ISDCSTRIrZL AlYD ENGILVEERIA'G CHEiUISTRY
April 15, 1930
181
Acid Numbers of Dark-Colored Resins' H a y w a r d H. C o b u r n EXPERIMEST.4L ST.4TIOS,
I
HERCULES POWDER
1; THE paint, varnish, and lacquer industries and in
other industries where natural or artificial resins are used, the chemist is frequently called upon to determine the acid number of these resins. Kormally this is accomplished b y titration of a n alcohol or benzene-alcohol solution of the resin with standard alkali, phenolphthalein being used as the indicator. However, when the alcohol or benzenealcohol solution is dark-colored, as is frequently the case, the phenolphthalein end point is so masked that accurate titration becomes very difficult. This inexactitude of the ordinary method and the inherent difficulties of the well-known electrometric titration led the writer to investigate the Albert method ( 1 , 2 ) . This method consists in titrating in a two-phase medium a benzene-alcohol layer and a n aqueous layer. I n order to improve the sharpness of the separation, the Ion-er layer is saturated with sodium chloride. The color bodies which would mask the end point remain in the upper layer, while the indicator color appears in the aqueous layer, and its change is consequently clear and sharp. The procedure used by the writer contains no change in the principle of the method, but some improvements have been made in the operating technic. Approximately 1 to 1.5 grams of rosin are dissolved in 25 cc. of a neutral mixture of 2 parts benzene and 1 part alcohol. When solution has taken place, 25 cc. of a neutral saturated sodium chloride solution and several grams of solid sodium chloride are added. The solid sodium chloride prevents the dilution of the salt solution by the water added with the alkali and acid during the titration. Phenolphthalein is added and the solution is titrated with 0.5 AT sodium hydroxide Kith vigorous shaking until the lower (aqueous) layer turns red and further shaking does not cause 1
Received November 16, 1929.
COMPANY, K E N V I L ,
N. J.
this color to disappear. A considerable excess of reagent will cause no inconvenience. The solution is now titrated with 0.5 N sulfuric acid until the red color of the lower layer just disappears. I n this titration, since the excess sodium hydroxide in the aqueous layer is being titrrtted, the flask should be given a rotary swirling motion, but it is not necessary to shake so vigorously as t o break u p the two layers. If, after the addition of the sodium hydroxide, the separation into two layers is not sharp, neutral alcohol may be added to the solution. Results
The acid numbers obtained by this method on a number of different grades of rosin and those obtained from determinations made on the same samples by direct titration with alkali are given below. S.4MPLE
A
SAMPLE B
SAMPLE DIRECT TITRATION
149.1
118.7 106.4 118 5
150.1
c
112.8 112.5
SAMPLE D 184.0
iOS. S
MODIFIED A L B E R T MISTHOD
149.0 149.0
103.6 103,6 103.0
113.7 1.13.5
183.7
Acknowledgment
The writer is indebted to Jean Piccard for his assistance in the preparation of the article, and to the Chemische Fabriken Dr. K u r t Albert, G. m. b. II., Wiesbaden-Biebrich, Germany, for their kindness in sending him a copy of their Albertschrift Nr. 15. L i t e r a t u r e Cited (1) Chemische Fabriken Dr. Kurt A l b e r t , Alberlschriff, Nr. 15, p. 5 5 , (2) Stock, Favben-Zfg., 34, 1727 (1929).
Duplex Weighing Tube' W. E. Eskew and F r a n k C. V i l b r a n d t UNIVERSITY OF N O R T H CAROLINA,
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