VII. Determination of Acid Number
Ha r o l d We i n b e r g e r a n d Wm. Ho w l e t t Ga r d n e r, The Polytechnic Institute of Brooklyn, Brooklyn, N . Y . for unadulterated materials are not true constants, but repre which have been accurately developed for a specific substance have been generally applied to other materials without a com
plete study of the new applicability. The literature is filled with conflicting data (6, 85), which may in part be accounted for by the use of different methods.
Me t h o d s f o r Ac i d Nu m b e r s
In the determination of acid numbers a wide variety of methods and various modifications have been employed. In some cases a method has been developed to meet the peculiar characteristics of a particular substance, but even here there has been a tendency to make the method general. Since many of the methods used for resins were first developed for fats, oils, and waxes, it is desirable when considering methods for acid numbers of resins to include these latter substances.
The high coloration of the extracts of some of the sub
stances, notably orange shellac, and the peculiar solubilities of other materials account to a large extent for the multi
plicity of methods which have been developed (6). Even when employing the original method, which consists in titrat
ing an alcoholic extract with alkali direct, numerous sets of directions are given.
Dieterich, who has made an extensive study of resins, recom
mends for storax (7) that a 1-gram sample be dissolved in 100 cc.
of cold 9G per cent alcohol and titrated with 0.5 N alcoholicpo- tassium hydroxide, using phenolphthalein as indicator. This method is v eiy similar to the standard method adopted by the American Societv for Testing Materials for linseed oil (2). Rich
mond, on the other hand, recommends the use of a solution of 1 aqueous alkali and states that at least 2 cc. of a 1 per cent solution of phenolphthalein should be used. Gardner and Coleman (10, mixture, refluxing for 30 minutes, and allowing to cool before titrat
ing with 0.1 N sodium hydroxide.
Steel and Sward (29) have used this alcohol-benzene mixture for vege
table oils. K e t t l e (18), on the other hand, has employed a mix
ture of 90 per cent of benzene with 10 per c e n t of a lc o h o l for oils.
Singh (27) dissolves 1 gram of shellac in 100 cc. of boiling alcohol, refluxing for 5 minutes. He filters and cools before t i t r a t i n g with standard aqueous potash, using phenolnhthalein as i n d i c a t o r . Parry (22) claims that the presence of lae dye in some samples obscures th e end p o in t w it h p h e n o l phthalein. Kranz and Majrich (/4)find that a-naphtholphthalein, alkali blue, and phenolphthalein are the most practical indicators for resins when titrated with 0.1 N potassium hydroxide. For shellac Nagel and Körnchen (19) recommend alkali blue 6R, using alcohol solutions. Whitmore, Weinberger, and Gardner (32) use thymol blue.
To overcome the inaccuracies caused by high coloration of solutions, Dieterich (6) has developed a method where the resin is dissolved in a standard amount of alkali and back-titrates with standard acid. The alkali serves both to neutralize the resin and as a solvent. The method cannot be used with substances which contain esters which are readily saponified. For dammar and sandarac (5) he dissolves 1 gram in 20 cc. of 0.5 N alcoholic potassium hydroxide, adds 50 cc. of benzene, and after standing for 24 horn's titrates the excess with 0.5 N sulfuric acid. For ammoniacum a preliminary refluxing with water and alcohol for 15 minutes is recommended (9) and part of the filtrate is treated with 0.5 N alcoholic alkali. In this case only 5 minutes’
contact is required before titrating with acid.
For similar reasons potentiometric methods have been em
ployed. Kremann and Muss (15, 16) titrated fatty acids dis
solved in alcohol with 0.5 N alcoholic sodium hydroxide in this manner; Seitz and McKinney (26) fatty acids and lubricating oils. Gardner and Whitmore (11) studied the applicability of the method to resins, including shellac; and Caldwell and Mat- tiello (S) linseed oil and its fatty acids. The latter authors found they could use successfully not only 95 per cent alcohol, but also butyl alcohol, and mixtures of equal volumes of alcohol and ben
zene as solvents. These methods are unquestionably the most accurate that can be employed, since they are totally free from inaccuracy in judging the end point of the indicators or due to color of the solution. They are not as convenient as the other methods, however, since the electrodes have to be carefully pre
pared (8, 11). For investigation purposes they are unexcelled, since the titration curves give at the same time considerable in
formation concerning the nature of the material being titrated.
By comparing results obtained by other methods with those ob
tained by the potentiometer, considerable improvement in tech
nic and means of judging end points can be effected.
A fourth type of method for acid number is typified by the giving a clear end point, since the phenolphthalein is retained in the aqueous layer while the resin and its salts remain in the upper organic solvents. The same end point may be obtained several
cations. This is particu larly true when studying shellac varnishes.
A N A L Y T I C A L E D I T I O N Vol. 5, No. 4 acid. Stock, who modified the original method to a slight extent,
determined values for a variety of resins (SO). It is clear from his value for shellac that he was not dealing with a usual sample.
Cobum (4) with similar modification used this method for dark- colored samples of rosin. His values agree better than Stock’s with these obtained by the usual method of titrating spirit solu
tions directly with alcoholic alkali.
It can be seen from this review that many of the methods which have been used differ widely both in technic and in principle. In only a few cases have attempts been made to compare results obtained by more than one or two methods, so that we are left in serious doubt when attempting to estab
lish limits of variation for many substances, especially by consulting the literature for possible climatic variations over long periods of time. The adoption of standard methods and systematic recording of results in the literature are highly de
sirable. They have been accomplished to some degree in the case of some oils (S'), but should be rapidly extended to include many other substances.
Ac i d Nu m b e r s f o r Sh e l l a c
The values given for shellac do not vary as widely as do many of those given for other substances, but in some in
stances the influence of method is obvious. Parker (21) ob
tained values of 60 and 66 for button and flake shellac; Wil
liams (33) 47.6 to 64 for the same varieties and garnet. His widest variation occurs with the flaked variety. Kremel (17) obtained 63.5 for a yellow lac; Schmidt and Erban (6) values from 60 to 65; and Dieterich 59 to 63. Singh (27) used aqueous alkali for titration and obtained values of 60 to 65.
His values are the only example of shellac derived from a re
corded sourcc. On drying or melting, he found that the values were reduced to 52 to 60. Umney (31) obtained values of 53 to 78 for all grades of shellac, and a value of 44 for stick- lac. Coffignier (6), using the conventional method, found 60 for shellac and 35 for stick-lac. Rudling (25) obtained 53 to 59 for seed-lac and button-lac, Parry (22) 55 to 65, and Ulzer and Defris (6) 65.4. Nagel and Körnchen (19) stated that the acid numbers of all grades range from 60 to 70 and for rosin-free samples 60 to 65. Gardner and Whitmore (11) showed values of 67.5 to 73; and Whitmore, Weinberger, and Gardner (32) 70 to 75. For bleached shellac Kremel (17) obtained 73.7, Coffignier (6) 81, Gardner and Whitmore (11) 85.7, and Whitmore, Weinberger, and Gardner (32) 93.6.
For refined wax-free bleached shellac the latter authors ob
tained 107.5 to 117.6.
The general uniformity of results ranging between 60 and 65 may be due to the common use of alcohol solutions and phenolphthalein as indicator. The values which are lower than 60 almost invariably occur for the poorer grades of shellac and may in part be accounted for by difficulty in judging the end point, which is masked by the deep wine color of the alkali salt of the yellow dye erythrolaccoin present in the orange grades. It is also possible that in these grades the acid value has been reduced, as Singh (27) has shown, by excessive heat or drying. Since the resin content is given in only a few cases, it is difficult to judge what the true value should be (32). The higher range of values of Gardner and Whitmore is unquestionably due to the more accurate method employed in judging the end point.
Di r e c t Me t h o d f o r De t e r m i n i n g Ac i d Nu m b e r s o f Sh e l l a c
It had been the previous experience in this laboratory that by following the potentiometric titration with outside indi
cators, for an accurate end point with phenolphthalein a fairly distinct pink must be taken. Because of the difficulty in judging such an end point, especially in the presence of the aforementioned dark purple color, the authors have substi
tuted thymol blue as the indicator. Although the
potentio-metric titration curves show a sharp inflection for all grades of shellac, nevertheless even with thymol blue there is a slight range in color change from yellow through green to blue at the end point. From a comparison with fatty acids of similar acid strength it might be expected that both these indicators would show a rapid rate of color change. What is sometimes overlooked in change of solvents in a determination is that the indicator range may also be changed and not in the same proportion. It would appear that the use of 95 per cent alcohol depresses the range of alkalinity of the two indicators. B y taking as the end point the first indication of a permanent blue with thymol blue when using two to three drops of a 0.04 per cent alcohol solution of the indicator on a spot plate with one to two drops of the solution, the authors have been able to obtain values which closely approach those obtained with the potentiometric method. This would ac
count for the relatively higher results of Whitmore, Wein
berger, and Gardner (32).
The exact method employed consists of the following:
Dissolve 5.000 grams of shellac, which has been carefully rolled, in 50 cc. of neutral 95 per cent ethyl alcohol. Titrate with stand
ard 0.5 N alcoholic potassium hydroxide, using thymol blue (thymosulfonphthalein 0.04 per cent in 95 per cent alcohol) as outside indicator. A porcelain spot plate is convenient for this titration. The end point is taken as that point where one to two drops of the solution on a glass rod gives the first temporary blue coloration to the indicator. The alcoholic alkali should be stand
ardized against aqueous acid each time before using to correct for any error in change in density due to fluctuations in temperature.
Identical results are obtained by this method when 50 per cent of benzene is added to the solution before titration.
Since all the acid constituents of shellac are spirit-soluble, it is unnecessary to use any other solvent. At least 5 grams should be taken as a sample in order to obtain uniformity of material for duplicate determinations. Proper rolling of sample is very important. As can be seen from Table I, excellent checks are obtained with duplicate samples.
Ta b l e I . In f l u e n c e o f Am o u n to f Be n z e n e Us e di n Al b e r t Me t h o d
Sample: T. N . orange shellac Ac i d Nu m b e r
Ra t i o Weinberger-Gardner
C«H*:CjHiOH Albert method direct method
3:1 6 9 .5 7 6 .1
3:1 6 9 .0
2:1 6 7 .2 7 6 .0
2:1 6 7 .2
1:1 6 6 .6 7 6 .0
1:1 6 6 .8 7 5 .9
1:1 6 6 .0
1:1 6 6 .5
1:1 66 .4
0 :1 . . 7 6 .0
In d i r e c t Me t h o d
Because of the somewhat greater ease in judging the end point of thymol blue when titrating with acid, an attempt was made to determine the acid number by the indirect method:
Dissolve 5 grams of shellac in 25 cc. of neutral alcohol, add 25 cc. of 1.0 N alcoholic potassium hydroxide, and titrate the solu
tion immediately with 0.5 N alcoholic sulfuric acid.
As can be seen from Table II, this method gives high re
sults. For confirmation that saponification takes place in such a rapid manner one sample was «allowed 10 minutes’
contact with the alkali before titrating. This raised the acid number by 20, and leaves no doubt as to the inapplicability of these methods to shellac.
T a b l e I I . P r e s e n c e o f S a p o n i f i c a t i o n i n I n d i r e c t M e t h o d Sample: Superfine (composite) orange shellac
Di r e c t Me t h o d In d i r e c t Me t h o d
7 0 .2 7 7 .2
7 0 .2 7 5 .6
9 0 .7a
® Titrated after standing for 10 minutes.
July 15, 1933 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 269
The determination was conducted in the following manner:
Dissolve 5.0000 grams of shellac in 100 cc. of a mixture of two parts of benzene and one part of alcohol. When solution has taken place, add 100 cc. of neutral saturated sodium chloride solution, with several grams of the solid salt and a few drops of phenolphthalein solution. Titrate the solution with 0.5 N acjueous sodium hydroxide with careful but thorough agitation, (lo o vigorous agitation will cause the shellac to become wetted with water and adhere to the sides of the flask. A swirling mo
tion was found to be the best.) Add hydroxide until the aqueous layer has a definite red color and then titrate with 0.5 N sulfuric acid until the pink just disappears. Repeat the addition of alkali and back-titration with acid until two or more consecutive read
ings give the same value for acid numbers.
Varying the amount of liquids did not affect the results.
With seed-lac, the red lac dye present prevents the obtaining of an end point with phenolphthalein. Thymol blue and
In seeking an explanation for the results obtained with the Albert method, 5 grams of shellac were dissolved in alcohol and just neutralized with alkali by following the procedure in the direct titration method. Twice the amount of benzene was added to this solution, and then the saturated salt solu
tion. Upon addition of phenolphthalein the aqueous solution was found to be alkaline. Back-titrating with aqueous acid gave a value identical with that originally obtained by the Albert method. From this it was clear that the difference in values was an inherent function of the two methods.
At, first it was thought that the difference might be caused by a difference in range of end point of the indicators in the two different solvents, alcohol and saturated brine solution.
However, the authors were able to show that by adding water to a neutralized alcoholic solution of shellac and boiling off the spirits, an aqueous solution neutral to both thymol blue and phenolphthalein was obtained. Addition of a drop of al
kali to this solution rendered it alkaline to either of these indi
cators. By adding amyl alcohol to the neutral aqueous solu
tion derived in this manner by the direct titration method, a two-liquid layer system is formed and the aqueous layer readily becomes alkaline.
B y using filtered alcoholic solutions of shellac it can be demonstrated that clear aqueous solutions of the potassium salt of shellac can be obtained in this manner similar to those obtained with wax-free bleached shellac (12). Clear solu
tions, however, are obtained only if sufficient alcoholic alkali is added to correspond to the acid numbers obtained by the direct method. Since any free shellac is highly insoluble in water, these experiments would favor the contention that the higher acid numbers of Gardner and co-workers represent the truer values for shellac.
Co n c l u s i o n s
The results obtained by the Albert method might be ex
plained in the following manner: Since it has been shown that shellac behaves as a weak organic acid (11, 12), its potassium salts would have a tendency to hydrolyze in water. Because of the marked insolubility of shellac (acids) in water, this hydrolysis proceeds only to the normal degree in aqueous solution; however, when there is a two-liquid layer system in which shellac is very soluble in the organic layer, any free shellac which is formed as a result of hydrolysis will be removed from the aqueous layer and hydrolysis can acid number of shellac, extreme care should be taken to con
sider some of the peculiarities of this material. Because of its difference in source and nature, methods which may suc
cessfully apply to other resins will not necessarily give ac
curate results with shellac. It would appear from this study that only the group of methods known as the direct methods and the potentiometric methods are open to selection. In developing such a method, consideration should be given to the amount of sample taken for analysis, care of rolling, the neutrality of the alcohol, constancy of titer of alkali, proper selection of indicator, and standardization of method in judging end point.
Since this work was undertaken, the importance of acid numbers for shellac has become recognized in this country.
Further study is in progress upon this subject.
Li t e r a t u r e Ci t e d Gum Resins,” 2d ed., tr. by Stock, Scott, Greenwood, London.
1920.
270
(17) Kremei, A., Notizen z. Priifg. d. A rzneim ittel, p. 33, m. Berück
sich der H erausgabe e. neuen österreichischen Pharm aeopoe, Frick, Vienna, 1889. Bureau of the U. S. Shellac Importers’ Association.