocedure is the same as for the unknown, except that no sampfe is added and the tubes are not heated The heating is omitted because decomposition occurs a t 180" ii the anhydride is heated alone, whereas it apparently does not occur if fat or oil is also present. Standardization is necessary only once a month if the acetic anhydride is stored in a dark, glass-stoppered bottle. If the sample contains free fatty acid a correction must be applied. Weigh 3.0 * 0.5 grams of sample into a 500-ml. Erlenmeyer flask, add 200 ml. of water, and continue as above.
Oil or Fat Olive oil Cottonseed oil
Castor oil
Coconut oil Cocoa butter Rapeseed oil
Neat's-foot oil Lard
Cocking-Andre-Cook Method
9.9 12.5 13.1 9.8
147.6 147.9 148.7 148.1 3.4 3.4 6.6 7.2 5.9 6.2 20.3 20.4 3.5 2.3
Proposed Method
11.3 11.6 12.9 12.1 12.6 147.7 146.6 2.9 6.2 8.1 9.5 6,O
8.6 21.4 20.0 5.5 5.5
Sodium hydroxide, substituted for potassium hydroxide, has the advantages of being more economical and more easily prepared, carbonate-free, from oil lye. The quantity of the sample does not vary with the hydroxyl number, so that the analyst does not have to know the nature of the oil under investigation. The use of carbon dioxide-free water throughout the procedure has been omitted, since the error caused by the quantity of carbon dioxide in good distilled water is negligible.
EXPERIMENTAL RESULTS AND DISCUSSION
The accuracy of the method was tested by comparing it with results obtained by the method of Cocking (9), Andre ( I ) , and Cook (3). Eight fats and oils were run by both methods with the results given in Table I. Each result listed under the Cocking-Andr6-Cook method represents an independent acetylation. When the original method of Roberts and Schuette was applied to cottonseed and castor oils, the mean results were 11.2 and 147.5,respectively. Results by the proposed method vere slightly higher than by the Cocking method. Other investigators have found a very similar relationship when the original method of Roberts and Schuette was compared with the Cocking method (4, 5 ) . The proposed method differs from the original method of Roberts and Schuette in the following respects. By the allowance of more liberal ranges in the weights of the acetic anhydride and the samples and in the normality of the
LITERATURE CITED
And&, E., Compt. rend., 172, 984 (1921); BdE. s o t . chim., [4] 29, 745 (1921). Cocking, T. T., Chemist and Druggist, 74, 87 (1913). Cook, L. W., J . Am. Chem. Soc., 44, 392 (1922). Roberts, W. L., J . Assoc. Oficial Agr. Chem., 18, 434 (1935). Ibid.,19, 420 (1936). Roberts, W. L., and Schuette, H. A., ISD. EBG. CHEM.,ANAL. ED., 4, 257 (1932).
691
