Determination of Pvrethrin I J
Investigation of Seil Color Reaction in Mercury Reduction Method CL,IRENCE S. SHERllIAN AND ROBERT HERZOG Ti.Y .
Cooper Vnion Institute of Technology, New Yorl,.
REVIETV of tlie literature relating to the evaluation of pyrethrum products indicates that, although there are a number of methods for determining pyrethrin I (IZ), serious objections have been raised to most of them ( 2 , 3, 6, IO). The most recent and most promising is the one proposed b y Wilcoxon (13) and modified by Holaday ( 4 , 5 ) . Results with the Wilcoxon method h a r e shown nonlinearityi. e., the use of different sized samples of the same extract gives different results for the percentage of pyrethrin I (8). The present authors have found this nonlinearity to hold in the analysis of commercial pyrethrum oleoresin, thus checking the observations of Martin (8). Furthermore they have carried out a series of determinations, b y means of the Wilcoxon method, using aliquots of a carefully purified sample of chrysanthemum monocarboxylic acid that \vas isolated from the commercial pyrethrum oleoresin in the manner used b y Wilcoxon. Results are linear over a large range of sample sizes. Holaday's revised method of analysis of commercial insecticides (5) removes substances other than the pyrethrin I acid that might give reaction with the potassium iodate or other reagent used in titrating mercurous chloride formed by the action of pyrethrin I acid on DenigBs' reagent (acid mercuric sulfate). Evidently interfering substances of this sort are present not only in commercial insecticides but in the pyrethrum oleoresin as well, and it is these that give the nonlinearity that has been noted with the use of the unmodified Wilcoxon method. The titration involved in the Holaday and Wilcoxon methods is based upon procedures elucidated b y Jamieson (7) and Oesper (9). T o check the possibility that filter paper in the titrated solution might have some effect on the end points obtained, titrations of pure mercurous chloride were carried out with and without the presence of filter paper. The results from a large number of titrations indicate that the presence of a filter paper larger than 7 cm. (S. & S. 597) makes the end point difficult to determine and not completely permanent.
Seil Color Reaction The basis of the Wilcoxon method is the reaction between pyrethrin I acid (chrysanthemum monocarboxylic acid) and DenigBs' reagent. This reaction was first noted in the literature b y Seil (11) and subsequently was commented upon b y Audifiren (1). Aside from the work of these men and of T57ilcoxon, nothing has been done in connection with the reaction, As the name indicates, the reaction is accompanied by some striking color changes, the cause of which is not known. I n fact, all the specific data given in the literature about the reaction can be summarized as follows:
(even though there are several substances capable of reducing mercuric mercury under the conditions of the reaction); and ( 5 ) the intensity of the color developed is roughly proportional to the amount of mono- acid used, but because the color is fleeting the reaction is not suitable for quantitative colorimetric analysis. T o attempt to arrive a t some conclusion regarding the mechanism of this color reaction, a number of experiments were carried out using practically pure chrysanthemum monocarboxylic acid isolated from pyrethrum oleoresin (kindly supplied b y S. B. Penick and Company). RESVLTS. The time required for the color change cycle t o occur, when approximately 20 mg. of pyrethrin I acid mere used, v a s as follow: Time after Adding Deniggs' Reagent
.Win.
See.
0 ... ...
30-40
1 2
15-25
35-50
0 10
40
40 ... ...
Color
(Observed by Transmitted White Light) F a i n t white oloudineas appears transiently Pink Red Light red-violet Blue-violet Dull blue Blue-violet a t top: greenish blue a t bottom
The intensity of color developed is roughly proportional to the amount of mono- acid used in the reaction. When fairly large amounts of the mono- acid are used, yellowgreen crystals come down during the color change period. These crystals have been identified as mercurous sulfate (plus colored contaminating material). In all cases a fine, powdery dark blue preci itate was resent on the bottom of the reaction vessel at the e n f o f the c o i r change period. On filtering this and washing it thoroughly with water, definite positive tests for mercury and sulfate ion were obtained. The blue material turned black on prolonged exposure to the atmosphere. Samples of the reaction mixture when the color was pink or red were unaffected when centrifuged (with a hand centrifuge); likewise Kith samples which had reached the blue-violet stage, In the case of samples which had reached the dull blue stage (after standing a t least 10 minutes) centrifuging caused the settling out of a quantity of blue powder. When saturated sodium chloride solution was added to the reaction mixture after it had stood for one hour, a white flocculent precipitate of calomel formed and the blue color vanished. On adding a sufficient quantity of saturated sodium chloride solution at any stage of the color changes (0 to 10 minutes) the color was changed instantaneously to blue, and a white precipitate of calomel plus this blue material came down; on standing awhile (or centrifuging briefly) the blue materia1 disappeared and only the white precipitate remained. If an insufficient quantity of saturated sodium chloride solution (insufficient to convert all the mercurous ion to calomel) is used in the above tests, the blue material formed remains permanently as a blue suspension or coloration. The above results completely confirm the existing data on the Seil color reaction, and also make possible setting u p the following logical hypothesis as to the mechanism of the color reaction. The nature of the color changes and the behavior of the colored material on centrifuging are definitely indicative of the formation during the reaction of a colloidal dispersion, the particles of which, as time passes, become larger in size and finally produce a coarse suspension whose settling can be hastened by centrifuging, unlike a colloidal dispersion. The addition of saturated sodium chloride solution at any stage of the reaction not only produces calomel but accelerates the
(1) The reaction is accompanied by a series of color changes from pink to blue-violet, and on standing greenish shades develop and a yellow-brown precipitate comes down; (2) on filtration the solution contains mercurous salt as shown by precipitation of calomel on the addition of hydrochloric acid; (3) if sufficient mono- acid is used, a crystalline precipitate of mercurous sulfate is deposited-usually contaminated with colored material; (4)the color reaction appears to be highly specific for the monocarboxylic acid (19)because it is not given by the pure dicarboxylic acid (pyrethrin I1 acid) or by any other substance thus far tested
136
MARCH 15, 1940
A X I L Y T I C I L EDITION
137
TABLEI. EFFECTOF PRECIPITANTS Time Elansed between .Iddin= DGnigBs’ Reanent PreSample andosodium Chloride cipitant Solution Added so.
.Ui,z. 1 2
3 4 7
6
-i
9
10 11
12
Sample Comments
Saturated Budiuin Chloride Solution 0 ( S a C I added before 0 2 Blue suspension formed, paitly addinp Deniges’ settled i n a fely hours reagent) 0 0 3 K h i t e precipitate plus iinely divided white suspension I .0 Same a s T o . 2 0 0.2 White precipitate, fairly well settled in 5 min., plus permanent blue suspension J 0 3 Slight white suspension, mainly a well-settled white precipit a t e ftinged with blue) 1.0 Complete precipitation, all 3 white 0.2 Pame as T o . 4 15 0 .i Same a s KO.5 13 1.0 Same as S o . 6. with better 1.5 settling 30 0.2 Same a s S o . i 0.5 Same a s KO.8 30 1.0 Same as S o . Y 30
1’
3
2‘
J
TO.
cc.
Saturated l l a g n e s i u m Chloride Solution 0.5 K h i t e suspension plus bluewhite pr,ecipitate 1.0 Same a s 1 , but less suspended material
agglomeration of the colloidal particles into their final coarse form (the blue powder). Since the addition of sufficient sodium chloride solution results in the disappearance of the blue coloration, while insufficient sodium chloride (although producing some calomel) allows the coloration to persist, it is concluded that the blue coloration is due to the presence of either metallic mercury or a mercury compound (or both) in dispersion, which is converted to the stable calomel on addition of saturated sodium chloride solution. The fact that the various colors produced are almost identical with those of metallic gold and platinum sols of different particle size confirms the supposition of changing particle size, but is not ground for assuming t h a t the colloidal dispersion is metallic mercury (because colloidal form and chemical constitution are not so closely related). HoTyerer, the change of the blue powder (which gives positive tests for mercury) to black on exposure to air is indicative of the presence of metallic mercury in this blue powder.
3‘ 4’
Time Elansed between Adding D e k g e s ’ Reagent Prea n d Sodium Chloride cipitant Solution Added Com:nents .Ifin. cc. Saturated Xlagnesium Chloride Solution iCont’d 5 1.5 Almost all-white preciDitate plus small a m o u n t of SUIpended material 5 2.0 Same as 3’, but precipitate all white
3‘
3
6‘
3
-,
h,
3 7
9‘
5
10’ 11‘
,
12‘
J
Saturated Zinc Chloride Solution 0.2 Blue and 6-hite precipitate plua blue suiipension 0.5 W i i t r precipitate plus fine blue suspension 1 0 S a m e a s 6’ 1 . .5 All white precipitate plus small amount of white suspension Sarurated Ammonium Chloride Solution 0.1 Bluish Precipitate plus large amount of white suspension 0.6 S a m e as 9‘ 1.5 Blue-white precipitate plus fine white suspension Sodium Chloride Control 1.1 .Ill-white precipitate, clear supernatant liquid
mately 1 cc. of saturated sodium chloride solution per 20 mg. of pyrethrin I is to be recommended (as in the Wilcoxon procedure, I S ) . Summar?
In the mercury reduction method for cletermining pyrethrin I excess filter paper in the titration mixture is to be avoided, as its presence tends to produce difficult and less readable end points. The application of the Wilcoxon method to samples of practically pure chrysanthemum monocarboxylic acid and of a commercial pyrethrum oleoresin indicates that linearity of results exists in the first case, rind a marked nonlinearity exists in the second case, confirming previous work. The color changes observed in the Seil color reaction are concluded to be the result of the formation of a colloidal dispersion of metallic mercury or of some mercury compound USE OF PRECIPITANTS OTHER THANSODIUMCHLORIDE. which, on standing, undergoes spontaneous successive increases in particle size until a coarse suspension (blue-colored) A number of experiments were carried out to determine the is formed. effect, on the appearance of the calomel precipitate, of the adI n the ~17ilcoxonprocedure the use of precipitants other dition of saturated sodium chloride solution a t various stages than sodium chloride has no advantage. However, the of the reaction, and also to investigate the effect of other time elapsed between the addition of DenigM reagent and the chlorides. addition of sodium chloride solution can be reduced to 15 minutes (from the time of 1 hour specified by J17ilcoxon), Procedure. I n each case 5 cc. of a solution of the sodium salt of chrysanthemum monocarboxylic acid (equivalent t o approvided the mixture is centrifuged briefly following the proximately 20 mg. of pyrethrin I) were treated with 3 cc. of sodium chloride addition. DenigBs’ reagent. The precipitants were added after varying time intervals, and the resulting precipitate and solution were hand-centrifuged for 0.5 minute. Results obtained are given in Literature Cited Table I.
Results. The data obtained indicate that sodium chloride is the best precipitant for complete conversion to calomel (all white) and for elimination of fine suspended material. Absence of suspended material is desirable because i t makes filtration easier. The data on the use of different amounts of sodium chloride solution with various elapsed times after addition of Denig&s’ reagent show that (1) undesirable fine white suspensions are produced for elapsed times under 15 minutes; (2) for elapsed times of 15 minutes or more, a completely settled white flocculent precipitate is produced (no advantage accrues evidently for elapsed times of greater than 15 minutes); (3) for best precipitation, the use of approxi-
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
rludiffren, J . pharm. chim., 19, 535-6 (1934). Gnadinger. C., S o a p , 12, No. 2, 97 (1936). Graham, IKD. ENG.CHEM., Anal. .Ed., 7, 222 (1935). Graham, J. J. T., J . Assoc. Oficial Agt. Chem., 21,413-15 (1938). Holaday, D. A.. IND. EKG.CHEJI.,Anal. Ed.. 10, 5 (1938). Hubsher, J., Siiddsut. Apoth.-Ztg., 75, 655-6 1:1935). Jamieson, G. S., “Volumetric Iodate Methods”, New York. Chemical Catalog Co., 1926. Martin, J. T., J . Agr. Sci., 28,111, 456-71 (1938). Oesper, R. E., “Newer Methods cif Volumetric .4nalysis”, New York, D. Van Nostrand Co., 1938. Poole, O., S o a p , 12, No. 2, 97 (1936). Seil, H. A., Ibid., 5 , No. 10, 89 (1934). Soap Blue Book, pp. 184-9, 1939. Wilcoxon, F., Contrib. Boyce Thompson I n s t . , 8, No. 3, 175-81 (1936).
