A Titrimetric Step in Determining Rotenone HOB ARD A . JONES Bureau of Entomology a n d Plant Quarantine. U. S. Department of Agriculture, Washington, D. C.
added, then 25 cc. more are added dropwise and the solution ik again cooled, and finally 50 cc. are added more rapidly and the qolution is again cooled. The material is filtered through a Gooch crucible, with filter paper, and washed with about 250 cc. of water in small portions. The outfide of the crucible is rinsed with rrater and the cont,ents are dissolved in 25 cc. of chloroform. This solution may be accomplished by placing the crucible and cont,ents in a beaker, adding the chloroform, and leaving the rrucible in the beaker during the titration. To the chloroform solution 50 cc. of freshly boiled water are added, and the mixture is t,itrated viith 0.1 N alkali, with phenolpht.halein as indicat,or. The mixture must be thoroughly agitated, particularly near the end point, to ensure that all the acid is extracted from the chloroform layer. Each cubic centimeter of 0.1 N alkali is equivalent to 39.4 mg. of rotenone. A blank should be run on the chloroform used. The usual allowances (3) for added rotenone and for solubility in carbon tetrachloride are made-any rotenone added in the original crystallizat.ion is subtracted from the result, and 0.07 gram is added to allow for solubility in the 25 cc. of carbon tetrachloride used.
-4 titrimetric step in the procedure for determining rotenone uses a rotenonedichloroacetic acid solvate and makes the method a volumetric one. The acid solvate is crystallized quantitatively from solution in the acid by addition of water. separated. and titrated with alkali. A s applied to the carbon tetrachloride solvate obtained in the usual crystallization procedure, the method effects a great saving in time, and in accuracy and precision it is equal to the gravimetric method.
T
HE solvate of rotenone with acetic acid has previously
been reported ( 2 ) . Recently a study was undertaken of the possibility of using an acid solvate such as this as the basis for a method of determination involving titration of the combined acid. Since this particular solvate has a low proportion of acid ( 2 nioles of rotenone to 1 of acetic acid), solvates were sought with other organic acids. Propionic and alpha-chloropropionic acids were found to combine with rotenone in the same ratio as does acetic acid. Monochloroacetic acid gave a material with a very low percentage of acid of no definite molecular ratio. Trichloroacetic acid may form a solvate containing an equimnlecular proportion of rotenone and acid, but in all attempts to prepare i t the percentage of acid was slightly higher than this, perhaps owing to tenacious retention of excess acid. Dichloroacetic acid, however, formed a definite solvate containing 1 mole of rotenone to 1 of acid. It was found possible, by dissolving rotenone in dichloroacetic acid and carefully adding water, to convert the rotenone quantitatively to this acid solvate. On filtering and m-ashing with water, the excess acid was readily removed, and titration of the solvate with standard alkali gave the theoretical percentage of acid for the molecular ratio mentioned. The method has been adapted to determining the purity of the crude carbon tetrachloride solvate obtained in the gravimetric crystallization method (3),and the procedure thus becomes an entirely volumetric one. 4 saving in time of from 6 hours to 1 day is effected in the determination.
Discussion of the Method I t was found that, when the carbon tetrachloride d v a t e was dissolved directly in dichloroacetic acid, the results of the determination were invariably loiv. When the carbon tetrachloride was removed by eraporation with acetone, correct results were obtained. Consequently, in making determinations on unsolvated rotenone, to which the method is also applicable, the evaporation with acetone is unnecessary. TABLEI. PCRITYOF CARBONTETRACHLORIDE SOLVATES BT A L C O H O L RECOVERY AND BY TITR.4TION Sample
Solvate NO.
By .4lcohol Recovery
% Derris root
1 2
3
Cube root
4
Pure solvate
6
5
86.5
89 81.5 87.5 84 5 100
BY
Titration
% 88.5, 8 8 , 88. 86 PS 90 5 , 91 91 85, 85 90, 91 86 98, 98, 100
Perhaps the most important point in the procedure is that
of obtaining actual crystallization of the acid solvate rather than precipitation of an amorphous material. If the latter occurs, excess acid is retained tenaciously. The 10 cc. of water first added are sufficient to make the solution supersaturated in the cold, if the rotenone content is a t least 1 gram, as required in the carbon tetrachloride crystallization. After the addition of a seed crystal, sufficient time should he alloffed to see that crystals have actually formed. Crystal. of the acid solvate for seeding may readily be obtained by dissolving pure rotenone in the 80 per cent acid and slonl? adding water. Seeding is not necessary in the determination as slow addition of more water l\ill finally induce the material to crystallize, but i t does expedite the procedure. i l ~ further water is added slowly, a point is reached, depending on the amount of rotenone present, a t which a thick mass of crystalline material separates. One adrantage of the method is that neutral insoluble materials present n i t h the carbon tetrachloride solvate in no way interfere n i t h the determination, as they do in the gravimetric method using the alcohol recovery test. Insecticidal dusts containing sulfur, which is frequently mixed with derris or cube, may be analyzed by this method without interference from the sulfur.
The extraction of the root eample and crystallization from carbon tetrachloride at 0" C. are carried out by the method already published ( 3 ) . The carbon tetrachloride solvate obtained in this way is filtered and washed by suction as usual. Then, without further drying, it is dissolved in about 25 cc. of acetone in a 250-cc. flask. This is readily accomplished blplacing the crucible in a funnel and washing the contents through into the flask with small lots of acetone. The solvent is evaporated completely on the steam bath. The residue is treated with 10 cc. of 80 per cent (by volume) dichloroacetic acid and warmed gently until the residue just dissolves. The solution is then cooled in an ice bath for a few minutes, 10 cc. of cold water are added slowly with swirling, a few seed crystals of rotenonedichloroacetic acid solvate are added, and the flask is again cooled in the ice bath for 2 or 3 minutes. Separation of a few small needle crystals will usually be noted at this point. If not, water is added a drop or two at a time, with intermittent cooling, until a few crystals are noted. Water is then added 10 to 15 drops at a time, with about 1 minute's cooling bet.ween additions, until 25 cc. have been 684
DECEMBER 15, 1938
ANALYTICAL EDITION
585
Results
Adaptation as a Direct Method
In tests on specially prepared carbon tetrachloride solvates (Table I), the method gave values for purity about 2 per cent higher than those by the older alcohol recovery test. Since the alcohol recovery test as used in this laboratory is known to give results about 1 per cent lower than the correct value (1). the titration method appears t o be at least as accurate.
Attempts were made to precipitate the acid solvate froni a dichloroacetic acid solution of whole derris and cube extracts. Such a procedure would greatly shorten the determination of rotenone. However, the resinous material formed was difficult t o filter and retained excess acid which it was practically impossible to remove entirely. Xumerous materials were added to overcome this, but none was satisfactory. Naphthalene was the best of such materials tried, and in one sample gave fair but not consistent results. When samples of higher nonrotenone-resin content were tried, the results were again too high. It is possible that future work will reveal a method for applying this idea directly to whole extracts, but no such procedure can be recommended at present.
TABLE 11. ROTESOSEIN ROOTSAMPLES AS DETERMINED B Y THE GRAVIMETRIC AND THE VOLUMETRIC METHODS Koot
Derris
Cube Tirnbo
Sample NO.
Gravimetric Method
%
%
3002 3006 3126 3307 3004 3005 3230
2.0 3.6 5.8
2.2, 2.0, 2.0 4 . 0 , 3 . 9 , 3.8 5.8,5.7 7.2,7.4 2.9, 2 . 9 5.6, 5.6 3 8,3 8
7.4
2.9 5.6 3 9
Volumetric Method
Results of rotenone determinations on samples of powdered root were in good agreement with those by the older gravimetric procedure (Table 11). The precision of the method, as judged by the replicate determinations (Tables I and 11), appears to be as good as that of the gravimetric procedure.
Literature Cited (1) Jones, H. A., IND. ENQ. CHEM.,Anal. E d , 9, 206-10 1937) (2) Jones, H . A., J. Am. (hem. Soc., 53, 2 7 3 8 4 1 (1931). (3) Jones, H. A., and Graham, J J T J -4ssoc 0 f i c ~ a . ldgr Chem
21, 148-51 (1938). RECEWFOD .4ugust 13, 1936.
A Simple Melting Point Outfit J E S S E W E R X E R , Columbia University, N e w York, N. Y .
I
S ORGANIC laboratory technique, it is common to take melting points in a beaker while stirring the heated liquid. The outfit herein described is a modification of this wellknown procedure. To make for permanence of setup, greater ease of manipulation, decreased dangers of breakage, and no contamination with rubber, a piece of 4-mm. Pyrex tubing is sealed onto a 400-cc. beaker, as shown in Figure 1. With some care and practice, it is a very simple matter to seal the tube onto the beaker without blowi n g . T h e t u b e is sealed on a t an angle slightly less than 45 degrees to the vertical and is tilted slightly forward, as shown in Figure 2. This slight forward tilt makes certain that the capillaries will be held in place by being gently wedged in between the thermometer bulb, the bottom of the side arm, and the top of the side arm, and does away with using wire springs or c o n s t r i c t in g the bottom of the side tube. This method FIGURE1
also allows several capillaries to be placed one on t o p of t h e other. so that several melting points can be taken a t one time with only one side tube. h small electric motor is advantageous, as it gives rapid agitation and is noiseless and inexpeneive. Both flywheel and centrifugal stirrers have been used satisfactorily with this type of outfit. For uniform heating, the beaker is supported on a sheet of asbestos. C o t t o n s e e d oil has FIGCRE2 been found very useful as a liquid for the bath. It is compounded with 1 per cent of hydroquinone as recommended by Gill and Ebersole (1) and further protected from dirt and rapid decomposition a t relatively high temperatures by covering the beaker with a thick sheet of asbestos having two small holes bored for the thermometer and stirrer.
Literature Cited ,1' Gill and Eberjole, IND. ENO.CHEV, 23, 1304 119311 R E C E I V EAugust D 18 1938.
