Determination of Menthol in Oil of Peppermint Acetyl Chloride as a Reagent BERT E. CHRISTENSEN AND LLOYD PENNINGTON Oregon S t a t e Cbllege, Corvallis, Ore.
THIS
readily accomplished by a second titration with silver nitrate study was undertaken to meet t h e demand of Oregon solution, using potassium chromate as the indicator. An allowState College Experiment Station for a simpler and more ance of 0.1 ml. was subtracted from the measured amount of silver rapid method for analysis of the oil of peppermint. I n a recent nitrate solution to account for the indicator blank (for formation p1,blication ( 1 ) Brignall cited this same problem, discussed the of silver chromate). Since limitations of the official method, and described a new proEquivalent XaOH used = equivalent HCl equivalent cedure based on the use of a n acetylating inisture of acetic HC2Hs0, (after hydrolysis) anhydride and n-butyl ether. Equivalent AgX03 = equivalent HCI A great deal of attention has been given t o estei,ification procedures for the assay of alcohols. For this purpose acetic equivalent of blank Equivalent HC1 absorbed = 2 - equivalent anhydride with and without pyridine has been used almost exclusively. Although acetyl chloride is a much more active acetylating agent, little study (4) has been given to the possiEquivalent of alcohol = equivalent of blank - equivalent of bilities of using it in quantitative work. NaOH - equivalent HCl absorbed Recently the authors made a rather extensive ini-estigation Equivalent of alcohol = equivalent of blank - equivalent of (9) of acetyl chloride as a reagent for determination of the equivalent of blank hydroxyl content of organic compounds, in which seventy difequivalent AgN03 NaOH 2 ferent alcohols and phenols were studied. dlthough acetyl The per cent of free alcohol will be given by the expression chloride gave approximately the same precision, i t was apparent that from [ 1 / 1 blank - ml. of NaOH F(m1. of AgNOJ] X N X molecular weight = % ' menthol the standpoint of speed i t was far 10 X weight of sample superior t o acetic anhydride as an where N = normality of NaOH and F = normality of NaOH AgNO, esterifying agent.
+
+
+
Reagents This equation is valid only for pure acetyl chloride or samples in which the equivalents of chloride per milliliter are equal t o half equivalents of base required to titrate 1 ml. of the hydrolyzed reagent. Otherwise it would be necessary to revise the equation and determine both a silver nitrate and a sodium hydroxide blank. Because of variation of the titer with temperature, etc., blank determinations should be made at the time of the run. These blank runs also serve as a good check on the analyst's precision.
Eaatman practical acetyl chloride was used in this work. The equivalents of silver nitrate required to measure the chlorinity of a measured charge of this reagent was equal to one-half the equivalents of base required to neutralize the same charge. Sodium hydroxide, 0.3 N (carbonate-free), made b y dilutin 17 ml. of 50 per cent sodium hydroxide to 1 liter with distillef
water. Potassium chromate indicator, 5 grams dissolved in 100 ml. of water.
ResuIts and Discussion
Apparatus
R e d t s of the analysis of nine samples of oil of peppermint by this procedure, b y the method of Brignall, and b y the official method are given in Table I.
The pipet (3) and reaction vessel have recently been described ( 2 ) .
Analytical Procedure OF OIL OF PEPPERMINT TABLEI. ANALYSIS
A 1-gram sample of dry filtered peppermint oil was wei hed in a 10-cm. (4-inch) test tube, a measured charge (0.5 ml.) ofacetyl chloride WBB added by pipet, and the small test tube was placed in a 20-cm. (&inch) test tube containing 5 to 10 ml. of water. A 15-cm. (6-inch) test tube inverted over the smaller test tube served to make a water seal (2,Figure 2). The tube was stoppered and placed in a water bath at 50" C. To ensure the best accuracy the pipet and reagent should be at room temperature, and the acetyl chloride for both the blanks and the regular runs should be measured at the same time. After 20 to 30 minutes the reaction vessel y a s removed and then inverted to hydrolyze the excess acetyl chloride, and the contents were transferred to a 250-ml. Erlenmeyer flask. The reaction vessel was finally rinsed with a small amount of alcohol and the last traces of ester were washed into the titration flask. The addition of a small amount of carbon tetrachloride (5 to 10 ml.) to highly discolored solutions removed the interfering materials and greatly aided the titration. Whenever the phenolphthalein end point faded, the base was added in small increments (0.03 rnl.) until the color persisted for 30 seconds. Since the oil of peppermint contains considerable amounts of pinene which absorbs some of the evolved hydrogen chloride, it is necessary to measure the chloride ion in order to determine the actual acetyl chloride involved in the acetylation. This wm
SampleNo.
1
2
3
4 5 6 Per Cent of Menthol
7
8
9
Authors' method 42.9 50.3 46.1 43.7 44.9 4 3 . 0 4 3 . 0 42.1 4 3 . 3 Brignell's method 42.9 5 0 . 3 45.5 4 3 . 8 44.9 44.2 4 3 . 3 4 2 . 8 4 3 . 0 Official methoda .. .. .. 43.7 43.4 42.2 42.7 42.1 a Analyses furnished by Oregon State College Experiment Station.
..
~
All values are averages of three or more determinations] which agreed within 0.5 per cent. Good agreement was obtained in most cases. Where considerable differences occurred, as in samples 3, 6, and 8, the determinations by the authors' method were repeated. I n all cases previous results were duplicated. To determine the total alcohols in peppermint oil, a n ester determination on a second sample is made in the usual manner. The per cent of total alcohol can then be calculated from the formula ( 1 ) : 54
ANALYTICAL EDITION
January 15, 1942
TABLE11. ANALYSISOF ESSENTIAL OILS Authors' Method Average deviation 1.1 0.3
% of alcohol
Oils Cedar oil 1 Cedar oil 2
constituent 13.9 6.7
-. _ ~.- " . N'WSti"P
Lavender oil Sandalwood Rosemary Cinnamon Citronella Cloves
% total alcohol % free
...
values 65.1 9.8
0.5 0.6
23:6 72.6
0.4 1.4
=
...
Brignall's Method Average deviation 0.8 0.8
% of alcohol oonstituent 12.7 8.7 16.6
63.5 10.1 13.3 35.0 73.9
...
0.7
0.1 1.3 1.1 0.1
( molecular weight of ester ) % ester molecular weight of alcohol
-t
Analyse- of other eqsential oils containing various alcohols werc attempted with this procedure. The results are given in Table 11.
55
As indicated, the results in most cases were not very satiefactory, probably because of the unsaturated nature of the alcohols in question, which apparently absorb the dry hydrogen chloride. During titration this hydrogen chloride is slowly released, giving rise to erroneous results. For this reason the authors do not recommend the procedure ab described for determination of free alcohols in oils other tlxtn peppermint,
Literature Cited (1) Brignall, T. W., IND. ENO.CHEM.,ANAL.ED.,13, 166 (1941) (2) Christensen, B E., Pennington, L., and Dimick, P. K., Ibid., 13, 821 (1941). (3) Linderstrgm-Lang, K., and Holter, H., Compt.-rend. trav. lab. Carlsberg, 19, S o . 4 (1931); Z . physiol. Chem., 201, 9 (1931). (4) Smith, D. >I., and Bryant, W.M .D., J . Am. Chem. SOC, 57, 61-5 (1935). PCBLIEHED with the approval of the Monographs Publications Committee Oregon State College. Research Paper No. 5 2 , School of Science, Department of Chemistry.
Acidimetric Orthophosphoric Acid Assay JOSEPH A. CALAMARI AND ROBERT HUBATA New York General Depot, Medical Section, New York, N. Y.
K
OLTHOFF (1) has shown that the presence of sodium chloride in the alkalimetric titration of orthophosphoric acid lowers the p H of both the first and second end points and that the acid may be accurately titrated as a monobasic a n d dibasic acid by using methyl yellow and phenolphthalein, respectively, as indicators for the two end points. I n the titration to the phenolphthalein end point, the solution is half saturated with sodium chloride to suppress the ionization of the dibasic salt, while the methyl yellow titration is conducted without the addition of sodium chloride. Similar procedures are used in the U. S. Pharmacopoeia
