Determination of Menthol in Peppermint
Oil
Acetic Anhydride and Pyridine as Reagent J S. JONES AND S. C. FANG, Chemistry Department, Oregon Agricultural Experiment Station, Corvallis, O r e ,
N = normality of standard base W = weight of sample in milligrams
THIS
study was undertaken to meet the need in a research program on peppermint oil production for simplification and increased accuracy in methods used for the quantitative determination of menthol. The method of Power and Kleber ( 7 ) , Kenerally accepted in the aromatic industry for that compound and “official” in the United States Pharmacopoeia, is both timeronsuming and of questioned reliability, since variable results o n identical samples of oil have been obtained all too frequently by it in different laboratories. Brignall (1) described a new procedure for the hydroxyl group hased on the use of an acetylating mixture consisting of acetic anhydride and n-butyl ether. In peppermint oils free menthol determinations by this method have averaged about 3% lower than those done by the method of Power and Kleber. Christensen and Pennington (%’) worked out a method involving the use of acetyl chloride in determining menthol in peppermint oil. By it, results of menthol determinations in this laboratory agree very well with those obtained by the Brignall procedure but fall distinctly low in comparison with those obtained by the official method. Other methods-vis., one by Delaby, Sabetay, and Brengnot (S), one by Freed and Wynnc (4), and one by Ogg, Porter, and Willits ( 5 ) ,each making use of a mixture of pyridine and acetic anhydride-have been proposed for the analysis of primary and secondary alcohols with application of heat. A micromethod, too, has been proposed by Petersen, Hedberg, and Christensen (6) using the same reagent without application of heat for determination of the hydroxyl content of pure organic compounds. The authors recently applied this last-mentioned technique on the macro scalr for the detcrmination of free menthol i n peppermint oils.
ANALYTICAL RESULTS
Free menthol was determined on samples of mint oil obtained by steam-distillation and refined by petroleum ether extraction Table I shows that an acetylation period of more than 24 hours is necessary and that one of 28 to 32 hours is ample for maximum values. At least 100% in excess of the theoretical amount of acetylating agent is required (Table 11). The amount of pyridine used may vary within a rather wide range but the ratio of acetic anhydride to pyridine must not be less than 5 to 5 (Table 111). In Table IV each value by the Christensen and Pennington and the official methods is the average of two determinations; each value by the authors’ method is the average of two or more determinations. Results by the official method and by the authors’ method are in close agreement, while results by the Christensen and Pennington method are from 1 to 47,lower. Finally the authors’ method was used for the recovery of menthol added in varying amounts to 500 mg. of peppermint oil containing 50.6% of free menthol. Recovery values shown in Table V, from single detcrminations, are considered satisfactory
Table
I.
ANALYTICAL PROCEDURE
The necessary reagents are: C.P.acetic anhydride, pyridine C.P. and water-free, and sodium hydroxide 0.5 N and carbonate-free. An approximately 0.6-gram sample of oil is introduced into a weighed 7.5-cm. (3-inch) test tube by means of a dropper and its weight again taken accurately to find the weight of sample used. Approximately 0.5 pram of acetic anhydride is introduced into the tube, which is then reweighed. Following the addition of 0.5 cc. of pyridine, the tube is sealed Tyith a cork which previously has becn dipped in melted paraffin. The tube is immediately shaken once, set aside a t room temperature for 48 hours, then opened and placed in an Erlenmeyer flask. Fifty cubic centimeters of water are added and the solution is titrated with standard sodium hydroxide. Near the end of the titration, flask and contents are heated for a few minutes to ensure complete hydrolysis of the excess acetic anhydride and then titration is carried to the end point of phenolphthalein as shown by persistence of the pink color for 1 minute, A blank titration is carried out a t the same time to determine the volume of standard base required t o neutralize the acid derived from 1 gram of acetic anhydride. Another sample is weighed and titrated with standard alcoholic sodium hydroxide with phenolphthalein as indicator t o determine the amount of free acid in the peppermint oil. The per cent of free menthol is then calculated by the formula
% free menthol
(A X R
- B1
+ B,)N
Effect
of Variations in Length of Acetylrting Period
Acetylation Period Hours
44 7
8 12 16 20 24 28 32 36
46.3 46.3 47.9 48.0 47.9 50.0 50.0 49.1 49.5 50.1 49.4
44 48 Each value from a single determination.
Table
II.
Effect of Variations in Amount of Acetylating Agent Used Acetic Anhydride Used, % of Theoretical Requirement
Table
%
4
40 a
Free Menthol Found0
111.
Free Menthol Found, % 38.9 47.8 47.9 49.8 50.1 50.1 49.5
Effect of Varying Acetic Anhydride-Pyridine
Rdtio
(Acetylating period 48 hours, room temperature, and a t least 100% excess acetic anhydride) Acetic .4nhydrideFree Menthol Pyridine Ratio Found, % Without pyridine 11.5 5:l 51.2 5:2 50.1 5:3 51.0 5:4 51 4 5.5 51 1 50 1 5:7.5 49 9 5:lO 49.1 5:15 48 8 5:20
X 156.16 X 100
W
in which A = weight in grams of acetic anhydride used R = ml. of standard base required t o neutralize the acid derived from l gram of acetic anhydride B1 = ml. of st.andard base required to neutralize the remaining acid B, = ml. of standard base required to neutralize the free acid of the sample 130
ANALYTICAL EDITION
February, 1946 *
IV.
Table
Results b y Different Procedures Christensen and Official or Power
Sample A B
F G 7
7A D7
D7A 82
Pennington Average menthol deviation 52.0 0.3 52.6 0.1 51.4 0.2 51.4 0.4 51.2 0.2 52.1 0.7 53.1 0.2 54.0 0.8 51.0 0.1
% of
Table
V.
and Kleber Average menthol deviation 54.0 0.2 53.2 0.5 52.2 0.3 53.1 0.4 52.8 0.6 54.7 0.1 56.0 0.6 58.0 0.2 51.9 0.4
yo of
Authors Average menthol deviation 53.9 0.1 54.2 0.2 52.4 0.2 53.3 0.1 52.7 0.2 54.8 0.1 58.1 0.0 57.6 0.1 51.2 0.0
% of
Recovery of Menthol
hlenthol Added
Menthol 4ctually Present
Menthol Found
Mv.
Mg,
Mo.
%
60.7 50.7 106.1 102.3 151.6 146.8 184.4 193.0
318.7 303.7 363.1 350.3 366,6 375.8 388.4 402.0
318.7 304.3 381.5 347,2 363.3 374.6 383.7 395.7
100.0 100.2 99.8 99.1 99.1 99.7 98.8 98.5
Recovery
131
amount. Since the menthol content of peppermint oil varies with the several factors pertaining to production and time of harvesting, it is necessary to add a substantial excess of this reagent. The amount of pyridine used may vary within a rather wide range, but the ratio of acetic anhydride to pyridine must not be greater than 1 to 1. A wider ratio necessitates a longer acetylation period. For peppermint oils containing free acid, the values thus obtained should be corrected. From this series of experiments it is clear that the Power and Kleber, or official, method for free menthol in peppermint oil is capable of yielding very satisfactory results, if the details of manipulation as given in the U. S. Pharmacopoeia are closely followed. The method devised by the authors, however, involving the use of acetic anhydride and pyridine as acetylating agents, is both more economical of reagents and substantially less timeconsuming per sample, particularly when a fairly large number of samples of oil are involved. Samples set aside for acetylation require no attention during the acetylation period. Duplicate results determined by the authors' method check more closely than those by the official method. LITERATURE CITED
SUMMARY
From the standpoint of time requirement and reproducibility of results, a very satisfactory procedure has been developed for the determination of free menthol in peppermint oil. It is based upon the utilization of a mixture of acetic anhydride and pyridine. The necessary acetylation period is somewhat longer for menthol than is required by the microprocedure with the same mixture for other alcohols unaccompanied by such a mixture of compounds as that which characterizes peppermint oil. The requirement for acetic anhydride is a t least 100% in excess of the theoretical
(1) (2) (3)
Brignall, T. W..ISD. ENG.CHEU.,ASAL. En., 13, 166 (1941). Christensen, B. E., and Pennington, L.. Ibid., 14, 54 (1942). Delaby, R., Sabetay, S., and Brengnot,,V.,Perfumery Essent. Oil Record, 26, 3 3 1 (1935).
(4) Freed, M.,and TT'ynne, A. M.,IND.ENG.CHEM.,ANAL.ED.,8, 278 (1936).
(5) Ogg, C. L.. Porter, W. L., and Willits, C. O., I h i d . , 17,394 (1945). (6) Petersen, T. W., Hedberg, K. W., and Christensen, B. E., Ibid., 15, 225 (1943). (7) Power, F. B., and Kleber, C., Pharm. Rundsch., 12, 163 (1594). PUBLISHED as Technical Paper No. 475 with the approval of the Director of the Oregon .4gricultural Experiment Station. Contribution of the Department of Agricultural Chemistry.
Accelerated Aging Test for Insecticidal Aerosols Containing DDT LYLE D. GOODHUE AND W. R. BALLINGER
U. S. Department of
Agriculture, Bureau of Entomology and Plant Quarantine, Beltsville, M d .
The introduction of DDT into the liquefied-gas aerosol has created the problem of stabilizing the aerosol solution and preventing corrosion of the container. The ease with which hydrochloric acid i s liberated from DDT in the presence of some iron salts has made necessary the development of an accelerated aging test for studying the effect of the different aerosol constituents on this reaction. A simple pressure test tube and a method of running'a test are described. The rate of decomposition varies greatly, depending mostly on the solvents used. Certain combinations have been developed which appear to be satisfactory.
T
HE first insecticidal aerosol (2) used by our armed forces consisted of a solution of pyrethrum and sesame oil in dichlorodifluoroniethane (Freon-12). This aerosol was very effective against mosquitoes, but the addition of D D T was found to increase its toxicity to flies and other insects (4). The incorporation of D D T in the aerosol presents several problems, including corrosion of the iron container and formation of tarry materials by decomposition of some of the aerosol constituents. D D T tends to liberate hydrochloric acid in the presence of some metals and their salts (1). Ferric chloride is a particularly good catalyst when the D D T is dissolved in some solvents, but in others very little action takes place. The solvent combination for the D D T aerosol solution is therefore very important. Since D D T is only slightly soluble in Freon-12, some cosolvent must be
used to keep even as little as 37, in solution. The choice of a cosolvent must take into consideration its effect on the decomposition of D D T in an iron aerosol bomb. In order to study some of these effects an accelerated aging test was developed. The method of making the test and some results are reported in this paper. APPARATUS
A simple pressure test tube was devised for these tests. A heavy glass tube was fitted to a tire valve as shown in Figure 1. The construction is described as follows: The frame, 1, was cut from 1/2-inch brass pipe and threaded on the inside of each end with S/s-inch pipe threads. h standard a/,-inch brass pipe plug, 2, was drilled, and a tire-valve stem, 3, was soldered in place, about 0.6 cm. (0.25 inch) of the stem extending through the plug. This extension passed through the neoprene washer, 4, and prevented the flow of the rubber, which was under pressure, from closing the hole. The glass test tube, 5, was made from tubing having an inside diameter of 10 mm. and a n outside diameter of 15 mm., and mas 15 cm. (6 inches) long. It was sealed a t one end, cut a t right angles and fire-polished a t the other end, and then annealed. The last step is very important. A calibration mark was etched on with a small grinding tool a t a point where the tube would contain 10 grams of solution. The test tube was held in place against the washer by another a/S-inch brass pipe plug, 6. A 0.47-cm. (3/ls-inch) depression was turned in the plug to accommodate &leather washer, 7, as a pad to protect the glass. The test strip, 8, was usually iron. Its size was 7.5 X 0.6 X 0.04 cm. (3 by 0.25 ,by 1/64 inch), affording twice the surface per unit volume of solution occurring in a 8.45-kg. (1-pound) aerosol bomb.
