A Drastic Saponification Method for Difficultly Saponifiable Esters TILLI.S31 E. SH.4EFER
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JEhN PICCSRD,' Hercules Powder C o . , \\ i l n i i t i g t o ~ i ,Del.
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HE problem of quantitatively saponifying esters that are resistant to alkali has been encountered many times. Kossel and Obermdller ( 8 ) , Kossel and Kruger ( 7 ) , Obermuller ( I O ) , and Beythien ( 2 ) all used sodium ethoxide dissolved in a ion--boiling solvent to effect the complete saponification of certain fats and of difficultly saponifiable waxes, such as ceresin. Pardee and Reid (12) and Pardee, Hasche, and Reid (11) used butyl alcohol as a saponification medium and obtained results that were in general a little higher than those obtained Then the saponifications mere carried out in the presence of ethanol. The reagent used by Steet (15) for his saponification 11-ork consisted of a solution of potassium hydrmide in ethylene glycol monoethyl ether. As a saponification reagent for twelve common esters, Redemann and L U C U(13) used a solution of potassium hydroxide in diethylene glycol. They obtained theoretical results by heating their ieaction mixtures a t 120" to 130" C. for only 3 minutes. Kesler, L0x1-y~and Faragher (3,4)saponified ethyl abietate by heating it w t h alcoholic potassium hydroxide a t 140" to 150" C. in a sealed glas3 tulje, getting a saponification d u e of 170.1 instead of the theoretical 169.8. (Throughout this paper the term '.saponification value" is used to denote the number of milligrams of potassium hydroxide required to saponify 1 gram of sample.) Allen, Meharg, and Schmidt (1) saponified certain resins by heating them in an autoclave for sereral hours n i t h 10 or 15 per cent sodium hydroxide, presumably not attempting to get quantitative results. Smith (14) has discussed comprehensively the effect of the use of various solvents in saponification. The writers wished to have available a quantitative method suitable for saponifying such terpene esters as bornyl and fenchyl phthalates as well as esters of abietic acid. One difficulty involved in accomplishing this consists in finding a solvent with a sufficiently high boiling point which does not get discolored by strong alkali, as do most primary alcohols other than methanol. h reagent obtained by dissolving sodium in tertiary butyl alcohol, used in glass apparatus and in accordance with the usual procedure, proved entirely satisfactory for the saponification of bornyl phthalate. I n two evperiments the results on carefully purified dibornyl phthalate were 254.7 and 254.7, respectively, instead of the theoretical value of 255.9. K h e n the usual saponification procedure was applied to this specimen, a saponification value of only 157 \vas obtained. Abietic acid esters are extremely resistant to alkali. Ethyl abietate has a theoretical saponification value of 169.8. When it v-as treated for 1 hour merely n-ith alchoholic potassium hydroxide, the usual procedure, a saponification value of 5 to 10 (3 to 6 per cent of theory) was obtained; when treated for 16 hours with the reagent made by the reaction of sodium n i t h tertiary butyl alcohol, the result was 67 (40 per cent of theory). Smith (14) obtained 1 0 ~saponification ~. values for methyl abietate and ethyl abietate when he saponified them in ethyl and nbutyl alcohol. A method involving the heating of samples n-ith alcoholic potassium hydroxide solution in a sealed glass tube would not constitute a satisfactory routine method even if the attack on the glass b y the alkali could be avoided. After a considerable amount of experimental n ork, the tei tiarj : Present addres-, C n i r e r s i t y of Minnesota, lllnneapolis, 3Iinn
butyl alcohol was replaced by cyclohexanol, the boiling point of which is 73" C. higher and which is almost equally stable in the presence of alkali. The insolubility of sodium cyclohexanolate in cyclobexanol n-as overconie by the addition of methanol, n-hich is ultimately removed by boiling. The following conditions for effecting quantitative saponification of very difficultly saponifiable materials seemed most appropriate: (1) Use of approximately 0.6 Ai sodium methylate in methanol and cyclohexanol as 8 reagent. This is a clear, homogeneous, reasonably stable solution x-hich attacks glass only slightly. (2) Use of an oil bath at 150" C., instead of a hot plate, in order t o prevent local overheating. This high temperature accelerates the saponification. (3) Use of a nitrogen stream to prevent oxidation of the sample or the reagent. (4) Complete removal of the methanol uGed in preparing the reagent. This is done in order to have a sufficiently high reaction temperature. ( 5 ) Reaction for a 16-hour period. (6) Reaction in the presence of a .light amount of moisture. OF METHYL ABIET-LTE TABLEI. S.LPOSIFICATIOX
Detn.
10.
13
4
b
5 10
'UVeight
of Sample Grams 0.5261 0.5516 0.5012 0,5297 0,9493 0.9917 0.9830 1.0130
Weight of Reagent Grams 9.184 9.388 8.607 9 662 8.413
9,647 9.406 9.693
.v
Alkali
Consumed
Excess Reagent
Used
CC.
76
1,650 1.746 1.679 1,676
283 27 1 2:; 297 117 113 109 11.0
2,990
3.112 3.104
3.Ii3
.iv.
Theory
Saponification Value Found
176.0 177.5 176.8 177.4 176.7 176.1 177.2 175.8
176.7 177 4
Kolthoff and Furman (6) discuss the effect of the presence of water in the saponification reaction \Then sodium alcoholate is used as a reagent, giving references to articles where water is considered essential to the r e a h o n ( I O ) and also where even traces of i t are con5idered harmful (e). I'ardee, Hasche, and Reid (11) added 0.5 cc. of water to each flask in order to facilitate the saponification reaction. For carefully purified methyl abietate, some of the writers' saponification results, obtained under the above conditions in the absence of water, vere 67.9, 99.6, 109.2, 97.7, 140.3, and 97.9. I n two experiments made under exactly the same conditions except that 0.5 cc. of water was added, the flasks were very badly attacked by the free alkali but the results were 164.0 and 160.3, respectively. I n all subsequent determinations water was added in the form of vapor by passing a slow current of nitrogen first through a mash bottle containing water and then through the reaction flasks. Under these conditions, t,he glass was not attacked and yet the saponification was complete, as shown by the satisfactory results in Table I.
A4pparatus -In oil bath which will maintain a constant temperature of
about 1.50" C. for long periods without requiring attention. .In apparatus consisting of t w o flat-bottomed concentric steel pans, differing both in depth and diameter, which were n-elded together and provided with a long pipe t o sewe as a condenser, llai proved highly satisfactory for this purpose. The space Iictn-een the pans contains about 3 liters of a suitable liquide.g., a mixture of commercial pinene and dipentene. An ordinary 1:trge oil bath provided with a device for maintaining a constant temperature n.ould be equally satisfactory.
ISDUSTRIAL AND ENGINEERING CHEhlISTRY
516
So-called acetylation flasks fitted T\ ith condensers through 25-mm. standard-taper ground-glass connections. The condensers-are nearly filled with water and have their vater inlets and outlets capped. Two small tubes enter the top of the condenser through a rubber stopper, one extending just through the stopper (short inlet) and one extending through the condenser and about 2 cm. below its lower end (long inlet). This apparatus is shown in Figure 1. Bailey weight buret.
VOL. 10, NO. 9
SUORT / N L € T -
"IET
?+ON=
Reagents 1. Solution of sodium methoxide in methanol and cyclohexanol, containing approximately 0.6 cc. of N alkali per gram. This is prepared as follows: Add approximately 9.3 grams of
sodium to 500 cc. of cyclohexanol, attach a reflux condenser, and add 250 cc. of absolute methanol in small portions. Reflux the solution overnight in order that the sodium methoxide may react completely with any esters JThich may be present, as impurities. Cool, and preserve the reagent in a tightly stoppered bottle. 2. Sulfuric acid, approximately 0.1 N. 3. Sodium hydroxide solution, approximately 0.1 N. 4. 1 per cent alcoholic solution of phenolphthalein. 5 . 1 per cent alcoholic solution of thymol blue. 6. Absolute alcohol, neutralized to phenolphthalein end point. 7. Moist nitrogen
Procedure Weigh samples of suitable size (approximately 1 gram in the case of abietic acid esters) into acetylation flasks. The viscous esters may best be placed in the flasks by means of a stirring rod. Fill the Bailey weight buret with reagent 1 and place weighed portions of about 10 cc. in each flask. Attach condensers to the flasks, and place the flasks in the oil bath at about 150" C. Pass a slow stream of moist nitrogen into the short inlets of t,he condensers for 30 minutes. This operation, performed in the hood because of methanol vapor, completely removes the methanol from the flasks. The methanol, along with a slight amount of cyclohexanol, is volatilized and forced out of the condenser through the long tubes, without having an opportunity to condense. Then rearrange the tubing connections and pass the moist nitrogen into the flasks through the long inlet tubes very slowly overnight. This causes the reaction to take place in an atmosphere free of oxygen and carbon dioxide. After bubbling through a wash bottle of water, the nitrogen stream is divided into four streams, each passing through four 1-mm. capillary tubes which are all of the same length (about 20 cm.) and then into the appropriate inlets at the tops of the condensers. After they have been heated overnight, remove the reaction flasks 'from the bath and separate them from the condensers. Add 50 cc. of absolute alcohol, previously neutralized to the phenolphthalein end point, to each flask at once. If the reaction mixture does not dissolve completely in the alcohol, attach the flasks to Allihn condensers through which water is flowing and heat on a hot plate until solution is complete. Titrate the solutions with 0.1 N sulfuric acid, using phenolphthalein as the indicator, and add enough 0.1 N alkali to make the solutions slightly alkaline. Add fragments of silicon carbide or porous plate to prevent bumping and boil the solutions for 30 minutes. This operation serves to hydrolyze completely any material present that can yield additional free alkali. Allow the solutions to cool, and titrate them with 0.1 N sulfuric acid and then to final end points with 0.1 N alkali. When this drastic saponification method is applied to darkcolored substances, it is preferable to use 0.5 cc. of a 1 per cent alcoholic solution of thymol blue as indicator and to make the titrations with the aid of a hand spectroscope. The concentration of the reagent should be determined under the same conditions as those which prevail in an analysis. If the reagent was properly stabilized, the concentration found will be substantially the same as that obtained if one titrates a sample of the reagent, without overnight heating, in the manner outlined above.
Calculation (Grams of reagent used X cc. of W alkali per gram) (cc. of NaOH X normality factor) - X 56.1 (cc. of H$Oa X normality factor) Grams of sample =. saponification value
+
1
c m.
FIGURE 1. DRASTIC SAPOSIFICATIOX APPARAT~;~
The specimen of methyl abietate which was used in this work had an acid number of 0.9. Determinations by the Zeisel method showed that it contained 9.77 per cent of OCH, (theory, 9.81 per cent OCH,). I t s density was 1.0442 at 20"/4", its index of refraction a t 20' mas 1.5336, its optical rotation was -45.6", and its thioqyanogen number was 93.5. When the drastic saponification method was applied to a specimen of commercial methyl abietate, the saponification numbers obtained were 164.6 and 165.7. The average of these values, 165.2, is 93.1 per cent of the theoretical. This specimen had previously been found t o contain 9.09 per cent of OCHI (Zeisel method), which indicates a purity of 92.7 per cent. The saponification numbers (drastic method) of n-butyl abietate were determined after a total of 16 and 40 hours' heating, respectively, with the following results: 16 hours 40 hours
133.3, 133.3 133.4, 135.2
Although these results indicate a purity of only 85 per cent, they furnish additional evidence as to the validity of the method. Furthermore, the authors' saponification method gives concordant results when it is applied to other esters of abietic acid. The drastic saponification method yields the theoretical saponification number-i. e., 185.&when it is applied to pure recrystallized abietic acid. When applied to rosins, the new reagent presumably reacts completely with all esters and anhydrides present.
SEPTEMBER 15, 1938
ANALYTICAL EDITION
1 per cent xhen i t is applied to reasonably light-colored products.
TABLE 11. SAPONIFICATION OF WOOD ROSIN FF Wood Rosin 155.2 163.9
Acid No. Saponification N o . (usual method) Saponification N o . (drastic method)
TABLE 111. Loss
IN
173.0, 173.8
I Wood Rosin 164.7 171.0
174.0, 173.8
WEIGHTOF PYREX FLASKS USEDIS DETERMINATIONS
S.4POSIFICATIOS T i m e of Heating HOUTS 1 20
40 2
20
31 7
Loss in Weight of Flasks Flask 2 Flask 1 Mg.
MQ.
1 2 1.3 3 1 1 1 1.0
0.9 1.0 3 0 0 9 1.4
The relatively slight attack of the reagent on the Pyrex glass flasks is shown by the results in Table 111. Unfortunately, a n ester of abietic acid which is commercially important, ester gum-i. e., the glycerol ester-cannot be analyzed by the method which has been devised because glycerol itself reacts n5th sodium methoxide t o form monosodium glyceroxide and perhaps also some disodium glyceroxide (9). The writers do not know how these compounds behave on prolonged heating a t 150” C., but evidence has been obtained which shows t h a t a variable loss of alkalir-ity occurs when the saponification reagent is heated lyith glycerol. This naturally tends to make the results variable and too high. With the exception of ester gum, and presumably also the glycol ester of abietic acid and other glycerol and glycol esters, this method appears to be applicable to all difficultly saponifiable esters. Obermuller (IO) was not troubled by a reaction between glycerol and sodium ethoxide, but presumably his saponifications were made a t a much lower temperature-viz., the boiling point of ethanol. Kogan (5) analyzed ester gum by determining the difference between saponification numbers obtained with 0.5 N and 4 N potassium hydroxide. The accuracy and precision of this method are both about
Summary
A saponification method Jyhich is considerably more drastic than any heretofore suggested is recommended for tlie analysis of natural and synthetic products containing difficultly saponifiable esters. This method consists in the use of sodium methoxide in methanol and cyclohexanol, the removal of tlie methanol, and the heating of the reaction mixture in an oil bath a t 150” C. for 16 hours in a current of moist nitrogen. The method has been applied to the quantitative saponification of pure methyl abietat’e and to the analysis of various impure a,bietic acid esters. It cannot be used in the presence of glycerol derivatiT-es and presumably not in the presence of glycol derivatives.
Literature Cited (1) Allen, I r e y , Jr., Meharg, 1’. E., and Schmidt, J. H., IND.ENG. CHEM.,26, 663-9, especially 666 (1934). (2) Beythien, Adolf, P h a r m . Zentralhalle, 38, 850 (1897) : (Chem. Zentr.. 1898, I, 274). (3) Kesler, C. C., private communicat,ion, Feb. 10, 1932. (4) Kesler, C. C., Lowy, A , and Faragher, W.I?., J . Am. Chem. Soc., 49, 2898-903, especially 2901 (1927). (5) Kogan, A , , Maeloboino Zhirocoe Delo, Nos. 9-10, 32-9 (1930); (Chem. Zentr., 1931, I, 1023). (6) Kolthoff, I. AI., and Furman, N. H., "Volumetric Analysis,” Vol. 1, p. 175, Nexv Tork, John Wiley & Sona, Inc., 1928. (7) Kossel, -i.,and Kriiger, >I., 2. phusiol. Chem., 15, 321-30; [ J . C h e m Soc. Abstracts, 60, 1143 (1891)l. (8) Kossel, A,, and Obermuller, K., 2.physiol. Chem., 14, 599-601; [ J . Chem. Soc. Abstracts, 58, 1474 (1890)l. (9) Lawrie, J. W., “Glycerol and the Glycols,” p. 209, New York. Chemical Cataloa Co.. 1928. (10) Obermuller, Kuno, 2. physiol. Cisem., 16, 152-‘9; [ J . Chem. SOC. Abstracts, 62, 139 (1892)l. (11) Pardee, A. M., Hasche, R . L., anti Reid, E. E., J. IND.ENG. CHEX, 12, 481-2 (1920). (12) Pardee, A. M., and Reid, E. E., Ibid., 12, 120-33 (1920). (13) Redemann, C. E., and Lucas, H . J., Ibid., .Inal. E d , 9, 521 (1937). (14) Smith, W.C., Ibid., 9, 469 (1937). (15) Steet, W.R., Analyst, 61, 687 (1936). Y
RECEIVED April 30, 1938.
The Determination of Water in Alcohol H. G. BOTSET, Gulf Research 87 Development Company, Pittsburgh, Pa.
IIS
A STUDY of methods for determining the water content of oil sands, a method was developed which may be applied to the determination of water in alcohol, and possibly in other substances as well. It is a modification of methods which have been used for some time (1, 2 , 3) , but is felt to be somewhat more rapid and to require simpler equipment. It is an application of the fact t h a t in the presence of certain organic liquids such as kerosene, carbon tetrachloride, xylol, etc., Tvater and alcohol are only partially miscible. I n the references cited the water content was determined by measuring the temperature a t which cloudiness, caused by phase separation, occurred. The method presented here does not depend upon a temperature effect, but upon the amount of water required to titrate a given volume of liquid.
Description of Method Ethyl alcohol and carbon tetrachloride are miscible, but 71-ater is insoluble in carbon tetrachloride. If, then, carbon
tetrachloride is added to a n alcohol-water mixture containing sufficient water, there >%-illbe a separation into two distinct layers, the act of separation producing initially a definite cloudiness throughout the liquid. If 10 ml. of absolute alcohol are mixed with 10 ml. of carbon tetrachloride and titrated with water, a n appreciable quantity of water (2.03 ml. a t 25’ C.) must be added before the appearance of cloudiness. If the alcohol already contains some water a smaller volume of u-ater will be required to titrate. The difference between the amount of water required t o titrate 10 ml. of absolute alcohol and t h a t required to titrate 10 ml. of alcohol solution containing a given quantity of water represents the amount of water eontained in the alcohol. Since a 10-ml. sample of alcohol solution contains less than 10 ml. of alcohol, the volume of contained water is not simply the difference between the titration value for this solution and for 10 ml. of pure alcohol, but is slightly less than this. T h e simplest way of determining the titration value for
